1.1

Question 1

What is the difference between heat and temperature?

Answer 1

Heat: a method of energy transfer that occurs because of a temperature difference between a system and its surroundings.
Temperature: a measure of the average kinetic energy of the constituent particles in an object or system.
- heat is a process of energy transfer, whereas temperature is a property of the object or system.
- Heat transfers can be determined from temperature changes, because a change in temperature reflects a change in the average kinetic energy of the particles.

Question 2

What is the relationship between temperature and kinetic energy of particles?

Answer 2

Temperature is directly related to the average kinetic energy of particles. As temperature increases, the average kinetic energy of the particles increases and the particles move more rapidly. As temperature decreases, the average kinetic energy decreases.

Question 3

Define kinetic energy, thermal energy, and potential energy in chemical systems.

Answer 3

Kinetic energy: the energy particles possess because of their motion and is given by: 𝐾𝐸=1/2𝑚𝑣².
Thermal energy: the total kinetic energy of all particles in a sample, so it depends on both the temperature and the number of particles present.
Potential energy: the stored energy associated with the position and arrangement of charged particles in a system.
In chemical systems, this mainly refers to the arrangement of nuclei, electrons, chemical bonds, and intermolecular forces.

Question 4

What is the difference between thermal energy and temperature?

Answer 4

Temperature measures the average kinetic energy of particles, whereas thermal energy is the total kinetic energy of all particles in the sample. Therefore, two samples can have the same temperature but different thermal energies if they contain different numbers of particles. This is why a swimming pool and a kettle can have the same temperature but very different amounts of thermal energy.

Question 5

Why is more heat energy needed to raise the temperature of a swimming pool by 1 K than the water in a kettle?

Answer 5

The swimming pool contains many more water molecules than the kettle. Since heat increases the kinetic energy of particles, much more heat is required to increase the average kinetic energy of all the particles in the larger system by the same temperature change.

Question 6

Define energy and state its SI unit.

Answer 6

Energy is the measure of the ability to do work, that is, to move an object against an opposing force. The SI unit of energy is the joule (J). One joule is the energy transferred when a force of 1 N moves an object through 1 m.

Question 7

In what ways can energy be transferred or stored?

Answer 7

Energy can be transferred in different ways including: heat, light, sound, electricity, work. Energy can also be stored in different forms of potential energy, including chemical potential energy.

Question 8

What is work and how is it different from heat transfer?

Answer 8

Work is an ordered transfer of energy that occurs when a force moves an object through a distance. - to move an object against an
opposing force.
Heat transfer is an energy transfer caused by a temperature difference. In thermochemistry, the main focus is on heat transfers accompanying chemical reactions.

Question 9

Why is heat described as a process of energy transfer rather than a stored property?

Answer 9

Heat is described as a process because it refers to energy in transit between a system and its surroundings due to a temperature difference. Once the transfer has occurred, the energy is part of the system’s internal energy rather than remaining as ‘heat’.

Question 10

What happens when heat is transferred to a system?

Answer 10

When heat is transferred to a system: the average kinetic energy of the particles increases, the temperature increases, the kinetic energy becomes more dispersed among the particles.

Question 11

At the same temperature, which atoms have the greater average vibrational speed: lithium or aluminium? Explain.

Answer 11

At the same temperature, particles have the same average kinetic energy, but lighter particles must move faster to have the same kinetic energy. Therefore, lithium atoms have the greater average speed because lithium atoms have a smaller mass than aluminium atoms.

Question 12

Which statements are true? I. The same amount of a gas will have greater thermal energy at a higher temperature. II. Thermal energy can be measured directly with a thermometer. III. A change in the thermal energy of a substance is reflected by a temperature change.

Answer 12

I and III only. I is true because higher temperature means greater average kinetic energy, so the total kinetic energy is greater. II is false because a thermometer measures temperature, not total thermal energy directly. III is true because a change in thermal energy changes the kinetic energy of particles and therefore affects temperature.

Question 13

What is chemical potential energy related to?

Answer 13

Chemical potential energy is related to the arrangement of charged particles in a system, especially nuclei and electrons. It depends on how particles are arranged in atoms, bonds, and intermolecular interactions.

Question 14

What is the relationship between potential energy and stability?

Answer 14

A system with lower potential energy is more stable, because the arrangement of particles is energetically favourable. A system with higher potential energy is less stable and less favourable. Lower potential energy → more stable Higher potential energy → less stable.

Question 15

What does a potential energy vs distance between atoms graph show?

Answer 15

It shows how the potential energy of two atoms changes as the distance between the nuclei changes. At large distances, the atoms do not interact significantly and the potential energy approaches zero. As the atoms approach, attractive forces lower the potential energy. At the minimum point, the potential energy is lowest and the arrangement is most stable. If the atoms move too close together, repulsive forces dominate and the potential energy increases sharply.

Question 16

What does the minimum point on a potential energy vs internuclear distance graph represent?

Answer 16

The minimum point represents the equilibrium bond distance, where attractive and repulsive forces balance. At this point, the potential energy is lowest, so the system is most stable and the bond is formed.

Question 17

Why does potential energy increase from position B to position A on a bonding curve?

Answer 17

As the atoms move closer than the equilibrium bond distance, repulsive electrostatic forces between the positively charged nuclei and between electron clouds increase strongly. These repulsive forces dominate over attraction and cause the potential energy to increase.

Question 18

Describe the change in potential energy required to separate two bonded atoms.

Answer 18

To separate two bonded atoms, energy must be supplied to overcome the electrostatic attractions between the nuclei and the shared electrons in the bond. Therefore, the potential energy of the system increases as the atoms are separated.

Question 19

What does the zero potential energy line represent on a bonding curve?

Answer 19

It represents the energy of two completely separated atoms with negligible interaction between them. Any bonded arrangement has lower potential energy than this separated-atom reference state.

Question 20

According to Coulomb’s law, what combination of charge and distance gives the strongest electrostatic force?

Answer 20

The strongest electrostatic force occurs when the particles have large charges and are very close together. Greater charge magnitude increases the force, and smaller separation distance also increases the force.

Question 21

Define the system in thermochemistry using proper IB wording.

Answer 21

The system is the area of interest or the part of the universe being studied, usually the sample, reaction mixture, or reaction vessel of interest. It includes the substances undergoing the chemical or physical change.
- energy is conserved in chemical reactions

Question 22

Define the surroundings in thermochemistry using proper IB wording.

Answer 22

The surroundings are everything outside the system. In theory, they are the rest of the universe. In practice, the surroundings include the solvent, container, air, and laboratory environment that can exchange energy with the system.

Question 23

What is an open system?

Answer 23

An open system can exchange both matter and energy with its surroundings. Example: a chemical reaction in an open test tube can transfer gases and heat to the surroundings.

Question 24

What is a closed system?

Answer 24

A closed system can exchange energy but not matter with the surroundings.

Question 25

What is an isolated system?

Answer 25

An isolated system can exchange neither matter nor energy with the surroundings.

Question 26

During the combustion of ethanol, which substances are part of the system?

Answer 26

The system includes the reactants and products involved in the reaction, so during the combustion of ethanol it includes: ethanol, oxygen, carbon dioxide, water. If the options given are ethanol, oxygen, and carbon dioxide, the correct answer is I, II and III.

Question 27

State the first law of thermodynamics.

Answer 27

The first law of thermodynamics is the law of conservation of energy. It states that energy cannot be created or destroyed; it can only be transferred or converted from one form to another. Therefore, the total energy of the system and surroundings remains constant. - total energy of the system and surroundings cannot change during a proess, the energy can be exchanged

Question 28

What does conservation of energy mean in chemical reactions?

Answer 28

Although energy can be transferred between the system and the surroundings, the total energy cannot change. Any energy lost by the system is gained by the surroundings, and any energy gained by the system is lost by the surroundings.

Question 29

Which statement is correct? If PE in the system decreases, PE in the surroundings must increase / TE in the surroundings must increase / PE in the surroundings must increase / TE in the system must increase.

Answer 29

If the potential energy of the system decreases, the thermal energy of the surroundings increases.

Question 30

What is enthalpy (H)?

Answer 30

Enthalpy is the chemical potential energy of a system associated with the arrangement of atoms, bonds, and intermolecular forces., associated with the arrangement of atoms, chemical bonds, and intermolecular forces. A system acts like a reservoir of chemical potential energy.

Question 31

What does ΔH represent?

Answer 31

ΔH represents the change in enthalpy during a reaction. It is the difference in enthalpy between products and reactants and corresponds to the heat transferred during the reaction at constant pressure.

Question 32

What does a positive ΔH mean?

Answer 32

A positive ΔH means heat is added to the system from the surroundings. The enthalpy of the system increases and the process is endothermic.

Question 33

What does a negative ΔH mean?

Answer 33

A negative ΔH means heat is released from the system to the surroundings. The enthalpy of the system decreases and the process is exothermic.

Question 34

Describe the energy changes in an exothermic reaction.

Answer 34

In an exothermic reaction: heat is transferred from the system to the surroundings, the temperature of the surroundings increases, the products have lower potential energy than the reactants, the products are therefore more stable, ΔH is negative.

Question 35

Describe the energy changes in an endothermic reaction.

Answer 35

In an endothermic reaction: heat is transferred from the surroundings to the system, the temperature of the surroundings decreases, the products have higher potential energy than the reactants, the products are therefore less stable, ΔH is positive. - products have more stored potential energy than the reactants and enthalpy change is positive

Question 36

Why does the temperature of water increase during an exothermic reaction in a calorimeter?

Answer 36

Because the reaction releases heat from the system to the surroundings, and the water in the calorimeter is part of the surroundings. The water gains thermal energy, so its temperature increases.

Question 37

Why does the temperature of water decrease during an endothermic reaction in a calorimeter?

Answer 37

Because the reaction absorbs heat from the surroundings, and the water in the calorimeter loses thermal energy. As a result, the water temperature decreases.

Question 38

Define heat capacity.

Answer 38

Heat capacity is the amount of heat required to raise the temperature of an object by 1 K. It depends on the mass and composition of the object.

Question 39

Define specific heat capacity.

Answer 39

Specific heat capacity is the amount of heat energy required to raise the temperature of 1 g of a substance by 1 K (or 1 °C).

Question 40

State the calorimetry equation used to calculate heat transfer.

Answer 40

Q=mcΔT where: Q = heat transferred, m = mass of substance, c = specific heat capacity, ΔT = temperature change.

Question 41

What specific heat capacity of water is used in IB Chemistry calculations?

Answer 41

4.18 J g−1 K−1

Question 42

How is the enthalpy change per mole of reaction calculated from calorimetry data?

Answer 42

ΔH=−Q/n where Q is the heat transferred and n is the number of moles reacting. The negative sign is used because the heat change calculated from the surroundings has the opposite sign to the enthalpy change of the system.

Question 43

What are the units of enthalpy change?

Answer 43

kJ mol−1

Question 44

How are heat transfer problems between two objects solved?

Answer 44

They are solved using the principle of conservation of energy: heat lost by hot object = heat gained by cold object. Using: Q=mcΔT for each object, the setup is: m1c1(Thot,initial−Tf)=m2c2(Tf−Tcold,initial). If both objects are made of the same material, the specific heat capacity cancels. The final temperature must always lie between the two initial temperatures.

Question 45

Explain what happens when a large hot iron cube is placed in contact with a smaller cold iron cube, assuming both objects are insulated from everything else.

Answer 45

Heat is transferred from the hotter iron cube to the colder iron cube because of the temperature difference. The hot cube decreases in temperature, and the cold cube increases in temperature, until both reach the same temperature, known as thermal equilibrium. Because the cubes are insulated from all other objects, the energy lost by the hotter cube is equal to the energy gained by the colder cube.

Question 46

What is thermal equilibrium?

Answer 46

Thermal equilibrium is the state in which two objects in thermal contact are at the same temperature, so there is no net heat transfer between them.

Question 47

How do temperature changes compare in kelvin and degrees Celsius?

Answer 47

A temperature change of 1 K is equal to a temperature change of 1 °C. Example: 310K−300K=10K=10°C.

Question 48

Which statements correctly describe an exothermic reaction?

Answer 48

For an exothermic reaction: potential energy in the system decreases, thermal energy in the surroundings increases, total energy in the universe remains constant.

Question 49

Which statements are true for an endothermic reaction? i. The reactants are more stable than the products. ii. Thermal energy in the surroundings decreases. iii. Potential energy in the system increases.

Answer 49

i, ii and iii are all true.

Question 50

Compared to two separate chlorine atoms, a Cl2 molecule has what kind of potential energy and stability?

Answer 50

A Cl2 molecule has lower potential energy and is relatively stable, because the bond formation lowers the energy of the system.

Question 51

How are reactions classified as exothermic or endothermic?

Answer 51

Reactions are classified based on the direction of energy transfer between the system and the surroundings. Exothermic reaction: heat is transferred from the system to the surroundings. Endothermic reaction: heat is transferred from the surroundings to the system.

Question 52

Why does the enthalpy of the system change during a chemical reaction?

Answer 52

The enthalpy of the system changes because chemical bonds and intermolecular forces are broken and formed during the reaction. These changes alter the chemical potential energy stored in the system, producing an overall enthalpy change Δ𝐻.

Question 53

Why are most chemical reactions exothermic?

Answer 53

Most chemical reactions are exothermic because more energy is released when new bonds form in the products than the energy required to break bonds in the reactants. Examples include: Most combustion reactions, All neutralization reactions. - reactants have more stored potential energy than products

Question 54

What happens to potential energy in exothermic and endothermic reactions?

Answer 54

Exothermic reaction: Potential energy of the products is lower than the reactants. Energy is released to the surroundings Δ𝐻<0. Endothermic reaction: Potential energy of the products is higher than the reactants. Energy is absorbed from the surroundings Δ𝐻>0.

Question 55

What do the axes represent on an energy profile diagram?

Answer 55

y-axis: potential energy (enthalpy, 𝐻). x-axis: reaction coordinate (progress of the reaction). The diagram shows how the potential energy of the system changes during a reaction.

Question 56

What does the vertical difference between reactants and products represent on an energy profile diagram?

Answer 56

The vertical difference represents the enthalpy change of the reaction (Δ𝐻). Δ𝐻=𝐻𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠−𝐻𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠. Exothermic: products lower than reactants. Endothermic: products higher than reactants.

Question 57

What happens to the solvent temperature during an exothermic reaction in aqueous solution?

Answer 57

Heat released by the reaction is transferred to the water (solvent). The average kinetic energy of water molecules increases. The temperature of the solution rises. Heat is then transferred from the water to the wider surroundings until equilibrium is reached.

Question 58

What happens to the solvent temperature during an endothermic reaction in aqueous solution?

Answer 58

Heat required for the reaction is absorbed from the water (solvent). The average kinetic energy of water molecules decreases. The temperature of the solution decreases. Heat is eventually transferred from the wider surroundings back to the water.

Question 59

Why are temperature changes in solution useful for measuring enthalpy changes?

Answer 59

When reactions occur in aqueous solution, the water acts as the surroundings. Any heat released or absorbed by the reaction causes a change in the temperature of the water, which can be measured to determine the enthalpy change.

Question 60

What experimental observations indicate that a reaction is exothermic?

Answer 60

Temperature of the surroundings increases. The container or solution feels warm. Heat may be released visibly (e.g., light or flame in combustion).

Question 61

What experimental observations indicate that a reaction is endothermic?

Answer 61

Temperature of the surroundings decreases. The container or solution feels cold. In extreme cases frost may form or nearby water may freeze. Example: ammonium thiocyanate reacting with barium hydroxide.

Question 62

Why can some endothermic reactions cause water outside the container to freeze?

Answer 62

Some reactions absorb large amounts of heat from the surroundings. If the heat absorbed is large enough, it can remove thermal energy from nearby water, causing it to freeze.

Question 63

What is the thermite reaction and why is it strongly exothermic?

Answer 63

Thermite reaction: 𝐹𝑒2𝑂3 + 2𝐴𝑙 → 2𝐹𝑒 + 𝐴𝑙2𝑂3. This reaction releases a large amount of heat because very stable bonds form in aluminium oxide, making the reaction strongly exothermic.

Question 64

Why is heat described as a process of energy transfer rather than a substance?

Answer 64

Historically heat was thought to be a liquid called caloric. Modern thermodynamics shows that heat is not a substance, but rather a process of energy transfer caused by a temperature difference.

Question 65

What happens to the average kinetic energy of molecules when a liquid evaporates?

Answer 65

During evaporation, the highest-energy molecules escape from the liquid first. This decreases the average kinetic energy of the remaining molecules, causing the temperature of the liquid to decrease.

Question 66

What determines whether a reaction is exothermic or endothermic?

Answer 66

The relative energetic stability of reactants and products determines whether a reaction is exothermic or endothermic. If the products have lower potential energy than the reactants, the reaction is exothermic. If the products have higher potential energy than the reactants, the reaction is endothermic. - depends on direction of energy transfer between syste and surroundings

Question 67

What is meant by energetic stability in chemistry?

Answer 67

Energetic stability refers to the potential energy of a system. A system with lower potential energy is more stable. A system with higher potential energy is less stable. Chemical reactions often proceed in the direction that reduces potential energy and increases stability.

Question 68

Why do many reactions occur spontaneously in the direction of lower energy?

Answer 68

Systems tend to move toward lower potential energy states because these states are more stable. This is similar to a physical system such as a ball rolling downhill — the lowest energy state is the most stable configuration.

Question 69

Why are most combustion reactions exothermic?

Answer 69

Most combustion reactions are exothermic because: The energy released when strong bonds form in the products (such as CO₂ and H₂O) is greater than the energy required to break bonds in the reactants. This leads to an overall decrease in potential energy and a negative ΔH.

Question 70

What does it mean that stability is a relative concept?

Answer 70

Stability is relative to a particular reaction or comparison. For example: Hydrogen peroxide is stable relative to hydrogen and oxygen, but unstable relative to its decomposition products (water and oxygen).

Question 71

What does Hess’s law imply about the enthalpy changes shown in multi-step energy diagrams?

Answer 71

Hess’s law states that the total enthalpy change for a reaction is independent of the pathway taken. Therefore: ΔH_total = ΔH1 + ΔH2 The overall enthalpy change equals the sum of the enthalpy changes of the intermediate steps.

Question 72

Why is the sign of ΔH not always a complete guide to whether a reaction will occur?

Answer 72

Although reactions tend to proceed toward lower potential energy, some endothermic reactions can occur spontaneously. This can happen when there is a large increase in energy dispersal (entropy). The sign of AH is a guide for the likely direction of change but it is not a complete guide other limitation of using AH values as a guide to change is that it does not indicate anything about the rate of reaction.

Question 73

Why can some highly endothermic reactions occur spontaneously?

Answer 73

Some endothermic reactions occur because they greatly increase the dispersal of matter and energy. For example, reactions producing multiple gas molecules increase disorder and energy dispersal, which can drive the reaction forward.

Question 74

Why does ΔH not indicate how fast a reaction occurs?

Answer 74

ΔH only describes the energy difference between reactants and products, not the rate of the reaction. Some reactions that are energetically favourable occur very slowly because they require an initial energy input to start. - Some reactions, which should take place on energetic grounds, do not occur at a noticeable rate in practice as the reactants need to be given some initial energy to react.

Question 75

What is activation energy?

Answer 75

Activation energy E_a is the minimum kinetic energy that reactant particles must have for a reaction to occur. It represents the energy barrier that must be overcome for bonds in reactants to begin breaking.

Question 76

Why is activation energy required for chemical reactions?

Answer 76

Activation energy is required because existing bonds in the reactants must first be broken before new bonds in the products can form. Breaking these bonds requires an initial input of energy.

Question 77

How does a match start the combustion of methane?

Answer 77

A match provides the activation energy needed to start the reaction. The heat supplies sufficient energy for methane and oxygen molecules to break initial bonds and begin forming new bonds in the products.

Question 78

What do energy profile diagrams show?

Answer 78

Energy profile diagrams show: the potential energy of the system how energy changes during the reaction the activation energy the enthalpy change (ΔH). The axes are: y-axis: potential energy (enthalpy) x-axis: reaction coordinate.

Question 79

Why does diamond not spontaneously convert into graphite even though graphite is more stable?

Answer 79

Diamond is energetically unstable relative to graphite, but the conversion requires breaking strong covalent bonds. Because the activation energy is extremely high, the reaction occurs so slowly that it is not observable under normal conditions.

Question 80

What information does a reaction energy profile diagram show and how are its axes labelled?

Answer 80

A reaction energy profile diagram shows how the potential energy (enthalpy) of a system changes during a chemical reaction. Axes: y-axis: potential energy / enthalpy (H) x-axis: reaction coordinate (progress of the reaction) The diagram shows: reactants products activation energy (Ea) enthalpy change (ΔH) The vertical difference between reactants and products represents ΔH.

Question 81

How do energy profile diagrams differ for exothermic and endothermic reactions?

Answer 81

Exothermic reaction Products have lower potential energy than reactants Energy is released to the surroundings ΔH is negative Endothermic reaction Products have higher potential energy than reactants Energy is absorbed from the surroundings ΔH is positive Thus the relative stability of reactants and products determines the sign of ΔH.

Question 82

What is activation energy and why is it required for a chemical reaction to occur?

Answer 82

Activation energy (Ea) is the minimum energy required for reactant particles to reach the transition state and begin reacting. On an energy profile diagram: Ea is the energy difference between the reactants and the peak of the curve. Activation energy is required because: existing bonds in reactants must first be broken before new bonds form even energetically favourable reactions will not occur unless this energy barrier is overcome This explains why some reactions with negative ΔH can still occur very slowly.

Question 83

How can an energetically unfavourable endothermic reaction still occur spontaneously?

Answer 83

An endothermic reaction may still occur spontaneously if the energetically unfavourable increase in enthalpy (ΔH > 0) is compensated by a large increase in the dispersal of energy and matter. This often happens when a reaction produces more particles (especially gases), which increases the distribution of energy among particles and the surroundings. Thus, although the reaction absorbs heat, the greater dispersal of energy can drive the reaction forward.

Question 84

How does activation energy differ for exothermic and endothermic reactions on an energy profile diagram?

Answer 84

Exothermic reaction Products have lower potential energy than reactants Activation energy for the forward reaction is smaller Activation energy for the reverse reaction is larger ΔH<0 Endothermic reaction Products have higher potential energy than reactants Activation energy for the forward reaction is larger Activation energy for the reverse reaction is smaller ΔH>0 The difference between the forward and reverse activation energies corresponds to the enthalpy change of the reaction (ΔH).

Question 85

standard enthalpy change of a reaction (ΔH°)

Answer 85

The standard enthalpy change of reaction, ΔH°, is the heat transferred when a reaction occurs at constant pressure under standard conditions, with all substances in their standard states. Standard conditions: - Pressure: 100 kPa - Concentration of solutions: 1 mol dm⁻³ - Substances: present in their standard states - Standard state: The pure form of a substance under standard conditions, usually specified at: 298 K (25 °C) 1.00 × 10⁵ Pa - The units of enthalpy change are kJ mol⁻¹. It is essential to include state symbols in thermochemical equations because enthalpy changes depend on the physical states of the reactants and products.

Question 86

thermochemical equation

Answer 86

A thermochemical equation is a chemical equation that shows: the balanced chemical reaction the physical states of reactants and products the enthalpy change (ΔH) associated with the reaction Example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = −890 kJ mol⁻¹ Meaning: 1 mole of methane reacts with 2 moles of oxygen forming 1 mole CO₂ and 2 moles H₂O 890 kJ of heat energy is released Photosynthesis example: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g) ΔH = +2802.5 kJ mol⁻¹ This means 2802.5 kJ of energy is absorbed when one mole of glucose is produced under standard conditions.

Question 87

enthalpy changes determined experimentally from temperature changes

Answer 87

Enthalpy changes can be determined by measuring the temperature change of a known mass of a substance, usually water, during a reaction. The heat transferred is calculated using: 𝑞 = 𝑚𝑐Δ𝑇 Where: q = heat transferred (J) m = mass of substance (g) c = specific heat capacity (J g⁻¹ K⁻¹) ΔT = temperature change (K or °C) For water: 𝑐 = 4.18 J g⁻¹ K⁻¹ The enthalpy change per mole of reaction is then calculated using: Δ𝐻 = −𝑞/𝑛 The negative sign is used because the heat absorbed by the surroundings is equal and opposite to the heat released by the reaction.

Question 88

factors determine the temperature change of a substance when heat is added

Answer 88

The increase in temperature (ΔT) depends on: - the mass (m) of the substance - the amount of heat added (q) - the nature of the substance The specific heat capacity (c) is defined as: the heat required to raise the temperature of 1 g of a substance by 1 K. - Different substances have different specific heat capacities because the number and mass of particles in a given mass differ, affecting how energy is distributed among them.

Question 89

enthalpy change of combustion measured experimentally

Answer 89

The enthalpy change of combustion of a fuel (e.g. ethanol) can be measured using a metal calorimeter, typically made of copper. Procedure: Burn a known mass of the fuel. The heat released warms a known mass of water in the calorimeter. Measure the temperature increase of the water. Calculate heat absorbed by the water using: 𝑞 = 𝑚𝑐Δ𝑇 Calculate the enthalpy change per mole of fuel burned. Copper is used because it is a good conductor of heat, allowing efficient transfer of heat from the flame to the water.

Question 90

experimental enthalpy values often different from literature values

Answer 90

Experimental values are often less exothermic (less negative) than literature values due to several sources of error: - heat lost to the surroundings - heat absorbed by the calorimeter - incomplete combustion of the fuel - the experiment may not be performed under standard conditions Example: Experimental value for ethanol combustion ≈ −1100 kJ mol⁻¹ Literature value ≈ −1367 kJ mol⁻¹

Question 91

enthalpy changes of reactions in solution measured

Answer 91

Reactions in solution are measured using a polystyrene cup calorimeter (coffee-cup calorimeter). Features: the polystyrene cup acts as a thermal insulator it minimizes heat loss to the surroundings Procedure: Measure the initial temperature of the solution. Mix the reactants. Record the temperature change. Calculate heat transfer using: 𝑞 = 𝑚𝑐Δ𝑇 The molar enthalpy change is then calculated using the limiting reactant.

Question 92

recorded maximum temperature in calorimetry experiments often lower than the true value

Answer 92

Heat loss begins as soon as the temperature of the system rises above the temperature of the surroundings. Therefore: the recorded maximum temperature is lower than the true temperature change - To correct for this: the cooling curve is extrapolated back to the moment the reaction started This provides a better estimate of the true temperature change (ΔT).

Question 93

assumptions commonly made in calorimetry calculations

Answer 93

Typical assumptions include: - No heat loss from the system - All heat released or absorbed by the reaction is transferred to the water - The solution behaves like water - Density of dilute solutions ≈ 1.00 g cm⁻³ Under these assumptions: Δ𝐻𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 = −Δ𝐻𝑤𝑎𝑡𝑒𝑟

Question 94

relationship between enthalpy changes of the system and surroundings

Answer 94

In calorimetry: Δ𝐻𝑠𝑦𝑠𝑡𝑒𝑚 + Δ𝐻𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔 = 0 Therefore: Δ𝐻𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 = −Δ𝐻𝑤𝑎𝑡𝑒𝑟 Meaning: Exothermic reaction: water temperature increases Endothermic reaction: water temperature decreases

Question 95

Why is the measured maximum temperature lower than the true maximum in calorimetry?

Answer 95

Because heat loss to the surroundings begins as soon as the temperature of the reacting mixture rises above that of the surroundings. Therefore the maximum recorded temperature is lower than the temperature that would have been reached in a perfectly insulated system.

Question 96

How is heat loss corrected for in a calorimetry temperature–time graph?

Answer 96

The cooling part of the graph is extrapolated back to the time the reaction started. This estimates the temperature that would have been reached if no heat had been lost to the surroundings.

Question 97

State the main assumptions made in calorimetry calculations.

Answer 97

no heat is lost to the surroundings all heat released/absorbed by the reaction is transferred to the water/solution the calorimeter absorbs negligible heat unless included the solution behaves like water density of dilute solution is 1.00 g cm⁻3 specific heat capacity is 4.18 J g⁻1 K⁻1 the reaction goes to completion

Question 98

Why are experimental enthalpy of combustion values usually less exothermic than literature values?

Answer 98

Because some heat is lost to the surroundings, some heat is absorbed by the calorimeter, combustion may be incomplete, some fuel may evaporate, and the experiment may not be under standard conditions.

Question 99

What is the formula for percentage error?

Answer 99

% error = |experimental value − accepted value| / |accepted value| × 100

Question 100

Why is a negative sign used in ΔH = −Q/n?

Answer 100

Because the heat change measured for the surroundings is opposite in sign to the enthalpy change of the system. If the surroundings gain heat, the system loses heat, so ΔH is negative.

Question 101

Why is polystyrene used in a solution calorimeter?

Answer 101

Because polystyrene is a good thermal insulator and has a low heat capacity, so it minimizes heat exchange with the surroundings.

Question 102

Why is copper used in a combustion calorimeter?

Answer 102

Because copper is a good conductor of heat, allowing efficient transfer of heat from the flame to the water.

Question 103

What must always be included in a thermochemical equation?

Answer 103

The balanced chemical equation, the physical states of reactants and products, and the enthalpy change, ΔH.

Question 104

What can be deduced if the temperature of the surroundings increases during a reaction?

Answer 104

The surroundings gain thermal energy, the system releases heat, the reaction is exothermic, ΔH is negative, and the products have lower potential energy than the reactants.

Question 105

What can be deduced if the temperature of the surroundings decreases during a reaction?

Answer 105

The surroundings lose thermal energy, the system absorbs heat, the reaction is endothermic, ΔH is positive, and the products have higher potential energy than the reactants.

Question 106

Why is 1 K temperature change equal to 1 °C temperature change?

Answer 106

Because the kelvin and Celsius scales have the same size unit; only their zero points differ.

Question 107

Why are experimental enthalpy values often less exothermic than literature values?

Answer 107

Because heat is lost to the surroundings during the experiment. This reduces the measured temperature change, leading to an underestimated heat transfer and therefore a less negative enthalpy change. - for simplifying assumptions that make heat measurements practical and accurate. When solutions are dilute, their physical properties can be safely assumed to be equivalent to that of the pure solvent (usually water)

Question 108

Why is the equation ΔH = −Q/n used in calorimetry?

Answer 108

Because the heat absorbed by the surroundings (water) is equal in magnitude but opposite in sign to the heat released or absorbed by the reaction.

Question 109

What assumptions are made in calorimetry calculations?

Answer 109

no heat is lost to the surroundings all heat released/absorbed by the reaction is transferred to the water the solution behaves like water density of dilute aqueous solution ≈ 1.00 g cm⁻³ Sometimes also assumed: specific heat capacity = 4.18 J g⁻¹ K⁻¹

Question 110

Why is the solution assumed to be dilute in calorimetry calculations?

Answer 110

If the solution is dilute, most of its mass is water. Therefore the specific heat capacity and density of the solution can be approximated as those of water (c=4.18 J g⁻1 K⁻1, ρ≈1.00 g cm⁻3), allowing the heat transfer to be calculated using q=mcΔT.

Question 111

transition state

Answer 111

The transition state is the highest-energy point along the reaction pathway, where bonds are partially broken and partially formed. It is unstable and cannot be isolated.

Question 112

activation energy in terms of the transition state

Answer 112

Activation energy is the minimum energy required for reactant particles to reach the transition state and begin reacting.

Question 113

activation energy in collision theory terms

Answer 113

Activation energy is the minimum amount of energy needed for reactant particles to have a successful collision and start the reaction.

Question 114

Why can the transition state not be isolated?

Answer 114

Because it is an unstable, very short-lived arrangement of atoms at the highest-energy point of the reaction pathway.

Question 115

What does an energy profile show?

Answer 115

An energy profile shows how the potential energy of a system changes during a reaction. It includes the reactants, products, activation energy, enthalpy change, and the transition state.

Question 116

Why do exothermic reactions usually have a lower activation energy than endothermic reactions on an energy profile?

Answer 116

In an exothermic reaction, the reactants are higher in energy and therefore closer in energy to the transition state, so the forward activation energy is smaller. In an endothermic reaction, the reactants are lower in energy and further from the transition state, so the forward activation energy is larger.

Question 117

What assumption is made when using a temperature correction graph in calorimetry?

Answer 117

It is assumed that the rate of cooling is constant, so the cooling line can be extrapolated back to the time the reaction started.

Question 118

What assumption is made about the calorimeter container in simple calorimetry calculations?

Answer 118

The specific heat capacity of the container is ignored, so all the measured heat transfer is assumed to affect only the water or solution.

Question 119

What is meant by assuming the reaction is complete in calorimetry?

Answer 119

It means all of the limiting reactant is assumed to react fully, so the calculated number of moles used in the enthalpy calculation is correct.

Question 120

What are the main sources of error in simple combustion calorimetry?

Answer 120

The main sources of error are heat loss to the surroundings and incomplete combustion of the fuel.

Question 121

How can heat loss be reduced in a combustion calorimetry experiment?

Answer 121

Heat loss can be reduced by placing the calorimeter close to the flame, using a lid, and shielding the apparatus from draughts.