Energetics

Question 1

Define standard enthalpy change of reaction

Answer 1

The energy change when molar quantities of reactants as specified by the chemical equation react to form products, with all reactants and products in their standard states, at a specified temperature,
usually 298 K.

Question 2

Define standard enthalpy change of formation

Answer 2

The energy change when 1 mole of a substance in its standard state is formed from its constituent elements in their standard states, at a specified temperature, usually 298 K.

Question 3

Define standard enthalpy change of combustion

Answer 3

The energy released when 1 mole of a substance is completely burnt in excess oxygen, with all reactants and products in their standard states, at a specified temperature, usually 298 K.

Question 4

Define standard enthalpy change of neutralisation

Answer 4

The energy released when 1 mole of water is formed in the neutralisation reaction between an acid and a base, all in their
standard states, at a specified temperature, usually 298 K.

Question 5

Define bond energy

Answer 5

Bond energy is the average energy required to break 1 mole of a covalent bond between two atoms in the gaseous state.

Question 6

Define standard enthalpy change of atomisation of element

Answer 6

The energy required to form 1 mole of gaseous atoms from the element, all in their standard states, at a specified temperature, usually 298 K.

Question 7

Define standard enthalpy change of atomisation of compound

Answer 7

The energy required to form gaseous atoms from the 1 mole of the compound, all in their standard states, at a specified temperature, usually 298 K.

Question 8

Define lattice energy

Answer 8

The energy released when 1 mole of solid ionic compound is formed from its constituent gaseous ions.

Question 9

Define first ionisation energy

Answer 9

The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cations.

Question 10

Define first electron affinity

Answer 10

The first electron affinity is the energy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous anions.

Question 11

Define standard enthalpy change of hydration

Answer 11

The energy released when 1 mole of free gaseous ions in its standard state is dissolved in water to give a solution of infinite dilution, at a specified temperature, usually 298 K.

Question 12

Define standard enthalpy change of solution

Answer 12

The energy change when 1 mole of substance in its standard state is completely dissolved in a solvent to give a solution of infinite dilution, at a specified temperature, usually 298 K.

Question 13

Formula of heat change

Answer 13

q = - (mass)(heat change)(specific heat capacity)
q = - (heat capacity)(heat change)

Question 14

Define standard state

Answer 14

The standard state of a substance at a specified temperature is its pure form at 1 bar (=10^5 Pa)*, and 1 mol dm^–3 for solutions.

Question 15

What is the activation energy of a reaction?

Answer 15

It is the minimum energy which the reacting particles must possess in order to form the products.

Question 16

Explain the energetic stability og a substance relative to its constituent elements using the signs of its standard enthalpy change of formation.

Answer 16

If the value is positive, the compound is less stable than its constituent elements.
If the value is negative, the compound is more stable than its constituent elements.

Question 17

Why are weak acids and bases have a slightly less exothermic standard enthalpy change of neutralisation?

Answer 17

This is because they do not dissociate completely in dilute aqueous solution; hence, during neutralisation, energy is absorbed to dissociate the undissociated weak acid or base to form H+ or OH-, respectively, thus less energy is released, and the value is less exothermic.

Question 18

State the value of the enthalpy change of combustion of O2, CO2 and H2O and any relationships between their combustion and formation values.

Answer 18

Enthalpy change of O2, CO2 and H2O all = 0 (as they are the products or reactants of combustion)
Enthalpy change of combustion of C = Enthalpy change of formation of CO2
Enthalpy change of combustion of H2 = Enthalpy change of formation of H2O

Question 19

Define specific heat capacity.

Answer 19

It is defined as the amount of heat required to raise the temperature of 1g of the substance by 1K.

Question 20

Define heat capacity.

Answer 20

It is defined as the amount of heat required to raise the temperature of a certain mass of the substance by 1K.

Question 21

State the assumptions when calculating heat change.

Answer 21

To ensure accurate results, all the heat changes of the reaction must be used to raise (or lower) the temperature of the solution
1. Negligible heat loss (or heat gain) from the surrounding air
2. Negligible heat loss (or heat gain) from calorimeter (heat capacity of calorimeter omitted)
3. Density of solution is the same as that of water (1.oo gcm-3) since the solution is dilute

Question 22

What does Hess’ Law state?

Answer 22

Hess’ Law states that the enthalpy change of a reaction is determined only by the initial and final states and is independent of the reaction pathway taken.

Question 23

State the equation for a given reaction when we know the combustion data of all the reactants and products.

Answer 23

Enthalpy change of reaction = Sum of enthalpy change of combustion of reactants - Sum of enthalpy change of combustion of products

Question 24

State the equation for a given reaction when we know the formation data of all the reactants and products

Answer 24

Enthalpy change of reaction = Sum of enthalpy change of formation of products - Sum of enthalpy change of formation of reactants

Question 25

What is the difference between atomisation and vaporisation?

Answer 25

Vaporisation only brings an element or a compound from the liquid to gaseous state. But for substance with simple molecule structures, vaporisation does not break any covalent bonds only overcome intermolecular forces of attraction.

Question 26

What is the relationship between enthalpy change of atomisation of gaseous diatomic molecules and the bond energy of that molecule?

Answer 26

Enthalpy change of atomisation = 1/2 of bond energy

Question 27

State the factors affecting lattice energy.

Answer 27

1. The charges on the ions 2. The sizes of ions or inter ionic distance With the equation: |LE| = |q+ x q-/ r+ + r-|

Question 28

Explain why first electron affinity is exothermic and second electron affinity is endothermic.

Answer 28

1at EA is usually exothermic as more energy is released form the attraction between the nucleus and electron added, than that required to overcome the repulsion between the electron added and those electrons already present in the atom. 2nd EA in the other hand is usually endothermic as more energy is required to overcome the repulsion between the added electron and the electrons already present in the monoanion compared to the energy released from atraction between the nucleus and electron added.

Question 29

What affects the standard enthalpy change of hydration?

Answer 29

The magnitude of the enthalpy change of hydration of an ion depends on its charge density. The higher the charge density of the ion, the stronger the ion dipole interaction and enthalpy change of hydration will be more exothermic.

Question 30

What is the formula linking enthalpy change of solution, lattice energy and enthalpy change of hydration?

Answer 30

Enthalpy change of solution = -LE + Enthalpy change of hydration cation + anion OR |LE| - |Enthalpy change of hydration of cation + anion|

Question 31

What is a spontaneous process?

Answer 31

It is one that takes place naturally in the direction states without a need for continuous input of energy form outside the system.

Question 32

Define entropy.

Answer 32

Entropy, S, is a measure of the randomness or disorder in a system, reflected in the number of ways that the energy of a system can be distributed through the motion of its particles. Its unit of Jmol-1K-1

Question 33

How does temperature affect the entropy of a chemical system?

Answer 33

As temperature increases, the average kinetic energy of the particles and the range of energies increase. This is reflected in the broadening of the curve (of a Maxwell Boltzmann energy distribution curve) as the energy distribution of the particles become more spread out. There are hence more ways in which energy can be distributed among the particles in the hotter system, hence entropy increases.

Question 34

How does change in phase affect entropy?

Answer 34

When a solid melts or sublimes, or when a liquid vaporises, the particles in the final state move more freely and are more disordered than in the initial state. This increases the number of ways that the particles and the energy can be distributed. Hence there is an increase in entropy.

Question 35

How does the change in the number of particles (esp for gaseous systems) affect entropy?

Answer 35

When a chemical reaction results in an increase in teh number of gas particles, the number of ways that the particles and the energy can be distributed increases, hence there is an increase in entropy.

Question 36

How does mixing of particles affect entropy?

Answer 36

(For mixing of gas particles) As the volume available for each gas increases, there are more ways to distribute the oaroficles and their energy, hence entropy increases. If they are mixed at constant volume, entropy does not change as the volume available to distribute the particles of each gas is the same. (For mixing of liquid particles) When liquids with similar polarities are mixed together, entropy increases as total volume increases hence there are more ways to distribute the particles and their energy. (For dissolution of an ionic solid) Entropy increases because ions in the solid are free to move in the solution. But entropy also decreases as the water molecules that were originally free to move become more restricted in motion as they arrange themselves around the ions. But the first factor is more significant the overall entropy change in positive.

Question 37

State the formula for Gibbs free energy change.

Answer 37

Gibbs free energy change = Heat change - T(Change in entropy)

Question 38

What does the value of Gibbs free energy change represent?

Answer 38

When it is negative , the reaction is thermodynamicslly feasible and is spontaneous or exergonic. When it is zero, the system is at equilibrium and there is no net reaction in the forward or backward direction. When it is positive, the reaction is thermodynamically not feasible and said to be nonspontaneous or endergonic and is spontaneous in the reverse direction.