C5 - Energy changes

Question 1

Why do Chemical reactions occur?

Answer 1

• Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons. This is done by chemical bonding (we have already seen ionic and covalent bonding) where old bonds are broken, and new bonds are formed
• This process involves the transfer of energy into and out of reaction mixtures. The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy
• In the majority of reactions, the energy is in the form of heat energy, although sometimes other types of energy are produced such as light or sound

Question 2

What is A Exothermic Reactions?

Give examples of the Exo Reactions and their uses

Answer 2

• In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases.
  e. g. Combustion, oxidation, and neutralisation
• Hand warmers and Self-heating

Question 3

What are Endothermic Reactions?

Give examples of endo reactions and their uses

Answer 3

• In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases
• e.g. Electrolysis, thermal decomposition reactions and photosynthesis
• Sports injury treatment often use cold packs

Question 4

Endo vs Exo

Reactions

Answer 4

Exothermic Vs. Endothermic Reactions

• The changes in heat content can be measured with a thermometer
• Note that the overall amount of energy does not change as energy is conserved in reactions. This means that it cannot be created or destroyed but it can be transferred
• So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred
• The following are some examples of heat changes in reactions
  
  • Neutralisation reactions:
    
    • These always give energy out
  • Displacement reactions:
    
    • These can either take energy in or give it out
  • Combustion reactions:
    
    • These always give energy out
• In summary:
  
  • In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls
  • In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases

Question 5

What is Activation Energy and how does it differ between reactions?

Answer 5

• Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy. Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• Different reactions have different activation energies, depending on the chemical identities involved

Question 6

What is a reaction profile?

What do the arrows indicate?

What does the difference between the products and reactants in height mean?

Answer 6

• Reaction profiles (sometimes called energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
• The energy of the reactants are on the y-axis and the reaction pathway is shown on the x-axis
• The difference in height between the energy of reactants and products represents the overall energy change of a reaction

Question 7

How are exothermic reactions depicted on a reaction profile?

Answer 7

.

• Energy is given out in exothermic reactions
• The energy of the products will be lower than the energy of the reactants, so the change in energy is negative

Question 8

How are Endothermic reactions depicted on a reaction profile?

Answer 8

• Energy is taken in endothermic reactions
• The energy of the products will be higher than the energy of the reactants, so the change in energy is positive

Question 9

What happens when bonds are broken?

Answer 9

Energy is transferred when bonds are broken or are formed.

During a chemical reaction:

• bonds in the reactants are broken
• new bonds are made in the products

The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.

A reaction is:

• exothermic if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
• endothermic if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants

Question 10

Is Endothermic reaction a positive or negative change in energy and why?

Answer 10

Endothermic

• If more energy is absorbed than is released, this reaction is endothermic
• More energy is required to break the bonds than that gained from making the new bonds
• The change in energy is positive since the reactants have more energy than the products
• Therefore an endothermic reaction has a positive change in energy

Question 11

Is Exothermic reaction a positive or negative change in energy and why?

Answer 11

Exothermic

• If more energy is released than is absorbed, then the reaction is exothermic
• More energy is released when new bonds are formed than energy required to break the bonds in the reactants
• The change in energy is negative since the reactants have less energy than the products
• Therefore an exothermic reaction has a negative change in energy

Question 12

How do we do Bond Energy calculation?

Answer 12

• Each chemical bond has a specific bond energy associated with it
• This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed

Energy change = Energy taken in(reactants) - Energy given out(Products)

Question 13

Hydrogen and chlorine react to form hydrogen chloride gas:

H₂ + Cl₂ ⟶ 2HCl

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic

Answer 13

Question 14

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr ⟶ H₂ + Br₂

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.

Answer 14

Question 15

What is an electrolyte?

Answer 15

A solution that can conduct electricity, for example a solution of an ionic compound

Question 16

What is a cell and what can it be used for?

How does a battery work?

Answer 16

• A simple cell is a source of electrical energy
• The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit

Question 17

Give an example of a cell and how it works

Answer 17

• A common example is zinc and copper
• Zinc is the more reactive metal and forms ions more easily, readily releasing electrons
• The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes
• The electrons then flow around the circuit to the copper electrode which is now the more positive electrode
• The difference in the ability of the electrodes to release electrons causes a voltage to be produced
• The greater the difference in the metals reactivity then the greater the voltage produced
• The electrolyte used also affects the voltage as different ions react with the electrodes in different way

Question 18

How long can a cell produce electricity for and why?

Answer 18

• A cell can only produce electricity for a certain period of time. Eventually, the chemicals in the cell run out and the reaction stops.
• Cells only produce electricity if we use metals with different reactivities.

Question 19

What is the Reactivity series?

Answer 19

Question 20

What does it mean if there is a greater difference between the reactivity of the metals

Answer 20

if there is a greater difference between the reactivity of the metals, the greater the potential difference produced by the cell.

The electrolyte also affects the p.d.

Question 21

What are batteries?

Answer 21

A battery contains two or more cells connected in series to produce a greater voltage.

Question 22

Why are alkaline batteries non-rechargeable?

Answer 22

At some point, the reactants in the batteries run out and no more electricity is produced.

Over time the electrodes degrade as the reactions that occur there are irreversible.

There is no way that we can reverse these reactions, so these are non-rechargeable.

Question 23

Why can rechargeable batteries be recharged?

Answer 23

They can be recharged as we are able to reverse the chemical reactions when we apply a current.

• This reverses the chemical reactions taking place allowing the cycle to be repeated

Question 24

What are Hydrogen fuel cells?

What does the diagram look like?

Answer 24

• A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
• These cells are becoming more common in the automotive industry to replace petrol or diesel engines
• As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
• Different electrolytes and fuels can be used to set up different types of fuel cells
• An important cell is the hydrogen-oxygen fuel cell which combines both elements to release energy and water

Question 25

Advantages and disadvantages of Hydrogen Fuel cells?

Answer 25

**Advantages** * They do not produce **any** **pollution** * They produce **more** **energy** per kilogram than either petrol or diesel * No power is lost in transmission as there are no moving parts, unlike an internal combustion engine * No batteries to dispose of which is better for the environment * Continuous process and will keep producing energy as long as fuel is supplied **Disadvantages** * Materials used in producing fuel cells are **expensive** * **High pressure** tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle * Fuel cells are affected by **low** temperatures, becoming less efficient * Hydrogen is **expensive** to produce and store

Question 26

Compare Hydrogen Fuel cells with rechargeable batteries.

Answer 26

* Hydrogen Fuel cells will produce electricity for as long as you provide hydrogen while rechargeable batteries run out and need to be recharged. * Hydrogen fuel cells do not get less efficient the longer they run where as rechargeable batteries store less electricity the more charging cycles they go through and eventually need to be replaced. * Hydrogen fuel cells can produce water for astronauts on spacecrafts. * Hydrogen fuel cells run on Hydrogen which in an explosive gas and is very difficult to store safely while no dangerous fuels are required with rechargeable batteries. Some types of rechargeable batteries can catch fire if not manufactured correctly. * Hydrogen Fuel cells produce a relatively low p.d. while rechargeable batteries produce a larger p.d than a hydrogen fuel cell.

Question 27

Electrode reactions in Hydrogen Fuel cells?

Answer 27

* The cell consists of an electrolyte which is usually phosphoric acid and porous carbon electrodes coated with a catalyst. * Hydrogen enters at the **anode** where it is oxidised and oxygen enters at the **cathode** where it is reduced. * The following reaction occurs at the anode: **2H₂ → 4H⁺ + 4e^–** * At the cathode the following reaction takes place: **4H⁺ + O₂ + 4e^– → 2H₂O** * The overall reaction is: **2H₂ + O₂→ 2H₂O** * The electrons move around the external circuit from the cathode to the anode. * This movement of electrons is used to drive an electric motor.