C5 Energy Changes

Question 1

What is the conservation of energy principle?

Answer 1

Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place

Question 2

What is an exothermic reaction? Give examples

Answer 2

A reaction where energy is transferred to the surroundings so that the surroundings temperature increases - combustion, oxidation reactions and
neutralisation (acid + alkali) reactions. Negative sign of energy change.

Question 3

What is an endothermic reaction? Give examples

Answer 3

A reaction where energy is taken in from the surroundings so the surroundings
temperature decreases - thermal decomposition, reaction of citric acid and sodium hydrogencarbonate. Positive sign of energy change.

Question 4

What is activation energy?

Answer 4

Minimum amount of energy that particles need to react

Question 5

What is a reaction profile?

Answer 5

Reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.

Question 6

What occurs in a chemical reaction in terms of
bond energies? Describe exothermic and endothermic reactions in terms of bond
breaking/forming.

Answer 6

Energy is supplied to break bonds and energy is released when bonds are made;
exothermic - energy released from forming bonds is greater than that needed to break the bonds; endothermic - energy needed to break bonds is greater than energy released making them

Question 7

What is the equation to find enthalpy change inn terms of bond energies?

Answer 7

Energy of reaction = sum of bonds broken - sum of bonds made

Question 8

What is a battery?

Answer 8

A battery consists of two or more cells connected in series.

Question 9

What is a cell?

Answer 9

A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.

Question 10

What determines the voltage obtained from a cell?

Answer 10

Identities of metals used as electrodes and the identity and concentration of an electrolyte.

Question 11

State the advantages and disadvantages of using cells and batteries.

Answer 11

(+) more or less cheap, some are rechargeable, a convenient source of electrical energy

(-) harmful chemicals

Question 12

Describe rechargeable and non-rechargeable cells

Answer 12

Rechargeable - chemical reactions are reversed when an external current is supplied

Non-rechargeable - reactants are used up, cannot be recharged

Question 13

What is a fuel cell? What is the overall reaction in a hydrogen fuel cell? What are the half equations?

Answer 13

Fuel cells are supplies by fuel and oxygen to oxidise the fuel to generate electricity.

2 H2 (g) + O2 (g) → 2 H2 O (l)
In summary, the two half-reactions in a hydrogen fuel cell are the oxidation of hydrogen gas at the anode and the reduction of oxygen gas at the cathode.

Question 14

What is the overall reaction in a hydrogen fuel cell? What are the half equations?

Answer 14

Cathode: 2 H2 → 4H+ + 4 e-
Anode: O2 + 4 H+ + 4 e- →2 H20
Overall: 2 H2 + O2 → 2 H20

Question 15

What are the advantages and disadvantages of hydrogen fuel cells?

Answer 15

Advantages: no pollutants, no recharging

Disadvantages: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis

Question 16

Endothermic reaction:

Answer 16

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases. ​In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Question 16

Activation energy:

Answer 16

The minimum amount of energy for particles to collide with in order for a successful reaction to occur.

Question 17

Exothermic reaction:

Answer 17

​An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. ​In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.

Question 18

Overall energy change of the reaction:

Answer 18

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.

Question 19

Reaction profile:

Answer 19

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.