SC19 Heat Energy Changes in Chemical Reactions

Question 1

SC19a
1) What is an exothermic reaction?
2) What is an endothermic reaction?
3) What are 2 examples of exothermic and endothermic reactions?

Answer 1

1) Energy is transferred from the stores of energy in chemical bonds to the surroundings.
2) Energy is transferred from the surroundings to the stores of energy in chemical bonds.
3) Exothermic reactions: combustion (burning), and neutralisation reactions between acids and alkalis.
Endothermic reactions: electrolysis, and thermal decomposition reactions.

Question 2

SC19b
1) What are the differences between exothermic and endothermic changes in terms of temperature?
2) Describe exothermic and endothermic reactions in terms of energy changes when bonds are broken and formed.
3) What is activation energy?
4) Where is the activation energy on a diagram?

Answer 2

1) In exothermic reactions, the temperature of the surroundings increases. In endothermic reactions, the temperature of the surroundings decrease. Therefore, exothermic reactions get hotter, and endothermic reactions get colder.
2) Energy is transferred to the reactants to break their bonds, so breaking bonds is endothermic.
Energy is transferred to the surroundings as bonds form, so making bonds is exothermic.
3) The activation energy is the minimum amount of energy needed by colliding particles for a reaction to happen.
4) The graph for an exothermic reaction typically has a “hill” or a curve rising up and then descending.
The activation energy arrow points from the energy level of the reactants to the peak of the “hill.” This indicates the energy needed to start the reaction.
The graph for an endothermic reaction also has a “hill,” but the overall slope rises after reaching the peak.
The activation energy arrow points from the energy level of the reactants to the peak of the “hill.” This shows the energy required to initiate the reaction.

Question 3

1) What does the peak on a reaction profile represent?
2) What does a reaction profile diagram look like for an exothermic reaction?
3) What does a reaction profile diagram look like for an endothermic reaction?
4) What is the difference between a reaction profile diagram for a catalysed and uncatalysed reaction?

Answer 3

1) The peak is the activation energy. The higher the peak, the higher the activation energy.
2) The line showing the reactants is higher than the line showing the products, as energy transferred to the surroundings.
3) The line showing the reactants is lower than the line showing the products, as energy transferred from the surroundings to the particles.
4) An uncatalysed reaction with have a higher activation energy than the catalysed reaction, so the peak will be higher on the diagram for the uncatalysed reaction.

Question 4

How do you calculate the energy change for a reaction?

Answer 4

To calculate an energy change for a reaction:
1. Add together the bond energies for all the bonds broken in the reactants, this is the ‘energy in’ - an endothermic change
2. Add together the bond energies for all the bonds formed in the products, this is the ‘energy out’ - an exothermic change
3. Energy change = energy in - energy out
4. If the energy change is negative (eg. -10), the reaction is exothermic, as the ‘energy out’ is larger than the ‘energy in’.
If the energy change is positive (eg. 10) , the reaction is endothermic, as the ‘energy out’ is smaller than the ‘energy in’.

Question 5

Explain in terms of bond breaking and making why some reactions are exothermic

Answer 5

Breaking bonds needs energy, and is endothermic. Making bonds releases energy, and is exothermic. There is more energy given out than taken in.