Topic 19 - Heat Energy Changes in Chemical Reactions

Question 1

7.9 - What are some examples of exothermic and endothermic reactions?

Answer 1

During a chemical reaction, energy is transferred between reactants and the surrounding.
The energy transferred is usually thermal energy.
Temperature increases in an exothermic reaction.
Temperature decreases in an endothermic reaction.

Precipitation reactions are when an two soluble reactants form an insoluble product.
They can be both exothermic and endothermic.

Neutralisation reactions are between an acid and a base.
Displacement reactions are between a metal/halogen and a compound of a less reactive metal/halogen.
These are exothermic reactions.

Question 2

7.9 - How can you model energy change in reactions?

Answer 2

Use a reaction profile to show energy stored before and after a reaction.

Question 3

7.10 - What is an exothermic reaction?

Answer 3

Energy is transferred from stores of energy in chemical bonds to the surroundings.

Question 4

7.11 - What is an endothermic reaction?

Answer 4

Energy is transferred from the surroundings to stores of energy in chemical bonds.

Question 5

7.12 - What happens to the bonds in different reactions?

Answer 5

In endothermic reactions, bonds are broken, by energy from surroundings being transferred to the reactants.
In exothermic reactions, bonds are made, by energy is transferred to the surroundings as bonds form.

Question 6

7.13 - How does the bonds determine if a reaction is exothermic or endothermic?

Answer 6

Exothermic - if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants.
Endothermic - if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants.

Question 7

7.14 - How do you calculate energy change in a reaction?

Answer 7

The energy needed to break one mole of a covalent bond is bond energy.
For example, to break C-O you need 358kJ/mol, so that much is taken in and given out when bonds are broken and made.

Question 8

7.15 - What is activation energy?

Answer 8

Minimum amount of energy needed for a reaction to happen.

Question 9

7.16 - How do you draw reaction profiles for endothermic and exothermic reactions?

Answer 9

For exothermic reactions, there is more energy in the reactants than products.
For endothermic reactions, there is more energy products than the reactants.
The peak of the curved line connecting the products and reactants is the activation energy.