2.8 Energy Changes in Chemistry

Question 1

What is the change in heat during an exothermic reaction?

Answer 1

They give out heat to their surroundings.

Question 2

What is the change in heat during an endothermic reaction?

Answer 2

Heat is taken in from the surroundings.

Question 3

What is the definition of ‘activation energy’?

Answer 3

Activation energy is the minimum energy needed by particles for a reaction to occur.

Question 4

What do the y and x axis show in an energy profile diagram?

Answer 4

Vertical y-axis shows increasing ‘energy’ content.

Horizontal x -axis shows the ‘progress of reaction’ where reactants form products.

Question 5

Describe the general reaction profile diagram for an exothermic reaction

Answer 5

In an exothermic reaction heat is given out overall.
Its reaction profile diagram shows that overall there is a negative energy change.
The diagram shows that in the first stage, reactants take in energy equal to the activation energy, Ea, (upward arrow).
In the second stage, energy is then given out to form products.
For an exothermic reaction more energy is given out than is taken in, so overall energy is given out (downward arrow).
For an exothermic reaction the overall energy change has a negative value.

Question 6

Describe the general reaction profile diagram for an endothermic reaction

Answer 6

Its reaction profile diagram shows that overall there is a positive energy change.
The diagram shows that in the first stage, reactants take in energy equal to the activatior energy, Ea, (first upward arrow).
In the second stage, energy is then given out to form products.
For an endothermic reaction less energy is given out than is taken in, so overall energy is taken in (second upward arrow).
For an endothermic reaction the overall energy change has a positive value.

Question 7

Describe the bond making/breaking in exothermic reactions

Answer 7

For exothermic reactions- more energy is released in making new bonds than was taken in to break bonds. The energy change has a negative sign.

Question 8

Describe the bond making/breaking in endothermic reactions

Answer 8

For endothermic reactions- more energy is taken in to break bonds than is released making new bonds. The energy change has a positive sign.

Question 9

What is the exam technique for answering questions to explain exothermic/endothermic reactions?

Answer 9

1. State that bond breaking takes in energy and name the reactants in which the bonds are broken.
2. State that bond making gives out energy and name the products in which the new bonds are made.
3. Compare the two energies and state which is greater and so explain if overall energy is given out or taken in.

Question 10

What is the definition of ‘bond energy’?

Answer 10

Bond energy is the amount of energy required to break one mole of a covalent bond.

Question 11

What is the equation for energy change?

Answer 11

Energy change = Energy taken in to break bonds in reactants - Energy released making bonds in products