Topic 5: 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. Negative 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 in terms of bond energies?

Answer 7

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