C5 Energy changes

Question 1

What is an exothermic reaction?

Answer 1

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. The surroundings feel hot as a result.

Question 2

What is an endothrmic reaction?

Answer 2

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases. The surroundings feel cold as a result.

Question 3

Give examples of exothermic reactions.

Answer 3

Exothermic reactions include combustion, many oxidation reactions and neutralisation.

Question 4

Give uses of exothermic reactions

Answer 4

Everyday uses of exothermic reactions include self-heating cans and hand warmers.

Question 5

Give examples of endothermic reactions.

Answer 5

Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate.

Question 6

Give uses of endothermic reactions

Answer 6

Some sports injury (“ice”) packs are based on endothermic reactions.

Question 7

What is the activation energy?

Answer 7

The activation energy is the minimum amount of energy that particles must have to react. It is the energy required to break all the bonds in the reactants.

Question 8

What is a reaction profile?

Answer 8

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.

Question 9

What does a reaction profile for an exothermic reaction look like?

Answer 9

Question 10

What does a reaction profile for an endothermic reaction look like?

Answer 10

Question 11

What is bond energy?

Answer 11

The bond energy is the energy absorbed when one mole of a particular bond is broken AND it is the energy released when one mole of the same bond is broken.

Question 12

How are bond energies used to calculate the overall energy change for a reaction?

Answer 12

1. First work out the equation for the reaction.
2. Now work out all the bonds broken
3. Multiply the number of each type of bond broken by the bond energy for that bond and work out the total energy supplied to break bonds
4. Now work out all the bonds made
5. Multiply the number of each type of bond made by the bond energy for that bond and work out the total energy released as bonds are made.
6. Finally remember that: total energy change = bonds made - bonds broken (make - break).
7. If the value is negative the reaction is exothermic and if it is positive the reaction is endothermic.

Question 13

What is a simple calorimeter?

Answer 13

It is a piece of equipment used to measure the temperature change of a reaction.

Question 14

How would you set up a simple calorimter in the school lab?

Answer 14

Note - Sytrofoam is an American term for polystyrene