Energy Changes in Reactions (2)

Question 1

Define

exothermic reaction.

and give two examples.

Answer 1

a reaction which gives out heat energy to the surroundings

e.g. combustion, neutralisation

Question 2

Define

endothermic reaction.

and give an examples.

Answer 2

a reaction which takes in heat energy from the surroundings

e.g. thermal decomposition

Question 3

What is a

reaction profile?

Answer 3

a graph that shows how the energy in a reaction changes as the reaction progresses

the graph starts at the energy level of the reactants and finishes at the energy level of the products
these two points are usually joined by a smooth curve

Question 4

What is

energy measured in?

Answer 4

joules (J)

Question 5

What type of

process is bond breaking?

and why?

Answer 5

endothermic because energy must be supplied to break existing bonds

Question 6

What type of

process is bond formation?

and why?

Answer 6

exothermic because energy is released when new bonds are formed

Question 7

How would you

calculate the overall enrgy change for a reaction?

(using bond energies) (4 steps)

Answer 7

1. Draw out the displayed formulas of the molecules in the reaction so that you can see all the bonds that are being broken and made.
2. Work out the amount of energy used in bond breaking by adding up the bond energies of all the bonds in all the reactants.
3. Work out the amount of energy given out from bond making by adding up the bond energies of all the bonds in all the products.
4. Work out the overall energy change by subtracting the energy of bond making from the energy of bond breaking.

If the overall energy change is negative, the reaction is exothermic.