2: Enthalpy profile diagrams

Question 1

Draw the enthalpy profile diagram for the combustion of methane.

The change in H is +/- 890 KJ mol^-1

Answer 1

Question 2

Draw the enthalpy of combustion profile diagram for the decomposition of calcium carbonate.

Knowing that the change in H is +/- 178KJmol^-1

Answer 2

Question 3

Define activation energy

Answer 3

Activation energy is the minimum energy required to start a reaction by the breaking of bonds.

Question 4

Give an example of an exothermic reaction which needs an acitvation energy to start it.

Answer 4

Natural gas needs a spark to overcome the activation energy of the reaction. Once the energy barrier has been overcome, the net output of energy provides more energy that can be used to overcome the activation energy for the reaction to continue.

Question 5

What would happen if there were no activation energies?

Answer 5

If the activation energy did not exist, exothermic reaction would take place spontaneously. Fuels would not exist as they would spontaneously combust.

Question 6

Draw a general enthalpy profile diagram for an exothermic reaction that includes the activation energy.

Answer 6

• Activation energy is positive and the arrow points up.
• The reaction is exothermic so the arrow goes down

Question 7

Draw the general enthalpy profile diagram for an endothermic reaction.

Answer 7