Unit 3 - Industrial Chemistry (Chemical Energy)

Question 1

Why must temperature be monitored during a reaction?

Answer 1

To ensure that the energy given out during the process doesn’t cause the temperature of reaction vessels to rise to unsafe levels.

Question 2

Which chemical reactions involve an enthalpy change (the difference in between the products and the reactants, in the form delta H)?

Answer 2

Breaking and forming bonds. The enthalpy change takes the form of heat given out or absorbed. The heat energy given out or taken in by one mole of a substance is measured in kilojoules per mole (kJ mol^-1).

Question 3

What is the formula for the enthalpy change of a reaction?

Answer 3

Eh = c x m x delta T.
(Eh ~ enthalpy change {kJ or kJ mol^-1}, c ~ the specific heat capacity of water {4.18 kJ kg^−1 °C^−1}, m ~ the mass of water {kg e.g. 100cm^3 = 0.1kg}, delta T ~ the change in temperature {degrees celcius}).

Question 4

What is the enthalpy of combustion?

Answer 4

The heat energy given out when one mole of a substance burns completely in oxygen. Combustion reactions are exothermic so the value for the enthalpy change is always negative.

Question 5

What can enthalpies of combustion be used for?

Answer 5

Comparing which fuels or substances release the most energy when they are burned.

Question 6

What measurements must be taken when calculating the enthalpies of combustion?

Answer 6

• The starting temperature of the water.
• The final temperature of the water.
• The mass of the burner before the experiment.
• The mass of the burner after heating.
• The temperature change.

Question 7

What does Hess’ Law state?

Answer 7

The enthalpy change for a chemical reaction is independent of the route taken. This means that the enthalpy change for the overall process will be identical regardless of how many steps are taken. The enthalpy change for the reaction that forms products C+D directly
will be the same as the sum of the enthalpy changes for the production of C+D via an indirect route where intermediate products are formed and subsequently react to produce C+D.

Question 8

Is bond breaking exothermic or endothermic?

Answer 8

Endothermic

Question 9

Is bond forming exothermic or endothermic?

Answer 9

Energy is released when new chemical bonds are formed, so bond making is an exothermic process.