Energetics

Question 1

What is enthalpy change?

Answer 1

The heat change accompanying a chemical reaction at constant volume or constant pressure.

Question 2

What is the standard enthalpy change of reaction?

Answer 2

The enthalpy change which occurs when substances react under standard conditions in quantities determined by the standard equation.

Question 3

What is the standard enthalpy change of formation?

Answer 3

The enthalpy change which occurs when one mole of the compound is formed from its element under standard conditions, and with everything in its standard state.

Question 4

What is the standard enthalpy change of combustion?

Answer 4

The enthalpy change which occurs when one mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.

Question 5

What does Hess’ Law state?

Answer 5

The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.

Question 6

You are calculating bond enthalpy, and your value is not the same as the quoted value. Why?

Answer 6

Because the quoted values are averaged across a lot of molecules.

Question 7

You are doing the burner practical (combustion), and calculating enthalpy change. Your value is not the same as the quoted value. Why?

Answer 7

• Energy is lost to the surroundings
• Incomplete combustion

Question 8

You are doing the polystyrene cup practical, and calculating enthalpy change. Your value is not the same as the quoted value. Why?

Answer 8

Energy is lost to the surroundings.

Question 9

Define an exothermic reaction in terms of making and breaking bonds.

Answer 9

The energy released from forming new bonds is greater than the energy needed to break existing bonds.

Question 10

Define an endothermic reaction in terms of making and breaking bonds.

Answer 10

The energy needed to break existing bonds is greater than the energy released from forming new bonds.