4: Energetics

Question 1

Define enthalpy change.

Answer 1

The heat energy change under constant pressure.

Question 2

What does standard states mean?

Answer 2

The states you would expect to find a substance in under standard conditions.

Question 3

What happens to energy in an exothermic reaction?
How does this affect temperature surroundings?

Answer 3

Energy is released to the surroundings.
Temperature goes up in surroundings.

Question 4

What happens to energy in an endothermic reaction?
How does this affect temperature of the surroundings?

Answer 4

Energy is absorbed into the reaction.
Temperature of the surroundings goes down.

Question 5

Give an example of an exothermic process?

Answer 5

Combustion

Question 6

Give an example of an endothermic process?

Answer 6

Thermal decomposition.

Question 7

What are bond enthalpies?

Answer 7

The energy stored by a chemical bond.

Question 8

Is bond making or breaking exothermic?
Why?

Answer 8

Bond making is exothermic.
Forming bonds requires energy

Question 9

Is bond making or breaking endothermic?
Why?

Answer 9

Bond breaking.
Energy must be absorbed into the reaction to provide energy too break the bond.

Question 10

What are mean bond enthalpies?

Answer 10

Mean bond enthalpies are average values of energies required to break bonds from a wide range of compounds. A mean is taken because the value of a bond, such as C-C for example, is different in different compounds.

Question 11

Give an equation for calculating enthalpy changes.

Answer 11

Enthalpy change = Enthalpies of products - enthalpy of reactants.

Question 12

Define standard enthalpy of formation.

Answer 12

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.

Question 13

Define standard enthalpy change of combustion.

Answer 13

The enthalpy change when 1 mole of a substance is completely burned in oxygen, with under standard conditions with all reactants and products their standard states.

Question 14

Define standard enthalpy change of reaction.

Answer 14

The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states

Question 15

Give the equation for heat lost/gained.

Answer 15

q = mct

Question 16

Define Hess’s Law.

Answer 16

The total enthalpy change for a reaction is independent of the route taken.

Question 17

Give the equation for standard enthalpy of formation.

Answer 17

Standard Enthalpy of Formation = Enthalpies of products - Enthalpies of reactants.

Question 18

Give the equation for standard enthalpy of combustion.

Answer 18

Standard Enthalpy of Combustion = Enthalpies of reactants - Enthalpies of products.

Question 19

When calculating standard enthalpy of formation, which reactants are not included in the equation?

Answer 19

Elements

Question 20

State why heat change calculated using a bomb calorimeter experiment is not an enthalpy change.

Answer 20

Pressure not constant in a bomb calorimeter.

Question 21

In calorimetry, suggest a change to the method that could reduce percentage uncertainty and explain why.

Answer 21

Use a larger mass of solid so the temperature change is greater.