Chapter 9 - Enthalpy

Question 1

What is the law of conservation of energy?

Answer 1

Energy cannot be created or destroyed, just transferred from one form to another.

Question 2

Define enthalpy.

Answer 2

The heat energy stored in a chemical system.

Question 3

Define enthalpy change.

Answer 3

The difference in enthalpy between products and reactants.

Question 4

Define exothermic.

Answer 4

When heat energy is transferred from the chemical system to the surroundings. ΔH is negative.

Question 5

Define endothermic.

Answer 5

When heat energy is transferred to the chemical system from the surroundings. ΔH is positive.

Question 6

Define activation energy.

Answer 6

The minimum energy required for a reaction to take place.

Question 7

What are the standard conditions?

Answer 7

Temperature - 298K
Pressure - 101kPa
Concentration - 1 mol dm-3

Question 8

What is meant by standard state?

Answer 8

The physical state of a substance under standard conditions.

Question 9

Define standard enthalpy change of formation.

Answer 9

The enthalpy change when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states.

Question 10

Define standard enthalpy change of combustion.

Answer 10

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states.

Question 11

What is the equation used to measure energy change?

Answer 11

Q = MCΔT

Q - Energy change with surroundings (J)
M - Mass (g)
C - Specific heat capacity (J/g-1/K-1)
ΔT - Change in temperature (K)

Question 12

Define standard enthalpy change of neutralisation.

Answer 12

The enthalpy change when an acid and a base react together to form one mole of water under standard conditions with all reactants and products in their standard states.

Question 13

Define standard enthalpy change of reaction.

Answer 13

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in their standard states.

Question 14

What are the causes for less energy being transferred than expected when working out ΔcH?

Answer 14

Heat loss to surroundings such as beaker or air
Incomplete combustion of alcohol
Evaporation of alcohol from wick
Non-standard conditions

Question 15

How can heat loss be accounted for using a graph of temperature against time?

Answer 15

Extrapolate the cooling curve back to when the zinc was added.

Question 16

Enthalpy of neutralisation always has the same value. What is it?

Answer 16

-57.5 kJ/mol-1

Question 17

Define average bond enthalpy.

Answer 17

The energy required to break one mole of a specific type of bond in a gaseous molecule.

Question 18

What is the rule for bond making and breaking?

Answer 18

Energy is required to break bonds, so bond breaking is endothermic.
Energy is released when forming bonds, so bond making is exothermic.

Question 19

Define specific heat capacity.

Answer 19

The energy required to raise the temperature of 1g of a substance by 1K

Question 20

Describe the experiment for spirit burner.

Answer 20

1. Measure out 150cm3 of water using a measuring cylinder and pour it into a beaker. Measure initial temperature to nearest 0.5C
2. Add methanol to spirit burner and weigh using mass balance.
3. Place spirit burner under beaker and light it. Stir water whilst burning with thermometer in place.
4. After 3 minutes, extinguish flame and immediately record maximum temperature of water.
5. Reweigh spirit burner containing methanol and assume wick has not been burnt.

Question 21

What is the formula for calculating enthalpy change from average bond enthalpies?

Answer 21

ΔrH = (sum of bond enthalpies in reactants) - (sum of bond enthalpies in products)

Question 22

What are the limitations of using average bond enthalpies to calculate enthalpy change?

Answer 22

The actual energy involved in breaking and making bonds is different due to bonds in different environments.

Calculations using average bond enthalpies need all species to be gaseous molecules. If water is (g) and not (l) then it is not accurate.

Because all ∆rH values should be in standard state, and H2O standard state is (l)

Question 23

Describe an experiment that can determine enthalpy change of reaction.

Answer 23

Add reacting solution to polystyrene cup, as it is a good insulator against heat loss to surroundings.
Record temperature change using thermometer.
Calculate heat energy change using Q=MCΔT

Question 24

What are 2 methods for reducing heat loss in the polystyrene cup experiment?

Answer 24

Add a lid to the cup.

Add insulating layer around cup.

Question 25

What are 2 methods of reducing heat loss in a spirit burner experiment?

Answer 25

Add a lid to the beaker.

Use draft shields around the apparatus.