AS - Energetics

Question 1

What is enthalpy change? What are the units?

Answer 1

The heat energy transferred in a reaction at constant pressure.

Units are KJ mol-1

Question 2

What are standard conditions?

Answer 2

100kPa and a stated temperature (usually 298K)

Question 3

What is an exothermic reaction? What is the enthalpy change for an exothermic reaction?

Answer 3

A reaction that gives out energy where enthalpy change is negative.

Question 4

What is an endothermic reaction? What is enthalpy change for an endothermic reaction?

Answer 4

A reaction that absorbs energy. Enthalpy change is positive.

Question 5

Is bond breaking endothermic or exothermic and why?

Answer 5

Endothermic as energy is required to break bonds.

Question 6

Is bond making endothermic or exothermic and why?

Answer 6

Exothermic because energy is released when bonds are made.

Question 7

Define standard enthalpy of formation

Answer 7

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

Question 8

Define standard enthalpy of combustion.

Answer 8

The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions.

Question 9

Define the term mean bond enthalpy.

Answer 9

The average energy required to break a certain type of bond, over a range of compounds.

Question 10

What is bond enthalpy?

Answer 10

The energy required to break bonds.

Question 11

Why are mean bond enthalpies always positive?

Answer 11

Because bond breaking is always endothermic.

Question 12

What does the energy required to break a bond depend on?

Answer 12

The environment it is in.

Question 13

In terms of energy absorbed and released, what basic equation denoted the enthalpy change of a reaction?

Answer 13

Total energy absorbed - total energy released

Question 14

Why are enthalpy changes calculated using mean bond enthalpies not exact?

Answer 14

Because mean bond enthalpies are average values.

Question 15

Are enthalpy change values from mean bond enthalpies more or less exact than those calculated using Hess’ law?

Answer 15

Less exact than Hess’ law.

Question 16

How can calorimetry be used to find enthalpy changes?

Answer 16

You can find out how much heat is given out by a reaction by measuring the change in temperature using a calorimeter.

As the fuel burns, it heats the water. You can work out the heat energy that has been absorbed by the water if you know the mass of the water, the temperature change and the specific heat capacity of water.

Question 17

What is the formula to calculate an enthalpy change from a calorimetry experiment? What does each letter mean and what are the units?

Answer 17

q=mc(deltaT)

Where:

q = heat lost or gained in joules (this is the same as enthalpy change is the pressure is constant)
m = mass of water in the calorimeter in grams 
c = specific heat capacity of water in joules per gram per Kelvin
DeltaT = the change in temperature of the water in Kelvin

Question 18

What does the specific heat capacity of water mean?

Answer 18

The amount of heat energy it takes to raise the temperature of 1g of water by 1K.

Question 19

Name two experimental problems that apply to all types of calorimetry.

Answer 19

1. Some heat will be absorbed by the container, rather than going towards heating up the water.
2. Some heat is always lost to the surroundings during experiment, no matter how well the container is insulated.

Question 20

Name two experimental problems that only apply to flammable liquid calorimetry.

Answer 20

1. Some combustion may be incomplete - which means less energy will be given out.
2. Some of the flammable liquid may escape by evaporation as they’re usually quite volatile.

Question 21

What is Hess’ law?

Answer 21

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

Question 22

What is the enthalpy change of formation for all elements?

Answer 22

0