Energetics

Question 1

What is enthalpy change?

Answer 1

Enthalpy change is the heat energy transferred in a reaction done at standard states under standard conditions at constant pressue measured in kJ/mol.

Question 2

What is an exothermic reaction?

Answer 2

Reactions that give out energy (enthalphy change is negative)

Question 3

What is an endothermic reaction?

Answer 3

Reactions that absorb energy (enthalpy change is positive)

Question 4

Give an example of an exothermic reaction

Answer 4

An oxidation reaction

Question 5

Give an example of an endothermic reaction

Answer 5

Thermal decompostion

Question 6

What is mean bond enthalpy?

Answer 6

The mean energy required to break bonds. Breaking bonds is always endothermic, so mean bond enthalpies are always positive.

Question 7

How can enthalpy changes be calculated?

Answer 7

Enthalpy changes can be calculated using mean bond energies.
Enthalpy change of a reaction = total energy absorbed-total energy released

Question 8

What is standard enthalpy of formation?

Answer 8

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

Question 9

What is standard enthalpy of combustion?

Answer 9

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.

Question 10

How does caliometry work?

Answer 10

To find out the enthalpy of combustion of a flammable liquid, you burn it inside some apparatus called a caliometer.
As the fuel burns it heats the caliometer; ideally, all the heat given out by the fuel would be absorbed by the water. In practise you always lose some heat- so it’s hard to get an accurate result.

Question 11

How can you calculate enthalpy change?

Answer 11

q=mcΔT
q= heat lost or gained in joules (same as enthalpy change)
m= mass of water (or other solution) in grams
c= specific heat capacity of water (4.18J/g/K)
ΔT= The change in temperature (in kelvin) of the water or solution

Question 12

What are some experimental problems with caliometry?

Answer 12

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 the experiment

Question 13

What are some experimental problems that only apply to flammable liquid caliometry?

Answer 13

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

Question 14

How can caliometry be used to calculate the ΔH for a reaction that can happen in a solution (neutralisation, dissolving or displacement)?

Answer 14

(For neutralisation)

1. Add a known volume of acid to an insulated container and measure the temperature
2. Then add a know volume of alkali, and record the temperature change (stir the solution to make sure it’s evenly heated)
3. You can work out the heat needed to raise the temperature of the solution using the formula q=mcΔT.

Question 15

What is Hess’ Law?

Answer 15

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