Enthalpy

Question 1

What is enthalpy?

Answer 1

The measure of heat energy in a chemical system.

Question 2

What units and symbols are enthalpy change given in?

Answer 2

ΔH / KJmol⁻¹

Question 3

What conditions do we use to determine a chemicals enthalpy change and why?

Answer 3

We use standard conditions because objects react differently depending on the conditions.
The conditions are:
100KPa (Or 1 atmosphere)
298K (Or 25°C)
1 mol dm⁻³ (Relevant for solutions only)
Standard state (The normal state of matter at 298K)

Question 4

What’s the symbol for standard conditions in enthalpy?

Answer 4

ΔᵣH⦵

Question 5

What are endothermic reactions?

Answer 5

Reactions that take in heat energy.

Question 6

Why is ΔH positive for endothermic reactions?

Answer 6

The products have more energy than the reactants.

Question 7

What are exothermic reactions?

Answer 7

Reactions that release heat energy.

Question 8

Why is ΔH negative for exothermic reactions?

Answer 8

The products have less energy than the reactants.

Question 9

What is standard enthalpy change of neutralisation?

Answer 9

The enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions.

Question 10

What’s the symbol for enthalpy change of neutralisation?

Answer 10

ΔₙₑᵤₜH⦵

Question 11

What is standard enthalpy change of combustion?

Answer 11

The enthalpy change when 1 mole of substance is completely burned to make CO₂ and H₂O under standard conditions.

Question 12

What’s the symbol for enthalpy change of combustion?

Answer 12

Δ𝒸H⦵

Question 13

What is standard enthalpy change of formation?

Answer 13

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

Question 14

What’s the symbol for enthalpy change of formation?

Answer 14

ΔfH⦵

Question 15

What is bond dissociation enthalpy?

Answer 15

The energy needed per mole to break attractive forces in bonds in gaseous compounds.

Question 16

How do we calculate enthalpy change (ΔH)

Answer 16

Energy needed to break bonds - Energy needed to form bonds

Question 17

How do we work out the enthalpy change of combustion for a fuel experimentally?

Answer 17

You can set up a spirit burner to heat up some water to a certain temperature.
Measure the fuel before it burns and after it has burnt. Once it has burnt, work out the mass of fuel burnt and use q=mcΔT to work out the energy change.
Then calculate the moles of the fuel and put it under the energy change to calculate the enthalpy.

Question 18

How can you increase the accuracy of an experiment to get the value for Δ𝒸H?

Answer 18

Add wind shields to the side of the burner.
Weigh the burner immediately rather than later.
Make sure the combustion is complete.

Question 19

How do we work out the enthalpy change of neutralisation experimentally?

Answer 19

In a polystyrene cup, add an acid and measure the temperature.
Then add an alkali and have it be mixed with an alkali.
Once the reaction is done, measure the temperature and work out the temperature change.
Then use q=mcΔT to calculate the energy change.
Work out the moles of the acids and alkalis and then have it be divided by energy for the enthalpy change.

Question 20

What is Hess’s Law?

Answer 20

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

Question 21

How do we work out enthalpy change of formation?

Answer 21

ΔfH Products - ΔfH Reactants

Question 22

How do we work out enthalpy change of combustion?

Answer 22

Δ𝒸H Reactants - Δ𝒸H Products

Question 23

What’s the formula for energy change when doing enthalpy questions?

Answer 23

q=mcΔT
q = heat energy lost or gained.
m = mass of water or solution
c = specific heat capacity (usually water so it's normally 4.18)
ΔT = Change in temperature.

Question 24

What is the definition of activation energy?

Answer 24

The minimum amount of energy required for a reaction to take place.