Chapter 9- Enthalpy

Question 1

How is enthalpy change calculated?

Answer 1

△H = H(products) - H(reactants)

Question 2

State the law of conservation of energy.

Answer 2

Energy cannot be created or destroyed.

Question 3

Describe an exothermic change.

Answer 3

Energy from the chemical system is transferred to the surroundings causing the surrounding temperature to increase. This is a negative △H.
More energy is required to break bonds than is produced when forming them.

Question 4

Describe an endothermic change.

Answer 4

The chemical system takes in heat energy from the surroundings causing the temperature of the surroundings to decrease. This is a positive △H.
More energy is produced when forming bonds than is required to break bonds.

Question 5

Define activation energy (Ea).

Answer 5

The minimum energy required for a reaction to take place.

Question 6

How does activation energy impact reactions?

Answer 6

Reactions with small Ea occur rapidly due to the energy required to break the bonds being readily available. Very large Ea may present a large energy barrier so the reaction takes place extremely slowly or not at all.

Question 7

What does △H^⦵ mean?

Answer 7

An enthalpy change under standard conditions.

Question 8

What are the conditions required for △H^⦵ to apply?

Answer 8

standard pressure- 100KPa
standard temperature- 298K (25^◦c)
standard concentration- 1 moldm^-3
standard state- physical state under standard conditions

Question 9

Define △rH^⦵.

Answer 9

The standard enthalpy change of a reaction- the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical reaction under standard conditions, with all reactants and products in their standard states.
Mg(s) + 1/2O2(g) → MgO(s) △rH^⦵= -602 KJmol^-1
2Mg(s) + O2(g) → 2MgO(s) △rH^⦵= -1204 KJmol^-1

Question 10

Define △fH^⦵.

Answer 10

The standard enthalpy change of formation- the energy change that occurs when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states. e.g.:
Mg(s) + 1/2O2(g) → MgO(s) △fH^⦵= -602 KJmol^-1
elements → 1 mole

Question 11

Explain △fH^⦵ for the formation of elements.

Answer 11

The standard enthalpy change of formation refers to the formation of one mole of an element from its elements for the formation of elements. This is no change so all elements have an enthalpy change of formation of 0 KJmol^-1.

Question 12

Define △cH^⦵.

Answer 12

The standard enthalpy change of combustion- the enthalpy change that occurs when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states. e.g.:
C4H10(g) + 61/2O2(g) → 4 CO2(g) + 5H2O(l) △cH^⦵=-2877

Question 13

Define △neutH^⦵.

Answer 13

The standard enthalpy change of neutralisation- the energy change that accompanies the reaction of an acid by a base to form one mole of H2O(l), under standard conditions, with all reactants and products in their standard states. e.g.:
H^+(aq) + OH^-(aq) → H2O(l)
HCl(aq) + NaOH(aq) → H2O(l) + NaCl (aq)

Question 14

Describe an experiment to determine enthalpy change of combustion.

Answer 14

The set up of the apparatus- This is a beaker of water with a thermometer in clamped to a clamp stand. The spirit burner with the fuel is then placed underneath.

Method- Set up the apparatus as described and use different fuels. q = mc△T is used. m is the mass of the water and the temperature change is that of the water too. The change in mass of fuel is used to calculate the moles of fuel burned. Then do (q/100)/moles to get △cH^⦵ KJmol^-1.

Question 15

How is energy change calculated?

Answer 15

Q= mc△T
Q- heat energy- J
m- mass- g
c- specific heat capacity- 4.18 kg^-1K^-1
△T- T(final) - T(initial)

Question 16

Why may experimental values for the enthalpy change of combustion be lower than the actual values?

Answer 16

• Incomplete combustion of fuel.
• The flame of the fuel was not put out immediately once an adequate temperature was reached so extra fuel was burnt without impacting the temperature.
• Some fuel evaporated before it was burnt.

Question 17

How could the accuracy of the experiment to determine enthalpy of combustion be improved?

Answer 17

Heat a larger volume of water.

Question 18

Describe an experiment to determine enthalpy change of neutralisation.

Answer 18

Set up of apparatus- Clamp the thermometer to the clamp stand and have this held in the polystyrene cup.

method- Set up apparatus as described. Add HCl to the cup and take the temperature every minute for the first three minutes. On the 4th minute add the NaOH and stir. Keep recording every minute until ten minutes, except for that of the 4th. Use q = mc△T with m being the mass of both solutions. Then divide by the moles of HCl. To improve a lid could be used of a digital thermometer.

Question 19

Describe endothermic and exothermic in terms of bond breaking and bond making.

Answer 19

Energy is required to break bonds making it endothermic.
Energy is released from forming bonds making it exothermic.
△fH = the sum of bond enthalpies in reactants - the sum of bond enthalpies in products.

Question 20

State Hess’ law.

Answer 20

If a reaction can take place by two routes and the starting and finishing conditions are the same, the total enthalpy change for each route is the same.
A + B = C

Question 21

What is the △cH^⦵ of Al2S3(s) in KJmol^-1?
 △fH^⦵ for each compound...
Al2S3(s) = -723.8
O2 = 0
Al2O3(s) = -1675.7
SO2(g) = -296.8

Answer 21

2Al2S3 + 9O2 → 2Al2O3(s) +6SO2(g)

2 x -723.8
0
2 x -1675.7
6 x -296.8

made from elements so…
step 2 - step 1 = -3684.6
-3684.6 / 2 = -1842.3
this has to be halved because △cH^⦵ is one mole of the substance/ fuel reacting completely with oxygen.