Apparatus
Measuring Cylinders
Pipettes
Thermometers
Polystyrene Cup
Safety
To ensure the experiments are carried out safely I will always wear a lab coat and goggles. My hair will be tied up. I will make sure my work area is tidy and will observe all other laboratory rules.
Method
-
Measure 25cm3 of acid into a polystyrene cup and take temperature.
-
Measure 5cm3 of alkali and record temperature. Add to the acid and take the maximum temperature.
-
Keep on adding 5cm3 of alkali and stir, recording the maximum temperature each time.
-
Keep doing this until you have added 50cm3 of alkali.
- Record results in a table.
- Plot a graph.
PROBLEMS
Although the results obtained are accurate there were a few problems/limitations that could be solved by using another method.
1. I found it hard to continually measure 5cm3 of alkali
2. It takes a lot of time to stir and record the maximum temperature so there might be some heat lost during the experiment even though the polystyrene cup is a good insulator.
Alternative Method
-
Measure 25cm3 of 1 mol dm¯ 3 acid into a polystyrene cup.
- Record its temperature.
-
Measure 25cm3 of 1 mol dm¯ 3 alkali into a measuring cylinder.
- Record temperature of alkali.
- Add alkali to acid in polystyrene cup, stir and record the maximum temperature.
- Repeat twice for each combination of acid and alkali to obtain accurate results that agree with each other
- Record initial and maximum temperature in a table.
Results
Heat of Neutralisations
1. HCl reacting with NaOH
∆H= m x C x θ
∆H= change in heat energy
m = mass (always 50g)
C = specific heat capacity (always 4.2 J/g/˚C)
θ = Temperature rise
25 x 1 moles = 50 x 4.2 x 7 kJ
1000 1000
0.025 moles = 1.47
1 mole = 1.47
0.025
= 58.8
The heat of neutralisation for hydrochloric acid reacting with sodium hydroxide is
–58.8 kJ (negative because it is an exothermic reaction)
2. HCl reacting with KOH
25 x 1 moles = 50 x 4.2 x 7 kJ
1000 1000
0.025 moles = 1.47
1 mole = 1.47
0.025
= 58.8
The heat of neutralisation for hydrochloric acid reacting with potassium hydroxide is
–58.8 kJ
3. HNO3 reacting with NaOH
25 x 1 moles = 50 x 4.2 x 7kJ
1000 1000
0.025 moles = 1.47
1 mole = 1.47
0.025
= 58.8
The heat of neutralisation for nitric acid reacting with sodium hydroxide is
-58 kJ
- CH3COOH reacting with NaOH
25 x 1 moles = 50 x 4.2 x 6 kJ
1000 1000
0.025 moles = 1.26
1 mole = 1.26
0.025
= 50.4
The heat of neutralisation for ethanoic acid reacting with sodium hydroxide is
-50.4 kJ
- CH3CH2COOH reacting with NaOH
25 x 1 moles = 50 x 4.2 x 6 kJ
1000 1000
0.025 moles = 1.26
1 mole = 1.26
0.025
= 50.4
The heat of neutralisation for propanoic acid reacting with sodium hydroxide is
-50.4 kJ
Analysis
From analysing my results it is clear that no matter what strong acid you react with what strong alkali you will always get the same heat of neutralisation. This also proves my prediction. In this investigation we reacted 2 strong acids:
Hydrochloric acid HCl
Nitric acid HNO3
with 2 strong alkali’s:
Sodium Hydroxide NaOH
Potassium hydroxide KOH
And we always obtained the same heat of neutralisation which was –58.8KJ. The temperature rise was also exactly the same, when reacting a strong acid with a strong alkali it was 7˚C and when reacting a strong alkali with a weak acid it was 5˚C.
When I reacted weak acids:-
Ethanoic acid CH3COOH
Propanoic acid CH3CH2OOH
with a strong alkali I got a smaller heat of neutralisation. IN both cases it was –50.4KJ. The heat of neutralisation is smaller when reacting any weak acid or alkali because energy is used and needed to fully ionise it. The reason why you obtain the same heat of neutralisation when reacting a strong acid with a strong alkali is because the same reaction is always taking place,
H + OH H2O
An acid and an alkali react together to form 1 mole of water.
I therefore from the results obtained I can conclude that you will always get the same heat of neutralisation if you react a strong acid with a strong alkali.
Evaluation
The 2nd/alternative method was certainly more accurate and reliable than the first as it took quicker to react because you didn’t have to add 5cm3 then wait until you found the maximum temperature rise. The longer you take, the more heat can be lost. I feel confident to say that the results obtained were extremely accurate because:-
- They matched my prediction
- I got the same heat of neutralisation when reacting a strong acid with a strong alkali.
However to back up the results one could carry out some further experiments. We could experiment with a larger range of acids and alkalis as I only used 4 different acids and 2 different alkalis. This would also support the results already obtained. We could also experiment with a different concentration as we only used acids and alkalis with a concentration of 1mol/dm-3.
