Thursday, 26 February 2015

4.4.1 - Laboratory Activity : Heat of Neutralisation between a Strong Acid and a Strong Alkali


Laboratory Activity 4.4.1:
Heat of Neutralisation between a Strong Acid and a Strong Alkali
Aim: To measure the heat of neutralisation of a strong acid by a strong alkali.
Problem statement: Will the heat of neutralisation of a weak acid with strong alkali be the same as heat of neutralisation between strong acid and strong alkali?
Hypothesis: The heat of neutralisation of a weak acid with strong alkali is different from the heat of neutralisation between strong acid and strong alkali.
Variable:
» Fixed variable : Volume and concentration of the hydrochloric acid, ethanoic acid and sodium hydroxide.
» Manipulated variable : Acids of different strength
» Responding variable : Heat of neutralisation

Material:
» Hydrochloric acid 2mol dm-3
» Sodium hydroxide 2mol dm-3
» Ethanoic acid 2mol dm-3

Apparatus:
» Polystyrene cup
» Thermometer
» Measuring cylinder
Procedure:

The following animation shows the arrangement of apparatus and the observation of the experiment.

1. 50cm3 of HCl 2mol dm-3 is measured with a measuring cylinder and poured into a polystyrene cup.
2. The initial temperature is measured with a thermometer and recorded.
3. 50cm3 of sodium hydroxide 2mol dm-3 is measured using a clean measuring cylinder and poured into the polystyrene. The temperature is measured with a clean thermometer and recorded.
4. The hydrochloric acid, HCl, is quickly poured into the polystyrene cup.
5. The mixture is stirred with thermometer, and the highest temperature achieved is recorded.
6. The experiment is repeated by replacing hydrochloric acid with ethanoic acid of the same concentration.
Data:

Type of acid Hydrochloric acid Ethanoic acid
Initial temperature of sodium hydroxide (°C) T1 T5
Initial temperature of acid (°C) T2 T6
Average temperature of acid and alkali (°C) T1+T22=T3 T5+T62=T7
Maximum temperature of the mixture (°C) T4 T8
Increase in temperature (°C) T4 – T3 = ∆θ1 T8 – T7 = ∆θ2
Calculation:

Neutralisation reaction between hydrochloric acid and sodium hydroxide
Total volume of the reacting solution = (50 + 50) cm3
Total mass of the reacting solution = 100g
Specific heat of solution = 4.2Jg-1 °C-1
Heat generated = mcΔθ = 100 X 4.2 X ∆θ1 = y joules

Concentration of NaOH = 2mol dm-3, volume of NaOH = 50cm3
Number of mole of OH used
= MV1000
= 2×501000
= 0.1mol

HCl(aq) 1 mole+ NaOH(aq) 1 mole NaCl(aq) 1 mole + H2O
0.01 moles of ion OH will liberate y J of heat energy.
Thus 1.0 mole of OH will produced (y/0.1) = 10y joules = (10y/1000kJ) = H1

Neutralisation reaction between ethanoic acid and sodium hydroxide
The heat of neutralisation between ethanoic acid and sodium hydroxide is calculated in the same way.
CH3COOH + NaOH → CH3COONa + H2O(l) , ∆H2 = xkJ mol-1
Analysis:

Neutralisation reaction between hydrochloric acid and sodium hydroxide
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) , ∆H1 = -57.0kJ mol-1
The neutralisation reaction between ethanoic acid and sodium hydroxide
CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l) , ∆H2 = -55.0kJ mol-1
The heat of neutralisation between a strong acid and strong alkali is always greater than the heat of neutralisation of a weak acid and a strong alkali.

The energy level diagrams for the neutralisation of acid and alkali it salt are shown in the following diagram.
Discussion:

Precautionary steps:
The initial temperature of solution was recorded after the thermometer is left in the solution for several minutes to obtain a constant temperature.
The final temperature recorded must be the highest temperature reached by the solution after mixing.



The experimental values of heat of neutralisation is always lower than their theoretical values. This is because some of the heat of neutralisation is lost to the surroundings and absorbed by the thermometer and the polystyrene.
Conclusion:

The heat of neutralisation between a strong acid and a strong alkali is bigger than the heat of neutralisation between a weak acid and a strong alkali.


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