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FC 024                        Wednesday 02/03/16
2 – 4pm
Experiment 4

Enthalpy of neutralisation

(Working in Pairs)

Pre-lab Questions:

Define the enthalpy of neutralisation

Write a balanced equation for the reaction of hydrofluoric acid and sodium hydroxide. Is this reaction exothermic or endothermic?

100cm3of 1M hydrofluoric acid at 20°C added to 100cm3 of 1M sodium hydroxide at 20°C produced a maximum temperature of 28°C for the mixture. Calculate ΔH.

*specific heat capacity of the solution (C) is 4.2 Jg-1K-1

Introduction

When an acid neutralises a base, a salt and water are formed. Aqueous hydrogen ions (H+(aq)) from the acid react with the hydroxide ions (OH–(aq)) from the base forming water, e.g.

H+Cl– + Na+OH– → Na+Cl– + H2O

The Na+ and Cl– ions remain unchanged; the only reaction is between the H+ and OH– ions:
H+ (aq) + OH– (aq)   →   H2O (l)
The neutralisation reaction will occur when H+ and OH– combine and energy is released until either H+ or OH– is entirely consumed. The reactant which is consumed first is called the limiting reactant.
The molar enthalpy of neutralization is defined as

ΔHneut =
qneut

n

Where qneut is the heat of neutralization, and n is the moles of the limiting reactant.
qneut = c m ∆T
Where m is the mass of solution
C is the specific heat capacity of the solution = 4.2 Jg-1K-1
∆T is the temperature difference

In this practical, we shall be measuring the enthalpy changes (ΔH) for different neutralisation reactions. Because the number of moles of water formed varies according to the acids and bases used, it is the convention to measure enthalpies of neutralisation (ΔH) in kilojoules per mole of water formed.

Procedure

Experiment 1: The reaction between Hydrochloric acid and sodium hydroxide

Measure 50cm3 of 2.0M hydrochloric acid into a polystyrene cup placed in a beaker and records its temperature. Pour 50cm3 2.0M sodium hydroxide into a measuring cylinder and record the temperature. Pour the sodium hydroxide into the acid, stir and take the final temperature.

(Note: if the acid and base were not at the same temperature initially, use the average of their initial temperature as the starting temperature)

Experiment 2

Repeat experiment 1 using 2.0M potassium hydroxide solution instead of sodium hydroxide solution.

Experiment 3

Repeat experiment 1 using 2.0M nitric acid (caution – corrosive) instead of the hydrochloric acid.

Experiment 4

Repeat experiment 1 using 2.0M sulphuric acid instead of the hydrochloric acid.

Experiment 5

Repeat experiment 1 using 2.0M sulphuric acid and 4.0M sodium hydroxide

Experiment 6

Repeat experiment 1 using 4.0M sodium hydroxide solution.

Table of Results

Acid
Base
Initial temp. 0C
(Average)
Final temp. 0C
(Average)
∆T 0C

Expt..1
2M HCl
2M NaOH

Expt. 2
2M HCl
2M KOH

Expt. 3
2M HNO3
2M NaOH

Expt. 4
2M H2SO4
2M NaOH

Expt..5
2M H2SO4
4M NaOH

Expt. 6
2M HCl
4M NaOH

Treatment of results

Write a balanced equation for each experiment including the states.
How much energy (qneut) was given to the 100cm3 of final solution during the reaction? (Assume that the specific heat capacity of the plastic cup is negligible and the specific heat capacity of the solution is 4.2 Jg-1K-1

How many mole of water was formed in each reaction (show your calculation).

Calculate the enthalpy change of neutralisation for each reaction (you’re your calculation).

Questions

Accepted values for the enthalpy changes of neutralisation of some acids and bases are shown below.

Reaction
ΔH   kJmol-1

HCl + NaOH → NaCl + H2O
-57.9

HNO3 + NaOH → NaNO3 + H2O
-57.6

HBr + NaOH → NaBr + H2O
-57.6

CH3COOH + NaOH → CH3COONa + H2O
-56.1

Compare the experimental values to the values in the table for the first two reactions. Account for any errors in your results.

Why is the enthalpy change for ethanoic acid slightly lower than the values for the first three reactions? (Hydrochloric, nitric and hydrobromic acids are all strong acids. Ka (ethanoic acid) =1.7×10-5.

Compare the temperature rise when 4.0M sodium hydroxide is used instead of 2.0M. Explain the relative values.

Experiment 4: Enthalpy of Neutralisation

Assessment

Behaviour in the lab.(respect of safety, focus, Attendance)                         3

Write-up and quality of report 10

Pre-lab questions

Q1                                                 1
Q2 2

Q3 2

Treatment of results

Balanced equation for each reaction 6

Energy for each reaction                         6

Moles of water in each reaction.             6

Calculate the enthalpy changes for each reaction 6

Q 5 3

Q6                                                                                     1

Q7 3

Observation, Discussion and Conclusion 6

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