Determining the Enthalpy of Neutralisation for Three Acids


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Determining the Enthalpy of Neutralisation for Three Acids

[IMAGE]Determine the Enthalpy of Neutralisation for the following
there Acids, H2SO4, HNO3 andH2SO4

Introduction

Acid and bases have a very important property that is that they are
able to cancel each other out when mixed together in the right
proportions, this reaction is called a neutralisation reaction, which
can be an exothermic reaction .

The standard enthalpy of neutralisation is the heat absorbed per mole
when an acid and a base react to form water under standard conditions.

In this experiment I will be investigating the values for enthalpy of
neutralisation of the three strong acids (HCl, HNO3 andH2SO4)

Scientific Background information

As explained before, neutralisation happens between an acid and a
base. Neutralisation is the formation of water from hydrogen and
hydroxide ions

H30+(aq) + OH-(aq) à 2H20(aq)

According to Arrhenius’ theory ‘neutralisation occurs because equal
mols of hydrogen ions in the acid are equal to the mols of hydroxide
ions in the base so the two react completely to form waterÂ’*1.

Neutralisation of HCl and NaOH:

NaOH(aq)         +          HCl(aq)              à             NaCl(aq)       +    H2O(l).

Neutralisation of HNO3 and NaOH

NaOH(aq) + HNO3(aq) à NaNO3(aq) +
H20(l).

Neutralization of H2SO4

NaOH(aq) + H2SO4 à Na2SO4(aq) + H20(l)

The enthalpy of neutralisation for strong acids are similar, because
‘strong acids fully disassociate in water therefore all hydrogen ions
and all hydroxide ions react to from water moleculesÂ’*2- taken from
Ramsden A level Chemistry

However enthalpy of neutralisation for H2SO4 would be different
compared to the the enthalpy of neutralisation for the other two

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acids. This is because H2SO4 is a dibasic acid which means for every
mol of H2SO4which dissolves 2 mols of H+ are dissacociated. For every
molecule of H2SO4 it will contain 2 hydrogens comaperd to HCl and HNO3
which contain only 1. This means twice as many H+ will be dissolved in
a given volume compared to HCl and HNO3. The enthalpy of
neutralisation, should therefore be different for H2SO4 as in
neutralisation energy is released when H+ + OH- à H20. Hence I H H2SO4
should release around twice the amount of energy in neutralisation
compare to HCl and HNO3. In order to confirm this In my experiment I
will use halve the conc of H2SO4 therefore this halves the amount of
H+ ions which are disassociated. Hence the amount of H+ dissolved
should be similar amount of H+ in HCL and HNO3, therefore the enthalpy
of neutralisation for all 3 acids should be similar.

Apparatus

· Polystyrene cup

· Glass Rod

· Thermometer

· Measuring Cylinder

· 1 moldm-3 of HNO3

· 0.5 moldm-3 of H2SO4

· 1 moldm-3 of HCl

· 1 moldm-3 of NaOH

· Stop clock

· Pen Paper, graph paper

· Ruler

· Calculator

Method

The heat released during a neutralisation (when 1 mole of water is
formed) can be found buy measuring the temperature rise produced in a
calorimeter. In this experiment I will be using a simple calorimeter
made of a polystyrene with a lid. ‘Expanded polystyrene is a good
thermal insulator and the temperature rise that occurs in the cup can
be measured before the loss of heat to the surroundings becomes
seriousÂ’*1 . I will carry out the neutralisation reaction in the cup
and taking regular temperature readings. Use this to plot a Temp vs
time graph. By extrapolating the graph find out the temperature rise
use this and the E= mc∆t to calculate the energy released to the water
and hence deduse the enthalpy of neutralisation.

The diagram below shoes how I will set it up:

[IMAGE]


Method:

1) Using a measuring cylinder, measure 25.0cm of Acid

2) Place 25.0cm3 of Acid in the polystyrene cup, measure the
temperature with a thermometer each minute for 4 minutes, using a stop
clock. ( I am measuring the temperature of the acid a couple of times
before I add the NaOH so that I get an accurate initial temperature as
this smoothes out any fluctuations in the temperature)

3) Using a measuring cylinder measure 25.0cm of NaOH

4) After the fourth minute add NaOH carefully and stir fully with a
glass rod. Carefully take the temperature reading of the solution
every minute until the 11th minute

5) Use this to plot at temperature-vs- time graph.

6) Repeat the experiment for the next acids.

Safety

· All the acids especially H2SO4 are toxic and skin irritants and are
the main hazard in the experiment. First of all I will be careful in
pouring the acid so that It does not go onto my skin when in contact
with acids the skin will irritate

· If the acid gets into contact with the eyes it will be difficult to
remove the acid, which would then cause serious damage to the eyes.
So for this reason I will wear safety goggles at all times during the
experiment, or if any explosions occur I will not damage my eyes.

Using the results

· Plot the temperature(°C) -vs- time(min) graph.

· [IMAGE]Use the diagram to draw a line of best fit for the
temperature of the solution before addition of alkali and draw the
line of Best Fit after addition of alkali as shown in the diagram
below

* Use this to find the initial temperature and the maxixmum
temperature, hence the temperature achange

Obtaining Results:

Results Tables

HCl

Conc: 1 moldm-3

Amount of acid added: 25cm3

Final temperature: 27.4°C

∆T : 5.9°C

Time (min)

Temperature of Solution / °C

0

21.0

1

21.5

2

21.5

3

21.5

4

22.0

5

24

6

25.0

7

26.5

8

27.0

9

26.5

10

25.5

11

24.5

12

24

13

23

[IMAGE]

HNO3

Conc: 1 moldm-3

Amount of acid added: 25.5

Final temperature:25.9°C

∆T : 5.9°C

Time (min)

Temperature Of Mixture/ °C

0

20

1

20

2

20

3

20

4

20

5

22.5

6

23

7

24.5

8

25

9

25.5

10

25

11

25

12

24

13

23.5

14

23

[IMAGE]


H2SO4

Conc: 0.5 moldm-3

Amount of acid added: 25

Final temperature: 26.7°C

∆T : 6.2°C

Time (min)

Temperature Of Mixture / °C

0

20.5

1

20.5

2

20.5

3

20..5

4

20.5

5

23

6

24.5

7

25.5

8

26.5

9

27

10

26

11

25.5

12

25

13

14

24.5

14

24

[IMAGE]


Analysis

Calculations:

HCl

Heat Transfer (E) = Mass x specific heat capacity x temperature
change

Heat = m x C x ∆T

Where: m= mass of acid + mass of alkali

C= specific Heat capacity 4.2Jg k.

∆T= temperature change (maximum temperature- initial temp)

We assume that the specific heat capacity of water is the same as the
acid and NaOH

Also that the 1cm3 of Acid or NaOH is = 1g

Mass = (25 + 25)

= 50., mass

From the graph

∆T= 5.9°C

E = 50g x 4.2 Jg k. x 5.9°C

= -1239.00J

Moles of HCL = volume (dm3) x concentration (moldm-3)

= 25/1000 x 1.0

= 0.025 moles

Standard Molar Enthalpy of neutralisation= -1.239J/0.025

= -49.560 kJ mol-1

HNO3

Heat Transfer (E) = Mass x specific heat capacity x temperature
change

From the graph

∆T= 5.9°C

E = 50 g 4.2 Jg k. x 5.9°C

= -1239.00J

Moles of HNO3= volume (dm3) x concentration (moldm-3)

= 25/1000 x 1.0

= 0.025 moles

Standard Molar Enthalpy of neutralisation= -1.239J/0.025

= -49.560 kJ mol-1


H2SO4

Heat Transfer (E) = Mass x specific heat capacity x temperature
change

From the graph

∆T= 6.2°C

E = 50 g 4.2 Jg k. x 6.2°C

= 1302.00 J

Moles of HNO3= volume (dm3) x concentration (moldm-3)

= 25/1000 x 0.5

= 0.0125 moles

Unlike the other acids, the molar ratio of H2SO4 with NaOh is 1:2,
shown by the following neutralisation reaction

H2SO4 + 2NaOH à Na2SO4 + 2H2O

Therefore the no. of mols. of H2SO4 = 0.0125 x 2

= 0.025mols

Standard Molar Enthalpy of neutralisation= -1.302J/0.025

= -52.08 kJ mol-1

As expected the Standard Molar Enthalpy of neutralisation were fairly
similar HCl HNO3 H2SO4 their Enthalpy of neutralisation were
-49.560, -49.560 and -52.08 kJ mol-1. this supports my theory that
Enthalpy of neutralisation of strong acids are the same, and for
dibasic acid H2SO4 release around twice the amount of energy in
neutralisation compared to HCl and HNO3.

However only to an extent does this support the theory as you can see
with the results the Enthalpy of neutralisation H2SO4 was different
to the Enthalpy of neutralisation for the other acids. However I
believe this is more to do with inaccuracies in my experiment, and the
different behaviour of diff acids.

Evaluation:

Looking at my results I can see there are some slight inaccuracies and
inconsistencies. Firstly the inaccuracy highlighted above and also the
values for the Enthalpy of neutralisation are slightly lower compared
to values of Enthalpy of neutralisation in the textbook. I believe
these are due to several errors in my experiment

1) Firstly the low values of Enthalpy of neutralisation which are
result of heat being lost the surroundings before the temperature rise
has been recorded. This is probably due to the fact that I didnÂ’t use
a lid on the polystyrene cup. If I was to repeat the experiment I will
make sure I do this to keep in as much heat energy in the cup.

2) Secondly my stirring for the diff acid experiment was not
consistence. I may of stirred it differently for all the others, this
would mean that more or less heat would of escaped, then required,
which affect my thermometer readings. There are

3) Thirdly In my attempt to reduce the time needed to complete my
experiment I didnÂ’t rinse the measuring cylinder which I was using to
measure out the volumes of Acid. Of course this must have led to some
contamination of different acids with the acid I was measuring. Next
time I will ensure that I wash all apparatus and rinse in appropriate
solutions each time I repeat the experiment for different acid.

Lastly the thermometer I was using was inaccurate by o.5C. The faulty thermometer would have no effect on the calculated

enthalpy of neutralization because the thermometer read 0.50 degrees low

consistently so it would have had no effect of the ∆T, however next time I will make sure I use a thermometer which is correct to avoid any confusion.

Bibliography and References

*1 : Taken from p9 194 of Advanced Level Chemistry for AQA: A2
Student

*2: Taken from Ramsden A level Chemistry Book

· A-Level Chemistry (A-Level Chemistry S.)
by E.N. Ramsden ISBN: 0748752994#

· Chemistry AS Level and A Level (Cambridge Advanced Sciences) by
Brian Ratcliff (Series Editor), Helen Eccles, John Raffan, ISBN:
0521544718

· A-level Study Guide Chemistry ('A'Level Study Guides) by Bob McDuell
ISBN: 1857583361


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