Ethanoic acid + Ammonium hydroxide
Once the acids and alkalis have been measured, then the acid should be placed in the polystyrene cup and its temperature taken. The temperature of the alkali should also be taken. Then the acid and the alkali should be mixed in the polystyrene cup. The thermometer should be put in the cup after the alkali has been put in, and the peak temperature, should be noted. The heat of neutralisation of an acid by a base is the amount of heat liberated when one mole of hydrogen ions from an acid reacts with one mole of hydroxide ions from a base. But in this case as the solutions are 2 molar then the heat liberated will be that when the 2 moles react. I predict that the stronger the acid or alkali the greater the heat change during the reaction.
All the acids and alkalis are 2 molar solutions. This means that each of the solutions contain 2 moles of solute dissolved in 10003cm (1dm3) of solution.
I predict that the strong acid and strong alkali added together will cause the largest heat change to occur and the weak acid and weak alkali to have the smallest heat change. The reason why the stronger acids and alkalis will have a higher heat change than the weaker ones is because a strong acid is one which fully dissociates into ions in an aqueous solution, and a weak acid only partially dissociates. Thus this allows more bonds to be created, therefore liberating more energy.
The above will be tested in such an order, as it is believed that this will be the final sequence in rank order of the increase in heat change from highest to smallest. The heat change in the strong acid and alkali will be the highest because there are more hydrogen and hydroxide ions in a strong acid than in a weak acid.
-57.3 kJ is an accepted constant value (‘Success in Chemistry’ by John Bandtock and Paul Hanson), of heat liberated when a strong acid is neutralised by a strong alkali. One mole of strong monobasic acid results in one mole of hydrogen ions, which can be neutralised by one mole of hydroxide ions from a strong base. So it can be inferred that when a weak acid or alkali (or even both) are mixed the heat of neutralisation is less than -57.3kJ. This is because the weak acid will not carry, as many hydrogen ions, and energy will consequently be used to liberate hydrogen ions from the undissociated acid in order to make them available for reaction with hydroxide ions.
In weak acids and alkalis the ions may not be totally independent as in strong acids and alkalis, so the following example demonstrates:
CH3COOH (aq) CH3COO- (aq) + H+(aq)
The above equilibrium may be in the ratio of 0.97:0.03 respectively, therefore meaning that
H+(aq) + OH- (aq) H2O (l)
the above takes place, and more bonds between CH3COOH and H+ have to be broken. This requires energy, 1.2kJ, and thus the total heat energy which is given out when the reaction
CH3COOH (aq) + NaOH CH3COO-Na+ + H2O (l)
occurs, is 1.2kJ less, giving –56.1kJ. The same is for all strong and weak acids and alkalis, the strong substances do not need the bonds to be broken as they are already fully dissociated.
When the acid and alkali are mixed, the start temperature will be taken, as well as a finish temperature. Then two repeat temperatures will be taken of each start and finish temperature. This allow for an average for the rise in temperature to be taken.
A preliminary experiment was carried out, and it was a thermometric titration of hydrochloric acid and sodium hydroxide. This allowed an insight into what to expect when two strong acid and alkalis are mixed.
Obtaining Evidence
As this experiment involved the usage of acids and alkalis, immense care had to be taken to prevent accidents from occurring. A number of safety precautions were taken. These were that eye protection had to be worn at all times. Also if any acid or alkali was spilt then it was cleared up immediately, and hands were washed thoroughly after the clearance. The acid and alkali were placed in contrasting measuring cylinders so that they were not confused; this was done to avoid a major setback such as the mixing of the wrong acid and alkali.
Calculations
Heat evolved = mass of solution x specific heat capacity x temperature rise
HE = m x c x θ
Density of solution = 1g/cm3 (value for water)
Mass = Volume x density
Mass = 40cm3 x 1g = 40g
Heat capacity of solution = 4.2j/kg (approx) (Value for water)
Heat evolved = 4.2 x 40 x θ (for each experiment)
Heat evolved (HCL +NaOH)
= 4.2 x 40 x 16.25
= 2730 Joules
Heat evolved (HCL + NH4OH)
= 4.2 x 40 x 13.50
= 2268 Joules
Heat evolved (CH3OOH + NaOH)
= 4.2 x 40 x 12.25
= 2058 Joules
Heat evolved (CH3OOH + NH4OH)
= 4.2 x 40 x 11.62
= 1952.16 Joules
To find Jmol-1
2 molar solution = 2 moles per 1000cm3
1000 x 2 = 2000cm3
40cm3 ÷ 2000cm3 = moles per 40cm3
= 0.08 moles
Heat evolved per 40cm3 ÷ moles = Jmol-1
HE ÷ 0.08 = Jmol-1
HCL +NaOH
2730 ÷ 0.08 = 34125 Jmol-1
HCL + NH4OH
2268 ÷ 0.08 = 28350 Jmol-1
CH3OOH + NaOH
2058 ÷ 0.08 = 25725 Jmol-1
CH3OOH + NH4OH
1952 ÷ 0.08 = 24402 Jmol-1
Analysing evidence
As can be seen from the results, the stronger the acid or alkali, the larger the heat change in the reaction. More heat is evolved from the sodium hydroxide and hydrochloric acid than any other solution. As the strength of the solution increases the more heat is evolved from it.
This can be seen from the bar chart, which illustrates the average amount of Jmol-1 released in the reactions. The reaction between hydrochloric acid (strong acid) and sodium hydroxide (strong alkali) had the most heat energy released, 34125 Jmol-1, although the reaction between the weak acid and alkali, ethanoic acid and ammonium hydroxide released 24402 Jmol-1.
There is a difference between these two reactions of 9723 Jmol-1, because of the equilibrium reactions that have to take place so that the hydrogen ions in the ethanoic acid the hydroxide ions in the ammonium hydroxide dissociate is an energy requiring process.
These processes are illustrated below:
CH3OOH (aq) CH3OO- (aq) + H+(aq) (energy requiring process)
NH4OH(aq) NH4+(aq) + OH- (aq) (energy requiring process)
The reason why the hydrochloric acid and sodium hydroxide release much more energy overall because the Na+ and OH- ions are already totally dissociated and thus independent, as is with the HCL, the H+ and Cl- ions are totally dissociated, and thus no energy is used to dissociate them.
The bar chart shows the amount of heat evolved in rank order. As can be seen, the weaker the concentrations the less heat is released.
The evidence that has been obtained from this experiment does support my prediction of the stronger the acid or alkali the greater the heat change in the reaction. This is because all the weak acids and alkalis released less heat per mole than the strong acids and alkalis. The difference of 9723 Jmol-1 between the highest heat change and the lowest heat change gives strong evidence of the prediction being correct.
Evaluation
The experiment was a success, as can be seen from the results which support the prediction. Although, there was room for substantial improvement.
The results obtained were considerably lower than the expected values. For example, the accepted theoretical value for hydrochloric acid + sodium hydroxide= -57.3kJ, whereas the result obtained was –34.125kJ. This can be explained through the faults of equipment and experimental procedure.
Firstly, heat energy escaped easily through the top of the polystyrene cup, and it was a substantial amount as it could be felt if your hand was placed above the cup. This would have undoubtedly lowered the value of the temperature change.
Secondly, the polystyrene cup was assumed to have a heat absorption capacity of zero. This is misleading as the polystyrene cup would have absorbed a small amount of heat energy, and this in turn would have lowered the temperature change.
Thirdly, the measuring cylinder was only accurate to 2.5 cm3 and a more accurate piece of equipment would have been more suitable to ensure that accurate measurements were taken. Also the thermometer was only accurate to 0.5oC and a more accurate thermometer would have allowed more accurate temperature changes to have been taken.
Even though the concentration of the hydrochloric acid had been tested in the preliminary experiment to ensure whether or not the concentration was accurately 2 molar, consequently which it was, the other acids and alkalis may not have been exactly 2 molar.
No anomalous results were obtained, but whilst undertaking the experiment some values that were obtained did not relate with the theory when compared with the previous results. It was later discovered that these incorrect results were due to small droplets of water being left in the bottom of the polystyrene cup and measuring cylinder, causing the concentration to lower of the acid and alkali. However these incorrect results were neglected, and a repetition was undertaken without the inaccuracy, and it was ensured that the cup and cylinder were thoroughly dried to obtain accurate results.
Some improvements to the experiment would be:
- A lid would be needed for the polystyrene cup so as to minimise the heat loss as much as possible. This would also be made of polystyrene.
- The use of a more accurate thermometer.
- The use of pipettes and burettes would be very helpful in carrying out the experiments to ensure that they were as accurate as possible.
- Undertake titrations of the acids and alkalis in order to check their concentrations.
To extend the investigation, some dibasic and tribasic acids and alkalis could be investigated. These are acids and alkalis that have more than one hydrogen or hydroxide ion. This can be seen below:
H2SO4 HSO4- + H+
HSO4 SO4- + H+
This would give an insight into how the heat evolved is related to the strength of an acid or alkali.