Heat (q) released by the reaction = - heat (q) absorbed by the solution
Heat released
We would record the temperatures of the acid and base solutions before mixing to get. The maximum temperature that was reached after the time of mixing would be recorded to be the solution’s final temperature. Start monitoring the temperature as soon as the solutions had been mixed.
Procedures:
The experiment was worked in pairs and I was grouped with Peter Chan, and the acid and alkali provided for Group 1 is hydrochloric acid and sodium hydroxide.
Part I - Neutralization between aqueous base and acid
Method 1:
- A dry polystyrene foam cup was getting ready and two clean burettes were set in stands respectively.
- Two burettes were filled with ~1.0M alkali solution and 1.0M acid respectively with filter funnels and the tips of the burettes were drained, then the initial burette readings were labeled and recorded.
- The specific volume of the alkali solution was drained in to the dry polystyrene foam cup according to the table in the table of the next page.
- The content in the polystyrene foam cup was let equilibrate for a few minutes and measured the temperature of the solution (initial temperature).
- The specific amount of acid was quickly drained into the polystyrene cup containing the alkali solution. And for the experiments 4 and 5, the alkali was added to the acid instead of the other way round.
- The reaction mixture was swirled gently, then the maximum temperature of the reaction mixture was measured immediately and the result was recorded for the maximum temperature reached.
- The content of the polystyrene foam cup was emptied and washed. And steps 3 to 6 were repeated with other combinations of volumes of alkali solution and acid.
- A graph was plotted and (1) the concentration of the given alkali solution and (2) the enthalpy change of neutralization were determined.
Method 2:
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25.00of the given alkali solution was pipette into a dry polystyrene foam cup.
- The content in the polystyrene foam cup was let equilibrate for a few minutes and the temperature of the solution was measured and the result was recorded.
- The acid was poured into the burette to the mark ‘0 cm’ through a filter funnel and make sure the tip of burette was drained before taking reading.
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2.00 of the acid were added from the burette into the polystyrene cup containing the 25.00alkali solution. The reaction mixture was swirled gently and the maximum temperature the solution reached was recorded.
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When the maximum temperature has been reached, another 2.00 was added into polystyrene cup and the maximum temperature was recorded again.
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Step 5 was repeated until a total volume of 40.0 of acid was added.
- A graph was plotted and (1) the concentration of the alkali solution and (2) the enthalpy of neutralization were determined.
Results:
Temperature of the laboratory:
Alkali used in the experiment: sodium hydroxide
Acid used in the experiment: hydrochloric acid
Part I
Method 1
(A large scale graph would be attached at page 11)
The maximum change in temperature was 5.8 degree Celsius.
The volume of alkali added at the time while maximum change in temp. = 14.70
The volume of acid added = 30.00 – 14.70 = 15.30
No. of mole of
No. of mole of
Molarity of
ΔH = -m x c x ΔT
= -30 x 4.184 x 5.8
= -728.016J
The enthalpy change of neutralization
Method 2
(A large scale graph would be attached at page 12)
The maximum temperature during the experiment was
The volume of acid (aq) added to the experiment while the maximum temperature reached was 25.5.
No. of mole of acid
No. of mole of alkaki
Molarity of alkali
ΔH = -m x c x ΔT
= -(25.50+25.00) x 4.184 x (28.3-22.4)
= -47.5 x 4.184 x 5.9
= -1246.623J
The enthalpy change of neutralization
Discussion:
1. Every chemical change is accompanied by a change in energy, usually in the form of heat. The energy change of a reaction that occurs at constant pressure (i.e. atmospheric pressure) is termed the heat of reaction or the enthalpy change. If heat is evolved, the reaction is exothermic and if heat is absorbed, the reaction is endothermic.
2. The first law of thermodynamics states that heat is not lost, gained, or destroyed. Only it is transformed from one form to another. The second law is shown by the material gaining the heat moving from the material losing heat. This movement is equal, meaning there is no loss or gain of heat. The combination of these two laws is
Heat lost = Heat gained
3. Although time was a set of data that we took in our lab procedures, it was not a factor in the equations we used for our calculations mention in the introduction section. This was because we assumed heat was instantly exchanged at the time of mixing. Thus, the ΔT we used in the calculations are the ΔT at mixing time. Notice that no reasonable temperature readings could be taken at the time of mixing, therefore we needed to assume that the temperature we measured was the highest temperature during the measure had no different to the ΔT at mixing time in the experiment.
4. Before the solutions were drained into the burette, we should make sure the filter funnel was cleaned and placed stable on the top of the burette. When draining the solution, the mouth of the burette MUST be below your eye-level and all the people around were wear the safety goggles and lab coats.
5. During the experiment set 1, 2 and 3 in method 1, acid were added into the polystyrene cup containing the alkali solution, and in the set 4 and 5, the alkali were added into the one containing the acid solution, it was because a large amount of acid or base might provide a higher degree of specific heat capacity to minimize the effects from the specific heat of the polystyrene foam cup or the thermometer. So we needed to do that step quickly and make the heat loss to the surrounding become the minimum.
6. We could use two polystyrene foam cups constructed of nested Styrofoam cups to reduce the heat loss to surrounding as Styrofoam was a good insulator.
7. The experiment was run at constant pressure, thus the heat changes observed would be equal to the enthalpies of reaction, ΔH for the reactions considered. We would obtain experimental data to directly calculate the ΔH for reactions in this experiment.
8. In the procedure of disposal of the chemicals, all reactants and products should be disposed of into the sink with water.
Conclusion:
The experiment was finished in two hours, as it was my first laboratory work in the University of Hong Kong, I felt a bit different to those in my secondary education. And I think the experiment was carried out successfully with my partner, Peter. Although it was our first time to cooperate with the others, we did the works in a smooth and correct direction without any unchangeable mistakes.
There should be an error happen in the set 2 of the method 1 in the experiment; the spot plotted on the graph was varying to the location we preferred, and it might make the answer a bit different to the correct one. The enthalpy change of neutralization should be much more negative, at least, smaller than -50kJ/mol.
Further Discussion for the QA section:
7. When calculating the enthalpy change of neutralization in the experiment, we would consider the heat absorbed by the cup, lid, thermometer and the rest of the surroundings negligible, as polystyrene foam is a good insulator, even though some heat would be absorbed by the cups and some will be released to the surroundings, it was small enough as compared to the heat absorbed by the solution in the calorimeter.
Also, we would like to assume that, the two solutions, sayandwere initially at the same temperature. This was a good assumption when the solutions had been prepared well ahead of lab time and had been allowed to equilibrate at room temperature.
Finally, we need to assume that the density of all solutions used in the experiment was the same as the density of water and the specific heat capacity of the solution is equal to that of the water, .
8. (A) Method 2 provides a more accurate result in the determination of the concentrations of alkali solution as the number of times we measured for the temperature (2once) was much more than the number of measurement, the value of the slope and the peak of the curve was more accurate than the one in method 1.
(B)But method 1 provides a more accurate result in the determination of the enthalpy change of neutralization as comparing to the method 2. It is because we needed to measure several times in the experiment method 2, so a higher amount of heat would be lost to the surrounding and the error in the calculation would be greater than that in the method 1.
9. The experimental results of our classmates were collected and they were recorded in the following table.
No group had done the experiment of combination of andas there were only 7 groups of people in the laboratory.
10. The change in enthalpy, ΔH, is a measure of the change in heat content and could be measured by subtracting the heat content of the reactants from the heat content of the products. And the enthalpy of neutralization was the heat produced when an acid and a base react together in aqueous solution to produce one mole of water. Strong acids (e.g. hydrochloric acid, sulfuric acid or nitric acid) and strong alkalis were completely dissociated in dilute solution, so the reaction between any strong acid and strong alkali might be represented as following:
If a weak acid or alkali were used, or if both were weak, then the enthalpy of neutralization was usually lower than. This was because weak acids and weak alkalis were only slightly ionized in aqueous solution, and energy was absorbed in ionizing the unionized molecules.
The enthalpies of reactions and Hess's Law would be used to calculate the enthalpy change for the reaction which was not directly observed. Recall that since enthalpy was a state function, the enthalpy change for any process would depend only on the starting and ending points, not on the reaction path followed. The enthalpy, ΔH, changes for the reaction, we used the acid-base combination of group 3 as an example:
, can be written as
Since ΔH’ is positive, the value had to smaller than the value of the combination of strong acid and alkali because, so the value of is less negative then that of.
Hazards:
The diluted hydrochloric acid and aqueous ammonia were corrosive to equipment and skin. They were also volatile, therefore, be very careful to keep their vapors out of the room. The dilute acid, base, and salt solutions might all be flushed directly down the sink with lots of water.
