I reckon that when a weak acid and weak alkali are added they’ll have a lower amount of heat given out than if two strong solutions were added.
When a weak acid and strong alkali react and vice versa the amount of heat given off will be lower than if a strong solutions were mixed depending on the weak solutions strength.
Results
Hydrochloric Acid (Strong) + Potassium Hydroxide (Strong)
Sulphuric Acid (Dibasic) + Sodium Hydroxide (Strong)
Ethanoic Acid (Weak) + Ammonia (Weak)
Ethanoic Acid (Weak) + Sodium Hydroxide (Strong)
Hydrochloric Acid (Strong) + Ammonia (Weak)
Heat of Neutralisation
Formula Needed: Heat = mass x specific heat capacity x Temperature rise
Moles = concentration x volume
1000
All solutions were 2 Molar.
Hydrochloric + Potassium Hydroxide
Heat = 100 x 4.18 x 12.85
= 5371.3 Joules
Moles of acid at end point
n = c x v
= 2.0 x 50/1000 = 0.1
Heat of Neutralisation = 5371.3/0.1 = 53713 Joules Mol –1
= 53.713 KJ/Mol-1
Sulphuric Acid + Sodium Hydroxide
Heat = 80 x 4.18 x 19 = 6353.6 Joules
n = c x v = 2.0 x 30/1000 = 0.06
Heat of neutralisation = 6353.6/0.06 = 105893.33 Joules Mol-1
= 105.89 KJ/Mol-1
Ethanoic Acid + Ammonia
Heat = 100 x 4.18 x 12.1 = 5057.8 Joules
N=c x v = 2.0 x 50/1000 = 0.1
Heat of neutralisation = 5057.8/0.1 = 50578 Joules Mol-1
= 50.578 KJ/Mol-1
Ethanoic Acid + Sodium Hydroxide
Heat = 100 x 4.18 x 17.65 = 7377.7 Joules
n = c x v = 2.0 x 50/1000 = 0.1
Heat of neutralisation = 7377.7/0.1 = 73777 Joules Mol-1
= 73.777 KJ/Mol-1
Hydrochloric Acid + Ammonia
Heat = 100 x 4.18 x 12.6 = 5266.8
n = c x v = 2.0 x 50/1000 = 0.1
Heat of neutralisation = 5266.8/0.1 = 52668 Joules Mol-1
= 52.668 KJ/Mol-1
Analysis
Hydrochloric Acid (Strong) + Potassium Hydroxide (Strong)
Neutralisation happened when 50cm3 of acid was added to 50 cm3 of alkali. Therefore they have a 1:1 ratio, this means that the acid has H+ ions and the alkali has OH- ions, they both nearly have the same amount as each other. This reaction gave out 53.713 KJ/Mol-1, so the reaction must be exothermic. The amount of heat energy given off when Hydrochloric acid and Sodium Hydroxide react is 57 KJ/ Mol-1. The reason why the heat of neutralisation is lower than for HCL + NaOH maybe because the NaOH may have had more OH- ions than KOH, the NaOH didn’t have to break bonds since it was almost completely ionised. When bonds are formed energy is giving off and when bonds are broken energy is needed to break them, these determine overall how much heat energy is giving off. KOH is still a strong alkali but not as strong as NaOH meaning that NaOH has more OH- ions than KOH when not mixed with an acid.
When more acid is added after the neutralisation has happened the temperature of the solution drops because the same amount of heat energy has to heat up more solution.
Sulphuric Acid (Dibasic) + Sodium Hydroxide (Strong)
Neutralisation happened when 30 cm3 of acid was added to 50 cm3 of alkali. Sulphuric Acid is dibasic, so has twice the amount of H+ ions, this is why the neutralisation point happens when only 30cm3 of acid is added. The acid doesn’t have to break many bonds because it is almost completely ionised and this also applies to the alkali, that is the reason it gave off 105.8 KJ/Mol-1. When the H+ and OH- combined they gave out heat energy, energy is needed to break bonds but the sulphuric acid and Sodium Hydroxide didn’t have to break many bonds, they mainly formed bonds resulting in such a high amount of heat energy given off.
When even more acid was added it made the solution cooler because the heat given off had to heat up a larger solution each time more acid was added.
Ethanoic Acid (Weak) + Ammonia (Weak)
Neutralisation occurred when 50cm3 of ethanoic was added to 50cm3 of ammonia. The heat of neutralisation point was 50.578 KJ/Mol-1. This is considerably lower than sulphuric acid and sodium hydroxide, the reason being because they were strong and these are weak. Meaning that the ethanoic and ammonia are not completely ionised, to make them completely ionised energy is needed to break bonds within the solutions to form H+ ions from the acid and OH- ions from the alkali. When the bonds now form to create H2O and a salt, heat energy is given off, but because the solutions became cooler more heat energy is needed to heat up the solution. These results in a lower amount of heat given off, than if the bonds were already completely broken into ions.
The results show the two solutions gradually getting cooler when more acid is added because the solution being added is cooler than the solution in the cup, so the same amount of heat has to heat up a larger solution.
Ethanoic Acid + Sodium Hydroxide
Neutralisation happened when 50cm3 of acid was added to 50cm3 of alkali. The amount of heat given off was 73.777 KJ/Mol-1. This is quite higher than when ethanoic and ammonia react. This is a big error since it should be around 52 KJ/Mol-1, this may have been caused by human error or the thermometer may have been inaccurate. The weak acid would be slightly split up into ions and would need energy to break bonds to form H+ ions. The alkali is strong and doesn’t need much energy to ionise it. There is only a limited amount of OH- and H+ to react, when they react that’s the amount of energy given out, but it has to heat a cooler solution because of the loss of heat when breaking bonds in the ethanoic acid.
The temperature after the point of neutralisation decreased because a cooler solution was being added so the same amount of heat energy had to heat up a larger solution.
Hydrochloric Acid (Strong) + Ammonia (Weak)
Neutralisation occurred when 50cm3 of HCL acid was added to 50cm3 of NH4OH alkali. The amount of energy given out was 52.668 KJ/Mol-1. The strong acid is almost completely ionised. The weak alkali isn’t and needs energy to get OH- ions. When the ions react to form water they give out heat energy, but there is a limit on how many H+ and OH- ions can react together. When bonds were being broken the solution became cooler, when bonds were being formed energy was being given out but since the solution became cooler when bonds were being broken, the heat now has to heat up a cooler solution.
After 50cm3 of acid was added the temperature started to decrease because a cooler solution was being added so the same amount of energy had to heat up a larger solution.
Conclusion
Strong Acid vs. Strong Alkali
I would like to conclude that when a strong acid and strong alkali are added together that a high amount of energy is given out depending on their strengths, but especially if they have many hydrogen ions because if there are more H+ ions they don’t need energy to get them since there already in the acid, this applies to dibasic acids.
My prediction was correct; I thought the strong acids vs. strong alkalis and dibasic acid vs. strong alkali would give out a lot of heat depending on how strong they were, when HCL + KOH reacted the temperature was lower than what I thought because the acid or alkali wasn’t as strong as I anticipated to be.
Weak Acid vs. Weak Alkali
I’d like to conclude that when a weak acid and a weak alkali are added together, a small amount of heat energy is given out. Since they are both not completely ionised both of the solutions need energy to break bonds, so therefore less energy is given out.
This supports my prediction since I thought that the amount of heat given off will be less than say if a strong acid and strong alkali was mixed together.
Weak Acid vs. Strong Alkali and vice versa
I would like to conclude that when a weak acid vs. strong alkali react the amount of heat energy given off is low, this also applies for a strong acid vs. weak alkali. This is because energy is needed to completely ionise the weak solution, so the solution then becomes cooler. When the H+ and OH- ions react heat is given off but it has to heat up a cooler solution since energy was needed to break the bonds before.
When NaOH and CH3COOH reacted it gave out 73.77 KJ/Mol-1, this is a high amount and didn’t support my prediction. I predicted that the amount given off would be low, like when I reacted HCL and NH4OH. The reason why NaOH + CH3COOH was so high was probably down to human or equipment error.
Evaluation
Overall I think that the experiment was successful. Although I did collect two anomalies on my graph, the one was on HCL + KOH and the other on H2SO4 + NaOH. They weren’t largely wrong but had a slightly higher temperature than what it should have been. This was most likely to have happened because of human error or an inaccurate reading by the thermometer.
When I added HCL and KOH the value was 53.713 KJ/Mol-1, the real value is 57.2 KJ/Mol-1. Therefore my results were fairly accurate even though I was using a very basic set of apparatus. The value I obtained for CH3COOH and NaOH was 73.777 KJ/Mol-1, the true value is 55.2. This is a large error, this may have happened because the thermometer was faulty since for that experiment I used a different thermometer. Also it could have been because of human error or inaccurate measuring equipment. The other combinations of acids and alkalis were quite accurate and only varied slightly to there true values. My results supported my predictions but not completely my prediction on weak acid and strong alkali and vice versa since the CH3COOH and NaOH results were largely inaccurate. Overall the results support my conclusion.
If I were to carry out the same experiment out again to improve it I would have all the amounts of acid needed in separate beakers (9 beakers of 10cm3 of acid) to avoid wasting time when carrying out the experiment. If possible it would make the experiment more accurate if the room temperature was kept the same. Also that I would always use the same set of apparatus, especially the thermometer. If the solutions were being measured with a pipette or burette it would be more accurate than if a measuring cylinder was used.
Further Work
Using different acids and alkalis I could do a similar experiment. Put 100cm3 of acid in a beaker and add a few drops of phenolphthalein and place a thermometer in it. Then add alkali every 5cm3 using a syringe, when neutralisation occurs the solution will become pink. Record the temperature every time the alkali is added. Then workout the heat of neutralisation, this value will be more accurate than the last experiment.
Bibliography
- Chemistry exercise book
- Chemistry Text book
- Teacher – Mr Mcloughlin
- Internet
- Encarta