Mix an acid and an alkali and measure the temperature change.
Temperature Change in a Reaction - Heat of Neutralisation Coursework
Planning:
Aim
To mix an acid and an alkali and measure the temperature change.
Factors
The factors that could make a difference are:
( A change in equipment, if you change the type of cup you use for your experiment such as if you use a beaker or a tin can instead of a polystyrene cup. The polystyrene cup is better because the temperature wouldn't decrease and because, in it as insulation to stop heat getting out.
( If you changed the type of acid or alkali you used. We used one type of acid and one type of alkali only, throughout our whole experiment.
( If you changed the amount of hydrochloric acid or sodium hydroxide. If the total number of each substance added together wasn't the same then this would cause a difference.
( If you changed the concentration of the acid because if you use more concentrated or stronger solution, the reaction will be faster.
( The temperature could make a difference because the higher the temperature, the faster the reaction will be.
( If you add a substance called a catalyst in then this would speed up the reaction without being used up.
The factor, which I am going to investigate, is the third one, which are the amounts of acid and alkali.
Prediction
I predict that the reaction will be high because the temperature will increase, when acid and alkali are mixed together. There will also be a change in the temperature when the two are mixed together, as it will quickly increase and this shows that a chemical reaction will take place because it is exothermic. I also predict that the temperature will increase when there is more solution in it and when the same amount of both alkali and acid is put in, so it is the same concentration.
Scientific Reasons
The reaction is: H Cl+ Na OH --> Na Cl + H2 0
The temperature will increase because a chemical reaction will take place and energy will be taken in.
A reaction that gives out heat energy is exothermic. The reaction supplies the energy as heat. This heat energy is given to the surroundings. The temperature of the reaction mixture might go up - making it feel hot. Examples are combustion - fuels burning and explosions - TNT exploding.
The reaction of magnesium with dilute hydrochloric acid is exothermic. The reaction tube feels warm because the reaction gives out heat energy to the water, the tube and your hand.
Exothermic reactions:
An example of an exothermic reaction
The energy changes involved
Energy level diagram
The energy released by the amounts of reactants shown in an equation is called the heat of reaction.
Chemical energy can be released as heat. In the above diagrams the reactants have an energy content, H1, and the products have an energy content, H2. During the reaction, some energy is given out by the reactants as they react. So the energy content of the products, H2, is lower than that of the reactants, H1.The heat of reactions is the difference between the energies of the reactants and products. It can be written as ' H.' ' H' means 'the change in energy content.' It must have a sign to show whether the energy content has increased or decreased. For an exothermic reaction, H is negative. The reactants lose energy to give the products.
Energy level diagrams like the one shown on the previous page can be used to show the energy changes.
All combustion reactions are exothermic. The heat of combustion of ethanol is 1366 kJ/mol. This information an be included in the equation:
C2 H5 OH (1) + 3O2 (g) --> 2CO2 (g) + 3H2O (1); H = - 1366 kJ/mol
?
Heat energy is given out
The energy change is for the amounts shown in the equation.
Heats of combustion reactions are measured by allowing the heat from the reaction ...
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Energy level diagrams like the one shown on the previous page can be used to show the energy changes.
All combustion reactions are exothermic. The heat of combustion of ethanol is 1366 kJ/mol. This information an be included in the equation:
C2 H5 OH (1) + 3O2 (g) --> 2CO2 (g) + 3H2O (1); H = - 1366 kJ/mol
?
Heat energy is given out
The energy change is for the amounts shown in the equation.
Heats of combustion reactions are measured by allowing the heat from the reaction to be absorbed by water.
Heats of reaction in solution are measured by allowing the heat from the reaction to be absorbed by or taken from the solution in an insulated container. In exothermic reactions, the heat given out is absorbed by the solution. The temperature rises. The insulation reduces the amount of heat energy 'lost' to the air.
If solutions are dilute, their specific heat capacities and densities are about the same as those of water. The density of water is about 1g/cm3, so the mass of a solution has about the same numerical value as its volume. The specific heat capacity is about 4.2 J (g º C).
A useful equation is
Heat transferred (J) = Mass of solution (g) x Temperature change (º C) x 4.2 J/(g ºC)
Measuring a heat of reaction in solution
The table below shows the heats of neutralisation when different acids and alkalis react together. Despite the fact that the equations look different, the heats of neutralisation are all very similar. This suggests that the same reaction is taking place in each mixture.
Reaction
H (kJ/mol)
HCl (aq) + NaOH (aq) --> NaCl (aq) + H2O (1)
- 57.9
HCl (aq) + KOH (aq) --> KCl (aq) + H2O (1)
- 57.8
HNO3 (aq) + NaOH (aq) --> NaNo3 (aq) + H2O (1)
- 57.6
HNO3 (aq) + KOH (aq) --> KNO3 (aq) + H2O (1)
- 57.7
HBr (aq) + NaOH (aq) --> NaBr (aq) + H2O (1)
- 57.6
If all acid solutions are ionized and contain hydrogen ions, H+ (aq), and all alkaline solutions contain hydroxide ions, OH (aq), the reaction taking place in each mixture is:
H+ (aq) + OH (aq) --> H2O (1); H - 57kJ/mol
The heat of neutralisation is the energy change when one mole of aqueous hydrogen ions reacts with alkali to give one mole of water.
Equipment
( Goggles
( 1 measuring cylinder
( 2 Polystyrene cups to keep the heat in the best
( Thermometer
( Hydrochloric acid (H Cl) - Acid
( Sodium hydroxide (Na OH) - Alkali
Method
. Using the measuring cylinder measure out 10 cm3 of hydrochloric acid.
2. Pour the hydrochloric acid in the polystyrene cup, when it is accurately measured.
3. Measure the temperature and record your results.
4. Using the measuring cylinder measure out 50 cm3 of sodium hydroxide.
5. Pour the sodium hydroxide alkali in the polystyrene cup, when it is accurately measured.
6. Measure the temperature and record your results.
7. Add the hydrochloric acid to the sodium hydroxide alkali, to see the change in the temperature.
8. Measure the temperature quickly and record your results.
9. Repeat this method but change the quantities. You should repeat it with 20 cm3 of acid and 40cm3 of alkali, then 30 cm3 of acid and 30 cm3 of alkali, 40 cm3 of acid and 20 cm3 of alkali and then 50 cm3 of acid and 10 cm3 of alkali. Repeat the whole experiment again, in order to get accurate and reliable results. You can also compare your two sets of results, to see if they resemble.
Fair Test
( We produced precise and reliable results by carrying out the experiment twice, to obtain accurate results throughout.
( All of the acids and alkali quantities had a total of 60 cm3, but the amounts were different.
( We measured the temperature for both the acid and alkali, and then the temperature for when they were both mixed together.
( We measured the acids and alkalis accurately, and checked that they were the exact measurements, to make it a fair test.
( We used the same acid and alkali in both sets of experiments.
( We took the temperature at its highest.
Safety
( Wear goggles throughout the experiment, because the chemicals might splash into your eyes.
( Keep the equipment and thermometer well away from the corner of the bench, so it doesn't roll off or break.
( If you accidentally break or spill something, inform you teacher immediately.
( If the chemical spills on your skin, wash it off immediately with cold water, to avoid your skin being burnt.
( Don't leave the tops of chemical bottles off, when you have finished pouring out the chemical because, they might spill on your clothes or burn you.
( Don't drop the thermometer in the beaker because it might break.
( Always stand when carrying out experiments.
Reliability
I will check my measurements by carrying out the same experiment twice, then comparing them to see which one is the most accurate. I will also make it a fair test, in order to make my results consistent and reliable.
Observing and Measuring:
This is my table of results to show the measurement of the normal temperature and when the acid and alkali are mixed together, which I have recorded below.
Table 1 for Experiment 1
Acid
(Cm3)
Alkali
(Cm3)
Normal temperature
(º C)
Temperature when mixed
(º C)
0
50
Acid: 23º C
25º C
Alkali: 24º C
20
40
Acid: 23º C
27º C
Alkali: 23º C
30
30
Acid: 23º C
29º C
Alkali: 23º C
40
20
Acid: 23º C
27º C
Alkali: 23º C
50
0
Acid: 23º C
25º C
Alkali: 22º C
Table 2 for Experiment 2
Acid
(Cm3)
Alkali
(Cm3)
Normal temperature
(º C)
Temperature when mixed
(º C)
0
50
Acid: 23º C
25º C
Alkali: 22º C
20
40
Acid: 23º C
26º C
Alkali: 22º C
30
30
Acid: 23º C
30º C
Alkali: 23º C
40
20
Acid: 23º C
26º C
Alkali: 21º C
50
0
Acid: 22º C
25º C
Analysing
The results seem the highest when acid and alkali are the same quantity, which is 30 cm3. This is when the temperature reaction is higher. The results in both tables were very similar, but some were 1º C out. Even when we increased the amount of acid and alkali the temperature remained fairly the same. If the quantities are the same, there is an increase in the temperature.
My results do fit my prediction because I said that the reaction would be high because the temperature will increase, when acid and alkali are mixed together. My prediction was also correct because I mentioned the fact that the temperature will increase when there is more solution in it and when the same amount of both alkali and acid is put in, so it is the same concentration. I was also correct when I said that there would be a change in the temperature when the two are mixed together, because it quickly increased and this showed the chemical reaction, which was endothermic. I noticed that if I added more acid the temperature gradually increases, and I if I add more alkali the temperature reduces.
Bases are antacids. They are the chemical opposites of acids. A base will neutralize an acid, and when this happens a salt is formed. Metal oxides and hydroxides are bases, and most of them are insoluble in water.
Bases, which dissolve in water, form alkaline solutions. The solutions turn litmus blue. They have a pH above 7. An alkali is a base, which is soluble in water. Common alkalis are sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia and sodium carbonate.
Bases can be recognized by what they do:
* A base will neutralize an acid to form a salt as the main product.
* Most bases are insoluble in water. Bases, which do dissolve, form alkaline solutions.
Acids react with bases, alkalis, reactive metals and carbonates. Acids have a low pH - neutralisation raises the pH, which destroys the acidity. It does this by removing H+ ions and converting them into water.
Examples: Acid + Base --> Salt + Water
Acid + Alkali --> Salt + Water
Acid + Metal --> Salt + Hydrogen
Acid + Carbonate --> Salt + Water + Carbon dioxide
Acid can be neutralised by any of these compounds:
Metal oxides
Metal hydroxides
Metal carbonates
Metal hydrogen carbonates
Ammonia solution
These compounds are all called bases. Any compound that can neutralise and acid is called a base. Alkalis can also neutralise acids, so they are bases. They are bases that are soluble in water.
Neutralisation always produces a salt, as these general equation show:
Acid + metal oxide --> metal salt + water
Acid + metal hydroxide --> metal salt + water
Acid + metal carbonate --> metal salt + water + carbon dioxide
Acid + metal hydrogen carbonate --> metal salt + water + carbon dioxide
Acid + ammonia solution --> ammonium salt + water
Evaluating:
Method
Everything went well because we followed our plan and the method and referred back to it. We also ensured that we followed all the safety and fair test factors, to make out experiment successful. We could have improved our experiment by taking the temperature at its highest in the first experiment.
Measurements
Our results were quite accurate, because we tried to measure the temperature at its highest and correctly. In our experiment there were results in the first experiment, which did not fit into the pattern. I think that we didn't wait long enough until the temperature had reached its highest. In the second experiment the volumes of acid, which didn't fit the pattern, were: 10, 20, 40 and 50 cm3.
I didn't get perfect result for the first experiment because they weren't reliable and were higher than experiment 1. Although the line graph was perfect and had the best fit, the results were lower than the second experiment.
Reliability
When I repeated my experiment my results were not the same. Our first experiments results were 1 or 2º C lower than experiment two. I think that experiment two results were the best because they were accurate and reliable as the temperatures were higher. The two anomalous results could be lower than expected because of the following reasons:
* I did not wait long enough for the thermometer to go to its highest.
* I did not measure the acid or alkali accurately
* I waited too long so the temperature had gone down again.
* Doing a lot more experiments at in between volumes e.g. 15, 25, 35 cm3 etc.
* Use accurate ways of measuring the volumes, such as a burette.
Supports conclusion - both graphs are the same shape, so the evidence is good enough to support my conclusion that the most heat is produced when the acid and alkali are equal in volume.
By Aisha Ahmed 10MR
