7. The solution is stirred*. A person watches the cross vertically during the reaction. (*In the experiment, we did not stir the solution.)
8. Stop the stop-watch when the cross on the paper just ‘disappeared’.
9. Record the time in the results table.
10. Repeat step 4-9 with 2 more times*. The water in step 4 should have a temperature of 40℃ and 60℃ respectively, such that the solutions of Na2S2O3 and HCl in step 6 will have a temperature of 40℃ and 60℃ respectively before poured into the beaker. (In the experiment, we do it 4 times more for 35℃, 45℃, 54℃, and 65℃.)
11. The whole results table is drawn. Time for the blot out of the cross is compared with the temperature.
Apparatus:
-
Beakers (100cm3 x 2, 250cm3 x 1)
-
Measuring cylinders (50cm3 and 10cm3)
- A stop-watch
- A white tile with a cross
- Bunsen Burner and matches
- Tripod and gauze
- A thermometer
- Boiling tubes x 2
Chemicals:
Na2S2O3(aq) (0.05M)
Dilute HCl(aq) (2.0M)
Results:
The time-temperature table is drawn below:
As you can see, the time of reaction and the temperature are inversely less proportionate.
The reaction rate-temperature table is drawn below:
As you can see, the reaction rate and the temperature are proportional.
Reaction rate rises more when temperature increases.
Discussion:
There are some points to note in this experiment. For example, some errors occurred during the experiments.
First, some heat is lost to the surroundings. It is inevitable but we can try to reduce the heat loss by wrapping the beaker with cotton wool. Unfortunately, we are not provided with this material.
Second, the temperature of the water bath is measured. We put the thermometer into the water bath instead of the boiling tubes which contain the reactants, this leads to errors on the temperature of the mixture. Transfer of heat from the water to the reactants inside the boiling tubes needs time. The temperature inside the boiling tubes is different to the temperature of the water in the large beaker. Our records may not be accurate.
Third, we assigned 1 student for looking at the mixture, while the other students record the time. This may lead to reaction time errors because there is reaction time between the student found the cross blotted out and shout out. It also present between he shouts out and the other student stop the stop-watch. As you can see, the reaction time caused errors. To minimize this kind of error, we should only assign one student for both looking at the mixture and record the time.
Answers to questions:
1. Is the reaction rate directly proportional to temperature? Explain briefly.
No, it isn’t.
2. Why do we use the same concentrations and quantities of reactants at the start in all cases?
We do this as to conduct a fair test. We only change the temperature of the mixture but not other factors, for example, the concentration of reactants because we do not want to change the moles of the reactants.
3. State ONE example in our daily life to show the understanding of the effect of temperature on reaction rate is very important.
If a bottle of fresh milk is exposed to normal condition, i.e. temperature of about 20℃, the freshness cannot last for long. With a long time, it may even deteriorate and turn sour. This means it has undergone chemical reaction with the bacteria in room condition. So the milk goes bad.
Normally, we will put opened milk in the refrigerators. As the temperature is low in the fridge, bacteria become inactive and thus the reaction rate which takes place inside the milk is very low. This can prevent the milk from going bad and prolong its storage time. This is why people often put opened milk in the fridge, and there are labels on the box of milk telling us avoid putting it under sunlight.
Conclusion:
In this experiment, we have found that the time of reaction is inversely less proportionate with the temperature of the reactants. As the temperature increased, the less the reaction time is required. The reaction rate was calculated by the reciprocal of reaction time a graph is plotted against temperature. It is not difficult to find out they are proportional, though it was not directly. From this experiment, we can conclude that when the concentration and quantity of the reactants are the same, the higher the temperature, the faster the reaction rate. The latter increase more as temperature increases.
