The activation energy is the minimum energy needed to cause a chemical reaction. A chemical reaction between two substances only occurs when an ion, molecule or atom of one collides with another ion, molecule or atom. Only a fraction of the total collisions result in a reaction, because usually only a small percentage of the substances interacting have the minimum amount of kinetic energy a molecule must possess for it to react. When the reactants collide, they may form an intermediate product whose chemical energy is higher than the combined chemical energy of the reactants. In order for this transition state in the reaction to be achieved, some energy must enter into the reaction other than the chemical energy of the reactants. This energy is the activation energy. Once the intermediate product, or activated complex, is formed, the final products are formed from it. The path from reactants through the activated complex to the final products is known as the reaction mechanism. Because the heat energy of a substance is not uniformly distributed among its atoms, ions, or molecules, some may carry enough heat energy to react while others do not. If the activation energy is low, a greater proportion of the collisions between reactants will result in reactions. If the temperature of the system is increased, the average heat energy is increased, a greater proportion of collisions between reactants result in reaction, and the reaction proceeds more rapidly. A catalyst increases the reaction rate by providing a reaction mechanism with a lower activation energy, so that a greater proportion of collisions result in reaction. Our experiments activation energy can be determined by measuring the reaction rate at different temperatures, plotting it on a graph and determining the slope of the straight line that best fits the points.
Therefore, I predict that the mixture of Sodium Thiosulphate and Hydrochloric acid will go cloudy in the beaker much quicker at 60°c and higher temperatures than at room temperature and lower temperatures, resulting in the black cross becoming invisible to the eye.
Plan (preliminary work)
Firstly you must measure out accurately 50cm of Sodium thiosulphate and 10cm of Hydrochloric acid. These must be measured in different, clean, measuring cylinders suitable for measuring 50cm and 10cm. After measuring them out carefully, you must pour each of the acids and sulphates into suitable instruments. In this case, sodium thiosulphate is placed in a conical flask and hydrochloric acid is placed in a boiling tube.
Next, you must take the temperature of the room and record this. This will be around 20°c I would have thought. Make sure all the equipment is in place e.g. the X board is directly under an empty beaker. Whilst taking part in this experiment, you must always remember that contaminating any area with substances can disrupt the whole experiment and prove it inaccurate.
Here is a diagram;
Finally, add the sodium thiosulphate and hydrochloric acid to the beaker and immediately start the stopwatch. You then need to mix the two substances together and watch carefully as the mixture gradually becomes cloudy and the X on the board is no longer visible. You then need to record your results;
How long does it take for the mixture to become cloudy so that you cannot see the X board at room temperature?
Answer:
Equipment needed:
1 thermometer
1 beaker
1 measuring cylinder
1 boiling tube
1 tripod
1 gauze
1 heatproof mat
1 stopwatch
1 Bunsen burner
X board
1 pair of tongs.
Dependant variable
I will measure the time taken for the cross to disappear, as it will show whether the rate of reaction has increased or decreased. The rate of reaction has increased if the time has decreased as the reaction occurred a lot quicker so therefore the time taken is a lot less. The rate of reaction has decreased if the time has increased as the reaction is a lot slower and therefore the time taken is a lot longer.
Controlled variable
I will keep the same volume of hydrochloric acid.
The same reaction volume.
The same size of cross.
The same size conical flask and boiling tube.
The same amount of mixing of the solution.
The same person will be timing the stopwatch.
The same person will be judging how long it takes for the cross to become invisible.
These controlled variables are to make the experiment a fair test.
Experiment; measuring the speed of the reaction
Our preliminary experiment, before changing the temperature of the sodium thiosulphate and hydrochloric acid, was to measure how long it would take for the mixture to go milky at room temperature. I set up the experiment like I have explained previously, and monitored the visibility of the cross beneath the beaker. The cross must disappear as slow as 2 minutes 30 seconds min. so that you have plenty of time to monitor observations. These are my results;
Concentrations for first experiment:
50cm – Sodium Thiosulphate
10cm – Hydrochloric Acid
observations;
20s slight bubbles
40s starting to go cloudy and
50s ‘’ ‘’
55s cannot see
As you can see, this concentration ratio of 5 part NaSO: 1 part HCl is too highly concentrated and therefore the reaction was too quick to monitor. However, you learn from your mistakes and now I know that I must dilute the Sodium Thiosulphate to gain a slower reaction time.
Concentrations for second experiment:
25cm – Sodium Thiosulphate
25cm – Water
10cm – Hydrochloric Acid
observations;
20s nothing
40s ‘’
60s ‘’
1.20m milky on surface
1.40m slight residue developing
2m drastically cloudy
2.2m slightly visible
2.35 cannot see it, invisible to the eye.
This experiment shows us that it takes 2.35 minutes to react fully. This makes us happy and therefore we are going to keep this concentration throughout the experiments. Now to get a more accurate set of results, you must do each experiment at least three times to work out an average.
Our results;
1st experiment – 2m 35s
2nd experiment – 2m 7s
3rd experiment – 2m 9s
We are a little worried with the results of out 1st experiment due to the fact it was so drastically out of range with out 2nd and 3rd results so we are going to do another one and use the 2nd, 3rd and 4th results to get a more secure and accurate average.
4th experiment – 2m 8s
Average: 2 minutes and 8 seconds at room temperature (20˚c)
Our next experiment is to measure the reaction rate at a temperature of 30°c. To do this you must heat both the test tube of hydrochloric acid and conical flask of sodium thiosulphate in a waterbath which is placed above a Bunsen burner on a tripod and gauze. Once both substances are a little above 30°c, place the conical flask over the black cross and pour the test tube into the conical flask.
observations
16s slight bubbling
26s mixture turning cloudy
36s “ “
46s “ “
52s cross invisible to the eye.
This experiment was repeated three times and these are our results;
1st experiment – 52s
2nd experiment – 1.3m
3rd experiment – 53s
Average: 52 seconds at 30°c
The next experiment is to measure the reaction rate at a temperature of 40°c. You must heat both substances to a little over 40°c and place them both over the black cross and pour together to form a mixture.
observations;
15s slight bubbling
25s “
30s going cloudy
44s invisible to the eye.
This experiment was repeated three times and these are our results;
1st experiment – 44s
2nd experiment – 37s
3rd experiment – 41s
Average: 41 seconds at 40°c.
Our next experiment is to measure the reaction rate at a temperature of 50°c. You must heat both substances to a little over 50°c and place them both over the black cross and pour together to form a mixture.
observations;
5s slight bubbling
10s bubbling
15s invisible to the eye.
This experiment was repeated three times and these are our results;
1st experiment – 15s
2nd experiment – 16s
3rd experiment – 15s
Average: 15 seconds at 50°c.
Our fifth and final experiment is to measure the reaction rate at a temperature of 60°c. You must heat both substances to a little over 60°c and place them both over the black cross and pour together to form a mixture.
observations;
2s bubbling
8s invisible to the eye.
This experiment was repeated three times and these are our results;
1st experiment – 8s
2nd experiment – 11s
3rd experiment – 10s
Average: 10 seconds at 60°c.
