Chemistry Internal Assessment IB June 2010 Saksham Malhotra
Design an experiment to investigate factors affecting the rate of chemical reaction. Research Question How is the rate of reaction affected when catalyst is used? Introduction For any reaction to take place, activation energy is required. So, for some reactions, the activation energy is quiet high and they require a catalyst. In simple terms, a catalyst increases the rate of reaction but does not take part in the reaction itself. It basically lowers the activation energy of any reaction. A catalyst works by either changing the structure of the molecules or by bonding to the reactant molecule. This will lead them to react or release energy. Without a catalyst, the reaction may not even take place or if it does react, it would take a lot of time. The reaction that was used in this experiment is the oxidation of potassium sodium tartrate by hydrogen peroxide using cobalt chloride as a catalyst. The tartrate ion reacts with hydrogen peroxide to release carbon dioxide, water and Methanoate ion. C 4H 4O6 2- (aq) + 3H 2O 2 (aq)
2CO2 (g) + 2HCO2-(aq)+ 4H 2O (l)
But this reaction without a catalyst takes a lot of time to fully react, so for this experiment we will use cobalt salt solution (in this case, Cobalt (II) Chloride) as a catalyst and note the time taken for the reaction.
The main motive behind doing this experiment is to prove that addition of a catalyst improves the rate of reaction. Therefore this experiment had two parts. First part was where no catalyst was used and the second part was where catalyst was present in the reaction. This reaction is also known as the traffic light experiment as the colour changes during this reaction represent the sequence of lights as seen in a traffic light. Cobalt (II) ions are pink in colour. The hydrogen peroxide initially oxidides the cobalt (II) to cobalt (III). This Co3+ is green in colour and because of this the colour changed to green. Then the cobalt (III) ions react with the tartrate ions and reduction takes place where the cobalt (III) changes back to cobalt (II), which is pink in colour. Hence, the reaction goes through a number of colour changes before it finally returns to pink. The catalyst here provides an alternative route of the reaction to take place. This route has lower activation energy and the reaction proceeds much more quickly.
Hypothesis Rate of reaction is affected by a number of things. Some of these improve the rate at which the reaction takes place while some decrease the rate of reaction. For this experiment we can hypothesize that addition of a catalyst will improve the rate of reaction by a significant amount. The difference in the rate of reaction with and without the catalyst might be huge.
Variables Type of Variables
Method of measuring variables
Independent Variable
A fixed amount (5cm3) of Cobalt (II) Chloride was used for all trials during this experiment.
Presence of Catalyst
Dependent Variable Rate of Reaction (Amount of CO2 produced per unit of time) Controlled Variable Size of Apparatus Temperature Amount of reactants
The amount of gas liberated was measured using a burette (Downward displacement of water) The same apparatus was used for all trials of this experiment and the temperature was maintained at 80oc. The amount of reactants used was also same throughout the experiment.
The table shows the variables involved in t his experiment
During the experiment, volume of the gas (CO2) was recorded along with the time taken to produce it. The amount of gas liberated during a specific time period will determine the rate of reaction for this experiment.
Apparatus Used
Uncertainty
Measuring Cylinder 50 cm3
± 0.5 cm3
Measuring Cylinder 10 cm3
±0.2 cm3
Beaker 250 cm3
±0.2 cm3
Bunsen Burner
-
Tripod Stand
-
Gauze
-
Thermometer
±0.01°C
Conical Flask
-
Burette 50 cm3
±0.1 cm3
Rubber Cork and Tubes
-
Stop watch
± 0.01 sec
Chemicals Used: •
• • •
Hydrogen Peroxide (20mol/dm3) Potassium Sodium Tartrate (Rochelle Salt) Cobalt (II) Chloride Solution (4%) Distilled Water
Procedure 1)
The apparatus was set up as shown in the figure below
2) 3) 4) 5) 6) 7)
8)
The first trial of this experiment was done without any use of catalyst. 4 grams of potassium sodium tartrate was added to 50 3 cm of distilled water and mixed. The mixture was placed on a tripod and gauze and heated till 80oc. Then 20 cm3 of 20 volume hydrogen peroxide was added to the tartrate solution. Temperature was noted and the mixture was heated again to 80oc. The mixture was allowed to react and time was recorded but there was no sigh of any gas being liberated. No colour change was observed and the level of water in the burette didn’t change. The steps from 3 to 7 were repeated again for two more trials. Then the experiment was performed again using the catalyst.
The amount of potassium sodium tartrate and hydrogen peroxide was same as used in the previous three trials. hydrogen 10)The mixture of potassium sodium tartrate and o peroxide was heated to 80 c. 3 11)5cm of cobalt chloride solution (red in colour) was added to this mixture and changes in colour and water level was recorded. 12)With the addition of the catalyst, the mixture turned red immediately and within a few seconds, the colour changed to yellow then green and then back to red. 13)During the colour changes, gas was released and measured using an inverted burette and the time taken for the gas to release was also recorded. 14)The method used to measure and collect the gas was downward displacement of water. 15)Steps 10 to 13 were repeated again for two more trials keeping the amount of reactants same for all trials. 9)
Data Recording Raw Data Collected The table below shows the change in the burette reading for the reaction where no catalyst was used. Burette Readings/cm3 (±0.1 cm3 ) Time/sec
Trial 1
Trial 2
Trial 3
10
50
50
50
20
50
50
50
30
50
50
50
40
50
50
50
50
50
50
50
60
50
50
50
From the table above we notice that the reaction did not take place for the first 60 seconds as no gas was produced and no colour change was observed. The three sets of experiments were kept aside for 2 hours and yet no signs of reaction were seen. Hence the reaction did not take place during the first 2 hours.
The table below shows the change in burette reading for the reaction where catalyst was used. Burette Reading/cm3 (±0.1 cm3 ) Time/sec
Trial 1
Trial 2
Trial 3
10
50.0
50.0
50.0
20
44.0
46.0
44.0
30
37.0
32.0
33.0
40
19.0
21.0
21.0
50
9.0
10.0
12.0
60
5.0
8.0
5.0
Uncertainty Table Volume of gas
Time
Instrument Uncertainty
Instrument Uncertainty
±0.1 cm3
± 0.01 seconds
Observer’s Uncertainty ±0.12 seconds
For Volume of Gas, formula used: Initial reading of water – final reading of water So the uncertainty for the volume of gas produced will be: 2*(± 0.1) = ± 0.2 cm 3
Therefore the total uncertainty for Burette is ±0.2 cm3 and for the stopwatch, it is ±0.13 seconds.
Processed Data The table shows the final set of processed data that was recorded for the experiment in which the catalyst was used. To calculate the values in the following table, the formula below was used:
Time/sec (±0.13 sec)
Volume of Gas Produced
Average Volume
(±0.2) Trial 1
Trial 2
(±0.2)
Trial 3
10
0.0
0.0
0.0
0
20
6.0
4.0
6.0
5.3
30
13.0
18.0
17.0
16.0
40
31.0
29.0
29.0
29.6
50
41.0
40.0
38.0
39.6
60
45.0
42.0
45.0
44.0
Graph of Time Vs. Volume of Gas produced
The chart uncertainty is quiet low so the error bars are very small.
Conclusion The experiment depicts clearly that the reaction that could not take place in two hours, just the addition of a catalyst could make the mixture react in about a minute. Clearly giving a solid proof that the addition of a catalyst improves the rate of reaction by a generous amount. So what we saw during this experiment was that when the potassium sodium tartrate was mixed with hydrogen peroxide, the
mixture does not react because the activation energy required was not attained. When the catalyst was added, brisk effervescences took place with the liberation of carbon dioxide gas and multiple colour changes took place. The reaction rate was very fast and clearly proved the hypothesis. With the addition of the catalyst (Cobalt ll Chloride), the activation energy i.e. the energy that must be overcome in order to the reaction to take place, is lowered. That means the rate of reaction increases. Also this reaction proves that the catalyst does not react with the reactants as when the reaction is complete, red colour of Cobalt (ll) Chloride is seen and after the reaction, the catalyst is back to its original form. The properties of a catalyst also tell that the catalyst can be used again for multiple reactions.
Evaluation During the reaction, the measurements were taken with good care and precision but the presence of random errors made the results a bit inaccurate. The following reasons may have caused the results to vary: •
•
•
•
•
The atmosphere temperature cannot be constant during the reaction and the same goes for atmospheric pressure. The variation in these two factors may have caused the results to differ. When raising the temperature of potassium sodium tartrate, 50 cm3 of distilled water was used before heating it. During the heating process, there was no lid or cover to prevent the water vapours to escape. This may have caused the amount of reactants to vary. When the reactants were mixed and catalyst was added, the rubber cork was placed. During the time when the reactants were mixed and cork was placed, some gas must have escaped as the reaction was fairly spontaneous. The rubber tube carrying the gas was not examined properly before the reaction. Presence of residue or any blockage could have been a hindrance for the passing CO 2 gas. The Hydrogen peroxide used for this experiment was pre made and bottled up. With time the concentration decreases and the concentration written on the label was 20 M but the actual concentration might not be exactly 20 M.
The presence of errors can thus be justified, as there were limitations during this experiment.
