Hypothesis
It can be hypothesized that as the temperature is increased the reaction rate too will increase. Firstly with an increase in temperature, the probability of more number of particles having greater energy than the activation energy also increases. Secondly the more frequent collisions having higher energy, the probability of effective collisions also increase thus increasing the reaction rate. But it must be noted that the graph of reaction rate against temperature will not be a linear graph alike the one drawn against concentration because although temperature is proportional to the reaction rate, it varies as a square function and thus we have a rising exponential curve.
Time
.
Temperature
Temperature affects the rate of reaction according to the graph shown above.
Variables
The key variables in this experiment are the other factors other than temperature that affect the reaction rate and to fulfill the experiment’s objective, all these factors need to be controlled:
- Volume and molarity of sodium thiosulphate
- Volume and molarity of hydrochloric acid
Planning (b)
Apparatus and Materials
- Two burettes
- 100ml. Beaker
- 5 test tubes,
- Stopwatch
- Graduated cylinder
- 1M Sodium Thiosulphate
- 1M Hydrochloric acid
- Thermometer
- Tripod and Wire Gauze
- Bunsen Burner
- Water Bath
- Labels
- Cross on paper
Procedure:
-
Using a measuring cylinder, measure 10cm3 of sodium thiosulphate solution and 40cm3 of distilled water and add them to a conical flask.
- Place the conical flask in a water bath and heat the contents to a particular temperature. Record the temperature.
- Place the flask on a white tile which is marked with a cross.
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Immediately, add 5cm3 of hydrochloric acid to the heated solution.
- Start a stop watch instantaneously.
- Swirl the contents of the flask and record the time taken for the cross on the tile to disappear.
- Repeat the experiment at different temperatures.
- Plot a graph of time against temperature.
Data Collection
It was observed that as soon as the contents of the two test tubes were mixed, the colour of the solution darkened and it gradually turned opaque and the cross was not viewable from the top.
Data Analysis
The graph for reaction rate against the temperature of the reactant mixture is given below:
The hypothesis has been proved right. The time taken for the reaction to be complete decreases with increasing temperature.
Evaluation
The graph of the rate against temperature is a rising exponential curve, which shows that temperature is proportional to the rate of a chemical reaction and is an exponential function of the rate.
The main principle on which it was based on was that in the main reaction, as sulphur is produced, it forms a dirty-blackish colloid and hence the cross below becomes invisible, thus providing a certain way to measure the end of the reaction. The equation for the reaction is given below:
Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(aq) + H2O(l) + S(s)
This equation can also be represented in the ionic-equation form in the following manner:
S2O32- + 2H+ → H2O + SO2 + S
Looking at the results obtained, it is interesting to note that for every ten-degree increase in temperature, the reaction rate nearly doubles. As mentioned earlier, as temperature increases, the average speeds of the reacting particles increase. And thus at higher temperatures, the collisions and the probability of effective collisions too are frequent and this results in an increase in the reaction rate.
Low temperature
High temperature
The graph above shows the Maxwell-Boltzman distribution for how the number of molecules having activation energy varies with temperature. This is what has exactly happened in this experiment. The solution has many molecules in it, some have quite high kinetic energy and some very low kinetic energy and the result is a distribution of these molecules in the form of a curve shown above. However at high temperatures this curve tend to flatten because the area enclosed by the curve has to be a constant as it represents the total no. of molecules, which is also a constant. And so it is seen that this flattening shows that a greater amount of molecules have energy greater than the activation energy.
The increase in rate due to the increase in collision frequency as temperature rises is not as much responsible as the increase in the amount of energy per collision..
Limitations
- The limitation of this experiment is that we arrive to the general conclusion about the relationship between temperature and rate of reaction through studying one individual reaction only, and no test is done to verify that this is not specific to this reaction.
- Also, the accuracy of the experiment relies heavily on the experimenter’s perception about the degree of disappearance of the cross, and this human limitation while taking timings is virtually impossible to totally overcome.
- While making the solution of sodium thiosulphate, a suspension of sulphur was noticed. This is due to the impurity present in the sodium thiosulphate and could also lead to inaccurate time measurements as reaction times are altered due to the presence of impurity.
- Also it is important to take into consideration the decrease in temperature that takes place during the course of the reaction for the first two readings. Swirling the contents of the conical flask leads to faster cooling. As a result the experiment is not performed at a constant temperature possibly leading to inaccurate results.
Precautions
Precautions must be taken so as to avoid the occurrence of errors:
- For safety reasons, use eye protection from any acid spills.
- To minimize the effect of human errors, the experiment should be repeated twice and the average readings should be noted.
- More accurate instruments must be used such as a burette with a better degree of accuracy for precise recordings and calculations.
- Each time for one set of readings, It is preferred that the same person records the time after the disappearance of the cross so as to make the experiment fair as otherwise different reaction times of different people will adversely affect the experiment.
Modifications
Several modifications can be made to this experiment to obtain a better result.
- The range of temperature could have been larger, taking into account temperatures lower than room temperature. This would enable us to judge the relation between temperature and rate of reaction more accurately.
- This experiment is based upon human perception of the time taken for the cross to disappear. This could be a major area of error. An improvement could be the addition of an indicator, which could indicate the end of the reaction.
- Instead of using a measuring cylinder to measure the volumes of sodium thiosulphate and water, a pipette could be used. This ensures that all the readings are taken using exactly same volumes of liquid.
Conclusion
Through this experiment, it is concluded that the absolute temperature is proportional to the reaction rate and the graph of reaction rate against temperature is a rising exponential curve. It was also inferred that for every 10οC rise in temperature the reaction rate doubles.
