The effect of hydrochloric acid
The results in figure 2.2 shows that change in concentration of hydrochloric acid, has no effect on the rate of reaction between sodium thiosulphate when kept at a constant volume and concentration . The rate of reaction is higher in in this reaction as the concentration increases.
Investigation 2: The effect of temperature changes
The collision theory:
The collision theory is stated that reactions occur when molecules collide with a certain minimum kinetic energy. The more frequent these collisions, the faster the reaction.
This energy is needed to overcome the energy barrier, the activation enthalpy, for the reaction. It is the energy that is needed to start breaking the bonds in the colliding molecules so that the collision can lead to a reaction.
Calculating activation enthalpy from rates of reaction (refer to page11-12)
Arrhenius equation:
In k = constant -Ea
RT
Where k is the rate constant, R is the gas constant, T the absolute temperature and Ea is the activation enthalpy.
In order to plot a graph these calculation are needed, a calculator is used to work out the log ln rate
e.g.
If you plot a graph of lnk rate against 1
Temperature (Kelvin)
The slope of graph = -Ea
R
The activation energy can be calculated from the gradient:
Therefore Ea (activation enthalpy) = -Slope × R R= 8.314 kJ mol-1
1000
The activation enthalpy from the effect of temperature on the rate is 49.4 kJ mol-1 .
Large activation energy (49.4 kJ mol-1) is only a small number of pairs of molecules that have enough energy to pass over it each second and turn into products, so the rate of reaction is slow. The data book literature value for the uncatalysed activation energy is 48 kJ/mol, this activation enthalpy is slightly lower than the one obtained from this experiment.
Activation energy is the minimum energy required before a reaction can occur. You can show this on an energy profile for the reaction. For a simple over-all exothermic reaction.
The Maxwell-Boltzmann Distribution
The Maxwell-Boltzmann curve describes the distribution of the kinetic energies of molecules. At each collision, energy is transferred between the particles, so the particles are continually changing their energies.
Maxwell-Boltzmann distribution diagram :
Only those particles represented by the area to the right of the activation energy will react when they collide. The great majority don't have enough energy, and will simply bounce apart.
Investigation 3: The effects of temperature changes and catalyst
The same calculations as above were carried out to find out the activation energy (refer to figure 4.3).
The activation energy for the effect of iron (lll) chloride on the reaction:
39.7 kJ mol-1
What are catalyst?
A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction. Catalysts are not used up in chemical reactions and you can always get exactly the same mass of catalyst as you had at the beginning. They are not changed chemically, though sometimes they may be changed physically.
Catalysts and activation energy
To increase the rate of a reaction you need to increase the number of successful collisions. One possible way of doing this is to provide an alternative way for the reaction to happen which has a lower activation energy.
In other words, to move the activation energy on the graph like this:
Maxwell-Boltzmann Distribution
Adding a catalyst has exactly this effect on activation energy. A catalyst provides an alternative route for the reaction. That alternative route has a lower activation energy. Showing this on an energy profile:
Preliminary work
Preliminary testing was carried out to see whether the methods used would be sufficient to carry out the actual experiments. When carrying out the investigations I used 0.1 mol dm-3 for sodium thiosulphate and hydrochloric acid this is because using low concentration minimised error in the reading for the time taken.
The effect of temperature changes:
This experiment was very straight forward, each temperature was repeated 3 times to obtain an accurate average and to reduce anomalies.
The effect of concentration changes
Before deciding which method was best, I experimented both methods:
METHOD A
conical flask drawing
Both experiments went according to plan, although method A used up a lot of volume, 50cm3 × 3 for each concentration (5 conc.). The expected volume needed for method A would have been 750cm3 of sodium thiosulphate and hydrochloric acid measured in a volumetric flask. Whereas method B, using test tubes I only used 10cm3 and repeated it three times, overall volume used for sodium thiosulphate and hydrochloric acid was 250cm3.
Evaluation
Overall the investigations were a success. sufficient data was obtained to create graphs and to support conclusions.
Percentage area
Percentage Error = Error ×100
Reading
Percentage error of 250cm3 volumetric flask:
Error = 0.2cm3, therefore % error = 0.2 × 100 = 0.08%
250
Percentage error of 25cm3 pipette:
Error = 0.06cm3, therefore % error = 0.06 × 100 = 0.24%
25
Percentage error of weighing scale: (sodium thiosulphate)
Error = 0.005, therefore % error = 0.005× 100 = 0.08% Error × 100
6.20 Mass
Potential errors:
All the systematic errors in the experiment are minimal. Another error which may have interfered with the experiment is human error, such as:
- Accurate measurements of volume
- Reading for the cross to become obscured
- Reading the thermometer
Potential improvements:
- To use better instruments to produce accurate measurements such as an electronic pipette which you can type and enter your desired volume.
- Repeating the same experiments with different acids, to check whether the pattern is general to all acids or not.