I will use the same standard each time for judging when the X has disappeared. The same person will be used to judge whether or not they feel the X at the bottom of the beaker has disappeared, as different people have different judgements.
I will make sure that the measuring instruments used to measure the Hydrochloric acid and Sodium Thiosulphate will not be mixed up.
To keep my investigation as fair as possible, I must change only one factor at a time. The factor I will be changing is the concentration of the hydrochloric acid. The factors that will be kept the same throughout the investigation are temperature; surface area and no catalysts will be used. The experiment will be repeated at least three times to get a more average result.
All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful.
There are six key factors that can change the rate of reaction:
- Concentration (of a solution)
- Temperature
- Surface area (of a solid)
- Pressure (of a gas)
- Light
- A catalyst
A collision theory can be used to explain how these factors affect the rate of a reaction. The two important parts of the theory are:
- The reacting particles must collide with each other.
- There must be sufficient energy in the collision to overcome the activation energy.
Prediction:
My prediction is that as the concentration of the hydrochloric acid is increased, the rate of the reaction will also increase. This means that the graph drawn up in my analysis will probably be curved. This can be justified by relating to the collision theory and the factors involved.
Concentration:
Increasing the concentration of a reactant will increase the rate of a reaction. For example if a piece of magnesium ribbon is added to a solution of hydrochloric acid the following reaction occurs:
Magnesium + hydrochloric acid magnesium chloride + hydrogen
Mg (s) 2HCL (aq) MgCL(aq) + H (g)
As the magnesium and acid come in contact, the acid effervesces and hydrogen is given off.
The graph above shows the volume of gas collected every 10 seconds when two different concentrations of hydrochloric acid are used. In experiment 1, the curve is steeper and has a greater gradient than experiment 2. In experiment 1 the reaction is complete after 20 seconds whereas in experiment 2 it takes 60 seconds. The rate of reaction is higher with 2.0M hydrochloric acid than with 0.5M hydrochloric acid. In 2.0M hydrochloric acid solution the hydrogen ions are more likely to collide with the surface of the magnesium ribbon than in the 0.5M hydrochloric acid.
Temperature
Increasing the temperature will increase the rate of reaction. Warming a chemical transfers kinetic energy to the chemical's particles. More kinetic energy means that the particles move faster. As they are moving faster there will be more collisions each second. The increased energy of the collisions also means that the proportion of collisions that are effective will increase.
Increasing the temperature of a reaction such as that between calcium carbonate and hydrochloric acid will not increase the final amount of carbon dioxide produced. The same amount of gas will be produced in a shorter time.
Surface Area
Increasing the surface area of a solid reactant will increase the rate of reaction. The reaction can only take place if the reacting particles collide. This means that the reaction takes place at the surface of the solid. The particles within the solid cannot react until those on the surface have reacted and moved away. Powdered calcium carbonate has a much larger
Surface area than the same mass of marble chips. A lump of coal will burn slowly in the air whereas coal dust can react explosively.
Pressure
Increasing the pressure on the reaction between gases will increase the rate of the reaction. Increasing the pressure has the effect of reducing the volume of the gas and so moving the particles closer together. If the particles are closer together there will be more collisions and therefore more effective collisions.
Light
Increasing the intensity of light will increase the rate of some reactions. This fact is important in photography. The photographic film is coated with chemicals that react when in contact with the light. Some laboratory chemicals, for example silver nitrate and hydrogen peroxide are stored in brown glass bottles to reduce the effect of the light.
Catalyst
A catalyst is a substance that alters the rate of a chemical reaction without being used up. The mass of the catalyst remains unchanged throughout the reaction. Hydrogen peroxide decomposes slowly at room temperature into water and oxygen. This reaction is catalysed by manganese (IV) oxide.
Manganese (IV)
Hydrogen peroxide water + oxygen
MnO
2H O 2H O + O
Most catalysts work by providing an alternative 'route' for the reaction that has a lower activation energy 'barrier'. This increases the number of effective collisions each second. Some catalysts slow down reactions. These are called negative catalyst or inhibitors. Inhibitors are added to petrol to prevent pre-ignition of the petrol vapour in the engine.
Apparatus:
- Pipette and Pump (25ml)
- Conical flask (250ml)
- White piece of paper marked with an 'X'
- Stop watch/clock
- Hydrochloric acid (2.0m, 1.8m, 1.6m, 1.5m, 1.4m, 1.2m)
- Sodium Thiosulphate (0.2m)
Safety:
Throughout the experiment, safety goggles will be worn. Hands will be washed after the experiment or any time during the experiment if the solutions being used have had any contact with the skin, (hands will have to be dried before continuing with the experiment in case water is dripping from the hands and drops into any of the solutions).
Coats and bags will be kept clear out of the way while the experiment is taking place. This will prevent any accidents relating to trips and falls from occurring.
Method:
As stated in the fair test, the volume of Sodium Thiosulphate is kept constant at 10cm3. The volume of Hydrochloric acid is also kept the same at 10cm3.
Using the pipette and pump, the Hydrochloric acid is measured to a volume of 10cm3 and pumped into the conical flask. Using another pipette and pump 0.2m of Sodium Thiosulphate is measured to a volume of 10cm3 and also pumped into the conical flask which is placed above the white tile with the piece of paper marked 'X'. The stopwatch will now be started. When the mixture has turned sufficiently cloudy so that the letter X can no longer be seen the stopwatch will be stopped and the time will be recorded. The experiment is repeated with all the concentrations. The whole procedure is then repeated.
Table of Results:
Analysis:
In this experiment I have found that as the concentration is increased the time taken for the reaction to take place decreases. This means the rate of reaction increases as it takes less time for a reaction to take place, so more takes place per second. So therefore in conclusion, the more concentrated a reactant is, the quicker the rate of reaction time will be.
I have come to this conclusion because of several reasons. Firstly, my results give pretty conclusive evidence that as the amount of Sodium Thiosulphate decreases, and the amount of water in the solution increases there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.
I drew up two line graphs, the first for the concentration of HCL against average time and the second for concentration of HCL against 1 over average time. For the first graph, I chose a curve for the line of best fit. For the second I chose to draw a straight line of best fit.
Although there is no particular pattern for either one of my graphs my results support the prediction I made that the greater the concentration of Sodium Thiosulphate the faster the rate of reaction time.
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.
Evaluation:
My experiment went according to plan but there were flaws in it. I think there is a human error factor involved when looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a cross, a light beam could be used and when the beam went out that would be the end point.
I feel that because my results have a certain amount of inconsistency between them it would be wise to repeat them again, at least another 3 more times if I had had more times.
My results are adequate in their accuracy and the points on the graph were plotted as accurately as possible. Although, I did get some anomalous results but that is to be expected, particularly after my group had different people judging to see whether the cross had disappeared or not. Despite this, all my ‘good’ results fitted my prediction well and the first graph had a strong trend.
My investigation could have been better if I'd tested my results to ensure that I had not made any mistakes, and if I'd carried out any preliminary work in order to familiarise myself with the project. Also, if I'd set-up and equipment and thought carefully about the whole investigation I would have been able to find a better way of doing the experiment.
The problem comes when using the overall average that any anomalous results that are included to produce this average make it less accurate and so less dependable.
My results seem reliable but you always have to consider more. Because I am basing my interpretation of their reliability on a hypothesis and my own personal view it is hard to tell. I see them as reliable but if my views and hypothesis are wrong then the results are not reliable. I would need more time to research this further in order to make a firm decision.
I think that the original idea of using a computerised light sensitive device was a good idea. I feel that with more time I could develop a way to use this. The results that I could get from this (if it worked) would be more reliable and ‘strong’.
To extend this work one could look into the affects of catalysts (and other variables) on reaction times.