- Concentration
Increasing the concentration of the reactants increases the rate of reaction in solution. This is because increasing the concentration increases the number of particles per cubic centimetre and hence the number of collisions between the particles. This results in an increase the rate of reaction. The greatest rate of reaction is usually as soon as the reactants are mixed, as they are both at their highest concentration. As the reaction precedes the concentration of the reacting substances decreases and the rate of reaction decreases (see diagram below).
- Pressure
Increasing the pressure of a gas phase reaction increases the rate. Increasing the pressure or concentration results in an increase in the number of particles per cubic centimetre, as the particles are forced closer together. This causes more collisions and increases the rate of reaction.
- Surface area
When one of the reactants is a solid, the reaction takes place on the surface of the solid. Increasing the surface area of solid reactants increases the rate, as there is a greater area for collisions to take place. For example, powdered reactants will react faster with acids than granulated reactants. The smaller the size of the granules, the greater the surface area.
- Light
The rates of some reactions are increased by exposure to light. Light has a similar effect, to increasing temperature.
- Catalysts
Catalysts are substances, which alter the rate of chemical reactions, without being chemically, changed themselves. Catalysts usually increase the rate of reactions. A catalyst, which slows down a reaction, is called a negative catalyst or inhibitor. Catalysts speed up reactions by providing an alternative pathway for the reaction that has a lower activation energy. More molecules possess the lower activation energy so therefore the rate of reaction increases. There are two types of catalyst, homogenous and heterogeneous catalysts. A homogenous catalyst is in the phase as the reactants, whereas a heterogeneous catalyst is not in the same phase (see diagram below).
For reactions in which gas is evolved, the reaction can be followed by measuring the volume of gas evolved over a period of time. By plotting a graph of time against the volume of gas, the rate of reaction can be determined at any given time, by measuring the gradient at that time. The rate of reaction is greatest when the gradient is steepest, i.e. at the start of the reaction. As the reaction slows down, the gradient becomes less steep. The reaction is complete when the graph becomes horizontal, i.e. there is no further increase in the volume of gas.
In this experiment the effect of concentration on the rate of reaction between calcium carbonate chips and Hydrochloric acid will be investigated.
CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g)
Aim: To investigate the effect of concentration on the rate of reaction between calcium carbonate (marble) chips and hydrochloric acid.
The concentration of the hydrochloric acid will be varied, and the reaction will be followed by measuring the volume of carbon dioxide gas evolved at given time intervals. The following variables will be kept constant in order to ensure that a fair test is conducted, temperature and surface area of the marble chips.
From preliminary work, the following concentrations of hydrochloric acid will be used, 0.5M, 1.0M, 1.5M and 2.0M, and the volume of carbon dioxide gas will be measured in intervals of thirty seconds.
Prediction: If the concentration of hydrochloric acid is doubled, the rate of reaction will also double.
Theory behind prediction: If the concentration of hydrochloric acid is doubled, the number of acid particles per cubic centimetre will double, hence there will be twice as many collisions occurring between the hydrochloric acid and calcium carbonate particles per cubic centimetre. This will result in the rate of reaction increasing by a factor of two.
This prediction will be tested by plotting graphs of time (s) against volume of carbon dioxide gas evolved (cm3) for the different concentrations of acid, and comparing the gradient of the graphs at a given time. If the rate of reaction doubles the gradient of the graph will double. The general shape of the graphs should be as shown below. The rate of reaction is greatest when the gradient is steepest, i.e. at the start of the reaction, as both the reactants are at their highest concentration. As the reaction precedes the concentration of the reacting substances decreases and the rate of reaction decreases, therefore the gradient becomes less steep. The reaction is complete when the graph becomes horizontal, i.e. there is no further increase in the volume of carbon dioxide gas evolved. When all the graphs are plotted on the same scale, the greater the concentration of the acid, the steeper the initial gradient of the graph. The reaction may finish sooner for the higher concentrations of acid, however the same amount of product formed will be the same in all cases (see graphs below).
Experimental
List of Apparatus:
- Balance
- Stop watch
- Conical flask
- Measuring cylinder
- Water bath
- Delivery tube
- Spatula
- 2.0g of calcium carbonate chips were weighed and placed into a conical flask. One end of a delivery tube was placed on the flask, the other end was placed into an upright measuring cylinder immersed in a water bath.
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25cm3 of 0.5M hydrochloric acid was measured and placed into the conical flask, simultaneously the stopwatch was turned on. It is important to keep the reactants separate whilst setting up the apparatus so that the starting time of the experiment can be measured accurately.
- After thirty second intervals the volume of carbon dioxide gas was measured. The measurements were made at eye level to avoid errors due to parallax. Care was also taken to insure there was no air bubbles inside the measuring cylinder. The measurements were continued until there was no change in the volume of gas evolved.
- This process was repeated using the following concentrations of hydrochloric acid: 1.0M, 1.5M and 2.0M. The experiment was repeated for each concentration of acid, so that the results obtained could be averaged. The same volume of acid and the same mass of marble chips were used for each concentration, to ensure that a fair test was carried out.
DISCUSSION
The results obtained are summarised in Table 1 and graphs 1 to 5.
The reactions were followed by measuring the volume of carbon dioxide gas evolved over a period of time. By plotting a graph of time against the volume of gas, the rate of reaction was determined by measuring the initial gradient of the graph. All the graphs show good agreement with the predicted shape. The rate of reaction is greatest when the gradient is steepest (0-30s), i.e. at the start of the reaction. This is because at the start of the reaction, the reactants are in their highest concentration and hence there are more collisions. As the reaction slows down, the gradient becomes less steep as their fewer collisions. The reaction is complete when the graph becomes horizontal, i.e. there is no further increase in the volume of gas. This was indicated by a halt in the production of bubbles.
From graph 5 it can be seen that the highest concentration of acid has the steepest gradient. This is because the greater the concentration of the acid, the more molecules there are per cm3. This results in more collisions between the acid particles and calcium carbonate and therefore an increase in the rate of reaction. The graph with least steep gradient is graph 1 (0.5 M). This is because graph 1 had the lowest concentraion of acid. The lower the concentration of acid, the fewer particles there are per cm3, resulting in a slower rate of reaction. I predicted that the graphs would all level off at the same volume. However my results do not support this. This is because, although an equal volume of acid was added in each case the the number of moles was different as the concentrations were not equal (n =cv). In order to obtain a graph consistent with the prediction, equal number of moles of acid need to be used for each concentraion. This would be achieved by using different volumes of acid. The lower the concentration the greater the volume of acid required.
I also predicted that if the concentration of hydrochloric acid is doubled, the rate of reaction will also double.
From the graphs the initial gradients (rate of reaction) were determined. The results are summarised in the table below.
From the table we can see that as the concentration doubled from 0.5 M to 1.0 M and from 1.0 M to 2.0 M, the rate of reaction is doubled. This is consistent with the quantitative prediction to within experimental errors. As the concentration of hydrochloric acid is doubled, the number of acid particles per cubic centimetre will double, hence there will be twice as many collisions occurring between the hydrochloric acid and calcium carbonate particles per cubic centimetre. This will result in the rate of reaction increasing by a factor of two. Overall the results show good agreement with my predictions.
CONCLUSION
Overall the results showed good agreement with the predictions. This is beacause the experimental setup is reasonably accurate, and only a few problems were encountered.
Although there are no anamolous results , a few points lie outside the line of best fit. The results where repeaed twice and the values were averged. In some cases there was not very good agreement between the two values. Due to time constraints the experiment could not be repeated for a third time. Had the experiment been repeated for a third time we could discard the value not consistent with the other two and hence obtain more accuarate results.
The errors present in the experimental setup include errors due to parralex( can be avoided my take measurements at eye level), the presence of bubbles in the measuring cylinder and the reaction of carbon dioxide with water (to form carbonic acid). All these errors would give innacurate measurements of the volume of gas. These could be overcome by using a syring the collect the gas, rather than collecting the gass under water. Other errors include judgement errors and errors due to reaction times.
This experiment could be exteneded by investigating the other variables which effect the rate of reaction, using different reactants and by using different experimental methods. Other methods include, sampling and titration and the use of a coloriemeter.
