To investigate how temperature has an effect on the rate of a reaction.

Chemistry Investigation

Investigating the factors which affect the rate of a reaction

Aim: To investigate how temperature has an effect on the rate of a reaction.

Diagram 1 - Apparatus for experiment:

Hypothesis:

I think that when the temperature of the hydrochloric acid is increased, the rate of the reaction between the hydrochloric acid and magnesium will increase. This is because when the temperature of the acid is increased, the atoms in the acid move faster gaining more kinetic energy. This increases the impact energy when the atoms in the acid collide with the magnesium atoms, providing enough energy to overcome the activation energy, this is the minimum amount of energy required for a product (Magnesium Chloride) to form.

Increasing the temperature will also increase the number of collisions per second between the magnesium and hydrochloric acid atoms, creating more opportunities per second for the reaction to occur which will also lead to a higher rate of reaction.

For these reasons I expect my final graph to look like the one below which shows that as temperature increases the rate of reaction increases.

Balanced Equation

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Method

Measure out 50cm³ hydrochloric acid using a burette to gain an accurate reading. Pour into a conical flask.

Using a Bunsen Burner, carefully heat the acid to the required temperature, stirring to make sure the temperature is evenly distributed.

Measure out 3cm magnesium ribbon and add this to the hydrochloric acid, starting the stopwatch as soon as the magnesium enters the acid.

Time the reaction until the magnesium ribbon has disappeared.

Record the results in a table.

Repeat the experiment to check the results.

Fair Test

The experiments will be a fair test as there will only be one variable in each. The possible variables in this investigation are:

Quantity of hydrochloric acid used
Concentration of the acid
Quantity of magnesium used
Surface area of the magnesium
Temperature of the hydrochloric acid
Temperature of the magnesium at the start of the reaction

The variable in this investigation will be the temperature of the hydrochloric acid. All other variables will be kept constant for each experiment as follows:-

50cm³ of 1 molar hydrochloric acid
3cm length of magnesium ribbon
Average room temperature of magnesium at start of reaction 22ºC

Apparatus required for both experiments:

1 Conical Flask

1 Bench mat

1 Bunsen Burner

1 Thermometer

600cm³ hydrochloric acid

30cm Magnesium Ribbon

1 Burette

1 Tripod

1 Gauze

1 Stopclock.

Preliminary Work

For my preliminary work, I tested the reaction between 50cm³ hydrochloric acid at 20ºC and a 3 cm length of Magnesium Ribbon. I used the method above and timed the reaction from the point at which the magnesium entered the acid to point where the magnesium had completely disappeared and the reaction had stopped. I found that the reaction lasted 80 seconds.

I also tested the reaction between hydrochloric acid at 60ºC and a 3 cm length of magnesium ribbon, using the same ...

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30cm Magnesium Ribbon

1 Burette

1 Tripod

1 Gauze

1 Stopclock.

Preliminary Work

I also tested the reaction between hydrochloric acid at 60ºC and a 3 cm length of magnesium ribbon, using the same method and timing the reaction. I found that the reaction lasted 32 seconds. These results helped me to form my hypothesis and give me a better idea of how to plan the full experiment.

Full Experiment

For the full experiment using the method given above I selected 5 temperatures to time the reaction over to give me enough points to plot a graph (see Table 1 below).

Conclusions and Analysis of Results

The results of my experiment are shown in Graph 1.

Graph 1 shows that when the temperature of the acid is increased, the rate of reaction is higher which agrees with my hypothesis.

It also shows that as the temperature range gets higher, increasing the temperature has less of an effect on the rate of reaction.

This could be because the hydrogen gas bubbles created in the reaction form very quickly on the surface of the magnesium ribbon when the temperature and rate of reaction is higher. These bubbles may stop some of the acid ions from reaching the surface of the magnesium ribbon preventing collisions. The larger hydrogen gas bubbles which collect on the flat surface of the magnesium ribbon could be lifting parts of it clear of the acid for short periods which would prevent the average the number of collisions per second increasing as quickly as expected so slowing the increase in reaction rate.

So the rate of reaction still increases with temperature but no by as much as it did for the same change at lower temperatures. These effects could explain why Graph 1 shows a slower increase in reaction rate with temperatures above 40ºC than below 40ºC.

This effect could be reduced by changing the shape of the magnesium. The magnesium ribbon's flat surface can trap hydrogen bubbles when it floats to the surface of the acid, so not much acid would be in contact with the magnesium, slowing the rate of reaction. If a sphere of magnesium were used, less hydrogen would be trapped, since there would be a higher surface area to volume ratio, which would increase the rate of reaction further.

Conclusion

I have made two main conclusions from carrying out this investigation:

When the temperature of the acid is increased, the rate of the reaction increases, for example the reaction between hydrochloric acid and magnesium at 60º is more than twice as fast as the reaction at 20ºC.
The rate of reaction increases more rapidly below 40ºC than above 40ºC.

The scientific knowledge supporting my conclusion

The Collision Theory states "the more collisions between particles in a given time, the faster the reaction". In my experiment the rate of reaction increases when the acid is heated because the acid ions and magnesium molecules move faster and are more likely to collide with each other giving the opportunity for a reaction to take place.

Also, at higher temperatures, the faster movement of the acid ions and molecules of magnesium gives them increased kinetic energy which means that when they do collide there is more chance that their combined energy will be enough to overcome the activation energy needed to create magnesium chloride, see Diagram 2 below.

Diagram 2 - Activation energy for the reaction of magnesium and hydrochloric acid

Both these effects of increased temperature act to give a higher reaction rate which agrees with my hypothesis.

Variation of Results and Reliability

The reliability of the results is dependent on the method used, the accuracy of the equipment used to carry out the measurements (ruler, stop clock, burette) and the care taken in making readings and setting up equipment.

If the experiment were repeated I would expect to get similar results although there may be some variation due to equipment set up and lab temperature.

There are two suspected anomalous readings in my results from this investigation taken in the first set, for the temperatures of 53ºC and 62ºC see Table 1 and Graph 1. The anomalous readings from the first set give slower reaction rates for increased temperature which is not a trend shown by any other results in Table 1 and Graph 1. The second set of results taken show the trend predicted in my hypothesis of increased reaction rate as temperature increases.

These anomalous readings could be due to a number of reasons as follows:-

Errors timing the reaction length with the stopwatch - deciding when to stop the stopwatch to mark the end of the reaction was difficult as the magnesium floated to the surface causing bubbles making it hard to know when the reaction had finished.
Variation in the shape of the magnesium ribbon - if the ribbon was twisted or misshaped it could make the hydrogen bubble effects discussed in my analysis above more significant.
Errors in measuring the length of magnesium, longer lengths would give an increased reaction time.
Failing to stabilise the temperature of the acid, if the temperature of the acid were higher at the bottom of the flask than at the top where the reaction is taking place this would give anomalous readings.
Errors reading the burette - if there were less acid this could increase the reaction time.
Temperature of the magnesium at the start of the reaction - if it had been cooler this could have slowed the initial rate of the reaction.

One or more of these effects could have combined to give the anomalous results.

The second set of readings were taken on a different day and I think that due to the fact that I was more familiar with the methods used in the experiment the readings taken are more reliable.

Accuracy of Results

Although there were some anomalous results in my first set of readings I think that overall the accuracy was sufficient for the experiment as the pattern shown by my results and my conclusions are supported by the collision theory.

Evaluation of the Experiment

I think that this experiment worked well, as the results fit with my hypothesis and are supported by the collision theory.

General Method Improvements

The list of reasons for anomalous readings given in the section above on Variation in Results could all be used to improve the method for example:-

Errors timing the reaction length with the stopwatch - deciding when to stop the stopwatch to mark the end of the reaction was difficult as the magnesium floated to the surface causing bubbles making it hard to know when the reaction had finished.

This effect could be reduced by weighing the magnesium down so that it is in contact with the acid as much as possible (for example in Diagram 3 below).

Diagram 3 - Improving the method by clamping the magnesium to a glass rod with a plastic clip

By clamping the magnesium below the surface of the acid with the glass rod and plastic clip, gas bubbles will rise quickly to the surface and will not lift the ribbon out of the acid affecting the rate of reaction and making it more difficult to tell when the reaction has completed.

Variation in the shape of the magnesium ribbon - if the ribbon was twisted or misshaped it could make the hydrogen bubble effects discussed in my analysis above more significant.

This effect could be reduced by making sure the lengths of magnesium were always the same length and shape before clamping into place, flattening out any twists or uneven surfaces with a roller.

Errors in measuring the length of magnesium, longer lengths would give an increased reaction time.

This effect could be reduced by taking greater care in measuring and cutting the lengths of magnesium for the experiment.

Failing to stabilise the temperature of the acid, if the temperature of the acid were higher at the bottom of the flask than at the top where the reaction is taking place this would give anomalous readings.

This effect could be reduced by making sure the acid is stirred very well whilst it is being heated to ensure an even temperature throughout.

All of these measures would improve the method and probably give more reliable results.

An alternative method of measuring the reaction rate at different temperatures could be used to compare with the results from this experiment. The hydrogen gas could be collected using a gas syringe and the amount of gas collected could be recorded every 10 seconds, see Diagram 4 below. This would give a rate of reaction which could be measured for different temperatures and this set of results could be compared to the results from this experiment. Although this is a more complicated method there is no need to determine the exact end of the reaction so one of the main causes of errors with my original method is removed.

Diagram 4 - Alternative method for measuring the rate of reaction by measuring the rate of hydrogen production

Further Research

This experiment could be repeated with a wider range of temperatures for the acid, more readings would give us a better indication of the relationship between temperature and rate of reaction.

The experiment could also be repeated with different shapes of magnesium, cubes, spheres, pyramids etc. to investigate how different ratios of surface area to volume for a given mass of magnesium affect the rate of reaction. I would expect that as the surface area to volume ratio increases for a given mass the reaction rate would increase as there will be more of the surface of the magnesium in contact with the acid.

The experiment could be repeated with different concentrations of acid this would show us how the reaction rate varies with acid concentration for a given temperature.

Rebecca Allott 10J Page