Investigate a factor which affects the rates of reaction between magnesium ribbon and hydrochloric acid.

INVESTIGATING REACTION RATES

Aim: To investigate a factor which affects the rates of reaction between magnesium ribbon and hydrochloric acid.

Background Knowledge

The rate of a chemical reaction is the speed that the reaction takes to finish. For a chemical reaction to take place reagent particles must collide with each other and the collision must have enough energy if a reaction is to take place. This energy is called the activation energy.

There are several things that can be done to speed up the reaction, which increase the rate.

Temperature

Increasing the temperature increases the energy particles possesses, and so more particles possess the minimum energy (activation energy) and so reaction is faster.

Concentration of acid

There are more reactant particles at a higher concentration in a volume. Therefore there is more chance of collision.

Dilute solution Concentrated Solution

Surface Area

Increasing S.A increasing rate of reaction. This is because smaller particles have a higher total surface area exposed for reaction and therefore more collisions occur, making the rate of reaction faster.

Magnesium Ribbon Powdered Magnesium

Catalysts

they provide a different pathway with a lower activation and so the reaction speeds up the reaction without being chemically changed itself.

The diagram above shows the energy level diagram for a reaction both with and without a catalyst. The catalyst has provided a lower energy barrier and therefore it is easier for the particles to change into products.

What Factor am I going to study?

I am going to study what effect concentration will have on the rate of reaction between magnesium ribbon and hydrochloric acid.

Prediction

I have stated in my background knowledge that the higher the concentration the faster the reaction, and so the rate increases. Therefore, I am going to predict that as the concentration of hydrochloric acid increases, the time taken for the magnesium to dissolve will decrease, and so the rate will increase.

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Trial Experiment

In order to get the best possible results I have to carry out a trial experiment to determine the range of values I am going to use for the concentration. In my trial experiment I used a 4cm length of magnesium ribbon and 50ml as the total volume of liquid, reducing the concentration by 20% each time.

I found that 4cm was a suitable of magnesium ribbon. I did decide to decrease the concentration by 10% it gave me many result, making it more accurate. I also decided to repeat my main experiment ...

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Trial Experiment

I found that 4cm was a suitable of magnesium ribbon. I did decide to decrease the concentration by 10% it gave me many result, making it more accurate. I also decided to repeat my main experiment twice. I will then take an average, as to increase the accuracy of my results. I will not include concentrations of 10% or 20% in my main experiment. This is because these reactions are very slow and cannot be finished within the time of our chemistry class.

Variables

Independent Variable: The factor I am going to change in this experiment is the concentration.

To work out the concentration of each acid solution I used the following equation,

Volume of acid

Total volume

Controlled Variable: To enable this is a fair test there are several things that must be kept the same, for example the temperature of the acid (room temperature), the length of magnesium ribbon (4cm) and the total volume of liquid (50ml).

Dependent Variable: In this case I will be measuring how long it takes for the magnesium to dissolve. I will do this by using a stopwatch.

Safety Precautions

As we are using acid in this experiment it is essential that all pupils must wear safety goggles to protect our eyes. But also, when handling the acid be careful not to spill any on your hands or the work surfaces.

Apparatus:

Safety goggles
25ml measuring cylinder (x2)
50ml beaker
250ml beaker (x2)
Stopwatch
Stirring rod
Dropper (x2)
Magnesium ribbon at lengths of 4cm (x18)

The total volume of liquid for each solution was 50cm3 and so I had to choose appropriate sizes of measuring cylinders and beakers. I used the two 25ml measuring cylinders to measure out the volume of the solution, one was used for the acid and one for the distilled water. Also, to make the volume of solution more accurate I used the droppers to measure out exactly 25cm3 per measuring cylinder, again one was used for the acid and one for the distilled water. I filled the 250ml beakers, one with the acid and one with distilled water, and brought it to my desk, this prevented me from having to go back and forth across the class to collect more.

Method

Collect all the apparatus in the list above.

Put on your safety goggles

Collect 64cm of Magnesium ribbon and using scissors cut it into 16 separate pieces, each measuring 4cm long.

Pour the acid and water into the separate 250ml beaker and bring it over to your desk.

Then measure out 50cm3 of acid use the 25ml measuring cylinder (using it twice) and pour this into the 50ml beaker.

Drop the 4cm strip of magnesium into the beaker and immediately start the stopwatch.

Hold the ribbon under the surface of the acid with the stirring rod, as to increase the surface area in contact with the acid, until dissolved.

As soon as the magnesium dissolved, stop the stopwatch and record the time in your results table. Rinse the beaker.

Now measure out 45cm3 of acid, again using the 25ml measuring cylinder (twice, this is so increase the accuracy) and 5cm3 of distilled water (using the second measuring cylinder) and pour both the acid and water into the 50ml beaker again.

Drop the 4cm strip of magnesium into the beaker, immediately start the stopwatch as before.

Repeating as before, using the stirring rod. As soon as the ribbon has dissolved, stop the timer and record the result in your table.

Repeat the process, decreasing the volume of acid each time by 5ml, and increasing the distilled water by 5ml, recording your results in the results table you have.

Repeat the experiment again, then take an average.

Work out the rate of the reaction and plot two graphs as shown below.

Average Rate

Time

Concentration Concentration

Strategy for Results

I am going to record the time it takes for the magnesium ribbon to dissolve when dropped into a beaker of hydrochloric acid. I will repeat this process several times using 8 different concentrations of acid. For each of the eight acid solutions I will carry out the experiment twice and then take an average time. Then I will plot a graph to show the average time against the concentration. For my second graph I will plot the rate of reaction on the y-axis and the concentration along the x-axis.

To calculate the rate I used this equation:

Rate (s-1) = time (s)

Results

For each of the different concentrations I carried out the experiment twice, as shown in the results table on the previous page. I had one rogue results, which was the 60% concentration. This is most likely a human error, the concentration may not have been accurate, the magnesium strip may not have been exactly the 4cm like the others or there may have been some of the previous solution in the beaker which could have affected this result. All the other results follow the predicted pattern. As the concentration of acid increases, the time it takes for the reaction to take place decreases. Using the results on the previous page I also have calculated the average time and the rate of reaction. I have used the following results to plot two separate graphs, which are one the next pages.

Conclusion

In my investigation I proved that if you increase the concentration of an acid the reaction will occur much faster. This is due to more collisions taking place between the acid particles and the magnesium atoms. My results also support my original prediction, that the higher the concentration, the faster the rate of reaction.

In my first graph you can see that the results slope downwards in a steep curve. This shows that as the concentration of the hydrochloric acid becomes weaker the average time for the reaction increases.

In the second graph the results show that as the concentration increases the rate also increases.

Evaluation

Overall, I think that my experiment went well. The method was simple and easy to follow and I didn’t encounter any problems whilst carrying out the experiment. It also proved that my prediction based on my background scientific knowledge was right. Therefore my method does work and give good results.

However, as human intervention is involved when starting and stopping the stopwatch my results may not be completely accurate. There are several ways I could change my experiment in order to make it more accurate. For example I could collect the volume of gas given off, or I could have used a burette. However, the best way to increase volume accuracy would be to use a pipette, as it is accurate to 0.02 cm3.

I used a wide range of different concentrations and obtained a variety of good results. They all follow the same general trend although I have on anomalous results at 60% concentration of the acid. My results prove that when the concentration of the solution is increased, the rate of reaction is also increased.

As I have already mentioned above, another way to carry out the experiment would be to measure the amount of hydrogen gas given off during the reaction. You could do this by following the same method but this time collect the volume of hydrogen gas given off in a gas syringe. Below is a diagram of the new apparatus needed.