Rates of Reaction

Introduction

For this investigation I will be carrying out experiments to see what kind of factors affect rate of reactions. Reactions happen inside and outside our body every single second. Reactions also occur all over the world in natural and man-made products. I will be investigating rate of reactions and what factors affect these.

Aim

My investigation is to investigate the rate of reaction when magnesium is put into Hydrochloric acid. I shall be keeping and changing different variables and make it as fair as possible. The main factor that I shall be investigating is how the concentration of the hydrochloric acid changes the rate of reaction to the magnesium.

Prediction

I predict that the higher the concentration of the acid, the faster the rate of reaction will be. I predict this because I believe that the higher the concentration of the acid, the greater the number of acid molecules (hydrogen ions) present in that volume. This means that there are more acid molecules colliding with the particles on the surface of the reactant (magnesium) thus increasing the rate of reaction.

When the reaction between hydrochloric acid and magnesium occur, the hydrochloric acid will dissolve the magnesium and produce hydrogen gas. For a chemical reaction to occur though, there must be reactants which when mixed may cause a chemical reaction. This in turn creates a product or outcome. In this experiment the reactants will be the hydrochloric acid and the magnesium ribbon. As soon as the magnesium is put into the acid and touches it the chemical reaction will start to take place.

When a chemical reaction occurs, the particles which combine need to meet up with each other (collide) so that they can swap or share electrons. If you want to speed up a reaction, you need to get these particles to hit each other more frequently. If the solution is made more concentrated it means there are more particles of reactant knocking about between the molecules, which make collisions between the important particles more likely. The main cause of acidity is hydrogen ions. In a more concentrated solution there are more hydrogen ions so therefore more collisions will occur. If hydrogen ions are the atoms that erode, then the more of them the more erosion that will take place. If there is more erosion taking place, the faster this chemical reaction will happen. So if the chemical reaction happens faster the hydrochloric acid will dissolve the magnesium quicker. So the stronger the concentration of the acid the faster the magnesium will dissolve.

Fair Test/Variables

To keep it a fair test there are a number of factors to consider. Some of them are I can control to the best of my abilities whilst other would be out of my reach.

The factors that I can control and keep the same will be the size of the magnesium pieces, beakers/measuring cylinders used, the volume of water and acid during the 2experiments to find an average.

Other factors that I cannot change is whether the temperature of the acid and water. Because I will be doing the experiments on different ...

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Fair Test/Variables

To keep it a fair test there are a number of factors to consider. Some of them are I can control to the best of my abilities whilst other would be out of my reach.

The factors that I can control and keep the same will be the size of the magnesium pieces, beakers/measuring cylinders used, the volume of water and acid during the 2experiments to find an average.

Other factors that I cannot change is whether the temperature of the acid and water. Because I will be doing the experiments on different days, the temperature could change and change my results. If the day were to be warmer then the acid and water would be warmer. If the solution is warmer then it has more energy. If it has more energy the particles will move quicker and if they mover quicker they will collide with the magnesium at a faster rate. This would speed up the chemical reaction and therefore change he time it takes for the magnesium to dissolve.

I will try to make the process of mixing the acid and water a fair as possible. I will put the amount of water in first, then acid and mix it up for 10 seconds. By doing this the acid and water would have mixed properly and the ratio and moles per cm² would be correct.

Apparatus

3 beakers, measuring cylinders, hydrochloric acid, water, magnesium, stopwatch, scissors, ruler.

Method

• Fill one beaker with 2M hydrochloric acid

• Fill one beaker with water

• Cut magnesium ribbon in to 5mmX10mm pieces

• Pour acid and water into 3rd beaker to right concentration

• Place magnesium piece into beaker

• Start stopwatch

• Stop stopwatch when magnesium dissolved

Plan

This is a table of the solution I am going to put the magnesium in.

Readings

As soon as I put the magnesium into the beaker and it touches the hydrochloric acid I will start the stopwatch. I will monitor the experiment and as soon as the magnesium disappears I will stop the stopwatch and record the time down. I shall be doing around 9 readings and repeat the experiment twice to get an average. By doing so I will make more results more reliable.

Safety

The safety of this experiment is very important. The things that I will need to do to keep my experiment safe for myself and other students around me are as follows: Wear safety goggles as I am using hydrochloric acid which can irritate the skin, Care to eyes and the skin besides all the other people is always vital and necessary, Care in using glassware since it is sharp when broken and can cut skin. Safe disposal of reagents and laboratory chemicals, Care when returning all used glassware and equipment at the end of the experiment.

Results

* Time taken for the magnesium to dissolve in seconds

Analysis

On the graph it shows a nice clean curve that passes through most points. Some though are just slightly off but I don’t think any are so off that it could be called an anomalous result. The curve sweeps across the page very rapidly and then begins to straighten off.

In the first part of the graph (0-60 seconds), it shows a rapid drop. At the beginning, it is almost vertical (15-30secsonds) but as you go along it begins to turn more and more to the right. This again backs up my prediction. Where it is near vertical, this is the part where the acid is most concentrated. This shows that the more concentrated the acid is the faster the magnesium will dissolve.

Round about 30-60 seconds though, it begins to get more horizontal and starts the flat out. I think this happened because as the concentration went down, the rate of reaction slowed down as well. Even though it is still eroding the magnesium away, it is taking a longer time than when it is most concentrated. This is because the weaker the acid is, the less acid molecules are in the solution (hydrogen ions). If there are less hydrogen ions there is less surface area that the acid can react with the magnesium. This results in fewer collisions with the acid molecules and the magnesium itself. Therefore the chemical reaction will have slowed down and thus the magnesium will take longer to dissolve.

Comparing this to where it is around 15-30 seconds, here the concentration is higher. The acid here will have more hydrogen ions and more surface area for the acid to react with the magnesium. This results in more collisions between the acid and magnesium, speeding up the chemical reaction and so for dissolving the magnesium quicker.

The graph shows this curve because when the weaker the acid gets, the more time it will take for the chemical reaction to happen/finish. If you combine both sections (15-30 and 30-60 as I said before) it will create a sweeping curve that will start to bend straighter and straighter. When the acid gets so weak and diluted there will come a time when the chemical reaction will not happen because there are simply not enough acid molecules for to dissolve the whole piece of magnesium. This would happen because when a reaction is taken place, electrons move and atoms combine. When all the hydrogen ions from the acid are taken by the magnesium there will not be any hydrogen ions for the magnesium to get a hold of and react. The equation for this chemical reaction would be –

Mg + 2HCl -> MgCl2 + H2

Magnesium + Hydrochloric acid -> Magnesium Chloride + Hydrogen

I think some of the points are not on the curve as they are slightly out because a number of reasons.

Firstly the temperature of the solution of water and acid. As I said before the warmer the solution the faster the rate of reaction.

Secondly the volume or mass of the magnesium would not have been the same. For each piece of magnesium put into the acid/water, each one of them would have a different mass as no piece could be cut to exactly the same. If a piece were to be slightly bigger then it would take longer for the magnesium to dissolve. This is because the acid needs to dissolve a bigger volume of magnesium and needs more acid molecules to react with it therefore it would take slightly longer. If a piece were to be smaller then there would be less volume for the acid to react with it and more acid molecules to get to it in the same concentration.

I also notice that sometimes the magnesium either sunk under or just floated at the top. This could have changed my results quite dramatically especially at the more concentrated end of the scale as there were not much difference between the beakers with acid concentration of 2M, 1.8M and 1.6M. I think that if it sunk under the acid then more acid to get to the magnesium. In other words there would be more surface area for the acid to react with the magnesium. This will cause more collisions between the hydrogen ions and the magnesium increasing rate of reaction. If the magnesium piece floated on the top, the acid would not be able to get to the top of it. Therefore of the acid would have had less surface area to react with the magnesium decreasing the rate of reaction.

When the acid did get weaker though it took longer to dissolve and that gap between each solution concentration got bigger and bigger, It wasn’t double concentration, double reaction time or half concentration half reaction time. As the less concentrated the acid was the longer and longer reaction time and this gap kept increasing.

Conclusion

My prediction was right. It was definitely true and the results now prove my prediction. The more concentrated the acid is, the faster the reaction will happen and the quicker the magnesium will dissolve. So more concentration = faster chemical reaction.

Evaluation

I think my results were pretty accurate and I was happy with them. I was glad I had enough time to do a second repeat of the experiment as I could check to see whether there was any anomalous results and make them reliable. There was a clear pattern on the graph that showed that my results must have been right (higher concentrated acid, faster reaction time, vice versa).

I think that if I had more time I would have done a 3rd repeat of the experiment just to make the averages more reliable. I think the method was sometimes a little inaccurate.

When mixing the water and acid, whichever you poured in last could have stayed at the top more and the solution might not have properly mixed.

When the magnesium was put in it either floated or sunk, or sunk half way through the experiment or even floated back up. This sometimes happened and the times were completely off from what it was.

The magnesium sometimes and often did float and fizz to the side of the beaker. I thought that if it were stuck at the side the acid could no have got to it more freely than if it was in the middle which could have changed the results as well.

For every concentration of acid solution used we kept the same beaker over and over again. We just poured the used solution out with some acid/water left clinging to the glass. We then poured the new solution in and the both could have mixed. Even though this might not affect it so mush it could have altered the entire percentage ration of acid/water.

If I were to do this experiment over again I would probably eliminate some of the notes above, this would make the test a lot fairer. I would definitely repeat it at least 3 times or even more to make the results more reliable. I don’t think I would change much else as most of the other factors I was happy with.