The effect of temperature on the rate of reaction

GCSE Science

Simon Flood

Mr Macintosh

11C

Chemistry Course work

11Hawking

School Number: 37625

Candidate number:

Investigation: The effect of temperature on the rate of reaction

We are conducting an experiment to ascertain whether Magnesium strips will dissolve quicker in a solution of Sulphuric Acid at various temperatures.

During the experiment I am trying to find out whether temperature will have an effect on the rate of a reaction, and also what the effect on the reaction is. Throughout the experiment I shall test the following reaction:

Magnesium + Sulphuric Acid → Magnesium Sulphate + Hydrogen

Mg (s) H2 (aq) MgSO4 (aq) H2 (g)

Preliminary Test:

Before starting our experiment we had to think ahead to any problems we may encounter during the experiment in and what would be the best way to resolve them. There were three main areas we decided it would be necessary to test, and these are listed below along with how we have decided to resolve them:

Exothermic Reaction: The majority of reactions give off excess heat, which basically means they are exothermic, and this would affect the results of our experiment as we are testing how temperature change can effect the rate of reaction. Basically this shows us that we need to find the activation energy. Activation energy is the minimum amount of energy put into the reaction to get the minimum out of it. Reactants have an amount of energy. You need to put the same amount of energy in to get the Activation energy.

Following is a table showing how different concentrations of sulphuric acid and lengths magnesium strip affect the temperature:

Previously we knew that the rate of a reaction will change during the reaction. It is greatest at the start, but declines as the reaction proceeds because reactants are being used up. This is why we had to carry out the preliminary experiment to be able to conclude with accurate results.

From this testing I concluded that the most suitable combination to use will be 1.0cm of magnesium and 40.0cm³ of sulphuric acid.

Magnesium Oxide: When magnesium is exposed to air, it reacts with oxygen within it and gains a layer of Magnesium Oxide which will affect the speed of the reaction (slow it down), therefore it will need to be removed. There are two ways in which the magnesium oxide can be removed, both are listed below:

Dipping the Magnesium strip in the acid solution.

Removing the layer by rubbing it off with sandpaper.

However the problem with dipping it in acid is that you then immediately have to wash it with water to avoid the whole piece of magnesium deteriorating and also you have no control over the extensity of corrosion. Therefore I chose to rub it away with sandpaper as you have more control over it and can make the experiment as fair as possible. For the dipping experiment we cannot make it fair as some of the Magnesium will corrode away before it is entered into the acid. One downfall of the sandpaper is however, that if it is rubbed too vigorously then it can ignite and put people in danger.

Magnesium Floats: When Magnesium is placed into a liquid it floats and the majority of the time has a tendency to stick to the side of the beaker, this will make it an unfair test as the surface area exposed to the acid will change from piece to piece and also the magnesium can not react with the maximum amount of sulphuric acid as possible, I will solve this simply by stirring the experiment during the experiment with a glass rod.

The apparatus I shall use for the entire investigation is listed below:

Apparatus:

Tripod
Gauze
Heat proof mat
goggles
Bunsen burner
-10°C - 100°C thermometer
50.0cm³ measuring cylinder
50.0cm³ beaker
100.0cm³ beaker
Clamp
stopwatch
Magnesium strips 0.5,1.0,1.5 cm
Sandpaper
Sulphuric Acid 20,30,40cm 3
Water

Diagram:

Method:

Cut fifteen 1.0cm Magnesium strips and rub away the remaining magnesium oxide layer with sandpaper (until it shines).

Place one of the 1cm Magnesium strips into a measured solution of Sulphuric acid (40.0cm³) which is at the room temperature (20oC) and time the reaction (until the Magnesium and Hydrogen bubbles disappear) using the stopwatch. Stirring the solution constantly makes it a fair test, (see preliminary testing), repeat this three times at room temperature and take an average reading. Also record the temperature after the reaction has taken place to make sure the acid has not increased by more than 1.0oC.
Set up the tripod, Bunsen burner and gauze, and heat water in the 100.0cm³ beaker, use this beaker as a water bath, and place a test tube containing 40.0cm³ of Sulphuric Acid in it also, and heat the acid until it reaches 30.0°C. When it reaches 30.0°C, pour the acid immediately into a 50.0cm³ beaker and add a 1cm piece of Magnesium and again time the reaction using the stopwatch. Repeat the experiment a further two times at this temperature to make the experiment fair and to make it possible to calculate any anomalous results.
Repeat the above experiment, heating the water containing a breaker of acid to 40°C, 50°C and 60°C and record the times, again repeat the experiment three times for each temperature- ie three pieces of mg for 40,50,60oC. When the test tube begins to be too hot to handle, use the clamp to hold it in the water bath.

Safety: I will be making the following measures for the following reasons to ensure that safety is at its highest possible level:

Wearing goggles: If the acid sprays up into your eyes it can be very dangerous, therefore eye protection is essential
Not directly heating the acid: using a water bath minimises the risk of the acid exploding which is very likely if you heated it directly.
Using a clamp: If we were holding the test tube when it was being heated to temperature of up to 60°C it could be very dangerous as if our fingers ...

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Safety: I will be making the following measures for the following reasons to ensure that safety is at its highest possible level:

Wearing goggles: If the acid sprays up into your eyes it can be very dangerous, therefore eye protection is essential
Not directly heating the acid: using a water bath minimises the risk of the acid exploding which is very likely if you heated it directly.
Using a clamp: If we were holding the test tube when it was being heated to temperature of up to 60°C it could be very dangerous as if our fingers overheated and we dropped the acid onto someone, someone could receive a very nasty burn along with a cut from the smashed glass.
I am using beakers with extra volume so that the liquids, especially the acid will be less likely to spill when we are moving them around and that the acid and water will not spill over the top when they are directly and indirectly heated.
Sitting down: If I were to sit down when the acid were boiling I could receive a very nasty sting and I would not be able to move quick enough to get out of the way.

Fair test:

I need to make sure that I take every measurement possible to keep this experiment fair. I only need to change the temperature in each experiment. Therefore I need to keep the following things the same:

Concentration: I have looked at the ideal concentration in my preliminary testing, and to control it I will make sure that I carefully measure all substances I use.
Particle size: this is a possible affect on the experiment and is controlled by the measures taken to control the concentration.
UV light exposure : does not really affect this experiment, and is unlikely to change between testing
Pressure: does not really affect this experiment, and is unlikely to change between testing
Catalysts: does not really affect this experiment, and is unlikely to change between testing

In addition to these steps, I am being careful about the amount of magnesium I file away with the sandpaper, and how long it is stirred for in each experiment.

Prediction:

In order for any chemical reaction to occur, the following factors must be taken into account:

the reagent particles must collide with each other.

the collision must have enough energy if a reaction is to take place. This energy is called the activation energy.

If the collision has enough energy, a reaction occurs. Magnesium chloride & hydrogen are formed.

If the collision does not have enough energy, no reaction occurs. The acid particle bounces away again.

I predict that the higher the temperature the faster the time for a reaction to have taken place, ie, the faster the Magnesium will dissove in the Sulphuric Acid. This is because when you heat particles you are giving them heat energy, which they can then transfer into kinetic energy. This extra movement means that they are more likely to collide. More collisions increase the rate of reaction. Four ways in which collisions can occur are listed below.

Temperature increases the rate of reaction: As a liquid is heated the particles move more quicker and therefore will collide more. This is due to the particles reacting with the substances at lower temperatures and therefore not having much energy, and the heating of these substances causes particles to take in more and more energy. After this has happened the particles will collide more due to them containing more energy and thus leading to the collisions gradually gaining more energy. Thus proving that the collisions will be more successful.

Concentration (pressure) increases the number of collisions: The more of the concentration that there is then the more particles will be moving about colliding with one another. If the concentration (of a reactant) increases, the rate of reaction will also increase. This is because a higher concentration of a substance will contain more particles, and therefore more successful collisions (i.e. collisions with enough energy for a reaction to take place) will occur per second. In our case dilute acid is the concentration and there are fewer acid particles, therefore there are less collisions between acid particles and magnesium atoms and therefore the reaction takes longer.

Size of Solid Particles (Surface area) increases collisions: If one reactant is a solid then obviously breaking it up will split these solid particles up again increasing collisions due to them being bigger. The greater the surface area, then the faster the rate of reaction will be, in the smaller particles the entire surface exposed for reaction is greater therefore more collisions occur and due to the increase in collisions, the rate of reaction is obviously going to be faster.

Catalyst increases the number of collisions: A catalyst works by giving the particles an extra surface to bounce off of and again increasing collisions. Catalysts offer an alternative path with a lower activation energy. Before reactants are able to convert into products, they must overcome an energy barrier. This is the slowest part of the reaction. The energy needed for a reactant to overcome the energy barrier is called the activation energy. Effectively, a catalyst lowers the activation energy, therefore speeding up the reaction (ie, the reaction rate increases).

Nb: a catalyst is a substance that changes the rate of a reaction, but remains chemically unchanged itself.

The Boltzmann theory relatively fits in with my predictions/scientific evidence, therefore I shall include it below:

We can now look at the Boltzmann theory graph: -

I have found looking at the GCSE double award Chemistry revision guide the Maxwell Boltzmann distribution.

Looking at the Boltzmann Distribution we can see that an increase in temperature increases the rate of reaction. Below is the diagram:

We can see this by the fact that:

-Molecules in larger numbers exceed the activation energy.

-The faster the molecules travel the more frequent the collisions are.

-The Boltzmann distribution curve is displaced with a lower peak to the right increasing the average energy.

The total area is the alike under each distribution curve:

-The temperature is the only factor that has changed.

-The molecules in numbers are correspondent to one another.

The Activation Energy (AE) is the point when a molecule has enough energy to create a product. All molecules before this point cannot create a product. By increasing only the heat we can give more molecules enough of the energy to make it possible to collide and produce an actual product. When this occurs the arch changes its size and overall structured shape.

Looking at this we can see why an increase in temperature increases the speed.

My second prediction was that for every 10oc rise the time will double due to the temperature being hotter. This would be because temperature increases the rate of reactions. If they are increased then the time should double due to this statement.

Observations

From this we can draw two graphs. One shows the effect of temperature and the other shows effect of temperature on rate. To find rate we needed to use the calculation 1/average time with the units of seconds to the –1(s-1)

From the table we can see that as the temperature increases the rate of reaction does the same. We can see this more clearly by looking at the graphs.

Analysis

Originally I constructed two predictions, both of which I would try and prove true. Both are below: -

The higher the temperature the faster the reaction will occur. (graph 1)
For every 10oc rise the time will double.(graph 2)

I shall try to prove prediction one true by explaining a few observations that I made and using scientific knowledge to prove them true.

Looking at the graph labelled “a graph to show the effect of temperature” I can see a smooth curve of best fit. This curve shows us the hotter the temperature the faster the reaction. But it also shows that although the time is decreasing and the temperature increasing, it also seems that as it is increasing it is also slowing down when compared to the previous result.

This may be because our results were not one hundred percent fair but there is definitely a strong trend that could be followed up with further tests and experiments if we were to continue with the test.

Looking at the graph however we can see that the speed is increasing, as is the temperature as the experiment develops.

I can prove the Hypothesis true by taking certain segments out of the graph and comparing them with one another.

Take a look at the temperature block of 20-30oc. We can see by looking at this section that there is a clear large gap of about 15.5 seconds. Thus showing that there is a long wait for the reactions. Now if we take a look at block 50-60oc we can see a much shorter difference, so much shorter that only about a 5 second difference can be seen.

I could use more examples but the resulting outcome would be almost of an identical one.

Looking at my second hypothesis I shall now try and back it up.

Looking at the graph labelled “A graph to show the effect of temperature on rate.” I can see that this hypothesis can not be true.

Take the two blocks 22oc and 32oc. When compared we can see that 22 is 0.018s-1 and 32 is 0.0225s-1. 0.018 multiplied by 2 is 0.036, which is more than the 32-degree Celsius measurement. Just to show that this is not a coincidence I shall repeat it with two other random temperatures. Take 420c and 520c. 42 is 0.0269 and 52 is 0.0309. When 0.0269 is multiplied by two we get 0.0538 which again is more than double.

So although the hypothesis was almost correct we can possibly say it is due to the times not being doubled. However during working this out I have found that for every 100c increase the time increases by about 0.017-0.02 each time. This proving that we have found a pattern but not the one we had hoped for.

The more energy put into an experiment the more will be produced at the end of it, however the energy will lose its activation energy after a while and will start to produce less generally.

Overall we can see that prediction (hypothesis) one has been proved to stand UN-corrected/true.

However Prediction (Hypothesis) two could not possibly be proved true due to the times becoming quicker slightly but not so rapidly that it would be doubled. Looking at the molecule movements this can backed up along with activation energy and the Boltzmann Distribution.

Evaluation:

I was very pleased with the general outcome of my results, as, when transported onto a graph, the line of best fit took a healthy linear shape. Also none of my results were anomalies as the points lay very close to the line of best fit due to fairly accurate readings. My graph did not curve and flatten at the top to show the end of the reaction because I had relatively precise measurements of the acid which was needed to overcome the magnesium, thus showing that the reaction went to consummation forming a good linear graph. This backs up my theory that reaction rate increases with concentration and therefore the evidence gained from looking at the graph is strong enough to support this conclusion of mine.

My method was greatly satisfactory as by carrying it out, I was able to achieve fairly precise results, and there were no errors large enough to be identified as anomalous results. However, the points were slightly out – i.e. a line of best fit was needed. An explanation for this would be the extreme likelihood of experimental errors occurring, such as not measuring out the acid exactly, making sure the magnesium strips were exactly precise in their given measurements. However, these errors can, within reason, be corrected. Excess acid in the measuring cylinder could be removed using a pipette or by re-measuring, exact precise measurements with a more accurate ruler of the magnesium could both have lead to a slight improvement overall.

Before starting the experiment, I could have dipped the magnesium strips in acid to get rid of the magnesium oxide coating them, but this may have led to more experimental and scientific errors. Although the strips could be weighed to see if they are the same weight, it is unlikely that the surface area of each strip would equal that of the others due to it already reacting within the actual measurements of acid. This leading to three problems:

The acid is reacting with the magnesium strips which could lead to an increase in temperature overall leading to inaccurate results.
If it is dipped into the acid then the surface area may differ in weight overall leading to in-appropriate results.

( it is important that the surface area is the same for each strip as reaction rates are affected by surface area(s).

One of the strips could have more magnesium oxides than the next which could lead into an array of problems ranging from weight loss to reaction rates.

Another possible error could be starting and stopping the stopwatch, as human judgement tends to be variable due to brain reactions and it is therefore unlikely that the exact stopping and starting time(s) would be achieved consistently. This was partly ensured by the fact that the same person always took the timed readings for the whole of the investigation(they will not be entirely accurate all the time however). More accurate results/observations could have been achieved with practice at using the stop watch provided.

Further Work

To improve my results further, I could repeat the experiment three or four times and find the average, rather than only repeating the experiment once and finding averages of the minor experiments within this larger one.

Factors that I know, but did not take into account in this experiment, are that

(1)The reaction between magnesium and hydrochloric acid is an exothermic one (which I found out by placing a thermometer in the mixture during the reaction and observing that the level in the thermometer had increased)

(2)The reaction gives off a high percentage of hydrogen gas (this is due to the fact that when the magnesium reacts with hydrochloric acid, it displaces the hydrogen from the acid, as it is placed higher than Hydrogen in the reactivity series).

The problem with (1) is that, due to the reaction being exothermic, the reacting particles will gain more energy from the increasing temperature and thereby reach the required activation energy in a shorter time. This could increase the rate of reaction and lead to inaccurate results. I could investigate a method for keeping the reactants at a constant temperature.

Taking (2) into consideration, I could repeat this experiment, this time recording the amount of hydrogen gas given off from each concentration of acid. I could use a gas syringe to collect the gas that would be given off from the experiment between the two substances. I believe this is a better way to investigate the rate of reaction(on temperature) as it would be possible using these results only, to calculate the initial rate of reaction. Thus could prove to be a more accurate set of results rather than averaging the rate of the reaction after the actual experiment, as I would then know the real/actual rate of reaction for magnesium and hydrochloric acid combined.

Investigating the reaction of Magnesium with different types of acid each time could further enhance the investigation. Hydrochloric, Sulphuric and Nitric acid would all react differently and perhaps give a greater understanding into the reaction mechanism with a clear pattern linking them all together.