Find out the factors that affect the rate of reaction.

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THE EFFECT OF SURFACE AREA ON REACTION RATES

The facts

The more finely divided the solid is, the faster the reaction happens. A powdered solid will normally produce a faster reaction than if the same mass is present as a single lump. The powdered solid has a greater surface area than the single lump.

The explanation

You are only going to get a reaction if the particles in the gas or liquid collide with the particles in the solid. Increasing the surface area of the solid increases the chances of collision taking place.

Imagine a reaction between magnesium metal and a dilute acid like hydrochloric acid. The reaction involves collision between magnesium atoms and hydrogen ions.

Increasing the number of collisions per second increases the rate of reaction.

http://www.chemguide.co.uk/physical/basicratesmenu.html

SECONDARY BACKGROUNG INFORMATON FROM BOOKS

Name – chemistry matters GCSE edition

Author – Richard Hart

The effects of increasing the concentration in a reaction

If zinc is placed in dilute hydrochloric acid, it reacts to produce bubbles of hydrogen and zinc chloride:

Zinc + 2HCl(aq)                         ZnCl2(aq) + H2(g)

If the reaction is repeated with concentrated acid, the hydrogen is evolved much more quickly, making the liquid fizz. This is because the rate depends upon how frequently the molecules of the reacting substances collide. The concentrated acid has more molecules for a given volume than the more dilute acid. Because there are more molecules about, the frequent of collision is greater, and the reaction happens faster.

The effects of surface area in a reaction

If you have a reaction involving a solid, the reaction can only happen at its surface. Make more surface available, and the reaction will happen faster.

For example: calcium carbonate in the form of marble chip reacts with dilute hydrochloric acid to produce carbon dioxide:

Calcium    +         hydrochloric                                 calcium   +   water  +  carbon

Carbonate                acid                               chloride                   dioxide

CaCo3(s)  +  2HCl(aq)                          CaCl2(aq) + H2O(l) + CO2(g)

Suppose you start with one 10g lump of marble. Instead of using it as it is, you break it up into ten 1g lumps. You have the same amount of marble, but much more surface area for the acid to react with.

PRETEST

Before we did the actual experiment we had to do a pre-test to check if any problem occur when we do the proper experiment. For our pre – test we tried using the pressure sensor and the collecting method to see which one worked best. From the two different tests I found out that the collecting method worked best for this experiment. From using the pressure sensor we found that the pressure inside the conical flask reached to 13-15 Kilo Pascal’s which was too high. This caused the bung from the flask to pop-up. One reason for this was we used too much of a big piece of magnesium (8cm) and another reason was as we were using a syringe to put the hydrochloric acid in the flask, air was already trapped in the flask and it had no way to escape. This caused even more pressure to build up in the flask, this also caused the bung to pop of the flask. To avoid this from happening we tried using a 5cm piece of magnesium and also before we put the hydrochloric acid in we left the bung out and we put the bung on after all the hydrochloric acid had gone in so air would rush out and there wouldn’t be as much pressure in the conical flask. After we tried doing it that way still the pressure reached to high.

Here are the results of the pre-test using the pressure sensor: 

When we did the collecting method we did have 1 problem with collecting the hydrogen and this was that the time showed that the hydrogen took long to collect (we realised this after the 3rd test) the reason for this was as soon as we started to put the hydrochloric acid in I put the timer on. This meant that I started timing before even the reaction started to take place, so to stop this problem from happening we decided to put all the hydrochloric acid in and than start the timer that way the time wouldn’t be that long.

Here are the results of the pre-test using the gas collecting method: 

Another thing that we found when doing the pre-test is that all the magnesium should be submerged into the hydrochloric acid so all of it gets reacted with if only half of it would cause anomalous results to occur. From these pre-tests I have found out that using the collecting method is more convenient for this experiment.

AIM

In this investigation I am trying to find out how the surface area of magnesium (a metal) effects the rate of reaction with hydrochloric acid (a acid) the way that I am going to test this I am going to time how long it takes 40cm3 of hydrogen a gas to be collected in a measuring cylinder and then I will work out the rate of reaction (1/time) I am going to show the results on a graph.

PLAN / METHOD

• Fill a trough about half way up with water making sure that the beehive shelf you will be using will be under the water.
• Place the beehive shelf in the water.
• Fill a measuring cylinder that holds 40cm3 with water full to the top making sure that there are no air bubbles inside it.
• Hold the top with your hand making sure that none of the water falls out.
• Turn it upside down and place it over the top hole on the beehive shelf still making sure that there are no air bubbles, if there are air bubbles refill it and do it again.
• Get a conical flask with a bung connected to a delivery tube and another smaller tube that will be connected to the syringe.
• Place the delivery tube inside the side hole of the beehive shelf.
• Put 25cm3 of hydrochloric acid in a measuring beaker and suck 25cm3 of hydrochloric acid up into the syringe.
• Place the small opening of the syringe inside of the small tube connected to the bung.
• Do not push the syringe just yet.
• Get the appropriate size / pieces of magnesium.
• Put it into the conical flask making sure it is flat so it will be submerged in the hydrochloric acid.
• Get the timer ready.
• Push the syringe so the hydrochloric acid goes into the flask until all of it is in.
• Start the timer after all of the hydrochloric acid is in.
• Time how long it takes for 40cm3 of hydrogen gas to be collected in the measuring cylinder.
• Do the same experiment 2 more times and find the average
• For more accurate results you are going to do each experiments 3 times and find the average by adding the three results up and dividing by 3.
• Then find the rate of reaction by doing 1/average time.
• Repeat this method again using these different surface areas :

5/2, 5/4, 5/8, 5/6.

APPARATUS

• 15 strips of Magnesium that are 5cm long and are cut out appropriately for each experiment

(5/1,  5/2,  5/4,  5/8,  5/16)

• 375cm3 of hydrochloric acid – concentration 1m
• Ruler
• Scissors
• Conical flask
• Delivery  tube
• Trough
• Bung
• Timer
• Beehive shelf
• 1 measuring cylinder holding 100cm3
• Syringe holding 25cm3
• 1 measuring beaker holding 75ml

SAFETY

While I am doing this experiment I have to keep in mind the safety and precautions, for example while the reaction is taking place I need to wear goggles. I also need to be careful with the hydrochloric acid, as the concentration is quite high so touching it would be corrosive to the skin. Another main precaution is; after 40cm3 of hydrogen has been collected the reaction might not have fully reacted so I need to wait until the whole reaction has finished or pour it into another beaker and leave it to react this is because pouring it straight into the sink might cause problems, like explosions.

FAIR TEST

In our experiment the invariable will be the time, and the rate of reaction, but the variable will be the surface area of the magnesium so we will have to keep this fair throughout the whole experiment.  I will be using 5cm of magnesium for each experiment cut up appropriately because the different lengths might vary the results making them unfair. I will also keep the hydrochloric acid the same volume for each experiment because if I vary this it also might effect how fast the magnesium will react. I will need to keep the room temperature the same which is 270C this is because different temperatures might effect how slow and fast the reaction takes to occur causing odd results. When I am setting the experiment up I need to make sure that there are no air bubbles in the measuring cylinder because this will affect the time as it would take less time for hydrogen to be collected. I am going to keep how much gas I am going to collect the same throughout the whole experiment. To find the most accurate results, I have to make sure that the magnesium will be flat at the bottom so it will be submerged in the hydrochloric acid. This is because if it is not flat only part of the magnesium will be reacted causing anomalous results. I will have to do each experiment 3 times and find the average of the three results to get more accurate results. Another thing that we have to keep in mind while doing the experiment is that we shouldn’t mix or shake the flask while the reaction is taking place as this might effect how fast or slow the reaction takes to occur. The thing in this investigation that I am going to vary is the surface area this is to see if my results match the prediction that I made.

MY PREDICTION

From the secondary information that I have found out I can predict that in this experiment that I am going to investigate the more the surface area there is available the faster the rate of reaction. This is because the larger surface area there is the more chance there is of the hydrochloric acid molecules to be in contact with the magnesium. In the experiment the way I am going to increase the surface area is by cutting up the magnesium strips first starting by cutting in half than quarter than eighths an than the largest surface area will be cut into sixteenths.

Here is a diagram to show how the surface area will increase the rate of reaction:

The two pictures show an example of how the magnesium pieces will be cut up and how it will increase the rate of reaction between the magnesium and hydrochloric acid, in the first picture the magnesium has less surface area and in the second picture had a larger surface area. There is more chance of the hydrochloric acid molecules to react with the sides of the magnesium the reaction is faster as there will be more collisions per second. However in the first diagram the magnesium has surface area so there is less chance of the hydrochloric acid molecules to hit the sides of the magnesium so the rate of reaction is slower so there is less number of hits per second. From the statement that I have made above this is what I think the two graphs would look like

ANALYSIS

From the graph that shows the average time taken to collect 40cm3 of hydrogen in the reaction between magnesium and hydrochloric acid I can make a conclusion that the smaller the surface area the longer time it takes for hydrogen to be collected. This can be explained as collision theory, the more the Hydrochloric acid molecules can collide with the Magnesium the shorter time it takes for hydrogen and magnesium chloride to be formed. The reason for this is, If one of the reactants Magnesium, its surface area will affect how fast the reaction takes to happen. This is because the two types of molecules Magnesium and hydrochloric acid can only bump into each other for a reaction to occur. So the larger the surface area of the solid, the faster the reaction will be. Smaller particles have a bigger surface area than larger particles if they have the same mass of solid so smaller pieces make more surface area available for the Magnesium to hit and break the bonds of the Hydrochloric acid. The line for this graph is in an inversely proportional direction.

From the graph showing the rate of reaction between magnesium and hydrochloric acid the line is directly proportional this means the line goes from low to high. The graph shows that the smaller the surface area the less the rate of reaction.  The reason that the rate of reaction is less if the surface area is less is because it takes less time if there is less surface area available. The more finely divided the solid is, the faster the reaction happens. A powdered solid will produce a faster reaction than if the same mass is present as a single lump. The powdered solid has a greater surface area than the single lump. A reaction will only occur if the particles in the gas or liquid collide with the particles in the solid. Increasing the surface area of the solid increases the chances of collision taking place. From the prediction that I made I can say that it matches my analysis.

EVALUATION

I can say that both of my graphs show consistent results and are accurate, there are no anomalous results except for one which is in the rate of reaction graph, 2 of the same surface area have the same rate of reaction, they are (5/4) and (5/8) and they both have the rate of reaction, which is 0.06. I think that to make the results more accurate I could have had the rate of reaction to 3 decimal places to have more precise answers or I could have had my rate of reaction results in standard index form, which would make the numbers shorter and readable. If I would do this experiment again I think that the thing that I would change is the way I varied the surface area. Instead of cutting up the strips I could have used different forms of magnesium e.g. big lumps, small lumps, smaller lumps, smallest lumps and than powder all weighing the mass 2g this would make the range between the result much more different and clearer on the two graphs. The bad thing about the way I set up my experiment is that as there was no beehive shelf available so I had to hold the measuring cylinder this caused some of the hydrogen gas bubbles to come out of the side of the cylinder. Another reason was, while I was holding the measuring cylinder it wasn’t to at straight level as there was no beehive shelf so I had to read the numbers carefully until 40cm3 of hydrogen had been collected. Overall I think that my investigation did go well as my results did match my prediction and also I found out the answer to the aim of the investigation.