My aim is to make Epsom salts at 4 different speeds, by choosing one factor/variable which will change the speed, and to predict and measure the yield of Epsom salts using the mole.

Prediction

As the surface area of magnesium increases the rate of the reaction will also increase. This is due to the theory of SURFACE AREA. The rate of reaction increases when the surface are of a solid reactant is increased. More specifically if the surface area of a solid reactant is doubles than the reaction will take half the time. Although we can not measure the surface area of our solid reactant, so we can not prove this theory.

Why rate increases with surface area: The reaction between the example (magnesium and acid) is much faster when the metal is powdered.

The acid particles can collide only with those magnesium atoms in the outer layer of the metal ribbon.

When the metal is powdered many more atoms are exposed. So there is a greater chance of successful collision.

The acid can attack from every angle.

A closer look at a reaction:

In order for the magnesium and acid particles to react together:

• They must collide with each other
• The collision must have enough energy

This is shown by he drawings below:

The particles in the liquid move around continuously.

Here an acid particle is about to collide with a magnesium atom.

                                        

                                        

                                                                

                                        

If the collision has enough energy, reaction takes place. Magnesium chloride and hydrogen are formed.

If the collision does not have enough energy, no reaction occurs. The particles bounce away again.

If there are lots of successful collisions in a given minute, then lots of hydrogen is produced in that minute. In other words, the reaction goes quickly –its rate is high. If there are not many, its rate is low.

The rate of reaction depends on how many successful collisions there are in a given unit of time.

In a successful collision

• Bonds are broken (this needs energy)
• New bonds are formed (this releases energy)

Predication of the mass of Epsom Salts produced

• The number of moles of starting chemical (reactant).

0.5g of Mg with an atomic mass of 24.

0.5        = 0.208333

24

• Use the balanced equation for you reaction to find the ratio of the number of moles of reactant to the moles of product. Multiply by this number.

Mg (s) + H2S04 (aq)                 Mg S04 + H2

1                        :        1

0.208333 * 1 = 0.208333

• Work out the formula mass of the product. Multiply the number of moles in your answer in the second stage, by his formula mass. This is the predicted mass of the product.

Mg S04 formula mass = 24 + 32 + (4*16)

                          = 24 + 32 + 64 = 120

0.208333 * 120 = 2.5g of Epsom salts produced.

Bibliography

Complete Chemistry – Rose Marie Gallagher & Paul Ingram

Internet

Encarta Encyclopaedia CD ROM 99

Risk Assessment

Hazard                Magnesium

Risk                Highly Flammable

Precaution        Wear Eye Protection

If Swallowed

Inform teacher. Wash out mouth with at least 2 glasses of water.

Seek medical attention if discomfort continues

Enters Eyes

                Inform teacher. Flood eyes with eyewash for 10 minutes

If spilt        

                Inform teacher.

Hazard                Sulphuric Acid

Risk                Very corrosive – if greater than 1.5M

                Irritant – if greater than 0.5 M but less than 1.5M

                Causes severe burns

Precaution        Wear Eye Protection

If Swallowed

Wash out mouth with at least 2 glasses of water.

Seek medical attention if discomfort continues

Enters Eyes

                Inform teacher. Flood eyes with eyewash for 10 minutes

If spilt        

                Inform teacher. Remove contaminated clothing and drench in water

Method

• Measure out the required amount of magnesium and sulphuric acid. For our experiment we are using 40ml of sulphuric acid and using 1g of Magnesium. Using a measuring cylinder, size 50ml, to measure the Acid with an accuracy of ±0.5ml and a balance to measure the Magnesium with an accuracy of ±0.005g.
• Fill the upside down measuring tube, with water and support over the end of the delivery tube, using a clamp stand. We are using a 200ml-measuring cylinder, which has an accuracy of 1ml.
• Add the magnesium and sulphuric acid to the conical Flask. First add the acid, and then the magnesium, however keep the bung close by so it can be used as soon as the reactants have come into contact.
• Take measurements of the amount of hydrogen produced by reading from the upside down measuring tube. You must take the measurements every 30 seconds and this will be a ml of the amount of hydrogen given off by the experiment and collected in the upside down measuring cylinder.
• Repeat this 2 times for each type of magnesium. Then record all your results in a table and use a graph with a curve of best fit to find any correlation between the results.

To make sure my Experiment is reliable. One person from the group will be in charge of measuring out the reactants and someone else is timing. This is reducing the risk of different readings due to different measurements and reaction times. Although it is mostly inevitable.

Fair Test

To make the experiment a fair test the amount and concentration of the acid must be kept the same and be measured by the same equipment and person to keep the accuracy the same, 40ml and a concentration of 0.25M. The amount of Magnesium must also be kept the same, as the only variable is the surface area of the magnesium. The accuracies of the measuring devices should also be kept the same to stop major differences in the amount used.

Diagram

Pilot Run Results

The above pilot runs were done to check that the amount of magnesium and Sulphuric acid were right to get a product and for the reaction to be at the right speed for us to be able to accurately measure the readings.

We also had to use some pilot runs testing with concentration to use:

We decided to use a concentration of 0.25M, with 40ml of Sulphuric acid and 0.5g of Magnesium.

Amendments made

For the real experiment we decided to use smaller differences between the time intervals. This was because we found with the faster reactions the reaction was slowing dramatically towards the end and accurate readings weren’t taken. We will also take 3 readings of each surface area, then take an average. This will mean many anomalous results should be taken out. I have also decided to incorporate an extra set of result by preparing an extra surface area. This will mean chopping up the ribbon into equal sections and using this as an extra set of results.

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Averages

Above are the results for the Ribbon, turning and powder. We have decided to try out one run of the cut up ribbon to see if there is much difference between the ribbon results and the cut ones. We are doing this, as the differences in surface area between the two are very small. The only extra surface area we are creating is the very end of the magnesium.

We saw that the difference between the two was so small that carrying out this experiment would be a waste of time and resources and would not give us any conclusive evidence to back up our prediction.

Mass of Epsom Salts Produced

Ribbon produced 2.11g + excess water

Turnings produced 2.40g + excess water although once water was taken away produced 1.97g

Conclusion

From the graph we can see that as the surface area of the surface area of magnesium gets smaller (from ribbon to powder) the amount of hydrogen produced in 120 seconds increases. We can also se that at the end of each set of results they start to slow down. This shows that the reaction is slowing down and that the reaction is nearly complete. This means there is little produced from 75 seconds onward. The smaller intervals helped us to point out where they started to slow down. From the lines of best fit you can see that for the reaction to have continued at the original speed the powder would have take 52 seconds to produce 110ml of hydrogen. For the turnings it would take 88 seconds to produce 110ml whereas the ribbon would need 118 seconds. From this we could see that the surface area of the powder is over double the surface area of the ribbon. This is shown as the time take for the powder is under half the time taken by the powder.

The speed of the reaction is taken by finding the gradient of the line:

Ribbon – 0.9

Turnings – 1.3

Powder – 1.8

Here we can predict that the surface area of the powder is double the surface area of the ribbon as the speed is doubled. This relates to my prediction that ‘more specifically if the surface area of a solid reactant is doubles than the reaction will take half the time’. Although we cannot measure the surface, we can get a good idea that our experiment was successful, as there is a correlation between both the speed and the time taken for hydrogen to be produced.

As you can see the speed of the reaction increases as the surface area increases. This is due to the theory of SURFACE AREA. The rate of reaction increases when the surface area of a solid reactant is increased.. We can see this theory in action with the powder and ribbon.

        The Yield of Epson Salts for turnings and powder must first have the amount of excess water taken away. This was done in the lab for the Turnings but was not for the ribbon. So I have decided to use the amounts of excess water found on the turnings and take that away from the ribbon.

Turnings produced 2.40g + excess water although once water was taken away produced 1.97g

So the amount of excess water was 0.43g. This must be taken away from the ribbon to give a realistic amount of Epson salts produced.

Ribbon produced 2.11g + excess water

Realistic result = 2.11g – 0.43g = 1.68g

We can now use these results to find the % yield of each experiment.

Turnings = 1.97g

2.5g is a 100% yield. 1% would equal 0.025g.

1.97g/0.025 means the yield is 78.8%

Ribbon = 1.68g

1.68g/0.025 means the yield is 67.2%.

Evaluate

We can see that our line of best fit is a curve. This is because the experiment has slowed down as there is a limiting factor.  These anomalies are due to many factors:

• The time delays the time it takes for the hydrogen to travel down the delivery tube and show on the measuring cylinder.
• The upside down measuring cylinder meant we did not have markings for 1-20 cm3, meaning we could not take very accurate results, even with the most accurate measuring cylinder.
• The measuring of the sulphuric acid and magnesium was not always 100% accurate this was due to the accuracies of the equipment. The mass measurer had an accuracy of ± 0.005g and the measuring tube to measure that acid had an accuracy of ±0.5ml.
• The time for which we had to take the reading and the reading being taken varied. Meaning that sometime there was upto 10 seconds between when it should be read and when it was. This means that the readings could actually be closer to the next time interval. The human reaction time and the time taken to read off affected this.
• We carried out the experiment over a couple of lessons, meaning the temperature could have changed over the days, slowing down or speeding up the reaction
• The angle at which you look at the upside down measuring cylinder changed the reading. This meant there would be ± 2 ml to be accounted for.
• We also noticed with come of the magnesium floated on the top of the acid. This means only the under side could be attacked by the acid. This would halve the surface area and dramatically decrease the speed.
• Some of the Epson salts were left in the tray and not measured this could mean we had a higher yield than stated.
• On two experiments we were required to make up our own concentrations. This may have meant it was not completely accurate, increasing or decreasing the rate of reaction.

To combat these problems we could:

• Use a special upside down measuring tube, which is more accurate.
• Use a taller, and thinner, more accurate measuring cylinder to measure out the sulphuric acid to make it more correct.
• Measure the temperature throughout the experiment, so this can be taken into account when we evaluate the experiment.
• To use a method which meant that the measurement was taken when it was meant to be. This may mean taking a count down to when it should be taken so the person taking the result could have time to read it accurately.

Improved method

To improve my result I got from this experiment I would:

• Use a heated bath where the acid would be kept so that the temperature of the room will not make a difference to the rate of reaction. The collision theory, means that as temperature increases so does the amount of successful collision so the rate of reaction.
• Use a pipette when measuring out the acid, this will mean you can attain the exact amount needed. Using the same balance to measure the magnesium should also keep the amounts the same.
• Use one concentration of Acid made from the same set. This would mean keeping this acid set aside so all the same one is used.
• Using a burette to measure the amount of hydrogen produced, these are more accurate and can be read off easily.