Investigation of Energy Changes in a Displacement Reaction.

January 2002

Michael Lavery U5W

Chemistry Coursework – Investigation of Energy Changes in a Displacement Reaction.

Plan

Introduction

When a metal is added to a solution of another salt metal the reaction between the two brings about an exothermic reaction. In this investigation I aim to discover, as the mass of zinc increases, when energy given out stops rising i.e. how much zinc can react with 50cm3 of 0.5M Copper Sulphate. The reaction is exothermic because energy given out when bonds are made is greater than the energy taken in to break bonds.

Zinc + Copper Sulphate → Zinc Sulphate + Copper

↑ ↑

(metal) (metal salt)

Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

Variables

The only variable I will alter throughout the experiment is the mass of zinc used; altering only one variable keeps the test fair. I will keep the following variables constant: Volume of copper sulphate used (50cm3) and the concentration of copper sulphate (0.5M)

Method

Apparatus

Large glass beaker
Polystyrene cup
Thermometer
Measuring Cylinder
Copper sulphate (0.5M)
Zinc (powder)
Small glass beaker
Mass balance
Spatula

Diagram

Using the measuring cylinder measure out 50cm3 of copper sulphate and pour this into the polystyrene cup which is in the glass beaker (as shown in the diagram). To make sure the measuring cylinder gives an accurate measurement read the amount from the bottom of the meniscus on top of the liquid.
Record the temperature of this copper sulphate using the thermometer, for maximum accuracy read the thermometer at eye level.
Place the empty small beaker on the balance and when it’s mass has been measured press the “on” button to put the reading back to zero, this enables the mass of the contents of the glass beaker to be weighed when added with the mass of the glass being included.
Using a spatula measure out the desired amount of zinc into the small glass beaker (I started with 0.5g and raised the mass by this amount each time).
Add the zinc to the copper sulphate sample and stir continuously.
While stiring with the thermometer regularly check the temperature and when this temperature stops rising record the highest temperature reached before it begins to fall again.
Subtract the initial temperature and record the temperature rise.
Repeat the process and take an average of the temperature rises recorded as your final results.
Repeat the experiment with a larger mass of zinc until the temperature rise stops increasing.

Safety Precautions

Wear a lab coat.
Wear goggles.
Wash hands when finished working with chemicals.

Preliminary Experiment

In preparation for this investigation I carried out a similar experiment reacting magnesium with lead nitrate.

Magnesium + Lead Nitrate → Magnesium Nitrate + Lead

Mg (s) + Pb(NO3)2 (aq) → Mg(NO3)2 (aq) + Pb (s)

I found the following results:

I discovered that the experiment was exothermic because the temperature rose each time. I found that the more magnesium powder I added the more heat was released during the reaction. This ...

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Safety Precautions

Wear a lab coat.
Wear goggles.
Wash hands when finished working with chemicals.

Preliminary Experiment

In preparation for this investigation I carried out a similar experiment reacting magnesium with lead nitrate.

Magnesium + Lead Nitrate → Magnesium Nitrate + Lead

Mg (s) + Pb(NO3)2 (aq) → Mg(NO3)2 (aq) + Pb (s)

I found the following results:

I discovered that the experiment was exothermic because the temperature rose each time. I found that the more magnesium powder I added the more heat was released during the reaction. This shows that adding different masses of zinc to copper sulphate will give out different amounts of heat.

Prediction

I predict that as the mass of zinc being added to the copper sulphate rises the temperature rise during the reaction will rise and so the energy given out will rise. I also predict that there will be a point where the energy given out will stop rising as mass of zinc added increases.

Theory

Zn + CuSO4 → ZnSO4 + Cu

Bond breaking, always endothermic. Bond making, always exothermic.

(Temperature drops) (Temperature rises)

This displacement reaction is exothermic overall because the energy used to make bonds is greater than the energy used to break bonds.

This graph shows energy change during the reaction, overall energy is lost and so the reaction is exothermic. The more zinc that is added then the more copper sulphate will react and the more energy will be released as more bonds are being formed.

Zn + CuSO4 → ZnSO4 + Cu

I am using copper sulphate of molarity 0.5M so in every litre of solution there are 0.5 moles of copper sulphate. With this information I can calculate the moles of copper sulphate in 50cm3 of solution:

Moles of copper sulphate = 0.5M x 50 = 0.025 moles

100

If there are 0.025 moles in my 50cm3 sample then the maximum energy given out must occur when the copper sulphate is reacted with 0.025 moles of zinc, the following graph shows how the energy released in the reaction stops increasing when 0.025 moles of zinc has been added even though the amount of zinc added continues to rise.

To find the mass of zinc that produces the largest amount of energy released I must calculate what mass of zinc contains 0.025 moles.

Relative atomic mass of zinc

↓

Mass of 0.025 moles of zinc = 0.025 x 65

= 1.625g

So maximum energy will be given out when 1.625 g of zinc is added to 50cm3 of 0.5M copper sulphate.

When plotting a graph of mass of zinc against energy given out I can use the following equation:

Energy given out = m x s x temperature rise

↑ ↑

Mass of solution Specific heat capacity (4.2)

Results

Analysis

I have selected four results from my graph, two while the energy given out is still rising and two after it was stopped.

These results fit in with my prediction in that energy given out increases in proportion to mass of zinc and that at one point energy given out will stop rising as mass of zinc continues to rise.

I predicted that the maximum measurement would be taken when 1.625g of zinc is added to the copper sulphate solution, the highest measurement on my graph is 5.75g of zinc.

My results did not agree with my prediction and the measurements outlined in my theory because the zinc powder did not all react with the copper sulphate, only the outside of each individual piece of zinc reacted with the solution, the inside of the granule was left untouched. The following is a diagram of a granule of the zinc powder:

My theory stated that the maximum amount of energy would be released when 1.625g of zinc was added to the 50cm3 of copper sulphate. This is because 1.625g of zinc contains 0.025 moles of zinc which is the same number of moles as in 50cm3 of copper sulphate, I concluded that this would produce the maximum energy released because at this mass all the copper sulphate should have reacted and produced the maximum possible temperature change.

The results related to my theory showing that it was exothermic, I know this because the overall temperature rose each time I reproduced the reaction. The equation included in my theory regarding the calculation of energy given out was also shown to be correct as on the graph mass of zinc and energy given out are seen to be directly proportional.

Evaluation

On my graph I found two anomalous points that occurred when I reacted 1.5 g of zinc and 3.5 g of zinc. I think these points were out of trend due to human error and inaccurate equipment, the thermometer I was using was only accurate to one degree and also it was hard to read accurately as it is small and there was nothing to stop heat escaping out of the top of the polystyrene cup.

Errors

Only the surface of each individual granule of zinc came into contact and reacted with the copper sulphate.
The thermometer I used only read to 1°c.
The measuring cylinder I used was not very accurate.
The balances used to measure mass of zinc were not as accurate as possible.
Lots of heat was lost through the top of the polystyrene cup.
I may not have stirred the mixture enough during the reaction, which means that the reaction may not have reached its highest temperature.

Improvements

To improve my temperature readings I could have used a thermometer that reads to 0.1°c but this was not available to me as these cost £50 and the lab was not equipped with them.
I could have improved the measurements of my copper sulphate by using either a pipette or a burette, these pieces of equipment are more complicated but much more accurate.
If I had used a mass balance which read to three decimal places my measurements of zinc, especially the low ones, will have been much more accurate.
A lid on the polystyrene cup would have stopped so much heat escaping but stirring during the reaction would have been a problem.
During the experiment I only but a glass beaker around the cup to stop heat escaping, accuracy would have increased if more lagging was included.
More repetition of the experiment would improve accuracy because averages would be more reliable, repetition would also make anomalous points on the graph less likely.
More masses of zinc would improve my graph, if I had increased the amount of zinc by 0.25g instead of 0.5g the results may have related to my prediction and theory better.
Using different concentrations of copper sulphate as well as masses of zinc would have been time-consuming but would have produced better results.
Other types of displacement reactions to compare this with would help to prove my prediction; I could use different metals and different metal salts.
I could use a temperature probe connected to a computer with a magnetic stirrer in the solution, this would greatly improve accuracy, the diagram below shows how such an experiment would be set up.