Investigate the factors affecting the enthalpy change during a displacement reaction.
The Task
To investigate the factors affecting the enthalpy change during a displacement reaction.
Background Knowledge
In this investigation a displacement reaction will be observed. A displacement reaction occurs when more reactive metal displaces, (or removes) a less reactive metal from a solution of its salt. Also known as chemical bullying because a more reactive metal effectively bullies the original metal out of its compound.
In this investigation zinc and copper sulphate will be reacted together. Zinc is the more reactive metal here, so the copper is displaced from the copper sulphate solution.
Zinc + Copper sulphate ?inc sulphate + Copper
Zn (s) + CuSO4 (aq) ?nSO4 (aq) + Cu (s)
After the reaction it will be observed that the solution will change from blue to clear, colourless. The copper discharged will be observed as a salmon pink deposit in the base of the vessel the reaction occurs in.
Reactions like this can be used to purify metals that are low down in the reactivity series. A more reactive metal can simply be added to purify the less reactive metal, leaving it in its pure state.
Lead Oxide + Tin ?ead + Tin Oxide
In reality thought this process is hardly ever used, as the more reactive metal is the left in an impure form, and thus must be reduced by an even more reactive metal. As this is not economically viable metals are generally reduced using electrolysis, which leaves them in a pure state, and requires no expensive more reactive metals, just electricity and often heat.
The displacement reaction is a redox reaction. This means that on one side of the reaction electrons are being lost and on the other electrons are being gained.
The enthalpy change in a reaction is in effect the energy chance between the reactants and the products. If there is more energy in the bonds of the products than the amount in the bonds of the reactants then it can be seen that the reaction is endothermic. This means that it must take energy in the form of heat from its surroundings.
The opposite is also true. If the bonds between the reactants in a reaction have more energy than the bonds in the product then it can be seen that energy is given out. This takes the form of heat energy, so the reaction is said to be exothermic.
Energy is produced when bonds are formed, and energy is required when bonds are to be broken. This means that in an exothermic reaction more bonds are being formed than are being broken, oppositely in an endothermic reaction more bonds are being broken than are being formed.
To take an accurate reading of the enthalpy within a reaction, (difference in/delta H) can be found. This reading can be calculated off a graph, however it will be more accurate if the formula is used.
/\H = mc/\T
The enthalpy, (delta H) is the difference ion heat energy, in Joules, m is the mass of the solution, c is the specific heat capacity of water, and /\T is the difference in temperature.
Knowing this formula it is easy to find and compare the enthalpy change when variables are altered.
Secondary Sources
Work covered during the GCSE course. This helped as detailed notes had been taken referring to the nature of displacement reactions and energetics.
Experimental Prediction
I believe that when the concentration of copper sulphate solution is doubled and reacted with a surplus of zinc the enthalpy change will double. This means that the reaction will get twice as hot. I believe this is true, as the ratio of bond energy between the reactants and products ...
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Knowing this formula it is easy to find and compare the enthalpy change when variables are altered.
Secondary Sources
Work covered during the GCSE course. This helped as detailed notes had been taken referring to the nature of displacement reactions and energetics.
Experimental Prediction
I believe that when the concentration of copper sulphate solution is doubled and reacted with a surplus of zinc the enthalpy change will double. This means that the reaction will get twice as hot. I believe this is true, as the ratio of bond energy between the reactants and products will be the same, so as the quantity increases the difference between the energy of the reactants and products will get proportionally larger.
For example, when 1.0M copper sulphate solution is used the heat change may be 60 degrees Celsius. I believe that if this were true when a 0.5M solution was used, (half the molarity of the 1.0M solution), the temperature difference would be 30 degrees Celsius. This is half of the original value so the variables can be said to be directly proportional.
I believe that surface are will have a similar effect, (when in surplus copper sulphate solution), in that the larger the surface area to volume ratio of the Zinc the greater the enthalpy change, (as one doubles so will the other). When the surface area of the zinc is kept constant I believe that if the mass is doubled then the enthalpy change will also double.
I also believe that temperature, pressure and pH will affect the rate of the reaction. When the temperature of the reactants is doubled the enthalpy change will double. When the pH of the solution is doubled I believe that the enthalpy change will probably double, and I believe, (though hard to test) that when the pressure is doubled the enthalpy change in the reaction will double.
I also believe that some of the factors above may not affect only the enthalpy change, and may also affect the rate of reaction.
Safety
In this experiment an exothermic reaction will be witnessed. As this is the case it is important that there is a stand for the vessel in which the reaction occurs, as otherwise burning may occur or the vessel may be dropped. To limit this care should be taken while handling the reactants. Equally a polystyrene cup could be used. This would not conduct heat well, so would remain safe to hold even when the solution within is hot.
Care should also be taken in handling zinc, especially if it has been powdered to increase its surface area. This is because it is toxic and can be easily be ingested through the skin, thus acting as a poison. As a precaution rubber gloves should be worn at all times when handling the substance.
Safety goggles should also be worn at al times during the experiment, as any chemical in the eye would cause irritation. There is also a greater risk in this field because the copper sulphate will become hot as it reacts with the zinc.
Factors to Vary and Control
The factors that will vary the enthalpy change of the reaction are:
Concentration of copper sulphate solution,
Surface area of zinc,
Mass of zinc,
Pressure,
Light,
PH,
Temperature.
It is important to maintain a fair test while investigating a variable. This means that when one variable is changed all the others must remain constant. This ensures that results achieved are due to a specific variable, and not due to another that is also being altered. e.g. when concentration is changed, mass and surface area must remain constant, thus ensuring that accurate results are achieved, so a precise conclusion can be achieved.
Range and Number of Observations
It is important that all experiments undergone are repeated a minimum of 2 times so that a men average can be taken from the results. Ideally each experiment should be repeated about 5 times, thus helping to account for anomalies. It is also important that sufficient ranges of results are taken. For example I believe it is important to measure the temperature of the solution at intervals of every 10 seconds. This will help to show the precise cooling curve and should make extrapolations on a graph more accurate.
Each variable investigated should be tested at about 5 different values, so when concentration is tested at least 5 different concentrations should be used, e.g. 1.0, 0.5, 0.25, 0.125, 0.05125.
Accuracy of Observations
To achieve accurate results I will perform each experiment a minimum of 3 times. This will mean that I can take an average, thus helping to account for anomalies. I will also be careful to make sure that when values for molarities and temperatures are taken all readings are accurate to the same number of decimal places. This will help to achieve a more accurate overall picture, thus any conclusion drawn will be more accurate too.
Preliminary Work
Apparatus
Copper sulphate solution, Zinc powder, distilled water, polystyrene cup, lid, weighing boat, electric balance (accurate to 0.01g), thermometer (accurate to 0.1 degree Celsius), stop clock (accurate to 1second).
Method
Exactly 25 cm3 of 1.0M copper sulphate solution was measured out in a measuring tube. This was placed in the polystyrene cup. Between 5.75g and 6.25g of zinc powder was then placed in a weighing boat and weighed out using a top pan electric balance.
The thermometer was placed in the solution and the stop clock was started. At precisely 3 minutes the lid was taken off the cup and all of the zinc powder was added. The lid was then replaced on the cup and the cup was shaken gently in order to ensure the reaction began.
Reading were taken every 30 seconds starting from minute 0. After minute 3 the contents were stirred gently with a thermometer to unsure all the reactants had a chance to mix. Readings were taken up until minute 8 where the experiment was concluded.
To work out the exact mass of zinc used I weighed out the mass of zinc left in the weighing boat and then subtracted this from the total weight at the first weighing.
Results
I have drawn a graph showing these results. I have extrapolated back to minute 3 by using the cooling curve. This will give a more accurate demonstration of the enthalpy change, as the present highest temperature will be inaccurate due to the reading on the thermometer still rising when the reading for 3 minutes was taken.
Using the figures extrapolated on the graph I can ascertain the enthalpy change in the reaction. The temperature change can be found by subtracting the original temperature from the highest temperature. 72 - 18 = 54 degrees Celsius.
The heat energy evolved in this reaction can be calculated using the formula
q = mc/\T
q = heat energy (in kJ), m = mass of copper sulphate solution (assuming density is equal to 1g/cm3), c = specific heat capacity of the solution (assume this equals 4.2 kJ/kg/k), /\T = temperature change (estimated from extrapolation).
Experimental Procedure
Concentration Experiment:
Apparatus
Copper sulphate solution, Zinc powder, distilled water, polystyrene cup, lid, weighing boat, electric balance (accurate to 0.01g), thermometer (accurate to 0.1 degree Celsius), stop clock (accurate to 1second).
Method
Apparatus should be set up as in diagram. Exactly 25 cm3 of 1.0M copper sulphate solution was poured out into a measuring tube. This was placed in the polystyrene cup. Between 5.75g and 6.25g of zinc powder was then placed in a weighing boat and weighed out using a top pan electric balance. To work out the exact mass of zinc used I weighed out the mass of zinc left in the weighing boat and then subtracted this from the total weight at the first weighing.
The thermometer was placed in the solution and the stop clock was started. At precisely 3 minutes the lid was taken off the cup and all of the zinc powder was added. The lid was then replaced on the cup and the cup was stirred gently in order to ensure the reaction began, but still making sure none of the solution was spilled.
Reading were taken every 30 seconds starting from minute 0. After minute 3 the contents were stirred gently with a thermometer to unsure all the reactants had a chance to mix. Readings were taken up until minute 8 where the experiment was concluded.
Once the experiment is completed all of the equipment should be cleared away. Results should be collected and tabulated. From these graphs can be drawn. Conclusions can then be reached about how enthalpy change is related to the concentration of copper sulphate solution used.
Mass experiment:
Apparatus
Copper sulphate solution, Zinc powder, polystyrene cup, lid, weighing boat, electric balance (accurate to 0.01g), thermometer (accurate to 0.1 degree Celsius), stop clock (accurate to 1second).
Method
Apparatus should be set up as in diagram. An excess of copper sulphate solution should be taken. This should be a 1M solution and the volume should remain the same in all experiments. The only variable to be changed must be the mass of zinc powder used.
25 cm3 of 1M copper sulphate solution should be measured out in a measuring cylinder then poured into the polystyrene cup. The thermometer should be placed in the solution and the clock should be started. The temperature reading off the thermometer should be taken every 30 seconds.
Zinc powder should be weighed out on the electric balance. I will investigate the effects of many different masses of zinc. To start with I will use 6.0g of zinc. At minute 3 this should all be added. The solution should be stirred and readings should be taken off the thermometer every 30 seconds until minute 8 when the reaction is concluded.
Results should be collected and then tabulated along with those achieved using 5g, 4g, 3g, 2g and 1g of zinc powder. All experiments should be repeated a minimum of 3 times so that a mean average of the findings can be taken. All results should then be tabulated and formatted on a graph to help explain a trend in the relationship between mass of zinc and enthalpy change in the reaction. A conclusion should then be reached.
Calculations
Using the extrapolations on the graph showing the relationship between time and temperature the energy change evolved in each reaction can be calculated. The can be worked out by using the equation:
q = mc/\T
When, q = heat energy (in J), m = mass of copper sulphate solution (assuming density is equal to 1g/cm3), c = specific heat capacity of the solution (assume this equals 4.2 J/g/degrees Celsius), /\T = temperature change (estimated from extrapolation).
The results here show the heat energy in each reaction. To calculate the enthalpy change one must account for the respective moles involved in each reaction.
To equal out the number of moles simply multiply the results by the respective amounts required to make them all amounts of Joules/1 mole.
When this calculation is performed on all the heat energy results enthalpy change can be compared. In an accurate experiment the enthalpy change when copper sulphate solution is reacted with zinc powder should remain constant. By taking into account the number of moles in each equation when multiplying the results will be brought into proportion, so the enthalpy changes should be roughly equal.