We can find the number of moles of metal needed to react with 20cm3 of solution by doing the following:
Finding how many moles of Copper Sulphate there are in 20cm3 of 0.2 molar solution.
Moles in solution = molarity x volume of solution
Moles of CuS04=x 20
= 4 x10-3 moles of CuS04 in 20cm3 of 0.2 molar solution.
Since we now know how many moles of Copper Sulphate we have, and since we know the reacting ratios are 1:1, we now know that 4 x10-3 moles of each metal is needed to react with
4 x10-3 moles of copper sulphate solution.
We can then find how many grams of each metal we need to react with 20cm3 of Copper Sulphate Solution.
Mass in g = moles of solid x mass of 1 mole (RAM)
So for magnesium we need:4 x10-3 x 24 = 0.096g
So for calcium we need: 4 x10-3 x 40 = 0.16g
So for zinc we need: 4 x10-3 x 65= 0.26g
So for copper we need: 4 x10-3 x 64 = 0.256 g
And for iron we need: 4 x10-3 x 56 = 0.224g
We have now worked out a way to make sure that our test will be completely fair in terms of how much of each metal we are using. We will be using 4 x10-3 moles of metal. While the mass of 4 x10-3 moles will differ from metal to metal, the fact remains that all the metals will contain the same number of particles.
However, there are other factors we must take into consideration when we do our investigation. For instance, temperature. The temperature of the room in which we do the investigation must remain constant. This does not appear to be much of a problem as the temperature of the lab is controlled by the air conditioner. So we can be assured that temperature of the room will not be an issue. Another issue is the temperature of the copper sulphate solution. This too, will not be an issue, because the Copper Sulphate Solution is left out in the lab, and so will have the same temperature as the room, whose temperature is controlled by the air conditioner.
Another precaution we will have to take is with the container we do the reactions in. Since I have to do 20 tests, I have decided to use 5 different containers, i.e. One for each metal to be tested and we will do all 4 tests for each metal in one container. We will ensure that before we repeat each test, we will wash out the container.
One other precaution I must take is to make sure that all the heat is kept in the container. There is no point in doing the tests if what I want to measure (heat or temperature rise) just dissipates. As a precaution against this I have decided to use expanded polystyrene cups with lids. This is because expanded polystyrene cups are very good insulators and are excellent at keeping heat in. Also I have decided to shake the cup so the metal is effectively mixed with the copper sulphate. This will make my readings more fair.
The independent variable:
The variable we must change is that of which metal we will test. We have five metals; iron; calcium; magnesium; zinc; and copper.
The dependent variable
The dependant variable will be temperature. We will measure the rise in temperature.
EQUIPMENT REQUIRED:
· 5 expanded polystyrene cups with lids
· Thermometer (Range -10oC to 100oC)
· 25cm3 measuring cylinder
· Top pan balance (Accurate to 3 decimal places)
ALSO REQUIRED:
· 500cm3 0.2 Molar Copper (II) Sulphate Solution
· 4 sets of the following:
Þ 0.096g Magnesium
Þ 0.16g Calcium
Þ 0.26g Zinc
Þ 0.256g Copper
Þ 0.224g Iron
THE TESTS:
from coursework.info
Set up the equipment as shown above.
The first thing we must do before we begin our experiment is to take a baseline temperature for the copper sulphate solution. We should do this because we then have something to subtract from our temperature readings when we perform our experiments to find the temperature rise.
We will then begin the experiment. We will add whatever metal we are testing to the copper (II) sulphate solution in the amount specified above. For example, if we were testing magnesium, then we would measure out 0.096g of magnesium on the top pan balance and add it to the copper sulphate solution and shake the cup gently, all the while looking reading off the thermometer. Every thirty seconds we will note down the temperature achieved by the metal reacting with copper sulphate solution.
We will test each metal for four minutes with 4 attempts. This will allow us to get a good spread of readings.
Every time we finish one attempt of a metal, we will wash out and dry the cup and do the experiment again in the same cup.
After we have the complete readings for each metal within a table, we will find the maximum temperature achieved from each attempt and put it in a table along with all the other metals. Form here we can then go on to find the maximum temperature rise achieved in each attempt by taking the maximum temperature away from the baseline temperature of the copper sulphate and the take an average maximum rise in temperature for each metal.
PREDICITION:
I predict that the reactivity series for those 5 metals will look like this:
Calcium
Magnesium
Zinc
Iron
Copper
The reason for my putting copper at the bottom of the reactivity series is obvious. Copper is not likely to react with its own salt solution (Copper (II) Sulphate)
From previous experiments, we have seen that when calcium reacts with cold water the reaction can be described as vigorous, fizzing and giving off hydrogen gas. The same can be said for when calcium reacts with dilute acid. It is because of these two facts that I have placed calcium at the top of my reactivity series.
Previous investigations have also shown that magnesium will not react with cold water, but it will react with steam to produce magnesium oxide and hydrogen gas. However, the reaction of magnesium with dilute acid is vigorous producing lots of hydrogen. The fact that it will not react vigorously with cold water means that I must place it second in my reactivity series.
Zinc and iron both have very similar reactions with steam and dilute acid. For this reason, a test of a different nature is needed. Iron can be displaced from iron sulphate using zinc, showing conclusively that zinc is more reactive than iron.
In a displacement reaction, a more reactive element will displace a less reactive one from its salt.
I am almost positive that copper will be at the bottom of this reactivity series because it is highly unlikely to react with its own salt solution and also because copper hardly react with water or dilute acid.
With the exception of copper, I believe that if we place any of the metals in copper sulphate solution then we will have a displacement reaction. The more reactive metal will enter solution as 2+ ions, giving their electrons up to the less reactive copper cations, which will turn to copper atoms and settle at the bottom of the container.
Copper sulphate solution
The introduction of calcium; magnesium; iron or zinc where any of the these four metals is ‘X’
in the diagram above.
I predict that the equations of the metals reacting with copper sulphate will be:
For calcium : Ca(s) + Cu2+(aq)SO42-(aq)à Ca2+(aq)SO42-(aq) + Cu(s)
For magnesium: Mg(s) + Cu2+(aq)SO42-(aq)à Mg2+(aq)SO42-(aq) + Cu(s)
For zinc: Zn(s) + Cu2+(aq)SO42-(aq)à Zn2+(aq)SO42-(aq) + Cu(s)
For iron: Fe(s) + Cu2+(aq)SO42-(aq)à Fe2+(aq)SO42-(aq) + Cu(s)
For copper: Cu(s) + Cu2+(aq)SO42-(aq)à Cu2+(aq)SO42-(aq) + Cu(s) [NO REACTION]
Let us take the example of the thermit reaction. This displacement reaction between aluminium and iron oxide generates a lot of heat, leaving behind aluminium oxide and iron. This is because iron is much lower down in the reactivity series than aluminium.
So we can see that if there is a large difference between the reactivity of metals involved in a displacement reaction, a lot of heat can be produced.
Let’s now apply this principle to calcium. Calcium is very reactive, both with water and with dilute acid. Copper is exactly the opposite, and will react with neither water nor dilute acid. So we can expect that a displacement reaction between calcium and copper sulphate will generate a fair amount of heat.
Predicted graph for temperature rise with metal displacement
reaction involving copper sulphate
The bond breaking which will occur will be caused by the displacement reaction ripping apart the bonds between the copper cations and the sulphate anions and because breaking bonds requires energy, the temperature of the reaction will fall. However, the more reactive metal will then make bonds between it and the sulphate anions, making for a release of energy, whichl we will see as a rise in temperature. Because the energy released by the making of the
bonds will be greater than that of the energy taken in to break the bonds, we will note the entire displacement reaction as causing an increase in temperature.
The more reactive the metal, the stronger the bonds it will make to the sulphate anions, and the more energy will be released when the metal forms those bonds. Thus energy given out by the reaction is proportional to the bond strength.
I believe that to some degree, the size of the atom has a part to play in the determination of the reactivity of the element. However, this pattern seems to break down when it comes to the transition metals.
The pattern does seem to work for the elements that aren’t transition metals. Let us take for example, the sizes of the atoms for magnesium and calcium. We know that calcium is more reactive than magnesium, and we can show this using their atomic structures as guides
As you can see, calcium’s outer shell electrons are further away than magnesium’s, and so this means that the nucleus has a weaker pull to those electrons and is more ‘willing’ to let them go. However, in magnesium, the atom’s outer shell electrons are close to the nucleus, and so magnesium has a greater attraction to it’s outer shell electrons and is not as willing to let them go as calcium. So it is less reactive than calcium.
The reaction will be:
Mg + Cu SO4 → Cu + Mg SO4
Magnesium + Copper sulphate → Copper + magnesium sulphate
The magnesium, being the more reactive metal, has displaced the copper and made an ionic bond with the sulphate group. You get the copper appearing as a black layer of microscopic particles on the surface of the magnesium.
You can look at this lot as two types of event: bonds being broken and bonds being made.
Bonds being broken take in energy. The ionic bond between the copper ion and the sulphate group is broken. The metallic bond between the magnesium atoms is broken. The magnesium atoms must become ionised to take part in the reaction – that takes in energy, too.
Bonds being made give out energy. The ionic bond between the magnesium ion and the sulphate group is made. The copper ions must gain electrons and make metallic bonds with each other to become the black solid. These changes give out energy, too.
In this reaction, the amount of energy taken in by the bonds being broken is less than the energy given out by the bonds being made. So on balance we see energy being given out as heat. It is an exothermic reaction.
Every reaction involves energy changes. In order for a reaction to take place old bonds need to be broken and new bonds need to be formed. This involves energy.
To break bonds, energy is needed. This is why many reactions need to be heated to get them started. Those that happen at room temperature are still using the heat from the surroundings to get started, they just don't need as much. The process of breaking bonds is endothermic (needs heat).
When bonds are made, energy is released. This is an exothermic reaction (heat is given out).
In the reaction you describe, the zinc is more reactive than the copper. As a result, it acts a bit like a bully, demanding that the copper give it the sulphate ions. Energy is needed to break the bonds between the copper ions and the sulphate ions. When the zinc bonds to the sulphate ions, energy is given out. The more zinc that is there, the more ‘demanding' it is because every zinc ion wants to bond with a sulphate ion and so they are competing with each other as well as with the copper.
A displacement reaction is an example of a redox reaction. In a redox reaction one reactant loses electrons (oxidation) while the other gains the electrons (reduction). For example zinc metal will displace copper ions from solution because zinc is higher than copper in the electrochemical series. Zinc ions and copper atoms (metal) are formed - the zinc atoms lose electrons while the copper ions gain the electrons.
