Displacement of Metals

Planning:      

             The aim of this experiment is to investigate reacting masses. When iron reacts with an excess of copper sulphate solution, copper metal precipitates. During this investigation, I will have to find out whether there is any pattern between the amount of Iron that is put in and the amount of Copper that is precipitated.

    Metals are amongst our most valuable resources. But many metal ores are in short supply, especially those that give up their metals very readily, like copper and tin for instance.

    Why is it easier to get some metals from their ores than others? It depends on their reactivity. A reactive substance is one which readily takes part in a chemical reaction. Gold, copper, tin and iron are elements. When they react, they form compounds. A reactive metal forms compounds more easily than an unreactive one. By knowing the reactivity of a metal we can understand how to extract it from its metal ore as well as how to use it.

    In order to investigate the reactivity of metals, one method is to look as the reaction between a metal and an acid. For example, if pieces of magnesium, iron and copper are put into different beakers containing a dilute hydrochloric acid, they may react. Actually, from the rate of bubbling, it is clear that magnesium reacts faster than iron, and that iron reacts faster than copper. This consequently shows us that magnesium is most reactive and copper the least reactive.

    Another method is to see how metals react with cold water or steam. Some metals react too violently with acids in order for the reaction to safe. Therefore, water can also be used. If you put lithium and calcium in two separate beakers of water, they will react. Calcium reacts rapidly, giving off a steady stream of hydrogen bubbles. Lithium reacts even more rapidly: hydrogen is given off fast enough for a flame to burn steadily.

    However, just as acid was no help in deciding whether copper or iron is more reactive, nor is cold water because they take such a long time to react. But reactions speed up if heated. Consequently, we find out that, while copper doesn’t even react with steam, iron does. The products formed when a metal reacts with hot water are hydrogen and a metal oxide.

    Many metals burn when heated directly in air. There is only one product of this reaction – the oxide of the metal. These reactions are too quick to be of much help in judging the metal’s reactivity, but the results do fit roughly into the reactivity series. The less reactive metals do not burn when heated in air. A layer of black copper (II) oxide will, though, form on the surface of copper when it is heated.

     However, the best way to compare metal reactivity is by using displacement reactions. When Magnesium is heated in steam, hydrogen is formed.

Magnesium + steam à magnesium oxide + hydrogen

Mg(s) + H₂0(g) àMgO(s) + H₂(g)

             When this happens, it is said that magnesium has displaced the hydrogen. The magnesium has taken the place of the hydrogen in the oxide compound.

             A reactive element forms compounds more readily than an unreactive one. In this reaction, magnesium is becoming part of a compound, magnesium oxide. The displaced hydrogen is leaving its compound, water. This shows that magnesium is more reactive than hydrogen.

             Displacement reactions can also occur between metals. For example

Copper (II) oxide + Magnesium à Magnesium oxide + Copper

CuO(s) + Mg(s) à MgO(s) + Cu(s)

    This is a very vigorous reaction. The copper is displaced by the magnesium. This consequently tells us that magnesium is more reactive than copper. Just to check, one can heat magnesium oxide with copper. Nothing happens. Magnesium cannot be displaced by copper.

    Displacement reactions give much more information about reactivity than comparing reactions of metals with acid or water. If metal A displaces metal B, then Metal A is more reactive than metal B. While if metal B displaces metal A, then metal B is more reactive of the two.

    Now it is easier to sort out the relative reactivity of most of the elements. A knowledge of the order of the metals in the reactivity series can be sued to predict and explain chemical reactions involving metals. Here is a list in order of most reactive to least reactive:  

Sodium             Na

Calcium           Ca

Magnesium       Mg

Aluminium        Al

Zinc                 Zn

Iron                   Fe

Lead                 Pb

Copper             Cu

Mercury            Hg

Silver               Ag

Platinum           Pt

One famous displacement reaction demonstrates that aluminium is more reactive than iron. It is called the Thermit reaction. Aluminium powder is mixed with iron(III) oxide powder. The mixture is heated to started the reaction. Once started, the reaction is very rapid. A large amount of heat is given off. The products are aluminium oxide and some very hot iron. This reaction has been used to produce molten iron, to weld lengths of railways line together.

Displacement reactions like the Thermit reaction can be very exciting, but they are also dangerous.  The Thermit reaction produces molten iron which can do a lot of damage. A safer way for performing displacement reaction in the laboratory is by using solutions of metal salts, instead of metal oxide. A piece of pure metal is put into a solution of the metal salt of a different metal. If the pure metal is more reactive than the metal in the salt, then the latter is displaced. The metal in the salt appears in the liquid as pure metal.

For example, copper is less reactive than iron. If some iron filings are put into a beaker containing Copper (II) sulphate solution, the iron displaces the copper from its compound. The blue solution turns almost colorless. Some of the copper sticks to the iron filings, while some falls to the bottom of the beaker. The iron slowly disappears, as it takes the place of copper in the solution – iron (II) sulphate is formed.

Iron + Copper (II) sulphate à Copper + Iron sulphate

Fe(s) + CuSO₄(aq) à Cu(s) + FeSO₄(aq)

Of course, if you try putting a piece of copper into some iron (II) sulphate solution, nothing will happen. Copper cannot displace iron.

          Now, in this experiment, I will be investigating whether there is any direct relationship between the mass of iron put in, and the mass of copper put out. I will be varying the quantity of iron filings that I will put into the solution, while I will be keeping constant the volume of Copper (II) sulphate solution the same.

In my opinion, there should be a relationship. I believe that if more iron is put in, then more copper will be precipitated.  

Obtaining evidence:       

For this experiment, you will need 5 test tubes, a 250ml beaker, an electric heater, iron filings, 15cm³ of 1M Copper (II) sulphate solution, propanone and a scale.   

1.      Take one test tube and weigh it. Record its mass. Then take all of the five test tubes and fill them with 15cm³ of Copper (II) sulphate solution.

2.      Now, take a piece of paper and weigh it on the scale. Then pour the quantity of iron filings that you want to react with the first test tube, 0.2 grams for example.

3.      Fill up the 250ml beaker with water, and begin to heat it on the electric heater, without making the water boil.

4.      Then pour the iron filings in one of the test tubes. Place the test tube into the beaker of hot water and wait for it to finish reacting. The hot water will speed up the reaction.

5.      Once the reaction is over, take the test tube out of the beaker and decant the excess liquid (i.e. Iron (II) sulphate solution) into a sink. Be careful not to pour the precipitant as well.

6.      Pour a bit of propanone into the test tube and then replace it into the hot water beaker. Wait until all of the fluids in the test tube are evaporated.

7.      Once the inside of the test tube is dry, take it out of the beaker of water. With a paper towel, dry the outside of the tube. Finally, weigh the test tube. Record the result.

8.      Repeat steps 2 to 7 as many times as you want (five times is recommended), each time using a different mass of iron filings.

9.      Once you have all of your results, draw a graph and write up your conclusion.

Results:

Analyzing Evidence:  

            During this experiment I studied a very simple displacement reaction: the displacement of copper because of iron. This reaction can be explained in the following equation:

Iron + Copper (II) sulphate solution à Copper + Iron (II) sulphate solution

Fe(s) + CuS0₄(aq) à Cu(s) + FeSO₄(aq)

During this reaction, I noticed that the solution turned to lighter color – a greenish blue. This is because, even though, iron (II) sulphate solution is a colorless liquid, an excess of Copper (II) sulphate was used. This is so that all of the iron that was put into the solution would react with it

As I mentioned in my hypothesis, I said that the mass of iron put into the copper (II) sulphate should be proportional to the mass of copper that is precipitated. This prediction is confirmed by the first graph, containing the calculate results. In fact, we can states that the points can be joined with a perfectly straight line. My personal results, however, were not totally the same as the predicted ones.

                  Looking at the graphs, we can say that my results were quite accurate considering that most of my results found themselves near to, if not on, the line of best fit. However, there was one anomalous result which I circled and wrote ‘error’ next to it. It is my opinion that this result was a mistake, because it is not very near to the line of best fit, if you compare it to the other points. This mistake could have been easily caused by my carelessness in evaporating any excess fluids found inside the test tube. I probably did not leave the test tube in the hot water long enough for all of the propanone and the excess liquids to evaporate. This consequently greatly affected my result. In fact, from the result charts, we see that more than 100% of Copper would have been displaced according to this result, thing which is impossible.

                        Overall, though, I would declare that this experiment was a success, since most of my results were always very close to calculated ones. I will have to be more careful in carrying out the experiment next time.

Evaluating evidence:

In my opinion, this experiment can be improved in many ways. First of all, I found it quite difficult to decant the excess iron (II) sulphate solution, due to the fact that I had to be careful not to pour any of the precipitated copper, since this would affect my results negatively. In order to contrast this problem, I would use filter paper. Before decanting the test tube, I would weigh the filter paper. Then I could easily decant all of the iron (II) sulphate solution through the filter paper and funnel. If any copper remains in the test tube, I would wash it out with water. Then I would wait for the filter paper to dry, and then weigh everything again.

                        Furthermore, in order to hurry up the evaporating process, I would heat the test tube with a Bunsen burner, instead of hot water and an electric heater. In this way, the inside of the test tube will be dehydrated quickly, and there is no problem of having to dry the outside with a towel. In my opinion, it would be quicker as well as more efficient.

                Additionally, if I could have used a colorimeter, I could have measured the intensity of the color of the solution. This would have told me the concentration of Copper (II) sulphate in the solution due to the intensity of the blue color.

                        In addition to this, I would also recommend another similar experiment: the displacement of copper, using magnesium. Even though this experiment would still be using copper (II) sulphate solution, in my opinion, it would be a bit more interesting because the magnesium reacts more violently with the solution than iron. This is due to the fact that magnesium is higher up in the reactivity series. We could investigate whether the mass of magnesium used was also proportional to the mass of copper precipitated. 

                    Another experiment could also involve magnesium and copper oxide. If we heat up copper oxide and magnesium, we will certainly get a displacement reaction. Even though this is a quite dangerous experiment, and therefore strict safety measures would have to be taken, I believe that it would be very exciting as well as interesting to watch.

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