Redox reaction in the displacement of metals from its salt solution Archives

Displacement of Halogen from Halide Solution

Generally, halogens are good electron acceptors and therefore are good oxidising agents.
(a) When going down Group 17, the size of the halogen atoms increases. The nucleus is further away from the outermost occupied shell.
(b) Hence, the electronegativity of halogens or their ability to accept electrons to form negatively-charged halide ions decreases down the group.
(c) As a result, the strength of halogens as oxidising agents decreases down the group.
Chlorine, bromine and iodine are three
commonly used halogens in the laboratory. Each of them gives different colour in aqueous solution. However, the colour changes slightly with concentration.
Therefore, the presence of the halogens is confirmed using an organic solvent such as 1,1,1-trichloroethane, CH₃CCl₃.
This is done by mixing thoroughly 1,1,1-trichloroethane to an aqueous solution of a halogen. Two layers will be formed whereby the denser 1,1,1-trichloroethane layer will be at the bottom and the less dense aqueous layer will be at the top.

Based on the colour of the 1,1,1-trichloroethane layer, the halogen present is identified.

Halogen	Colour of halogen in aqueous solution	Colour of halogen in 1,1,1-trichloroethane
Chlorine	Pale yellow or colourless	Pale yellow or colourless
Bromine	Brown, yellowish-brown or yellow, depending on concentration	Brown, orange or yellow, depending on concentration
Iodine	Brown, yellowish-brown or yellow, depending on concentration	Purple

In a displacement of halogen, a more electronegative halogen displaces a less electronegative halogen from its halide solution.

The halide ions of the less electronegative halogen act as the reducing agent. They lose their electrons and are oxidised to form halogen molecules.
The electrons are accepted by the more electronegative halogen which acts as the oxidising agent. By doing so, the halogen undergoes reduction to form its halide ions.
In short, there is an electron transfer from the halide ions of the less electronegative halogen to the more electronegative halogen.

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Displacement of Halogen from Halide Solution Experiment

Aim: To investigate the oxidation and reduction in displacement of halogen.
Materials: Chlorine water, bromine water, iodine solution, 0.5 mol dm^-3 potassium chloride solution, 0.5 mol dm^-3potassium bromide solution, 0.5 mol dm^-3 potassium iodide solution, 1,1,1-trichloroethane.
Apparatus: Test tubes, test tube rack.
Procedure:

2 cm³ of 0.5 mol dm^-3 potassium bromide solution is poured into a test tube.
2 cm³ of chlorine water is added to the test tube and the mixture is shaken thoroughly.
2 cm³ of 1,1,1 -trichloroethane is added to the mixture. The mixture is shaken thoroughly.
After a few seconds, the colour of the aqueous and the 1,1,1-trichloroethane layers are observed.
Steps 1 to 4 are repeated using the halogens and halide solutions as shown in Table 3.11.

Results:

Mixture	Colour of aqueous layer	Colour of 1,1,1- trichloroethane layer	Inferences
Chlorine + potassium bromide	Yellow	Orange	Bromine is present. Displacement of bromine has occurred.
Chlorine + potassium iodide	Yellow	Purple	Iodine is present. Displacement of iodine has occurred.
Bromine + potassium chloride	Yellow	Orange	Bromine is present. No displacement reaction has occurred.
Bromine + potassium iodide	Yellow	Purple	Iodine is present. Displacement of iodine has occurred.
Iodine + potassium chloride	Yellow	Purple	Iodine is present. No displacement reaction has occurred.
Iodine + potassium bromide	Yellow	Purple	Iodine is present. No displacement reaction has occurred.

Discussion:

The results can be summarised as follows.
Chlorine is more electronegative than bromine and iodine. Therefore,
(a) chlorine displaces bromine from potassium bromide solution. Chlorine acts as the oxidising agent, whereas bromide ions act as the reducing agent.

(b) chlorine displaces iodine from potassium iodide solution. Chlorine acts as the oxidising agent, whereas iodide ions act as the reducing agent.
(a) Bromine is less electronegative than chlorine. Therefore, bromine cannot displace chlorine from potassium chloride solution.
(b) Bromine is more electronegative than iodine. Therefore, bromine displaces iodine from potassium iodide solution. Bromine acts as the oxidising agent, whereas iodide ions act as the reducing agent.
Iodine is less electronegative than chlorine and bromine. Therefore,
(a) iodine cannot displace chlorine from potassium chloride solution.
(b) iodine cannot displace bromine from potassium bromide solution.

Conclusion:
A more electronegative halogen can displace a less electronegative halogen from its halide solution whereby the more electronegative halogen acts as the oxidising agent and the halide ions of the less electronegative halogen act as the reducing agent.

Redox reaction in the displacement of metals from its salt solution

Generally, metals are good electron donors and therefore are good reducing agents. However, different metals have different strength as reducing agents.

The strength of metals as reducing agents can be compared by using the electrochemical series.

The electrochemical series lists metals according to their electropositivity, that is, according to their ability to lose electrons to form positive ions.
Redox reaction in the displacement of metals from its salt solution 1

Redox reaction in the displacement of metals from its salt solution 1

The higher the position of a metal in the electrochemical series, the more electropositive the metal is, the easier it is for the metal to lose its electrons. Thus, the better reducing agent the metal is.

On the other hand, the ability of a metal ion to accept electrons increases down the series. Thus, the strength of a metal ion as an oxidising agent increases down the electrochemical series.

In a displacement of metal, a more electropositive metal will displace a less electropositive metal from its salt solution.
(a) The more electropositive metal acts as the reducing agent. It loses electrons and undergoes oxidation to form positive ions.
(b) The ions of the less electropositive metal act as an oxidising agent by accepting the electrons. While doing so, the ions are reduced to metallic atoms.
(c) In short, there is an electron transfer from the more electropositive metal to the ions of the less electropositive metal.

Oxidation and reduction in the displacement of metals experiment

Aim: To investigate oxidation and reduction in the displacement of metals.
Materials: Zinc strip, copper strip, 0.5 mol dm^-3 copper(II) sulphate solution, 0.1 mol dm^-3 silver nitrate solution, sandpaper.
Apparatus: Test tubes, test tube rack.
Procedure:

2 cm³ of 0.5 mol dm^-3 copper(II) sulphate solution and 2 cm³ of 0.1 mol dm^-3 silver nitrate solution are poured into two separate test tubes.

A strip of zinc and a strip of copper are cleaned with sandpaper. The strips are then dropped into the test tubes as shown in Figure.
Redox reaction in the displacement of metals from its salt solution 2

Redox reaction in the displacement of metals from its salt solution 2

Any change in colour and whether any metal is deposited are observed.

Observations:

Test tube

Observations

The blue colour of the solution slowly fades until it becomes colourless.
The zinc strip dissolves.
A brown solid is deposited.

The colourless solution slowly turns blue.
The copper strip dissolves.
A silvery grey solid is deposited.

Discussion:

In test tube X, zinc displaces copper from its salt solution.
(a) Zinc is more electropositive than copper. Thus, zinc acts as the reducing agent, losing electrons to form zinc ions. By doing so, zinc is oxidised. This explains why the zinc strip dissolves.

(b) The electrons are accepted by copper(II) ions in the solution. Thus, copper(II) ions act as the oxidising agent and are reduced to metallic copper. The brown solid deposited in test tube X is copper metal.

(c) The decreasing amount of copper(II) ions in the solution causes the solution to slowly change colour from blue to colourless.
(d) The redox reaction that occurs can be represented by the following equation.

In test tube Y, copper displaces silver from its salt solution.
(a) Copper is more electropositive than silver. So, copper acts as the reducing agent, losing electrons to form copper(II) ions. In other words, copper is oxidised. This explains why the copper strip dissolves.

(b) The increasing amount of copper(II) ions in the solution causes the solution to slowly change colour from colourless to blue.

(c) The electrons are accepted by silver ions in the solution. Thus, silver ions act as the oxidising agent and are reduced to silvery grey silver.
(d) The redox reaction that occurs can be represented by the following equation.

Conclusion:
A more electropositive metal can displace a less electropositive metal from its salt solution whereby the more electropositive metal acts as the reducing agent and the ions of the less electropositive metal act as the oxidising agent.

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