Practical Activity
Reactivity trend in group 7
It is important in this practical activity to ensure the safe use and careful handling of liquids, including careful mixing of reagents. In particular, solutions of the group 7 elements are toxic, irritating or harmful.
This outlines one way to carry out the practical. Eye protection must be worn.
Aims
To use displacement reactionA reaction that occurs when a more reactive element replaces a less reactive element in a compound. to identify the trend in reactivity of the group 7The vertical column of non-metal elements next to group 0 in the periodic table, also called the halogens. (IUPAC group 17) elementA substance made of one type of atom only..
A more reactiveThe tendency of a substance to undergo a chemical reaction.halogenAn element placed in group 7 of the periodic table, which starts with fluorine and ends with astatine. The name 'halogen' means 'salt-producing' because halogens produce a range of salts when they react with metals. can displaceTake the place of another substance in a chemical reaction. For example, a metal can displace a less reactive metal from its oxide, removing oxide ions from the less reactive metal and becoming an oxide itself. a less reactive halogen from solutionMixture formed by a solute and a solvent. of its saltThe substance formed when the hydrogen ion in an acid is replaced by a metal ion.. For example, chlorine is more reactive than iodine. A solution of chlorine can displace iodine from potassium iodide solution:
chlorine + potassium iodide → potassium chloride + iodine
Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)
The reaction mixture turns darker and iodine solution forms.
Method
- use a dropping pipetteA piece of apparatus used to measure accurate and repeatable volumes of liquid. Also called a volumetric pipette. to add a few drops of potassium chloride solution, KCl(aq), to a column of three wells in a spotting tile
- repeat step 1 with two more columns of wells, using potassium bromide solution, KBr(aq), then potassium iodide solution, KI(aq)
- use a dropping pipette to add a few drops of chlorine solution, Cl2(aq), to a row of three wells in the spotting tile
- repeat step 3 with two more rows of wells, using bromine solution, Br2(aq), then iodine solution, I2(aq)
Results
Observe and record what happens in each well at step 3 or 4 in a suitable table. This table shows some sample results.
| KCl(aq) | KBr(aq) | Kl(aq) | Number of reactions seen | |
| Cl2(aq) | Not done | Solution darkens | Solution darkens | 2 |
| Br2(aq) | No visible reaction | Not done | Solution darkens | 1 |
| I2(aq) | No visible reaction | No visible reaction | Not done | 0 |
| Cl2(aq) | |
|---|---|
| KBr(aq) | Not done |
| Kl(aq) | Solution darkens |
| Number of reactions seen | Solution darkens |
| 2 | |
| Br2(aq) | |
|---|---|
| KBr(aq) | No visible reaction |
| Kl(aq) | Not done |
| Number of reactions seen | Solution darkens |
| 1 | |
| I2(aq) | |
|---|---|
| KBr(aq) | No visible reaction |
| Kl(aq) | No visible reaction |
| Number of reactions seen | Not done |
| 0 | |
Analysis
Count the number of reactions seen with each halogen solution.
Example
Use the results in the table to deduce an order of reactivity, starting with the most reactive halogen.
The order of reactivity is: chlorine > bromine > iodine. This is because chlorine could displace bromine and iodine, bromine could only displace iodine, but iodine could not displace chlorine or bromine.
Evaluation
Question
Suggest an explanation for why three combinations of halogen and salt solution were not done.
A halogen cannot displace itself from a solution of one of its salts, so these combinations were not done.
Question
Astatine is placed below iodine in group 7. Predict whether astatine will displace iodine from potassium iodide solution.
Astatine is less reactive than iodine, so it will not displace iodine from potassium iodide solution.
Example - Higher
a) Write an ionic equationA chemical equation that shows how positively charged ions join with negatively charged ions to make a compound. for the reaction between chlorine solution and bromide ions from potassium bromide solution.
Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq)
b) Write two half equationAn equation, involving ions and electrons, that describes the process happening at an electrode. for this reaction.
Cl2(aq) + 2e- → 2Cl-(aq)
2Br-(aq) → Br2(aq) + 2e-
c) Explain why this displacement reaction is also a redox reactionWhen reduction and oxidation take place at the same time. reaction.
Chlorine gains electrons and is reduceA substance is reduced if it loses oxygen or gains electrons. to chloride ions. At the same time, bromide ions lose electrons and are oxidiseChemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons. to bromine.
Hazards, risks and precautions
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Possible harm | Possible precaution |
| Chlorine solution | Chlorine gas is produced, which is toxic if inhaled. | Make sure the lab is well ventilated and avoid inhaling the gas. |
| Bromine solution | Toxic bromine vapour is produced. Solution irritates the skin. | Make sure the lab is well ventilated and avoid inhaling the vapour. Wear gloves. |
| Iodine solution | Harmful to the skin. Stains clothes. | Wear gloves. Avoid contact with clothing. |
| Hazard | Chlorine solution |
|---|---|
| Possible harm | Chlorine gas is produced, which is toxic if inhaled. |
| Possible precaution | Make sure the lab is well ventilated and avoid inhaling the gas. |
| Hazard | Bromine solution |
|---|---|
| Possible harm | Toxic bromine vapour is produced. Solution irritates the skin. |
| Possible precaution | Make sure the lab is well ventilated and avoid inhaling the vapour. Wear gloves. |
| Hazard | Iodine solution |
|---|---|
| Possible harm | Harmful to the skin. Stains clothes. |
| Possible precaution | Wear gloves. Avoid contact with clothing. |
