Group VII properties

1. Overview

Group VII of the Periodic Table, known as the Halogens, consists of reactive non-metals that exist as diatomic molecules. This topic explores the physical trends and chemical behaviors of these elements, focusing on how their reactivity changes as you move down the group, which is fundamental to understanding displacement reactions.

Key Definitions

• Halogen: An element found in Group VII of the Periodic Table.
• Diatomic: Molecules consisting of two atoms chemically bonded together (e.g., Cl₂).
• Halide: A compound formed when a halogen reacts with another element, usually a metal (e.g., Sodium Chloride).
• Displacement Reaction: A chemical reaction in which a more reactive element takes the place of a less reactive element in a compound.

Core Content

Characteristics of Group VII Elements

The halogens are non-metals that exist as diatomic molecules. This means they are never found as single atoms in nature; they travel in pairs: F₂, Cl₂, Br₂, I₂, and At₂.

Physical Properties at r.t.p. (Room Temperature and Pressure)

As you move down Group VII, the elements change state from gas to liquid to solid as their melting and boiling points increase.

Element	Formula	Appearance at r.t.p.	State at r.t.p.
Chlorine	Cl₂	Pale yellow-green	Gas
Bromine	Br₂	Red-brown	Liquid
Iodine	I₂	Grey-black	Solid (forms purple vapor on heating)

Trends Down Group VII

1. Density increases: The atoms become heavier and more closely packed.
2. Reactivity decreases: Elements at the top (Fluorine) are the most reactive; elements at the bottom (Astatine) are the least reactive.
3. Melting/Boiling Points increase: The molecules get larger, increasing the intermolecular forces that hold them together.

Displacement Reactions

A more reactive halogen will displace a less reactive halogen from its halide solution. This is used to prove the reactivity trend (Cl > Br > I).

1. Chlorine displacing Bromine:

• Word Equation: Chlorine + Potassium Bromide → Potassium Chloride + Bromine
• Symbol Equation: Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
• Observation: The colorless solution turns orange/yellow (due to the formation of aqueous Bromine).

2. Chlorine displacing Iodine:

• Word Equation: Chlorine + Potassium Iodide → Potassium Chloride + Iodine
• Symbol Equation: Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(aq)
• Observation: The colorless solution turns brown (due to the formation of aqueous Iodine).

3. Bromine displacing Iodine:

• Word Equation: Bromine + Potassium Iodide → Potassium Bromide + Iodine
• Symbol Equation: Br₂(l) + 2KI(aq) → 2KBr(aq) + I₂(aq)
• Observation: The colorless solution turns brown.

Note: If you add Iodine to Potassium Chloride, no reaction occurs because Iodine is less reactive than Chlorine.

Explaining the Reactivity Trend

Halogens react by gaining one electron to achieve a full outer shell.

• As you go down the group, the number of electron shells increases (atoms get larger).
• The outer shell is further from the positive nucleus.
• The attraction between the nucleus and the incoming electron becomes weaker.
• Therefore, it is harder to gain an electron, making the element less reactive.

Predicting Properties

You can predict the properties of other elements like Fluorine (above Chlorine) or Astatine (below Iodine):

• Fluorine: Predicted to be a very pale yellow gas, extremely low density, and the most reactive element in the group.
• Astatine: Predicted to be a black solid, very high density, and the least reactive element in the group.

Extended Content (Extended Only)

There are no specific supplemental requirements for this sub-topic in the current syllabus.

Key Equations

Reaction Type	General Equation
Displacement	Halogen A + Metal Halide B → Metal Halide A + Halogen B
Specific Example	Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(aq)

Symbol Meanings:

• Cl₂: Chlorine molecule
• KI: Potassium Iodide
• KCl: Potassium Chloride
• I₂: Iodine molecule
• (g): gas; (aq): aqueous (dissolved in water); (l): liquid; (s): solid.

Common Mistakes to Avoid

• ❌ Wrong: Writing halogens as single atoms in equations (e.g., Cl + KBr).
• ✓ Right: Always write halogens as diatomic molecules (Cl₂, Br₂, I₂).
• ❌ Wrong: Confusing the names "Bromine" and "Bromide."
• ✓ Right: Use "-ine" for the element (Bromine) and "-ide" for the ion/compound (Potassium Bromide).
• ❌ Wrong: Stating that Iodine is a purple solid.
• ✓ Right: Iodine is a grey-black solid. It only produces a purple vapor when it sublimes.

Exam Tips

• Color Changes: Questions often ask for the "final color of the solution." For displacement reactions, remember: Bromine in water is orange/yellow; Iodine in water is brown.
• Command Words: If asked to "Describe" the trend, state what happens (e.g., reactivity decreases). If asked to "Explain," you must talk about atomic size and the ease of gaining an electron.
• Real-world Context: Halogens are often discussed in the context of water purification (Chlorine) or antiseptic use (Iodine).
• State Symbols: Always include state symbols in displacement equations unless the question specifically says not to; they are often worth 1 mark.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State the colour and physical state of bromine at room temperature and pressure. [2]

(b) Predict the colour and physical state of astatine at room temperature and pressure. [2]

(c) Explain why astatine is predicted to have a higher density than iodine. [1]

Worked Solution:

(a)

1. Bromine is a red-brown liquid. This states the observed colour and physical state of bromine.

How to earn full marks:

• 1 mark for stating "red-brown" or "brown-red"
• 1 mark for stating "liquid"

(b)

1. Astatine is predicted to be a black solid. This predicts the colour and physical state of astatine based on the trend in Group VII.

How to earn full marks:

• 1 mark for stating "black" or "dark grey"
• 1 mark for stating "solid"

(c)

1. The density increases down the group. Astatine is below iodine in Group VII, so it has a higher density. This explains the density trend based on the position of astatine in the group.

How to earn full marks:

• 1 mark for stating that density increases down the group and therefore astatine will have a higher density than iodine.

Common Pitfall: Remember to link the position of the element in the group to the trend. Simply stating the trend without relating it to the specific elements won't earn you the mark.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Define the term halogen. [1]

(b) Chlorine gas is bubbled through a solution containing potassium bromide. State what you would observe. [1]

(c) Write a balanced chemical equation for the reaction in part (b). [2]

(d) Explain why chlorine is more reactive than bromine. [2]

Worked Solution:

(a)

1. A halogen is an element in Group VII of the periodic table. This is the definition of a halogen.

How to earn full marks:

• 1 mark for stating that a halogen is an element in Group VII of the periodic table.

(b)

1. The solution turns orange/brown. This describes the colour change as bromine is displaced.

How to earn full marks:

• 1 mark for stating that the solution turns orange/brown.

(c)

1. $Cl_2(g) + 2KBr(aq) \rightarrow 2KCl(aq) + Br_2(aq)$ This is the balanced chemical equation for the reaction.

How to earn full marks:

• 1 mark for correct formulae of all reactants and products.
• 1 mark for correct balancing.

(d)

1. Chlorine has fewer electron shells than bromine. This states the difference in atomic structure.

2. The outer electrons in chlorine are closer to the nucleus and more strongly attracted, so chlorine gains an electron more readily. This explains the reactivity in terms of electron affinity.

How to earn full marks:

• 1 mark for stating that chlorine has fewer electron shells or that the outer electrons are closer to the nucleus.
• 1 mark for stating that chlorine gains an electron more readily.

Common Pitfall: When explaining reactivity, don't just say "chlorine is more reactive." You need to explain why it's more reactive, linking it to the number of electron shells and the attraction between the nucleus and outer electrons.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

A student investigates the reaction between chlorine water and potassium iodide solution.

(a) Describe how the student could carry out this experiment safely in the laboratory. Include any necessary safety precautions. [3]

(b) The student adds chlorine water to potassium iodide solution and observes a colour change. (i) State the initial colour of the potassium iodide solution. [1] (ii) State the final colour of the solution after the addition of chlorine water. [1] (iii) Explain why the colour change occurs. Include the relevant ionic equation. [4]

Worked Solution:

(a)

1. Wear safety goggles to protect the eyes from splashes. This mentions a key safety precaution.
2. Carry out the experiment in a fume cupboard to avoid inhaling chlorine gas. This mentions another important safety precaution.
3. Wear gloves to avoid skin contact with the chemicals. This mentions another important safety precaution.

How to earn full marks:

• 1 mark for stating to wear safety goggles.
• 1 mark for stating to carry out the experiment in a fume cupboard.
• 1 mark for stating to wear gloves.

(b) (i)

1. The initial colour of the potassium iodide solution is colourless. This states the initial colour.

How to earn full marks:

• 1 mark for stating that the solution is colourless.

(ii)

1. The final colour of the solution is brown. This states the final colour.

How to earn full marks:

• 1 mark for stating that the solution is brown.

(iii)

1. Chlorine is more reactive than iodine and displaces iodine from potassium iodide. This explains the displacement reaction.
2. Iodine is formed, which is brown in solution. This explains the colour change.
3. $Cl_2(aq) + 2I^-(aq) \rightarrow 2Cl^-(aq) + I_2(aq)$ This is the correct ionic equation.
4. Chlorine gains electrons (is reduced) and iodide ions lose electrons (are oxidised). This explains the redox process.

How to earn full marks:

• 1 mark for stating that chlorine is more reactive than iodine and displaces it.
• 1 mark for stating that iodine is formed, which is brown in solution.
• 1 mark for the correct ionic equation.
• 1 mark for stating that chlorine is reduced and iodide ions are oxidised.

Common Pitfall: When writing ionic equations, make sure you only include the ions that are directly involved in the reaction. Spectator ions (those that don't change) should be left out. Also, double-check that your equation is balanced for both atoms and charge.

Exam-Style Question 4 — Extended Response [8 marks]

Question:

A student investigates the reaction of different halogens with solutions of halide salts. The results are shown in the table below:

Halogen	Potassium Chloride	Potassium Bromide	Potassium Iodide
Chlorine	No reaction	Reaction	Reaction
Bromine	No reaction	No reaction	Reaction
Iodine	No reaction	No reaction	No reaction

(a) Explain why chlorine reacts with potassium bromide but not with potassium chloride. [3]

(b) Predict the outcome of mixing fluorine gas with potassium chloride solution. Explain your prediction. [3]

(c) Suggest a chemical test to confirm the presence of iodide ions in potassium iodide solution before and after the reaction with bromine. State the expected observations. [2]

Worked Solution:

(a)

1. Chlorine is more reactive than bromine but less reactive than fluorine. This states the relative reactivity of chlorine.

2. Chlorine can displace bromine from potassium bromide because chlorine is more reactive. This explains the displacement reaction.

3. Chlorine cannot displace chlorine from potassium chloride because they are the same element, or because chlorine is not more reactive than itself. This explains why there is no reaction with potassium chloride.

How to earn full marks:

• 1 mark for stating the relative reactivity of chlorine compared to bromine and fluorine.
• 1 mark for stating that chlorine can displace bromine.
• 1 mark for stating that chlorine cannot displace chlorine because they are the same element.

(b)

1. Fluorine will react with potassium chloride solution. This predicts the outcome.
2. Fluorine is the most reactive halogen and will displace chlorine from potassium chloride. This explains the prediction.
3. The solution will turn a pale yellow-green colour as chlorine gas is formed. This states the expected observation.

How to earn full marks:

• 1 mark for predicting that fluorine will react.
• 1 mark for stating that fluorine is more reactive and will displace chlorine.
• 1 mark for stating that the solution will turn pale yellow-green.

(c)

1. Add silver nitrate solution, $AgNO_3(aq)$, to the potassium iodide solution. This states the chemical test.
2. A yellow precipitate of silver iodide, $AgI(s)$, will form. This states the expected observation.

How to earn full marks:

• 1 mark for stating to add silver nitrate solution.
• 1 mark for stating that a yellow precipitate will form.

Common Pitfall: When predicting reactions, always consider the reactivity series of the halogens. A halogen can only displace a halide ion if it's higher in the series (more reactive). Also, remember the colours of the silver halide precipitates: silver chloride is white, silver bromide is cream, and silver iodide is yellow.