Describe halogen reactions with hydrogen
Write hydrogen-halogen equations and compare their relative reaction conditions and vigor.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the general product when hydrogen reacts with a halogen X₂?
What you will learn
- Write balanced H₂ + X₂ equations
- Compare the reaction behavior down the group
- Link relative reactivity to the oxidising trend
All halogens combine with hydrogen to form hydrogen halides
The general equation is H₂(g) + X₂(g) → 2HX(g). For example H₂ + Cl₂ → 2HCl and H₂ + Br₂ → 2HBr. Iodine gives a reversible equilibrium at high temperature: H₂ + I₂ ⇌ 2HI.
The reaction becomes less vigorous down the group. Fluorine reacts extremely vigorously; chlorine reacts readily when initiated by light or heating; bromine needs heating; iodine reacts slowly at high temperature and does not go to completion under the usual equilibrium conditions. Do not treat these comparisons as safe experimental instructions.
A rate trend and an equilibrium limit are different
The weaker oxidising ability down the group helps explain why the halogens become less reactive towards hydrogen. For iodine, the reverse reaction also matters: the observed mixture can contain both H₂/I₂ and HI.
A balanced equation gives stoichiometry, not reaction rate or final yield. State the conditions or reversibility when the question supplies them.
Pause and explain
Which halogen reacts least vigorously with hydrogen in the standard comparison?
Worked example
Write the equation for iodine reacting with hydrogen and describe the important equilibrium feature.
Show the worked solution
- One H₂ molecule and one I₂ molecule form two HI molecules.
- The reaction is reversible at high temperature.
- The equilibrium mixture need not contain only HI.
Answer H₂(g) + I₂(g) ⇌ 2HI(g); at high temperature the reaction is reversible and an equilibrium mixture forms.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Balance the equation for hydrogen reacting with bromine.
Give me a hint
Each Br₂ molecule needs two H atoms, supplied by one H₂ molecule.
Compare my reasoning
- The product formula is HBr.
- Use two HBr to balance two bromine atoms.
- One H₂ supplies the two hydrogen atoms.
H₂(g) + Br₂(g) → 2HBr(g).
Look for these in your work
- Product formula correct
- Coefficients balanced
- Diatomic reactants retained
Compare the reaction of chlorine and iodine with hydrogen.
Give me a hint
State the direction of reactivity and distinguish iodine's equilibrium behavior.
Compare my reasoning
- Chlorine reacts more readily than iodine.
- Iodine reacts slowly at high temperature.
- H₂ + I₂ ⇌ 2HI is reversible and reaches an equilibrium mixture.
Chlorine reacts more readily; iodine reacts slowly at high temperature and forms HI reversibly: H₂ + I₂ ⇌ 2HI.
Look for these in your work
- Relative reactivity correct
- Iodine condition included
- Equilibrium represented
A data set records lower HI yield when the temperature is raised after equilibrium is established. What does this show about the forward reaction?
Give me a hint
A yield response gives equilibrium direction, not simply bond-breaking rate.
Compare my reasoning
- The forward reaction is exothermic if increasing temperature shifts equilibrium toward H₂ and I₂.
- The equation is reversible, so temperature can change the equilibrium mixture.
- This observation alone does not mean the forward reaction stopped.
The forward formation of HI is exothermic in that data set; raising temperature favors the reverse direction and lowers the equilibrium HI yield.
Look for these in your work
- Equilibrium shift interpreted
- Thermal direction inferred
- Rate and yield distinguished
Common mistakes
- Writing HX instead of 2HX
- Treating all halogen reactions with hydrogen as equally vigorous
- Writing iodine's reaction as complete and irreversible
- Ignoring the conditions that affect observed rate
What can you explain now?
What is the general product when hydrogen reacts with a halogen X₂?
Compare with the explanation
Hydrogen halide HX
The balanced equation is H₂ + X₂ → 2HX.
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Make it stick
What is the general hydrogen-halogen equation, how does vigor change down the group, and why is HI reversible?
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