Displacement reactions as redox reactions

3 marks · Normal

Chlorine gas is bubbled through a solution of sodium iodide. Write the balanced ionic equation for this reaction. Show, with working, the steps you use to remove spectator ions from the full ionic equation.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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2I^-(aq) + Cl_2(aq) \rightarrow 2Cl^-(aq) + I_2(aq)

Must show: removal of Na^+ spectator ions, correct stoichiometry throughout

Also accept: I^-(aq) + Cl_2(aq) \rightarrow Cl^-(aq) + I_2(aq) (with all coefficients divided by 2 and still balanced)

Common mistakes: including sodium ions in the final ionic equation; incorrect balancing of electrons or coefficients

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Writes the full ionic equation including all ions, e.g., 2Na^+(aq) + 2I^-(aq) + Cl_2(aq) \rightarrow 2Na^+(aq) + 2Cl^-(aq) + I_2(aq).

M2 · 1 mark · Identifies and removes spectator ions (correctly omitting Na^+).

A1 · 1 mark · Writes the correct ionic equation with correct state symbols and stoichiometry: 2I^-(aq) + Cl_2(aq) \rightarrow 2Cl^-(aq) + I_2(aq). Accept multiples as long as the equation is correctly balanced. (follow-through)

Worked solution

Step 1: Write the equation for chlorine reacting with sodium iodide:

Step 2: Split all soluble compounds (salts and acids) into ions for the full ionic equation:

Step 3: Identify and remove the spectator ions (Na^+), which do not change:

This is the balanced ionic equation for the displacement reaction.