A sealed flask contains an equilibrium mixture of hydrogen, iodine and hydrogen iodide.
H2(g)+I2(g)⇌2HI(g) \mathrm{H_{2}(g) + I_{2}(g) \rightleftharpoons 2HI(g)} H2(g)+I2(g)⇌2HI(g)A scientist replaces a small amount of the iodine with the same amount of iodine made from a radioactive isotope. The temperature stays the same.
After some time, radioactive iodine atoms are found in both the iodine molecules and the hydrogen iodide molecules. The total amount of hydrogen iodide has not changed.
What does this show?
Radioactive iodine has moved the position of equilibrium to the right.
The reaction had stopped before the radioactive iodine was added.
The forward and reverse reactions are both still taking place.
Radioactive iodine acts as a catalyst for the forward reaction.
47 exam-style questions on OCR GCSE Chemistry 5.3 Equilibria, covering 5.3.1 Reversible reactions and dynamic equilibrium and 5.3.2 Changing the position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.