Acidified potassium dichromate, K₂Cr₂O₇, also oxidises concentrated hydrochloric acid: K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 3Cl₂ + 7H₂O

1. aCalculate the oxidation number of chromium in K₂Cr₂O₇. [1]
2. bCalculate the oxidation number of chromium in CrCl₃. [1]
3. cUse the change in the oxidation number of chromium atoms to show that potassium dichromate is reduced during the reaction. [1]
4. dName another oxidising agent that can be used in the preparation of chlorine from hydrochloric acid. [1]

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