Shanghai World Foreign Language Middle School Chemistry Worksheet 1 – Redox Reactions Name: ______________________________________( ) Date: ___________ Class: _____ 1. Work out the oxidation number of the underlined element in the following compounds. (a) MnO2 (b) MnSO4 (c) KMnO4 (d) K2MnO4 (e) CrCl2 (f) CrCl3 (g) K2Cr2O7 2. In each of the following reactions, state which substance is oxidised, and which is reduced. (a) Ca(s) (b) 2CO(g) (c) Fe2O3(s) + + + O2(g) O2(g) 3CO(g) 2CaO(s) 2CO2(g) 2Fe(s) + 3CO2(g) 3. Iodine is extracted from seaweed in a redox reaction using acidified hydrogen peroxide. The ionic equation for the reaction is: 2I- (aq) + H2O2 (aq) + 2H+(aq) I2(aq) + 2H2O(l) (a) What is the oxidation state of iodine in seaweed? (b) There is a colour change in this reaction. Why? (c)(i) Is the iodide oxidised or reduced? (ii) Complete the half equation for this change: 2I- _______________ (d) In hydrogen peroxide, H2O2 the oxidation state of the hydrogen is +1. (i) What is the oxidation state of the oxygen in hydrogen peroxide? (ii) How does the oxidation state of oxygen change during the reaction above? (iii) Complete the half equation for hydrogen peroxide. H2O2 (aq) + 2H+(aq) +….. 2H2O(l) 4. The oxidising agent potassium manganate(VII) can be used to analyse the of iron(II) present in iron tablets. This ionic equation shows the ions taking part in the reaction. MnO2(aq) + 8H+(aq) + 5Fe2+(aq) Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) (a) What does the H+ in the reaction tell you about this reaction? (b) Describe the colour change in this reaction. (c) Which is the reducing agent in this reaction? (d) How could you tell when all the iron(II) had reacted? (e) Write a half equation to show how iron(III) reacts. 5. Cells can be set up with inert electrodes, and the electrolytes as oxidising and reducing agents, as shown in the diagram. Here, the potassium manganate(VII) is the oxidising agent and potassium iodide is the reducing agent. (a) Describe the colour change that would be observed in the left beaker. (b) Write an ionic equation for the reaction in the right hand beaker.