Definition 1 An element or compound is oxidized when it gains oxygen atoms

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1 Oxidation and Reduction Part I Learning Outcomes 1. Introduction to oxidation and reduction: simple examples only, e.g. Na with Cl 2, Mg with O 2, Zn with Cu Oxidation and reduction in terms of loss and gain of electrons. 3. Oxidising and reducing agents. 4. The electrochemical series as a series of metals arranged in order of their ability to be oxidised (reactions, other than displacement reactions, not required). 5. Electrolysis of (i) copper sulfate solution with copper electrodes and (ii) acidified water with inert electrodes. (Half equations only required.) Introduction In order to keep track of electrons during chemical reactions involving covalent compounds chemists introduced the idea of oxidation numbers/states. It is the charge that an atom appears to have when the electrons are distributed according to certain rules Definition 1 An element or compound is oxidized when it gains oxygen atoms

2 Definition 2 A COMPOUND IS OXIDISED WHEN IT LOSES HYDROGEN ATOMS Oxidation = addition of oxygen to a substance C + O 2 CO 2 Reduction is loss of oxygen or addition of hydrogen CuO + H 2 Cu + H 2 O Definition 3 An element is oxidised when it loses electrons An element is Reduced when it gains electrons Examples Na loses an electron [oxidised] Cl gains an electron [reduced] Chemical Rxs can be divided into two categories 1. Those which involve no electron transfer. e.g. Positive silver ions are attracted to negative chlorine ions resulting in solid silver chloride

3 2. Those which do Electrons are lost by Zinc and gained by Copper Electron transfer reactions are known as Oxidation-reduction reactions (Redox Rx) Examples 1. Burning 2. Rusting 3. Photosynthesis 4. Respiration If oxidation occurs it must always be accompanied by Reduction

4 Oxidation is the loss of electrons Reduction is the gain of electrons Oxidising & Reducing Agents An oxidising agent is a substance that allows oxidation to occur by gaining electrons itself A Reducing agent is a substance that allows Reduction to occur by losing electrons itself Uses of Oxidising Agents 1. Hydrogen Peroxide (H 2 O 2 ) 2. Bleaching Hair 3. Tincture of Iodine I 2 ) 4. Sterilizing Wounds 5. Chlorine (Cl 2 ) 6. Swimming Pool disinfectant 7. Bleach, Sodium Hypochlorite (NaOCl) 8. Household disinfectant Uses of Reducing Agents 1. Carbon Monoxide (CO)

5 2. Removes oxygen from Iron 3. Sulfur Dioxide (SO 2 ) 4. Paper manufacture Halogens as oxidising agents The group VII Halogens can all act as oxidising agents Halogens as oxidising agents Fluorine is the most powerful oxidising agent of the halogens Fluorine releases Chloride, Bromide and Iodide from solutions of their salts (oxidises them) Chloride is next best and releases, Bromide and Iodide from solutions of their salts (oxidises them) Chlorine Example The release of Bromine from a solution of its salt by the oxidising agent Chlorine

6 Another Chlorine Example The release of Iodine from a solution of its salt by the oxidising agent Chlorine Bromine Example The release of iodine from a solution of its salt by the oxidising agent Bromine

7 Solutions of Bromine, Chlorine and Iodine can oxidise Iron (II) ions to Iron (III) ions and also Sulfite ions to Sulfate ions in aqueous solution Example Iron (II) to Iron (III) Example: Chlorine oxidises Sulfite ions to Sulfate ions

8 The Electrochemical series Element Potassium Calcium Sodium Magnesium Sy K Ca Na Mg Aluminium Al Zinc Zn Iron Fe Tin Sn

9 Lead Hydrogen Copper Silver Gold Pb H Cu Ag Au Arranged in order of tendency to lose electrons (be oxidised) Metals react by losing electrons to form positive ions Atoms of alkali metals will lose electrons faster (more readily) than others...transition metals A metal on the table will displace one lower down the table from a solution of its salt. The more reactive metal will be oxidised and the less reactive reduced e.g Mg displace Cu and is oxidised in the process Use To coat a less expensive metal with a more expensive one Electrolysis Electrolytes are substances which conduct electricity when dissolved in water or when molten. Acids like Hydrochloric acid and Bases like Sodium Hydroxide

10 and salts like Sodium Chloride are all electrolytes. When a current is passed through an electrolyte then the substance is chemically altered Definition Electrolysis is the breaking up of a substance due to the passage of an electric current through it in aqueous solution or when molten Electrodes are connected to the negative and positive terminals of a DC power supply. The electrodes are dipped into an electrolytic solution. Positive ions (cations) are attracted to the negative electrode and Anions are attracted to the positive electrode. Oxidation occurs at the anode and reduction occurs at the cathode Electrolysis of Aqueous Solutions A complex situation because of the possibility that a number of different species are present resulting in several oxidation/reduction reactions occurring Water itself as well as any anions present may be oxidised. In fact water may also be reduced as well as any cations present Factors affecting Oxidation/reduction Electrode Type Inert versus copper

11 Ease of oxidation / Reduction H 2 O vs SO 4 2- Electroplating A practical application of electrolysis A layer of metal is put onto the surface of another metal Silver plates nickel knifes (EPNS) Chromium plates steel...improves resistance to corrosion Purification of Copper