Redox Reactions Explained - Chemistry Study Guide

  CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)  C6H12O6 + 6O2 → 6CO2 + 6H2O  heat copper + oxygen → copper(II) oxide heat 2 Cu + O2 → 2 CuO  heat copper(II) oxide + oxygen → copper + water   carbon dioxide + carbon...

  CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)  C6H12O6 + 6O2 → 6CO2 + 6H2O  heat copper + oxygen → copper(II) oxide heat 2 Cu + O2 → 2 CuO  heat copper(II) oxide + oxygen → copper + water   carbon dioxide + carbon → carbon monoxide iron(III) oxide + carbon monoxide → iron + carbon dioxide   What is a REDOX reaction? There are 2 ways of thinking of REDOX reactions The first way is by focusing on Oxygen Oxidation is the addition of oxygen to a substance and Reduction is the removal of oxygen from a substance.  What is a REDOX reaction? The second way is by focusing on Electrons Oxidation OIL RIG Is Loss of electrons Reduction Is Gain of electrons  Oxidation numbers Rules for assigning Oxidation Numbers The oxidation number or oxidation state of an atom in a compound is the charge that atom would have if compound consisted only of separate ions  Oxidation numbers 1. An element has its own oxidation number of zero. 2. For a simple ion, the oxidation number is the charge of that ion. 3. Some special example should be learned. 4. In a compound, the sum of the oxidation states is zero. 5. The sum of the oxidation numbers equals the charge on the complex ion. Oxidation numbers Oxidation numbers  Oxidation Numbers Oxidation numbers can be used to define the processes of oxidation and reduction. During oxidation, the oxidation number increases: H2 → 2H+ + 2e- oxidation 0 +1 number During reduction, the oxidation number decreases: Fe3+ + e- → Fe2+ oxidation +3 +2 number  Oxidation Numbers in Names Oxidation numbers can be used in the names of compounds to indicate which oxidation state a particular element in the compound is in. The oxidation state is usually put in brackets in roman numerals after the name of the element in question. Iron(II) hydroxide Iron(III) hydroxide Fe(OH)2 Fe(OH)3 Iron is in oxidation Iron is in oxidation state +2 state +3  REDOX and Electrons Magnesium burns in oxygen to form magnesium oxide. It is obvious that the magnesium has been oxidized, but what has happened to the oxygen? A redox reaction can also be explained in terms of the gain or loss of electrons. What happens to the atoms and electrons in this reaction? magnesium + oxygen magnesium oxide 2Mg(s) + O2(g) 2MgO(s)  Oxidation and electron loss When magnesium burns in oxygen to form magnesium oxide, what happens to magnesium and its electrons? oxidized (electrons lost) Mg + O Mg2+ O2- The magnesium has been oxidized. The Mg atom has lost 2 electrons to form a Mg2+ ion. Oxidation is the loss of electrons.  Oxidation and electron loss When magnesium burns in oxygen to form magnesium oxide, what happens to oxygen and its electrons? Reduction (electrons gained) Mg + O Mg2+ O2- The oxygen has been reduced. The O atom has gained 2 electrons to form a O2- ion. Reduction is the gain of electrons.  Blast Furnace – Extracting Iron Extracting iron Iron is used in bridges, buildings and other structures because it is strong. The blast furnace Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron(III) oxide, Fe2O3. The oxygen must be removed from the iron(III) oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.  Blast Furnace – Extracting Iron It is so hot in the blast furnace that carbon monoxide is the reducing agent which reduces the iron(III) oxide. The equation for the reduction of iron(III) oxide in the blast furnace is: iron(III) oxide + carbon monoxide → iron + carbon dioxide Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g) Molten iron is tapped off at the bottom of the blast furnace.  REDOX Example 1 Magnesium is oxidised when it reacts with oxygen to form magnesium oxide. Explain why this is an oxidation reaction. Magnesium + oxygen → magnesium oxide 2Mg + O2 → 2MgO Explanation: Magnesium gains oxygen and gain of oxygen is oxidation. or Magnesium also loses electrons and loss of electrons is oxidation. This is shown by the half equation below (higher tier): Mg → Mg2+ + 2e-  REDOX Example II Copper(II) oxide can be reduced to form copper when it reacts with hydrogen. Explain why copper(II) oxide is described as reduced. Copper(II) oxide + hydrogen → copper + water CuO + H2 → Cu + H2O Explanation: Copper(II) oxide loses oxygen and loss of oxygen is reduction. or Copper(II) ions gain electrons and gain of electrons is reduction. This is shown by the half equation below (higher tier): Cu2+ + 2e-→ Cu  REDOX Example III Nitrogen reacts with hydrogen to form ammonia according to the equation below. N2(g) + 3H2(g) → 2NH3(g) Q1. Explain why nitrogen is described as being reduced. Explanation: Nitrogen gains hydrogen and gain of hydrogen is reduction. This one cannot be explained in terms of oxygen (as there is no oxygen in the reaction) and all the substances involved are covalent so there is no loss of gain of electron. Usually, oxidation and reduction take place simultaneously in a reaction. We call this type of reaction a redox reaction.  REDOX Example IV Magnesium reacts with copper(II) sulphate solution according to the equation: Mg(s) + CuSO4(aq) → MgSO4(aq) → Cu(s) Q1. Explain, in terms of electrons, why this reaction is a redox reaction. HINT: The sulphate ion, SO42-, is the spectator ion in this reaction The ionic equation is: Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)  REDOX Example IV Part 2 The half equations are (higher tier): Mg → Mg2+ + 2e- Cu2+ + 2e- → Cu Explanation: Magnesium loses electrons and loss of electrons is oxidation. Copper(II) ions gain electrons and gain of electrons is reduction. Redox is when oxidation and reduction occur simultaneously in the same reaction.  What is a REDOX reaction? Which substances are oxidized and reduced in this reaction? oxygen removed reduction carbon lead oxide + carbon lead + monoxide oxygen added oxidation Reduction and oxidation always take place together. Why is this type of reaction called a redox reaction? redox = reduction and oxidation  What are oxidation and reduction? Oxidation and reduction can be used to describe any of the following processes: Oxidation Reduction addition of oxygen loss of oxygen e.g. 2Mg + O2 → 2MgO e.g. 2CuO + C → 2Cu + CO2 loss of hydrogen addition of hydrogen e.g. CH3OH → CH2O + H2 e.g. C2H4 + H2 → C2H6 loss of electrons gain of electrons e.g. Al → Al3+ + 3e- e.g. F2 + 2e- → 2F-  What are Oxidisation/Reducing Agents? Now we can think about the things CAUSING oxidisation and reduction. oxygen removed reduction carbon lead oxide + carbon lead + monoxide oxygen added oxidation Lead is being reduced by Carbon. Carbon is the reducing agent. Carbon is oxidised because of Lead Oxide. Lead Oxide is the oxidising agent. What are oxidising and reducing agent? Oxidizing agents… Reducing agents… …oxidize other species …reduce other species …accept electrons …donate electrons …are themselves reduced …are themselves oxidized For example, in the reaction below: 2NaCl + F2 → 2NaF + Cl2 Fluorine: ⚫ oxidizes Cl- (to chlorine gas) ⚫ is reduced (to F-) ⚫ accepts electrons (from Cl-) ⚫ is an oxidizing agent  Overall equation Zn + CuSO4 → ZnSO4 + Cu zinc copper(II) sulfate zinc sulfate copper Ionic equation Zn + Cu2+ → Zn2+ + Cu zinc copper(II) ion zinc ion copper Pink solid formed on zinc metal (∵ Cu metal is formed) Blue colour becomes lighter (∵ blue Cu2+ is being used up) Ionic equation Zn + Cu2+ → Zn2+ + Cu zinc copper(II) ion zinc ion copper Zn + Cu2+ → Zn2+ + Cu 30 protons 29 protons 30 protons 29 protons 30 electrons 27 electrons 28 electrons 29 electrons Zn atom loses two electrons Cu2+ ion gains two electrons ⇒ Zn is oxidised (to Zn2+) ⇒ Cu2+ is reduced (to Cu) ⇒ Zn is the electron donor ⇒ Cu2+ is the electron acceptor ⇒ Zn is the reducing agent ⇒ Cu2+ is the oxidising agent  The reduction and oxidation reactions are written as two half-equations: Type of reaction Half-equation Remarks on position of electrons Reduction: Cu2+ + 2e− → Cu Electrons on LHS are gained by Cu2+ Oxidation: Zn → Zn2+ + 2e− Electrons on RHS are lost by Zn Overall reaction: Cu2+ + Zn → Cu + Zn2+ (the electrons cancel out) Type of reaction Half-equation Remarks on electron transfer − − Reduction: Cl2 + 2e → 2Cl Cl2 gained electrons Oxidation: 2Br−→ Br2 + 2e− Br− lost electrons Overall reaction: Cl2 + 2Br− → 2Cl− + Br2 (the electrons cancel out) Reactions that are not redox reactions 1a. Neutralisation (acid-base) reactions HCl + NaOH → NaCl + H2O 1b. Acid-carbonate reactions 2HCl + MgCO3 → MgCl2 + H2O + CO2 2. Thermal decomposition of carbonates CuCO3 → CuO + CO2 3. Precipitation reactions AgNO3 + KCl → AgCl + KNO3