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Leaving Certificate Chemistry Oxidation and Reduction

Leaving Certificate Chemistry Oxidation and Reduction. Michael O’Leary Patrician Academy, Mallow. Earlier Theories. An early theory on oxidation was that when a substance combined with oxygen it is oxidised. e.g. The carbon is said to be oxidised.

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Leaving Certificate Chemistry Oxidation and Reduction

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  1. Leaving Certificate ChemistryOxidation and Reduction Michael O’Leary Patrician Academy, Mallow

  2. Earlier Theories An early theory on oxidation was that when a substance combined with oxygen it is oxidised. e.g. The carbon is said to be oxidised Reduction is the opposite of oxidation.When oxygen is removed from a substance, it is said to be reduced. e.g. The haematite, Fe2O3 is reduced

  3. ‘OIL RIG Theory A later theory stated that oxidation was loss of electrons e.g. Place a piece of zinc in a solution of copper sulfate and observe Copper has left the solution to become plated onto the zinc plate Here, also zinc has lost two electrons Zinc is oxidised

  4. ‘OIL RIG Theory This theory also stated that reduction was gain of electrons e.g. Cu2+ has gained 2 electrons Copper is reduced There is a complete transfer of electrons from zinc to copper

  5. ‘OIL RIG Theory Thus we have the theory OXIDATION IS LOSS OF ELECTRONSREDUCTION IS GAIN OF ELECTRONS

  6. Oxidising & Reducing Agents A substance that allows oxidation to take place by gaining electrons itself is called the oxidising agent. The oxidising agent gains electrons The oxidising agent is reduced Similarly, The reducing agent is oxidised

  7. Consider the reaction between carbon and oxygen According to the earlier theory, here the carbon is oxidised. But as CO2 is a covalent molecule and NO IONS here,there is NO complete transfer of electrons from Carbon to form Carbon Dioxide. i.e. CO2 is a covalent molecule where the electrons are shared. A new theory of oxidation and reduction is needed…

  8. Oxidation Number In order to overcome this problem, the concept of OXIDATION NUMBER was introduced. OXIDATION NUMBER is defined as - the charge that an atom has or appears to have when electrons are distributed according to certain rules.

  9. Rules for Assigning Oxidation Numbers • The oxidation number (O.N.) for atoms of elements in the uncombined state is zero (0)e.g. O.N. of sodium in Na is 0 (zero), the O.N. of hydrogen in H2 is 0 (zero) • In neutral molecules such as H20, the sum of the O.N.s is 0 (zero)Here, each Hydrogen is +1 and Oxygen is -2 • In complex ions such as CO32-, the sum of the O.N.s of the elements is equal to the charge on the ion

  10. Rules for Assigning Oxidation Numbers - continued • In ions, the O.N. of a monatomic ion equals the charge of the ion • e.g. the oxidation number of Na+ is +1; the oxidation number of N3- is -3 The O.N. of F- is -1 • The O.N. of oxygen is -2, except in peroxides (H2O2) when it is -1, and when in OF2 when it is +2 • The O.N. of hydrogen is +1 except in metal hydridese.g. NaH, MgH2 when it is -1 • The O.N. of chlorine is -1 except when combined with oxygen or fluorine

  11. Calculating Oxidation Numbers What are the oxidation numbers of the elements in CO2? The O.N. of oxygen is -2 Since there are 2 oxygen atoms, the total contribution of oxygen is -4, therefore the O.N. of the carbon atom is +4.

  12. Question What are the oxidation numbers of the elements in H2SO4? Solution: Start with elements whose O.N. can be deduced directly from the rules X = O.N. of sulfur 2(+1) + X + 4(-2) = 0 X = 8 - 2 X = 6 The sum of the O.N.s = 0 The O.N. of hydrogen is +1 The O.N. of oxygen is -2 H2SO4 +1 +6 -2

  13. Quiz What is the oxidation number of Hydrogen in H2O +1 Hydrogen in H2O2 +1 Oxygen in H2O2 -1 Oxygen in Cu2O -2 Calcium in CaCl2 +2 Nitrogen in NH3 -3

  14. And Finally What is the oxidation number of Carbon in CO32- +4 Hydrogen in OH- +1 Sulfur in SO42- +6 Hydrogen in MgH2 -1 Carbon in C6H12O6 0 Manganese in MnO4- +7

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