oxidation states review: redox reactions involve one substance being oxidised and another reduced....
TRANSCRIPT
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Oxidation States
Review:Redox reactions involve one substance being
oxidised and another reduced.What is oxidation?What is reduction?
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Review:
• Oxidation always involves loss of e-; it can also involve the gain of oxygen and/or loss of hydrogen
• Reduction always involves gain of e-; it can also involve loss of oxygen and/or gain of hydrogen.
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• The oxidation state describes the extent to which an element has been oxidised or reduced.
• The oxidation state can be identified by oxidation numbers.
• Oxidation numbers can be seen as a method to account for electrons taking part in a reaction.
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• Oxidation numbers can be used to establish what element has been oxidised and what element has been reduced in a reaction.
• There is an increase in oxidation number with oxidation
• A decrease in oxidation number with reduction
• Oxidation numbers are assigned to elements.
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www.youtube.com/watch?v=EHe8-AFMsMA
Short Video clip on the Oxidation Number trends in the Periodic Table.
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Oxidation number rulesRules Examples
1.When atoms exist as elements they have an O.N of 0.
Na, Cl2, Ne, C & H2 all have an O.N of zero.
2. The oxidation number of a monotomic ion is the same as the charge on the ion.
Cu2+ has O.N +2, Cl- has O.N -1.
3. Hydrogen in compounds has an O.N of +1 (except in metal hydrides, where its O.N is -1)
H in H2O, CH4, NH3 has O.N +1 (In NaH the O.N of H is -1)
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4. Oxygen in compounds has an O.N of -2 (except in H2O2, where its O.N is -1)
Oxygen in MgO, H2SO4, H2O & KMnO4 has O.N -2, in H2O2, O.N of O is -1
5. For polyatomic ions the sum of the O.N’s equals the charge of the ion.
For NH4+, the sum of
the oxidation numbers is +1, for SO4
2- the sum is -2.
6. The sum of the O.N’s in a molecule is zero.
The sum of the O.N’s for the elements in H2SO4, C4H10 & H2O is zero.
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Examples:• Calculate the ON of nitrogen in the following
compounds
O = -2 x 2 = -4
Charge = -1
N = +3
NO2-
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NH3
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N2O
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NO
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HNO3
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Review:
• An increase in oxidation number is…
• A decrease in oxidation number is…
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Recognising oxidation & reduction
PbO2(s) + C(s) -> Pb(s) + CO2(g)
?,2(-2) 0 0 ?,2(-2)4,2(-2) 0 0 4,2(-2)Carbon oxidised(ON increased from 0 to 4)Lead oxide reduced(ON reduced from 4 to 0)
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• Mg(s) + 2H+ -> Mg2+(aq) + H2(g)
0 2(+1) +2 0
Magnesium oxidised (ON, increased 0 to 2)Hydrogen ions reduced (ON, decreased +1 to 0)
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H+(aq) + OH-(aq) -> H2O(l)Assign oxidation numbers+1 (-2+1) (2+1)(-2)
no change in oxidation number .. Not redox
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Oxidation number quick check. Calculate the oxidation number of the
bold element
• Mn2+
• MnO2
• MnO4-
• S2Cl2• SCl2• SO2
• SO3
• S2O32-
• Cr2O72-
+2+4+7+1+2+4+6+2+6
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Work out the Oxidation state of each atom
• MnO4- Mn2+
• Cr2072- Cr3+
• Cl2 Cl-
• H2O2 H2O
• MnO2 Mn2+
• NO3- NO2
• NO3- NO
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Oxidants and Reductants
• An oxidant is defined as an electron acceptor.• An oxidant can also be called an oxidising agent.• An oxidant is therefore reduced.
• A reductant is defined as an electron donor.• A reductant can also be called a reducing agent.• A reductant is therefore oxidised.
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Review:• What is the oxidation number of:
a.) Manganese in manganese dioxide, MnO2.
b.) Nitrogen in nitrate ions, NO3-
• Give the oxidation number of each type of atom present in the following:a.) Mg b.) SO2 c.) MgO d.) S8 e.) Ag+ f.)NaHCO3 g.)H2SiO3
• Element M has O.N +3. The most likely formula for the oxide of M is:a.) MO b.) M2O c.) M2O3 d.) MO2
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Recall…
• The reactant which is oxidised is called the reductant (or reducing agent) since it reduces the other reactant.
• The reactant which is reduced is called the oxidant (or oxidising agent) since it oxidises the other reactant.
e.g. 2Mg(s) + O2(g) 2MgO(s)Mg is the because .Oxygen is the because .
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Oxidation in terms of oxygen transfer
Oxidation is the gain of oxygen, reduction is the loss of oxygen
Example:2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g)
oxygen transfer• Fe2O3 is reduced since it loses oxygen• C is oxidised since it gains oxygen to form CO2• Fe2O3 is the oxidant and C is the reductant.
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Oxidation in terms of hydrogen transfer
Oxidation is the loss of hydrogen, reduction is the gain of hydrogen
Example:2H2S(g) + O2(g) 2S(s) + 2H2O(l)
Hydrogen transferH2S is oxidised because H is lost
O2 is reduced because H is gained to form H2O
H2S is the reductant and O2 is the oxidant.
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Oxidation in terms of electron transfer
Oxidation is loss of electrons, reduction is gain of electrons
Example:Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)
2e-Zn metal loses electrons and is oxidised.Copper ions gain electrons to form copper metalZinc is the reductant and copper ions are the
oxidant.
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Questions
1. For the reaction 2PbO + C 2Pb + CO2a.) Use the transfer of oxygen definition of redox to identify the oxidant and reductant.b.) Use oxidation numbers to show the same oxidant and reductant would be identified.
Explain your answers.
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Questions
2. For the reaction Fe + Cu2+ Fe2+ + CO2a.) Use transfer of electrons to identify the oxidant and reductantb.) Show that, using oxidation numbers, the same oxidant and reductant would be identified.