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Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in oxidation number.
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The definition that covers all types of redox (reduction-oxidation) reactions is based on the transfer of electrons.
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Oxidation is the loss of electrons by a chemical species, and can be identified when:
a species loses electrons a species loses hydrogen atoms a species gains oxygen atoms the oxidation state of an atom increases. |
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Reduction is the gain of electrons by a chemical species, and occurs when:
a species Gaines electrons a species loses hydrogen atoms a species loses oxygen atoms the oxidation state of an atom decreases. |
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An oxidizing agent is reduced and a reducing agent is oxidised.
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An oxidising agent is a substance that oxidises another substance and is reduced itself by gaining electrons.
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A reducing agent is a substance that reduces another substance and is oxidised itself by losing electrons.
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Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox reactions.
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A half-equation is an equation that shows the changes that happen in a redox reaction dude to either oxidation or reduction only.
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The oxidation half-equation is a half-equation that shows only the chemical changes that happen in a redox reaction due to oxidation.
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The reduction half-equation is a half-equation that shows only the chemical changes that happen in a redox reaction due to reduction.
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Assign oxidation states to determine which atoms are being oxidized and which are being reduced.
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Write half-equations for oxidation and reduction as follows:
balance the atoms other than H and O balance each half-equation for O by adding H2O as needed balance each half-equation for H by adding H+ as needed balance each half-equation for charge by adding electrons to the sides with the more positive charge check that each half-equation is balanced for atoms and for charge |
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Equalise the number of electrons in the two half-equations by multiplying each appropriately.
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Add the two half-equations together, cancelling out anything that is the same on both sides.
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Balanced oxidation half-equations will have electrons on the product side. Balanced reduction half-equations will have electrons on the reactant side.
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Deduction of the oxidation states of an atom in an ion or compound.
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Oxidation state is a measure of how many electrons an atom has gained or lost when forming a compound.
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Atoms in the free (uncombined) element have an oxidation state of zero.
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In simple ions, the oxidation state is the same as the charge on the ion.
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The oxidation states of all the atoms in a neutral (uncharged) compound must add up to zero.
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The oxidation states of all the atoms in a polyatomic ion must add up to the charge on the ion.
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The usual oxidation state for an element is the same as the charge on its most common ion.
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Most main group non-metals, the elements at the bottom of group 14, and transition elements have oxidation states that vary in different compounds – depending on the conditions and other elements present.
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Deduction of the oxidation states of an atom in an ion or compound.
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Substance
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Usual oxidation state
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Exceptions
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Group 1 metals
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+1
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Group 2 metals
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+2
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Halogens
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-1
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(when Cl is combines with O or F)
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Oxygen
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-2
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Peroxides
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Hydrogen
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+1
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Metal hydrides
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A redox titration determines the concentration of an unknown solution by titrating it against a standard solution. A colour change associated with the redox reaction shows when the equivalence point has been reached.
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The activity series ranks metals according to the ease with which they undergo oxidation.
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Metals are ranked in the activity series according to how easily they lose electrons and are oxidized.
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The activity series can be used to predict whether or not a redox reaction that involves a metal can happen.
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The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree of pollution in a water sample.
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Biochemical oxygen demand (BOD) is the quantity of oxygen needed to break down organic matter in a sample of water over a 5-day period at a set temperature.
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A high BOD means that high levels of bacteria or algae are present in the water, resulting in a low level of dissolved oxygen.
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The Winkler method uses a series of three redox reactions to determine the concentration of dissolved oxygen in a water sample.
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Dissolved oxygen reacts with Mn2+ ions in basic conditions to form MnO2.
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MnO2(s) + O2(aq) +4OH-(aq) -> 2MnO2(s) + 2H2O(l)
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MnO2 reacts with I- in acidic conditions to form Mn2+
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MnO2(s) + 2I-(aq) +4H+(aq) -> Mn2+(aq) + I2(aq) +2H2O(l)
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I2 is titrated with S2O32- to give I- and S4O62-
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2S2O32–(aq) + I2(aq) ➝ S4O62–(aq) + 2I–(aq)
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The moles of dissolved oxygen in the original solution can be calculated from the moles of S2O32- used in the final reaction.
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n(S2O32–) = 4n(O2)
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