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3.2.1 redox reactions

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chem whatarethemechanismsofchemicalchange

Reactivity 3.2.1 - oxidation and reduction can be described in terms of electron transfer, change in oxidation state, oxygen gain/loss or hydrogen loss/gain

the term oxidation has been used for a long time to describe reactions where oxygen is involved

the term oxidation has been expanded to contain any reaction which involves the loss of electrons, as species generally lose electrons when reacting with oxygen

since the lost electrons lost cannot disappear, they are taken away by another species which is reduced

  • oxidation is the loss of electrons
  • reduction is the gain of electrons

oxidation state is the charge an atom would have if the compound were composed of ions. considering:

both elements have an oxidation state of 0 as the bonds in and are non-polar. the product is polar with possessing a partial charge and the possessing a partial negative charge

given the partial charges of the molecule, is assigned an oxidation state of and an oxidation state of

is oxidised as its oxidation state has increased
is reduced as its oxidation state has decreased

NOTE

oxidation and reduction occur during chemical change whenever there is a shift in electron density from one atom to another

it is possible for a species to be both oxidised and reduced, for example:

limitations of the oxidation concept
  • oxidation states have little structural or physical significance
  • the concept doesn’t transfer very well to covalently bonded atoms
  • in molecules with more than one atom of an element, the oxidation state is average value, can be fractional
oxidising and reducing agents
  • the oxidising agent, oxidises other species, accepts electrons
  • the reducing agent, reduces other species, supplies electrons

common examples of oxidising agents:

challenge questions
  1. assign oxidation states to the overall reactants and products of photosynthesis to show that it is a redox reaction.

the oxidation state of carbon changes from to , indicating that it has been reduced

the oxidation state state of oxygen changes from to and , indicating that it has been oxidised overall

  1. hydrogen peroxide, , can act as an oxidising agent and as a reducing agent. with reference to the oxidation states of its atoms, suggest why this is so and which behaviour is more likely.

the oxidation state of hydrogen is
the oxidation state of oxygen is

reduction of hydrogen peroxide is more likely as oxygen can be oxidised to or be reduced to . as oxygen is a very electronegative element, reduction is more likely, so it is more likely to act as an oxidising agent.