Activity . Heat a china dish containing about g copper powder (Fig. . ).
What do you observe? Figure . Figure . Figure .
Figure . Figure . Oxidation of copper to copper oxide The surface of copper powder becomes coated with black copper(II) oxide. Why has this black substance formed?
This is because oxygen is added to copper and copper oxide is formed. 2Cu + O Heat 2CuO ( . ) If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained. CuO +H Cu+H O Heat ( .
) If a substance gains oxygen during a reaction, it is said to be oxidised. If a substance loses oxygen during a reaction, it is said to be reduced. During this reaction ( . ), the copper(II) oxide is losing oxygen and is being reduced.
The hydrogen is gaining oxygen and is being oxidised. In other words, one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions. ( .
) Some other examples of redox reactions are: ZnO + C → Zn CO ( . ) MnO HCl MnCl H O Cl ( . ) Recall Activity . Recall Activity .
Recall Activity . Recall Activity . Recall Activity . , where you have mixed the solutions of lead(II) nitrate and potassium iodide.
What was the colour of the precipitate formed? Can you name the compound precipitated? Write the balanced chemical equation for this reaction. (iii) Is this also a double displacement reaction?
What causes this? The white precipitate of BaSO is formed by the reaction of – SO and Ba + . The other product formed is sodium chloride which remains in the solution. Such reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
In reaction ( . ) carbon is oxidised to CO and ZnO is reduced to Zn. In reaction ( . ) HCl is oxidised to Cl whereas MnO is reduced to MnCl .
From the above examples we can say that if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised. If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.