Oxides & Hydroxides (Edexcel International A Level Chemistry)

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Reactions with Acid

Overall equations 

  • We have seen that Group 1 and Group 2 metals can react with oxygen to form oxides 

metal + oxygen → metal oxide 

  • Oxides can react with water to form hydroxides 

oxide + water → hydroxide

  • If calcium oxide is added to water, you should know that calcium hydroxide is formed
    • Calcium hydroxide solution is also called limewater

  • When metal oxides react with dilute hydrochloric acid and dilute sulfuric acid, chloride and sulfate salts are formed

metal oxide + dilute hydrochloric acid → metal chloride + water

metal oxide + dilute sulfuric acid → metal sulfate + water

  • Similar reactions occur with hydroxides

metal hydroxide + dilute hydrochloric acid → metal chloride + water

metal hydroxide + dilute sulfuric acid → metal sulfate + water

Reactions of Group 1 oxides

With water

  • The Group 1 metal oxides will react with water to give a colourless alkaline solution 
    • For example

Na2O (s) + H2O (l) → 2NaOH (aq) 

  • The solution is alkaline as hydroxide ions are released which also occurs for the reaction of Group 2 oxides with water

O2- (s) +  H2O (l) → 2OH- (aq) 

Reactions of Group 1 hydroxides with dilute acid

  • The Group 1 metal hydroxide is acting as an alkali when it is added to dilute acid  
  • When an alkali reacts with an acid, a neutralisation reaction occurs forming salt and water
    • For example

NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l) 

NaOH (aq) + H2SO4 (aq) → Na2SO4 (aq) + H2O (l) 

Reactions of Group 2 oxides

With water

  • All Group 2 oxides are basic, except for BeO which is amphoteric (it can act both as an acid and base)
  • Group 2 oxides react water to form alkaline solutions which get more alkaline going down the group

Group 2 Oxides reacting with Water

Group 2 oxide Reaction with water Observations
MgO MgO (s) + H2O (l) → Mg(OH)2 (s)  MgO is only slightly soluble in water, therefore a weakly alkaline solution (pH 10.0) is formed
CaO CaO (s) + H2O (l) → Ca(OH)2 (s)  A vigorous reaction which releases a lot of energy, causing some of the water to boil off as the solid lump seems to expand and open (pH 11.0)
SrO SrO (s) + H2O (l) → Sr(OH)2 (aq)   
BaO BaO (s) + H2O (l) → Ba(OH)2 (aq)   

With acid

  • Group 2 sulfates also form when a Group 2 oxide is reacted with sulfuric acid
  • The insoluble sulfates form at the surface of the oxide, which means that the solid oxide beneath it can’t react with the acid
  • This can be prevented to an extent by using the oxide in powder form and stirring, in which case neutralisation can take place

Reactions of Group 2 hydroxides with dilute acid 

  • The Group 2 metal hydroxides form colourless solutions of metal salts when they react with a dilute acid
  • The sulfates decrease in solubility going down the group (barium sulfate is an insoluble white precipitate)

Group 2 Hydroxide Reactions with Dilute Acids

Group 2 hydroxide Reaction with dilute HCl Reaction with dilute H2SO4
Mg(OH)2 Mg(OH)2 (s) + 2HCl (aq) → MgCl2 (aq) + 2H2O (l) Mg(OH)2 (s) + H2SO4 (aq) → MgSO4 (aq) + 2H2O (l)
Ca(OH)2 Ca(OH)2 (s) + 2HCl (aq) → CaCl2 (aq) + 2H2O (l) Ca(OH)2 (s) + H2SO4 (aq) → CaSO4 (aq) + 2H2O (l)
Sr(OH)2 Sr(OH)2 (s) + 2HCl (aq) → SrCl2 (aq) + 2H2O (l) Sr(OH)2 (s) + H2SO4 (aq) → SrSO4 (s) + 2H2O (l)
Ba(OH)2 Ba(OH)2 (s) + 2HCl (aq) → BaCl2 (aq) + 2H2O (l) Ba(OH)2 (s) + H2SO4 (aq) → BaSO4 (s) + 2H2O (l)

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.