Reactions of Group 2 (AQA A Level Chemistry): Revision Note
Exam code: 7405
Reactions of Group 2
The group 2 elements react with oxygen, water, and dilute acids
Group 2 Reactions - Observations
| Reaction with oxygen | Reaction with water | Reaction with dilute HCl | Reaction with dilute H2SO4 |
|---|---|---|---|---|
Mg | Burns easily | Mg reacts very slowly with cold water and vigorously with steam | Reacts vigorously | Reacts vigorously |
Ca | Difficult to ignite | Reacts moderately, forms a hydroxide | Reacts vigorously | Reaction slowed by the formation of a sparingly soluble sulfate layer on the metal, stopping hydrogen bubbles from rising |
Sr | Difficult to ignite | Reacts rapidly, forms a hydroxide | Reacts vigorously | The reaction is quickly stopped by the formation of an insoluble sulfate layer on the metal |
Ba | Difficult to ignite | Reacts rapidly, forms a hydroxide | Reacts vigorously | The reaction is quickly stopped by the formation of an insoluble sulfate layer on the metal |
Examiner Tips and Tricks
Mg + steam: the required observations are a bright white flame/light and a white (or grey) ash/solid — and mark schemes explicitly do NOT accept "effervescence"/"fizzing" or "precipitate" for this reaction. This is the single most-flagged pitfall on the topic.
Reactivity increases down the group
Down the group, the atoms have more electron shells, so the outer electrons are further from the nucleus and are lost more easily
Reactions with water and oxygen
The reaction of group 2 metals with oxygen follows the following general equation:
2M (s) + O2 (g) → 2MO (s)
Where M is any metal in group 2
Remember that Sr and Ba also form a peroxide, MO2
The reaction of all metals with water follows the following general equation:
Except for Be, which does not react with water
M (s) + 2H2O (l) → M(OH)2 (s) + H2 (g)
Group 2 Metals reacting with Water and with Oxygen - Equations
| Reaction with oxygen | Reaction with water |
|---|---|---|
Mg | 2Mg (s) + O2 (g) → 2MgO (s) | Mg (s) + H2O (g) → MgO (s) + H2 (g) |
Ca | 2 Ca (s) + O2 (g) → 2CaO (s) | Ca (s) + 2H2O (l) → Ca(OH)2 (s) + H2 (g) |
Sr | 2Sr (s) + O2 (g) → 2SrO (s) Sr (s) + O2 (g) → SrO2 (s) | Sr (s) + 2H2O (l) → Sr(OH)2 (aq) + H2 (g) |
Ba | 2Ba (s) + O2 (g) → 2BaO (s) Ba (s) + O2 (g) → BaO2 (s) | Ba (s) + 2H2O (l) → Ba(OH)2 (aq) + H2 (g) |
Magnesium reacts extremely slowly with cold water:
Mg (s) + 2H2O (l) → Mg(OH)2 (s) + H2 (g)
The solution formed is weakly alkaline (pH 9-10) as magnesium hydroxide is only slightly soluble
However, when magnesium is heated in steam, it reacts vigorously with steam to make magnesium oxide and hydrogen gas:
Mg (s) + H2O (g) → MgO (s) + H2 (g)
Reactions of Group 2 metals with acid
The Group 2 metals will react with dilute acids to form colourless solutions of metal salts
For example, they will form colourless solutions of metal chlorides if reacted with hydrochloric acid
When metals react with an acid, the by-product of this reaction is hydrogen gas
Group 2 Reactions with Dilute Acids - Equations
| Reaction with dilute HCl | Reaction with dilute H2SO4 |
|---|---|---|
Mg | Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) | Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (g) |
Ca | Ca (s) + 2HCl (aq) → CaCl2 (aq) + H2 (g) | Ca (s) + H2SO4 (aq) → CaSO4 (s) + H2 (g) |
Sr | Sr (s) + 2HCl (aq) → SrCl2 (aq) + H2 (g) | Sr (s) + H2SO4 (aq) → SrSO4 (s) + H2 (g) |
Ba | Ba (s) + 2HCl (aq) → BaCl2 (aq) + H2 (g) | Ba (s) + H2SO4 (aq) → BaSO4 (s) + H2 (g) |
When some of the Group 2 metals react with sulfuric acid rather than hydrochloric acid, an insoluble sulfate forms
Going down the group, the Group 2 sulfates become less and less soluble
Calcium sulfate is sparingly soluble, but strontium sulfate and barium sulfate are insoluble
The reaction of the metals with dilute HCl follows the following general equation:
M (s) + 2HCl (aq) → MCl2 (aq) + H2 (g)
The reaction of the metals with dilute H2SO4 follows the following general equation:
M (s) + H2SO4 (aq) → MSO4 (aq) + H2 (g)
Remember that SrSO4 and BaSO4 are insoluble
Examiner Tips and Tricks
Learn the general equation for the reaction with water and for magnesium with steam. You could be asked about the reactions of the oxides and hydroxides with acids.
Group 2: Oxides, Hydroxides & Carbonates
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 a base)
Group 2 oxides react with 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) | Reacts readily and releases a lot of energy |
BaO | BaO (s) + H2O (l) → Ba(OH)2 (aq) | Reacts very readily and releases a lot of energy |
Remember that:
oxide + water → hydroxide
You should know that calcium hydroxide, when in solution, is also called limewater
Reactions of Group 2 oxides 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
Remember that:
metal oxide + dilute hydrochloric acid → metal chloride + water
metal oxide + dilute sulfuric acid → metal sulfate + water
Reactions of Group 2 hydroxides
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 (s) + 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) |
Remember that:
hydroxide + dilute hydrochloric acid → chloride + water
hydroxide + dilute sulfuric acid → sulfate + water
Reactions of Group 2 carbonates
All Group 2 carbonates (except for BeCO3) are insoluble in water
All Group 2 carbonates will form soluble chloride salts, water, and carbon dioxide gas when reacted with dilute hydrochloric acid
The carbonates of Ca, Sr, and Ba form as an insoluble sulfate layer on their solid carbonates, which stops any further reaction after the initial bubbling (effervescence) of carbon dioxide gas is seen
Group 2 carbonates reacting with dilute acids
Group 2 carbonate | Reaction with dilute HCl | Reaction with dilute H2SO4 |
|---|---|---|
MgCO3 | MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l) + CO2 (g) | MgCO3 (s) + H2SO4 (aq) → MgSO4 (aq) + H2O (l) + CO2 (g) |
CaCO3 | CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g) | CaCO3 (s) + H2SO4 (aq) → CaSO4 (s) + H2O (l) + CO2 (g) |
SrCO3 | SrCO3 (s) + 2HCl (aq) → SrCl2 (aq) + H2O (l) + CO2 (g) | SrCO3 (s) + H2SO4 (aq) → SrSO4 (s) + H2O (l) + CO2 (g) |
BaCO3 | BaCO3 (s) + 2HCl (aq) → BaCl2 (aq) + H2O (l) + CO2 (g) | BaCO3 (s) + H2SO4 (aq) → BaSO4 (s) + H2O (l) + CO2 (g) |
Remember that:
carbonate + dilute hydrochloric acid → chloride + water + carbon dioxide
carbonate + dilute sulfuric acid → sulfate + water + carbon dioxide
Related topics
Examiner Tips and Tricks
Calcium sulfate is classified as sparingly soluble (or slightly soluble). It sits between the soluble magnesium sulfate and the insoluble barium sulfate in the Group 2 trend (solubility of sulfates decreases down the group).
In chemical equations, particularly where it is formed as a product of a reaction (e.g., mixing calcium ions and sulfate ions, or reacting calcium carbonate with sulfuric acid), it is represented as a solid (s).
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