Which of the following is a precipitation reaction?
I. 2 Mg (s) + O2 (g) → 2 MgO (s)
II. SO3 (g) + 2 H2O (l) → H3O+ (aq) + HSO4- (aq)
III. Pb2+ (aq) + CrO42- (aq) → PbCrO4 (s)
IV. 2 H2O (g) → 2 H2 (g) + O2 (g)
V. Ag+ (aq) + 2 NH3 (aq) → [Ag(NH3)2]+ (aq)
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From the given options, the precipitation reaction is represented by the equation III. Pb2+(aq) + CrO42-(aq) → PbCrO4(s).
In a precipitation reaction, two aqueous (dissolved) ionic compounds react to produce a solid precipitate. In this case, the soluble lead (Pb2+) and chromate (CrO42-) ions combine to form the insoluble lead chromate (PbCrO4) solid.
The other options do not represent precipitation reactions:
I. 2 Mg(s) + O2(g) → 2 MgO(s) is a synthesis reaction where solid magnesium reacts with oxygen gas to form solid magnesium oxide.
II. SO3(g) + 2 H2O(l) → H3O+(aq) + HSO4-(aq) is an acid-base reaction where sulfur trioxide reacts with water to produce hydronium ions (H3O+) and bisulfate ions (HSO4-).
IV. 2 H2O(g) → 2 H2(g) + O2(g) represents the decomposition of water vapor into hydrogen gas and oxygen gas.
V. Ag+(aq) + 2 NH3(aq) → [Ag(NH3)2]+(aq) is a complex ion formation reaction where silver ion (Ag+) reacts with ammonia (NH3) to form a coordination complex ion [Ag(NH3)2]+.
To determine which of the given reactions is a precipitation reaction, we need to understand what a precipitation reaction is.
A precipitation reaction is a type of chemical reaction where two aqueous solutions react to form an insoluble solid compound called a precipitate. This solid compound is formed when the cations (positively charged ions) and anions (negatively charged ions) in the reactants combine and exceed their solubility limit, causing them to come out of solution.
Now let's analyze the given reactions one by one:
I. 2 Mg (s) + O2 (g) → 2 MgO (s)
This is not a precipitation reaction because it involves the combination of a solid (Mg) and a gas (O2) to form another solid (MgO), but no aqueous solutions are involved.
II. SO3 (g) + 2 H2O (l) → H3O+ (aq) + HSO4- (aq)
This is not a precipitation reaction either. It is an acid-base reaction where SO3 reacts with water to produce hydronium ions (H3O+) and bisulfate ions (HSO4-).
III. Pb2+ (aq) + CrO42- (aq) → PbCrO4 (s)
This reaction is a precipitation reaction. It involves the reaction between lead ions (Pb2+) in aqueous solution and chromate ions (CrO42-) in aqueous solution to form the insoluble solid compound lead chromate (PbCrO4).
IV. 2 H2O (g) → 2 H2 (g) + O2 (g)
This is not a precipitation reaction but a decomposition reaction. It represents the breakdown of water vapor into hydrogen gas (H2) and oxygen gas (O2).
V. Ag+ (aq) + 2 NH3 (aq) → [Ag(NH3)2]+ (aq)
This is not a precipitation reaction either. It is a complexation reaction where silver ions (Ag+) react with ammonia molecules (NH3) to form the complex ion [Ag(NH3)2]+.
Therefore, the only precipitation reaction among the given options is III. Pb2+ (aq) + CrO42- (aq) → PbCrO4 (s).