Question 7.1 [2 marks]
Define a Lowry-Brønsted base.
A Lowry-Brønsted base is defined as a substance that can accept a proton (H⁺ ion) in a chemical reaction. In other words, it is capable of increasing the concentration of hydroxide ions (OH⁻) in a solution by accepting protons from acids.
Question 7.2 [6 marks]
Rewrite the chemical equation and indicate with the help of lines and arrows the conjugated acid-base pairs. Clearly indicate the conjugated acid and base pairs for the forward and reverse reactions. You may use colour to help distinguish the labels.
The original reaction is:
HSO₄⁻ + HCO₃⁻ → SO₄²⁻ + H₂CO₃
In this reaction, we can identify the conjugate acid-base pairs:
- Forward Reaction:
- HSO₄⁻ (Bronsted Acid) ⇌ SO₄²⁻ (Conjugate Base)
- HCO₃⁻ (Bronsted Base) ⇌ H₂CO₃ (Conjugate Acid)
This can be represented as follows:
Forward Reaction
HSO₄⁻ + HCO₃⁻ ↔ SO₄²⁻ + H₂CO₃
(acid) (base) (base) (acid)
- Reverse Reaction:
- SO₄²⁻ (Bronsted Base) ⇌ HSO₄⁻ (Conjugate Acid)
- H₂CO₃ (Bronsted Acid) ⇌ HCO₃⁻ (Conjugate Base)
This can be represented as follows:
Reverse Reaction
SO₄²⁻ + H₂CO₃ ↔ HSO₄⁻ + HCO₃⁻
(base) (acid) (acid) (base)
Question 7.3 [2 marks]
Identify the substance(s) that are considered ampholytes from the above reaction.
In the given reaction, HCO₃⁻ (bicarbonate) is considered an ampholyte, as it can act as both an acid (donating a proton to form CO₃²⁻) and as a base (accepting a proton to form H₂CO₃).
Question 7.4 [3 marks]
Write down the balanced chemical equation for the reaction when Magnesium Oxide (MgO) reacts with Nitric Acid.
The balanced chemical equation for the reaction of magnesium oxide with nitric acid is:
\[ \text{MgO (s) + 2 HNO}_3 \text{ (aq)} \rightarrow \text{Mg(NO}_3\text{)}_2 \text{ (aq) + H}_2\text{O (l)} \]
Question 7.5
50 g of IMPURE calcium carbonate reacts with 200 cm³ of a dilute sulphuric acid with a concentration of 1.5 mol∙dm⁻³. All the calcium carbonate and sulphuric acid react completely leaving the impurities unreacted at the end. (a) Calculate the number of moles of sulphuric acid used and (b) the amount of calcium carbonate that reacted.
(a) To calculate the number of moles of sulfuric acid:
- Concentration (C): 1.5 mol/dm³
- Volume (V): 200 cm³ = 0.200 dm³
\[ \text{Number of moles} = C \times V = 1.5 \text{ mol/dm}^3 \times 0.200 \text{ dm}^3 = 0.3 \text{ moles} \]
(b) The balanced reaction between calcium carbonate (CaCO₃) and sulfuric acid (H₂SO₄) is:
\[ \text{CaCO}_3 (s) + \text{H}_2\text{SO}_4 (aq) → \text{CaSO}_4 (aq) + \text{H}_2\text{O} (l) + \text{CO}_2 (g) \]
From the balanced equation, 1 mole of CaCO₃ reacts with 1 mole of H₂SO₄.
Given that 0.3 moles of H₂SO₄ was used, 0.3 moles of CaCO₃ also reacted.
Next, we will calculate the mass of calcium carbonate that reacted using its molar mass (CaCO₃ = 100 g/mol):
\[ \text{Mass of CaCO}_3 = \text{Number of moles} \times \text{Molar mass} = 0.3 \text{ moles} \times 100 \text{ g/mol} = 30 \text{ g} \]
So, 30 g of calcium carbonate reacted.
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