Chemistry • Year 11 • Module 3 • Lesson 5
Acid-Base & Acid-Carbonate Reactions
Apply your knowledge of neutralisation and acid-carbonate reactions to real data, antacid scenarios, and sequencing tasks. Band 4–5.
1. Interpret a gas-collection experiment
A student adds dilute hydrochloric acid (HCl) in 2 mL increments to 0.5 g of solid calcium carbonate (CaCO3) and collects the CO2 gas produced over a water trough. Volume of CO2 (mL) is recorded at each step. 8 marks
Figure 1.1. Volume of CO2 collected vs. HCl added (2 mL increments, 0.5 g CaCO3, excess acid). Adapted from student lab data.
1.1 Describe the trend in CO2 volume as HCl is added from 0 to 14 mL. Use figures from the graph. 2 marks
1.2 Write the balanced molecular equation (with state symbols) for the reaction taking place. 2 marks
1.3 Explain, using the graph data, why the CO2 volume stops increasing after approximately 8 mL of HCl is added. 2 marks
1.4 Predict the shape of the graph if the same experiment were repeated using Mg(OH)2 instead of CaCO3. State one similarity and one key difference. 2 marks
2. Interpret antacid comparison data
A pharmacist tests four antacid products by adding 0.5 g of each to 20 mL of 0.1 mol/L HCl and measuring time to reach pH 6 and whether visible gas is produced. 7 marks
| Antacid product | Active ingredient | Time to pH 6 (s) | Gas produced? |
|---|---|---|---|
| Mylanta liquid | Mg(OH)2 | 42 | No |
| Quick-Eze tablet | CaCO3 | 65 | Yes |
| Andrews Antacid | CaCO3 + MgCO3 | 58 | Yes |
| Baking soda solution | NaHCO3 | 28 | Yes |
2.1 Identify which product(s) undergo an acid-carbonate or acid-hydrogen carbonate reaction. 1 mark
2.2 Write the balanced molecular equation (with state symbols) for Mylanta reacting with HCl. 2 marks
2.3 A patient who has recently had abdominal surgery cannot comfortably expel gas. Using the data in the table, identify the most suitable antacid and explain your reasoning in terms of reaction type. 2 marks
2.4 Despite reaching pH 6 the fastest (28 s), baking soda is not recommended for a patient who cannot comfortably expel gas. Using the balanced equation from the lesson, explain why — and state the name and formula of the gas produced in this reaction. 2 marks
3. Sequence the balancing steps
The steps below describe how to write and balance the equation for nitric acid reacting with calcium hydroxide. They are shuffled. Write the correct order (1–6) in the “Order” column. 5 marks
| Order | Step |
|---|---|
| Check atom balance: confirm H, N, Ca, O are equal on both sides. | |
| Identify the cation (Ca2+ from Ca(OH)2) and anion (NO3− from HNO3) to name the salt: calcium nitrate, Ca(NO3)2. | |
| Identify the reaction type: acid + base → neutralisation. | |
| Add the coefficient 2 before HNO3 to supply 2 NO3− and balance H for 2 H2O. | |
| Write the unbalanced equation: HNO3(aq) + Ca(OH)2(aq) → Ca(NO3)2(aq) + H2O(l). | |
| Write the balanced equation with state symbols and add coefficient 2 before H2O: 2HNO3(aq) + Ca(OH)2(aq) → Ca(NO3)2(aq) + 2H2O(l). |
4. Case study — soil liming on NSW farms
Across the Tablelands of New South Wales, years of cropping and rainfall have driven soil pH to around 4.5 in many paddocks — too acidic for wheat and canola to thrive. Farmers apply finely ground agricultural lime (calcium carbonate, CaCO3) or slaked lime (calcium hydroxide, Ca(OH)2) to raise soil pH. 5 marks
4.1 Using correct chemical terminology, explain why adding CaCO3 to an acidic soil raises pH. Reference the products of the relevant reaction type in your answer. 3 marks
4.2 A colleague claims that Ca(OH)2 and CaCO3 will raise soil pH by identical chemical reactions. Is this claim correct? Distinguish the two reaction types and identify one practical difference a farmer might observe when applying each material. 2 marks
Q1.1 — Trend description
From 0 to 8 mL HCl, the volume of CO2 rises steeply and proportionally, increasing by approximately 30 mL per 2 mL of acid added, reaching ~120 mL. After 8 mL HCl, the volume plateaus at 120 mL and does not increase even as more acid is added (to 14 mL).
Q1.2 — Balanced equation
2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g) [1 for correct products; 1 for balanced with state symbols].
Q1.3 — Plateau explanation
At ~8 mL HCl, all of the CaCO3 has been completely consumed by the acid-carbonate reaction. With no carbonate remaining to react, no further CO2 can be produced. Additional HCl beyond this point has no CaCO3 left to react with. [1 mark: states CaCO3 fully consumed; 1 mark: links to no further CO2 production].
Q1.4 — Mg(OH)2 prediction
Similarity: the graph would still show a plateau after a certain volume of HCl is added (once Mg(OH)2 is fully consumed). Key difference: no CO2 gas would be collected at any point because Mg(OH)2 is a hydroxide that undergoes neutralisation (not acid-carbonate reaction) — the y-axis value would be zero throughout. [1 each].
Q2.1 — Acid-carbonate products
Quick-Eze (CaCO3), Andrews Antacid (CaCO3 + MgCO3), and Baking soda (NaHCO3) all produce gas. [Award 1 mark for all three named.]
Q2.2 — Mylanta equation
2HCl(aq) + Mg(OH)2(s) → MgCl2(aq) + 2H2O(l) [1 for correct products; 1 for balanced with state symbols].
Q2.3 — Post-surgical patient
Mylanta (Mg(OH)2) is most suitable. Its active ingredient is a hydroxide base; it undergoes neutralisation (acid + base → salt + water) and produces no CO2. All three carbonate/hydrogen carbonate antacids produce CO2 gas that must be expelled by belching — uncomfortable and potentially dangerous post-surgery. [1 product; 1 reaction-type reasoning].
Q2.4 — Baking soda and CO2 production
The balanced equation is HCl(aq) + NaHCO3(aq) → NaCl(aq) + H2O(l) + CO2(g). This is an acid–hydrogen carbonate reaction; one of the three SWC products is CO2 gas [1]. Carbon dioxide causes visible fizzing and must be expelled as belching — making baking soda unsuitable for a patient who cannot comfortably belch, just like Quick-Eze (CaCO3). The gas produced is carbon dioxide, CO2 [1].
Q3 — Correct sequence
Step 1: Identify the reaction type (neutralisation).
Step 2: Identify the cation and anion to name the salt (Ca(NO3)2).
Step 3: Write the unbalanced equation.
Step 4: Add coefficient 2 before HNO3.
Step 5: Write the fully balanced equation with state symbols.
Step 6: Check atom balance.
Award 1 mark per correct adjacent pair (5 marks for all 6 in correct order).
Q4.1 — Soil liming explanation (3 marks)
Agricultural lime (CaCO3) undergoes an acid-carbonate reaction with the H+ ions in acidic soil [1]. The equation is: CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g) [1]. H+ ions are consumed (used up) as products form, so the [H+] decreases and the pH rises [1].
Q4.2 — CaCO3 vs Ca(OH)2 (2 marks)
The claim is incorrect. CaCO3 reacts via an acid-carbonate reaction (products: Ca2+, H2O, CO2), while Ca(OH)2 reacts via neutralisation (products: Ca2+, H2O only; no gas) [1]. A practical difference: CaCO3 application produces visible fizzing/bubbling (CO2 gas) in the soil; Ca(OH)2 produces no fizzing [1].