Acids Reacting with Hydrogencarbonates

Introduction

Drop an effervescent vitamin tablet into water and it fizzes furiously. Mix baking soda with vinegar and foam spills over the jar. Add baking powder to a cake mixture and the cake rises in the oven. All three depend on the same reaction: an acid meeting a hydrogencarbonate. Hydrogencarbonates behave very like carbonates, but with an important difference in the numbers — and they are gentle enough to eat.

Core explanation

What is a hydrogencarbonate? The hydrogencarbonate ion is HCO₃⁻. It is a carbonate ion that has already picked up one hydrogen ion. The best-known compound is sodium hydrogencarbonate , NaHCO₃, sold as baking soda or bicarbonate of soda. Potassium hydrogencarbonate, KHCO₃, is also used.

The general equation.

hydrogencarbonate + acid → salt + water + carbon dioxide

The products are the same as for a carbonate.

Examples.

NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

2KHCO₃(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(l) + 2CO₂(g)

NaHCO₃(s) + CH₃COOH(aq) → CH₃COONa(aq) + H₂O(l) + CO₂(g)

The last is the baking soda and vinegar reaction, producing sodium ethanoate.

The ionic equation. Because HCO₃⁻ already carries one hydrogen, it needs only one more H⁺:

HCO₃⁻(aq) + H⁺(aq) → H₂O(l) + CO₂(g)

Compare the carbonate ion, which needs two: CO₃²⁻ + 2H⁺ → H₂O + CO₂. So, mole for mole, a hydrogencarbonate neutralises half as much acid as a carbonate but still releases one CO₂ molecule for each HCO₃⁻ ion.

A mild base. Sodium hydrogencarbonate solution is only weakly alkaline, with a pH of about 8.3. Sodium carbonate solution is noticeably more alkaline, at about pH 11. This mildness is why baking soda is safe to eat in small amounts and is used in food and medicine.

Everyday uses.

- Baking powder contains sodium hydrogencarbonate mixed with a dry solid acid. When water is added and the mixture is heated, the acid and hydrogencarbonate react; bubbles of CO₂ are trapped in the batter and make cakes rise. - Effervescent tablets contain a hydrogencarbonate and citric acid. Kept dry, they do not react, because the ions cannot move. In water they dissolve, the ions meet and the tablet fizzes. - Antacids often contain sodium hydrogencarbonate to neutralise excess hydrochloric acid in the stomach. The carbon dioxide released is one reason such remedies can cause burping.

Heating alone also releases CO₂. Sodium hydrogencarbonate breaks down on heating, even without acid: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g). This thermal decomposition also helps some baked goods rise.

Step-by-step reasoning

To write a hydrogencarbonate–acid equation:

1. Identify the metal ion (Na⁺ or K⁺) and the acid's anion. 2. Write the salt formula from the charges. 3. Write hydrogencarbonate + acid → salt + water + carbon dioxide. 4. Balance using the rule one HCO₃⁻ + one H⁺ → one H₂O + one CO₂. 5. Add state symbols.

Visual explanation

Picture a dry effervescent tablet as a packed crowd of HCO₃⁻ ions and citric acid molecules frozen in place. When it drops into water, the crowd breaks up and each HCO₃⁻ meets an H⁺. They join, split into a water molecule and a CO₂ molecule, and the CO₂ molecules gather into bubbles that stream to the surface.

Real-world analogy

Baking powder is like a flat-pack kit with the pieces sealed in separate bags. Nothing can be built while the bags are dry and closed. Adding water opens the bags, so the acid and hydrogencarbonate pieces finally meet and assemble into bubbles of gas.

Real-world example

Some fire extinguishers used to work by mixing sodium hydrogencarbonate solution with an acid, producing a jet of liquid driven by carbon dioxide. Today baking soda is still used to deodorise refrigerators and, in bakeries, to make soda bread rise without yeast, using buttermilk as the acid.

Why?

Why does a dry effervescent tablet not fizz in its tube? The acid and hydrogencarbonate are both solids, and ions in a solid cannot move to meet each other. Water dissolves them, freeing the ions so that H⁺ and HCO₃⁻ can collide and react.

Common misconception

"Sodium hydrogencarbonate is an acid because its name contains hydrogen." Despite the hydrogen in its formula, HCO₃⁻ acts as a base towards acids, accepting H⁺ to form water and carbon dioxide. Its solution is slightly alkaline, around pH 8.

Worked example

Question: How many moles of hydrochloric acid are neutralised by 2 moles of NaHCO₃, and how many moles of CO₂ are produced?

Reasoning: The equation is NaHCO₃ + HCl → NaCl + H₂O + CO₂, a 1 : 1 : 1 ratio for NaHCO₃, HCl and CO₂.

Answer: 2 moles of HCl are neutralised and 2 moles of CO₂ are produced.

Quick check

1. Give the formula of sodium hydrogencarbonate, the main ingredient of baking soda. Answer: NaHCO₃ — one sodium ion and one hydrogencarbonate ion, HCO₃⁻.

Exam focus

Remember that hydrogencarbonates give the same three products as carbonates. Balance carefully: one HCO₃⁻ reacts with one H⁺, not two. Examiners like everyday contexts — baking powder, antacids, effervescent tablets — so be ready to explain each with an equation and the role of CO₂.

Advanced insight

The hydrogencarbonate ion can act as an acid or a base: it can accept H⁺ to form carbonic acid, or give up H⁺ to form carbonate. This dual role makes the carbonic acid–hydrogencarbonate system the main buffer that keeps human blood at a pH close to 7.4.

Summary

Hydrogencarbonates, containing HCO₃⁻, react with acids to form a salt, water and carbon dioxide. Each HCO₃⁻ needs only one H⁺, unlike carbonate's two. Sodium hydrogencarbonate is a mild base used in baking powder, effervescent tablets and antacids, where the carbon dioxide produced causes rising or fizzing.

Practice questions

1. Complete the word equation: sodium hydrogencarbonate + sulfuric acid → ? Answer: Sodium sulfate + water + carbon dioxide. 2. Write the balanced equation for sodium hydrogencarbonate reacting with sulfuric acid. Answer: 2NaHCO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g). 3. Write the ionic equation for a hydrogencarbonate ion reacting with an acid. Answer: HCO₃⁻(aq) + H⁺(aq) → H₂O(l) + CO₂(g). 4. Explain how baking powder makes a cake rise. Answer: When wet and heated, the acid and sodium hydrogencarbonate in baking powder react to release carbon dioxide; the gas bubbles are trapped in the mixture and expand, making the cake rise.