Acids and Bases in Industry

Introduction

Sulfuric acid is so widely used that its production was once treated as a measure of a nation's industrial strength. Beyond the chemical works, acids and bases turn up in your kitchen cupboard, in the soap by your sink and in the water that comes out of your tap. This page surveys how industry uses the properties of acids and bases you have already studied: neutralisation, reactions with carbonates, and the ability to break down greasy compounds.

Core explanation

Sulfuric acid and the Contact process. Around 250 million tonnes of sulfuric acid are made worldwide each year. The Contact process has three main stages:

1. Sulfur is burned in air to make sulfur dioxide: S + O₂ → SO₂. 2. Sulfur dioxide reacts with more oxygen over a vanadium(V) oxide catalyst to make sulfur trioxide: 2SO₂ + O₂ ⇌ 2SO₃. This is a reversible reaction; conditions around 450 °C and slightly above atmospheric pressure give a good yield at a reasonable rate. 3. Sulfur trioxide is absorbed into concentrated sulfuric acid, and the product is then diluted with water in a controlled way. SO₃ is not added directly to water, because that reaction is so exothermic that it forms a corrosive acid mist.

Uses of sulfuric acid. Roughly half is used to make fertilisers, especially by treating phosphate rock to make soluble phosphates and by making ammonium sulfate. Other uses include car battery electrolyte, making detergents, paints and pigments, processing metal ores and removing rust from steel before coating (pickling).

Other industrial acids and bases. Nitric acid is used for fertilisers and explosives; hydrochloric acid for cleaning steel and making PVC; sodium hydroxide for making soap, paper and aluminium; calcium hydroxide and calcium oxide for cement, mortar and treating acidic waste.

Cleaning products. Different dirt needs different chemistry:

- Alkaline cleaners (oven cleaners, drain unblockers) often contain sodium hydroxide. Strong alkali breaks fats and grease into soap-like, water-soluble substances (saponification) and dissolves hair and food blockages. They are corrosive to skin and eyes. - Acidic cleaners (kettle descalers, toilet cleaners, bathroom limescale removers) contain acids such as citric, methanoic or sulfamic acid, or dilute hydrochloric acid. They dissolve limescale, which is calcium carbonate: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. - Mild alkalis such as sodium carbonate (washing soda) soften water and help detergents work.

Acidic and alkaline cleaners should never be mixed, and bleach must never be mixed with acidic cleaners because a toxic gas can be released.

Water treatment. Drinking water must have a pH of roughly 6.5 to 9.5 in UK and EU standards. Acidic water corrodes pipes and can dissolve metals such as lead and copper, so treatment works add calcium hydroxide or sodium hydroxide to raise the pH. Alkaline industrial effluent may be neutralised with acid, often carbon dioxide or sulfuric acid, before release. Aluminium sulfate is added so that fine particles clump together and settle out.

Step-by-step reasoning

Choosing a cleaning product for a stain:

1. Identify the dirt: grease and fats, or mineral scale. 2. Grease reacts with alkalis, so choose an alkaline cleaner. 3. Limescale is a carbonate, which reacts with acids, so choose an acidic cleaner. 4. Consider hazard: use the mildest product that works and follow safety labels.

Visual explanation

Picture a flow chart for the Contact process: a sulfur burner feeds SO₂ into a converter with catalyst beds, arrows show heat being removed between beds, and SO₃ flows into an absorption tower where concentrated acid trickles down. A branching arrow from the product tank points to icons for fertiliser, battery, detergent and paint.

Real-world analogy

Choosing an acidic or alkaline cleaner is like choosing the right key for a lock. Limescale is locked against water but opens with acid; grease is locked against water but opens with alkali. Using the wrong key does nothing except add hazard.

Real-world example

Many towns in Scotland and Wales have soft, slightly acidic water from upland reservoirs. In older houses with lead pipes, this water could dissolve lead. Water companies raise the pH with lime and add orthophosphate, which forms a protective layer inside pipes, greatly reducing lead levels in tap water.

Why?

Why does vinegar remove limescale from a kettle but not grease from an oven? Limescale is calcium carbonate, a base, which reacts with ethanoic acid to form a soluble salt and carbon dioxide. Grease is a fat, which reacts with alkalis, not with acids, so vinegar has little effect on it.

Common misconception

"A stronger cleaner is always a better cleaner." A strongly alkaline oven cleaner will not remove limescale, and a strong acid is wasted on grease. The chemistry must match the dirt, and weaker products are safer when they work.

Worked example

Question: A limescale remover contains hydrochloric acid. Write the ionic equation for its reaction with limescale and explain the fizzing.

Reasoning: Limescale is CaCO₃. Acid supplies H⁺. Carbonates react with acids to give a salt, water and carbon dioxide.

Answer: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g); the fizzing is carbon dioxide gas escaping.

Quick check

1. What is the largest single use of sulfuric acid? Answer: Making fertilisers.

Exam focus

Know the three stages of the Contact process and the catalyst (vanadium(V) oxide). Be able to name uses of sulfuric acid, sodium hydroxide and calcium hydroxide. Explain cleaning products by matching acid to carbonate scale and alkali to grease.

Advanced insight

Concentrated sulfuric acid is also a powerful dehydrating agent: it removes water from sugars, leaving carbon. This property makes it useful as a drying agent in industry but also makes it especially damaging to skin, which it dehydrates as well as attacking chemically. Heat released when it mixes with water is why acid is always added slowly to water, never the reverse.

Summary

Sulfuric acid, made by the Contact process (sulfur to SO₂ to SO₃ over V₂O₅, then absorption), is used mainly for fertilisers, plus batteries, detergents and metal treatment. Alkaline cleaners break down grease; acidic cleaners dissolve carbonate limescale. Water treatment adjusts pH with bases such as lime to protect pipes and health, and neutralises effluents before release.

Practice questions

1. Name the catalyst used in the Contact process and the reaction it speeds up. Answer: Vanadium(V) oxide, V₂O₅; it catalyses 2SO₂ + O₂ ⇌ 2SO₃. 2. Why is sodium hydroxide used in oven cleaners? Answer: It is a strong alkali that breaks down fats and grease into water-soluble products (saponification). 3. Why do water companies raise the pH of acidic water supplies? Answer: Acidic water corrodes pipes and dissolves metals such as lead and copper; raising the pH reduces corrosion and metal contamination. 4. Explain why acidic and alkaline cleaning products should not be mixed. Answer: They neutralise each other, releasing heat and making both ineffective, and some mixtures such as bleach with acid release toxic gases.