Acids Reacting with Metals

Introduction

Drop a piece of magnesium ribbon into dilute hydrochloric acid and the liquid fizzes, the metal shrinks and the test tube warms up. Drop a piece of copper into the same acid and nothing happens at all. Acids react with many metals, but not all, and the difference tells us a great deal about how reactive each metal is. This page explains what is produced and why the reaction varies so much from metal to metal.

Core explanation

The general equation. When a metal reacts with a dilute acid:

metal + acid → salt + hydrogen

Unlike neutralisation with a base, no water is formed. The metal replaces the hydrogen in the acid, and the hydrogen is released as a gas.

Examples.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

In each case the salt takes its positive ion from the metal and its negative ion from the acid: magnesium chloride, zinc sulfate, iron(II) chloride.

What is happening to the particles? Metal atoms lose electrons to become positive ions, and hydrogen ions from the acid gain those electrons to become hydrogen molecules:

Mg(s) → Mg²⁺(aq) + 2e⁻ 2H⁺(aq) + 2e⁻ → H₂(g)

Overall: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g). The chloride ions are spectators. Because electrons are transferred, this is a redox reaction: the metal is oxidised and the hydrogen ions are reduced.

Reactivity decides the vigour. The more reactive the metal, the more readily it gives up electrons, and the faster it fizzes:

Metal Reaction with dilute hydrochloric acid --- --- Potassium, sodium, lithium Violent and dangerous — never carried out in school Calcium Very vigorous Magnesium Rapid fizzing, test tube becomes warm Zinc, iron Steady to slow bubbling Lead Very slow Copper, silver, gold No reaction

Metals below hydrogen in the reactivity series, such as copper, cannot push hydrogen out of an acid, so they do not react.

The test for hydrogen. Hold a lighted splint at the mouth of a test tube in which hydrogen has collected. Hydrogen burns rapidly with oxygen from the air, producing a squeaky pop . This is the standard test for hydrogen.

Nitric acid is an exception. Nitric acid often reacts with metals without releasing hydrogen, forming oxides of nitrogen instead, because it acts as an oxidising agent. For this reason hydrochloric and sulfuric acids are used when metal–acid reactions are studied.

Step-by-step reasoning

To predict and write a metal–acid reaction:

1. Check the reactivity series: is the metal above hydrogen? If not, there is no reaction. 2. Name the salt: metal name + acid's salt name (chloride, sulfate). 3. Write the salt formula from the ion charges. 4. Write metal + acid → salt + H₂, then balance. 5. Add state symbols: metal (s), acid (aq), salt (aq), hydrogen (g).

Visual explanation

Picture a magnesium ribbon at the particle level. Along its surface, magnesium atoms hand two electrons each to pairs of H⁺ ions in the solution. The H⁺ ions pair up as H₂ molecules and rise as bubbles, while the Mg²⁺ ions drift off into the solution. The ribbon gets thinner until it vanishes.

Real-world analogy

Imagine a game of musical chairs where the chairs are partners for the acid's negative ions. A strong player (a reactive metal) pushes the hydrogen out of its seat easily and quickly. A weak player (copper) cannot shift the hydrogen at all, so nothing changes.

Real-world example

Food cans made of steel are coated on the inside because acidic foods such as tomatoes would slowly react with the iron, releasing hydrogen, dissolving metal into the food and eventually swelling the can. Coatings of lacquer or tin act as a barrier.

Why?

Why does a reactive metal fizz faster? A reactive metal has outer electrons that it loses easily. The more easily electrons are transferred to H⁺ ions, the faster hydrogen molecules form and the more vigorous the effervescence.

Common misconception

"Metals react with acids to form a salt and water." That pattern is for bases. A metal has no oxygen or hydroxide to combine with the hydrogen ions, so the hydrogen is released as a gas instead of forming water.

Worked example

Question: Write a balanced equation with state symbols for zinc reacting with hydrochloric acid and name the salt.

Reasoning: Zinc forms Zn²⁺, chloride is Cl⁻, so the salt is ZnCl₂. Two Cl atoms on the right need 2HCl, which provides 2 H atoms to form H₂.

Answer: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g); zinc chloride.

Quick check

1. What is seen when a lighted splint is held in hydrogen gas? Answer: The hydrogen burns with a squeaky pop.

Exam focus

Learn "metal + acid → salt + hydrogen" and the pop test. Be ready to explain why copper does not react, using the reactivity series. For higher marks, write the ionic equation and identify the reaction as redox, stating which species is oxidised and which is reduced.

Advanced insight

The rate also depends on surface area, temperature and acid concentration: powdered zinc reacts faster than a lump, and a warmer, more concentrated acid gives faster fizzing. Measuring the volume of hydrogen collected over time is a classic way of studying reaction rates.

Summary

Metals above hydrogen in the reactivity series react with dilute hydrochloric or sulfuric acid to form a salt and hydrogen. The more reactive the metal, the more vigorous the reaction; copper, silver and gold do not react. The reaction is redox: the metal loses electrons and H⁺ gains them to form H₂. Hydrogen is identified by the squeaky pop with a lighted splint.

Practice questions

1. Complete the word equation: magnesium + sulfuric acid → ? Answer: Magnesium sulfate + hydrogen. 2. Write the balanced symbol equation for iron reacting with hydrochloric acid to form iron(II) chloride. Answer: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g). 3. Explain why copper does not react with dilute hydrochloric acid. Answer: Copper is below hydrogen in the reactivity series, so it cannot give electrons to H⁺ ions and displace hydrogen from the acid. 4. Write the ionic equation for magnesium reacting with an acid and state which species is oxidised. Answer: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g); magnesium is oxidised because it loses electrons.