How Do Acids React with Metals?

Many metals react with dilute acids. During the reaction, the metal atoms become part of a salt, while hydrogen ions from the acid form hydrogen gas.

The general word equation is:

acid + metal → salt + hydrogen

For example:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

This reaction can often be observed because bubbles of hydrogen gas form around the metal.

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Recognizing an Acid-Metal Reaction

When a reactive metal is placed into a suitable dilute acid, several observations may provide evidence that a chemical reaction is occurring.

You may observe:

• Bubbles or fizzing as hydrogen gas is produced.
• The metal gradually getting smaller or disappearing.
• The temperature of the mixture increasing.
• A new solution containing a salt forming.

The speed and intensity of these changes depend on the metal, acid, concentration, temperature, and other conditions.

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The Products

The two main products of a typical acid-metal reaction are:

a salt + hydrogen gas

For example:

hydrochloric acid + zinc → zinc chloride + hydrogen

Reactants:

• Hydrochloric acid
• Zinc

Products:

• Zinc chloride
• Hydrogen

The metal does not simply disappear. Its atoms become part of the salt.

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Word Equations

A word equation identifies the substances involved in a chemical reaction.

General pattern:

acid + metal → salt + hydrogen

Example

hydrochloric acid + magnesium → magnesium chloride + hydrogen

Another Example

sulfuric acid + zinc → zinc sulfate + hydrogen

Learning the salt produced by different acids makes these equations easier to predict.

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Predicting the Salt

The acid determines the second part of the salt's name.

For the common classroom reactions with hydrochloric and dilute sulfuric acid, this provides a useful way of predicting the product.

For example:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

The metal gives us:

magnesium

Hydrochloric acid gives us:

chloride

Therefore, the salt is:

magnesium chloride

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Hydrochloric Acid and Metals

Hydrochloric acid produces chloride salts.

Magnesium

hydrochloric acid + magnesium → magnesium chloride + hydrogen

Zinc

hydrochloric acid + zinc → zinc chloride + hydrogen

Iron

hydrochloric acid + iron → iron chloride + hydrogen

At this level, the iron salt is commonly specified more precisely as iron(II) chloride.

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Sulfuric Acid and Metals

Dilute sulfuric acid generally produces sulfate salts when it reacts with suitable metals.

Magnesium

sulfuric acid + magnesium → magnesium sulfate + hydrogen

Zinc

sulfuric acid + zinc → zinc sulfate + hydrogen

Iron

sulfuric acid + iron → iron(II) sulfate + hydrogen

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A Note About Nitric Acid

Nitric acid requires special care.

The simple rule:

acid + metal → salt + hydrogen

works well for many reactions involving dilute hydrochloric acid and dilute sulfuric acid.

Nitric acid behaves differently because it is an oxidizing acid. Its reactions with metals often produce nitrogen-containing gases rather than hydrogen.

For introductory acid-metal investigations, hydrochloric acid and dilute sulfuric acid are therefore usually better examples.

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Worked Example: Predicting Products

Complete:

hydrochloric acid + calcium → ?

Step 1: Identify the metal

The metal is:

calcium

Step 2: Identify the salt ending

Hydrochloric acid produces:

chloride

Step 3: Name the salt

The salt is:

calcium chloride

Step 4: Add hydrogen

Therefore:

hydrochloric acid + calcium → calcium chloride + hydrogen

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Worked Example: Sulfuric Acid

Complete:

sulfuric acid + magnesium → ?

Sulfuric acid produces:

sulfate

The metal is:

magnesium

Therefore:

sulfuric acid + magnesium → magnesium sulfate + hydrogen

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Chemical Equations

Word equations can also be represented using chemical formulas.

For example:

Mg + 2HCl → MgCl₂ + H₂

This represents:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Notice that hydrogen gas exists as:

H₂

rather than individual H atoms.

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Another Balanced Equation

Zinc reacting with hydrochloric acid:

Zn + 2HCl → ZnCl₂ + H₂

Zinc reacting with sulfuric acid:

Zn + H₂SO₄ → ZnSO₄ + H₂

Balanced equations show that the same number of each type of atom appears on both sides of the equation.

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What Happens to the Metal?

At the particle level, metal atoms lose electrons and become positive ions.

For example:

Mg → Mg²⁺ + 2e⁻

The magnesium ions then become part of the magnesium salt dissolved in the solution.

Meanwhile, hydrogen ions from the acid gain electrons and eventually form hydrogen gas.

A simplified representation is:

2H⁺ + 2e⁻ → H₂

The overall process involves the transfer of electrons.

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Testing for Hydrogen

Hydrogen gas can be identified using the lighted splint test.

A small sample of gas is collected and exposed to a lighted splint.

Hydrogen produces a characteristic:

"squeaky pop"

The sound occurs because hydrogen reacts rapidly with oxygen.

This test should only be carried out using appropriate small quantities and normal laboratory safety procedures.

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Comparing Different Metals

Different metals react with acids at different rates.

For example, under similar conditions:

magnesium generally reacts vigorously with dilute hydrochloric acid.

zinc reacts less vigorously.

iron usually reacts more slowly.

copper does not normally react with dilute hydrochloric acid.

These differences are related to the reactivity of the metals.

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The Reactivity Series

The reactivity series arranges metals according to how readily they undergo chemical reactions.

A simplified section is:

potassium

sodium

calcium

magnesium

aluminium

zinc

iron

hydrogen

copper

silver

gold

Metals higher in the series are generally more reactive.

Hydrogen is included as a useful reference point even though it is not a metal.

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Metals Above Hydrogen

Metals above hydrogen in the reactivity series can generally displace hydrogen from suitable dilute acids.

For example:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Magnesium is above hydrogen, so the reaction occurs readily.

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Metals Below Hydrogen

Metals below hydrogen generally do not displace hydrogen from dilute non-oxidizing acids such as hydrochloric acid.

For example:

copper + dilute hydrochloric acid → no reaction

Copper is below hydrogen in the reactivity series.

This means copper cannot normally displace H⁺ ions from dilute hydrochloric acid.

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Comparing Magnesium, Zinc, and Iron

Suppose equal-sized pieces of magnesium, zinc, and iron are placed into identical samples of dilute hydrochloric acid.

You might observe:

Magnesium

• Rapid bubbling.
• Metal disappears relatively quickly.
• Noticeable temperature increase.

Zinc

• Steady bubbling.
• Slower reaction than magnesium.

Iron

• Slower bubbling.
• Metal takes longer to react.

The observations provide evidence of differences in reactivity.

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Measuring Reaction Rate

Instead of simply describing a reaction as "fast" or "slow," scientists can collect quantitative data.

Possible measurements include:

• Volume of hydrogen produced in a certain time.
• Time required to produce a certain volume of hydrogen.
• Time required for the metal to disappear.
• Change in mass as hydrogen escapes.

For example:

The data could then be plotted on a graph.

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Comparing Metals Fairly

To compare the reactivity of metals, the investigation must be a fair test.

Only the type of metal should intentionally change.

Independent Variable

Type of metal

Dependent Variable

For example:

volume of hydrogen produced in 30 seconds

Control Variables

Keep constant:

• Acid type.
• Acid concentration.
• Acid volume.
• Temperature.
• Amount of metal.
• Metal surface area, as far as practical.
• Measuring method.

This allows differences in the results to be more confidently connected to the type of metal.

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Surface Area Matters

The surface area of a metal can affect how quickly it reacts.

A powdered metal has a much greater exposed surface area than one large piece of the same mass.

More exposed particles can collide with acid particles at the same time.

Therefore:

greater surface area → faster reaction

This means differently sized pieces of metal could make a reactivity comparison unfair.

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Acid Concentration Matters

A more concentrated acid contains more reacting particles within a given volume.

This can increase the frequency of successful collisions with the metal surface.

Therefore, increasing acid concentration can often increase the reaction rate.

When comparing metals, the acid concentration should therefore remain constant.

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Temperature Matters

Increasing temperature generally causes particles to move faster.

This can increase:

• Collision frequency.
• Collision energy.
• Number of successful collisions.

Therefore:

higher temperature → usually faster reaction

Temperature should be controlled when comparing the reactivity of different metals.

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Evidence of Chemical Change

An acid-metal reaction provides several possible signs of chemical change.

These can include:

Gas production

Bubbles form as hydrogen is produced.

Temperature change

The reaction may release thermal energy.

Metal disappearing

Metal atoms enter the solution as ions.

Formation of new substances

A salt and hydrogen gas are produced.

These observations distinguish a chemical reaction from a simple physical change.

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Reaction Rate and Reactivity Are Related but Different

Reaction rate describes:

how quickly a particular reaction occurs

Reactivity describes:

how readily a substance undergoes chemical reactions

A more reactive metal will often react more rapidly with the same acid under identical conditions.

However, reaction rate can also be affected by:

• Temperature.
• Concentration.
• Surface area.

Therefore, conditions must be controlled when using reaction rate to compare metal reactivity.

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Acid-Metal Reactions in Everyday Life

Acid-metal reactions are important outside the laboratory.

They can help explain:

• Corrosion in acidic environments.
• Damage to metal structures.
• Selection of materials for chemical containers.
• Industrial chemical processing.
• Production of hydrogen in some chemical processes.

Engineers must consider chemical reactivity when choosing metals for particular environments.

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Acid Rain and Metals

Rainwater is naturally slightly acidic, and environmental pollutants can sometimes increase its acidity.

Acidic conditions can contribute to the deterioration of some metals and other materials.

This is one reason materials used outdoors may need:

• Protective coatings.
• Paint.
• Corrosion-resistant alloys.
• Regular maintenance.

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Choosing Materials

Suppose an engineer needs a metal container for an acidic substance.

The engineer must consider whether the metal will react with the acid.

If the metal reacts readily:

• The container may corrode.
• The acid may become contaminated.
• Hydrogen or other gases might be produced.
• The container could eventually weaken.

Understanding chemical reactivity therefore helps engineers select suitable materials.

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Comparing Acid-Metal and Neutralization Reactions

These two types of reactions should not be confused.

Acid + Metal

acid + metal → salt + hydrogen

Example:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

Acid + Base

acid + base → salt + water

Example:

hydrochloric acid + sodium hydroxide → sodium chloride + water

The key difference is the second product:

metal reaction → hydrogen

neutralization → water

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Comparing Acid-Metal and Acid-Carbonate Reactions

These reactions also produce different gases or products.

Acid + Metal

acid + metal → salt + hydrogen

Acid + Carbonate

acid + carbonate → salt + water + carbon dioxide

Therefore:

metal → hydrogen gas

carbonate → carbon dioxide gas

Gas tests can help distinguish between them.

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Predicting Whether a Reaction Will Occur

For dilute hydrochloric acid or sulfuric acid, use the reactivity series.

Metal above hydrogen

Usually reacts and releases hydrogen.

Metal below hydrogen

Usually does not release hydrogen from these dilute acids.

For example:

zinc + hydrochloric acid → reaction

but:

copper + hydrochloric acid → no reaction

This makes the reactivity series a useful predictive tool.

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Worked Example: Comparing Metals

Three metals are placed separately into identical samples of dilute hydrochloric acid.

Results:

Assuming the test was controlled fairly, magnesium produced hydrogen most rapidly, followed by zinc and then iron.

The observations are consistent with:

magnesium being more reactive than zinc, and zinc being more reactive than iron.

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Common Mistakes

Saying Oxygen Is Produced

The gas normally produced when a suitable metal reacts with dilute hydrochloric or sulfuric acid is:

hydrogen

not oxygen.

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Forgetting the Salt

Remember:

acid + metal → salt + hydrogen

Both products should be included.

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Using the Wrong Salt Name

Remember:

hydrochloric acid → chloride

sulfuric acid → sulfate

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Assuming Every Metal Reacts with Dilute Acid

Some metals, such as copper, are below hydrogen in the reactivity series and do not normally react with dilute hydrochloric acid.

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Assuming All Metals React at the Same Rate

Different metals have different reactivities.

Magnesium, zinc, iron, and copper therefore behave differently when placed in the same dilute acid.

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Comparing Unequal Pieces of Metal

A large strip and a small piece do not provide a fair comparison.

Differences in surface area can change reaction rate.

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Confusing Hydrogen and Carbon Dioxide

Remember:

acid + metal → hydrogen

acid + carbonate → carbon dioxide

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Applying the Simple Rule to Every Acid

The rule:

acid + metal → salt + hydrogen

is particularly useful for common reactions involving dilute hydrochloric and sulfuric acids.

Oxidizing acids such as nitric acid can behave differently.

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Check Your Understanding

1. Complete:

acid + metal → ______ + ______

2. What gas is normally produced when magnesium reacts with dilute hydrochloric acid?

3. Name three observations that could indicate an acid-metal reaction is occurring.

4. Complete:

hydrochloric acid + magnesium → ______ + ______

5. Complete:

hydrochloric acid + zinc → ______ + ______

6. Complete:

sulfuric acid + magnesium → ______ + ______

7. Complete:

sulfuric acid + zinc → ______ + ______

8. What type of salts are produced by hydrochloric acid?

9. What type of salts are produced by sulfuric acid?

10. Describe the test for hydrogen gas.

11. What result indicates that hydrogen is present?

12. Why does a piece of magnesium become smaller as it reacts with acid?

13. What is the reactivity series?

14. Why is hydrogen included in the reactivity series?

15. Would magnesium normally react with dilute hydrochloric acid? Explain.

16. Would copper normally react with dilute hydrochloric acid? Explain.

17. Which would normally react faster with dilute hydrochloric acid: magnesium or iron?

18. Give one quantitative method for comparing the reactions of two metals with acid.

19. Identify the independent variable when comparing different metals.

20. Give three variables that should be controlled.

21. Explain why metal surface area should be controlled.

22. Explain why acid concentration should be controlled.

23. Explain why temperature should be controlled.

24. What is the difference between reaction rate and reactivity?

25. Complete:

acid + carbonate → ______ + ______ + ______

26. Which gas is produced by an acid-metal reaction?

27. Which gas is produced by an acid-carbonate reaction?

28. Explain one reason engineers need to understand acid-metal reactions.

29. A metal reacts faster with hydrochloric acid after the acid is heated. Does this necessarily prove that the metal became more reactive? Explain.

30. Design a fair experiment to compare the reactions of magnesium, zinc, and iron with dilute hydrochloric acid. Identify the independent variable, dependent variable, and at least three control variables.

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Key Terms

• Acid-metal reaction – reaction between an acid and a suitable metal that commonly produces a salt and hydrogen.
• Hydrogen – colourless gas commonly produced in reactions between suitable metals and dilute non-oxidizing acids.
• Salt – ionic compound formed during many acid reactions.
• Reactivity – tendency of a substance to undergo chemical reactions.
• Reactivity series – arrangement of metals according to their relative reactivity.
• Reaction rate – speed at which a chemical reaction occurs.
• Surface area – amount of a substance exposed to its surroundings.
• Concentration – amount of dissolved substance in a particular volume of solution.
• Lighted splint test – common laboratory test used to identify hydrogen by its characteristic squeaky pop.
• Corrosion – chemical deterioration of a material through reactions with its surroundings.

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Key Takeaways

• Many metals react with dilute acids to produce a salt and hydrogen gas.
• The general pattern is acid + metal → salt + hydrogen.
• Hydrochloric acid usually forms chloride salts.
• Dilute sulfuric acid usually forms sulfate salts.
• Bubbling, temperature changes, and disappearance of the metal can provide evidence that a reaction is occurring.
• Hydrogen can be identified using the lighted splint test, producing a characteristic squeaky pop.
• Different metals react at different rates because they have different reactivities.
• The reactivity series helps predict whether metals will react with dilute hydrochloric or sulfuric acid.
• Metals above hydrogen generally displace hydrogen from these acids.
• Metals below hydrogen, such as copper, generally do not.
• Reaction rate is also affected by temperature, concentration, and surface area, so these variables must be controlled when comparing metals.
• Acid-metal reactions are important in understanding corrosion, material selection, industrial chemistry, and laboratory reactions.

The key reaction pattern to remember is:

Acid + metal → salt + hydrogen