Acid-Base Reactions
3. Acids and Metal Oxides
Learning outcomes
- I can describe how acids react with metal oxides.
- I can identify the products formed during these reactions.
- I can explain why metal oxides are considered basic.
- I can write word equations for acid-metal oxide reactions.
- I can relate these reactions to neutralization.
Acids and Metal Oxides
Acids react with many metal oxides to produce a salt and water. This is an important type of neutralization reaction because the metal oxide acts as a base and neutralizes the acid.
The general reaction is:
acid + metal oxide → salt + water
For example:
sulfuric acid + copper(II) oxide → copper(II) sulfate + water
Unlike the reaction between an acid and a metal, hydrogen gas is not produced.
What Is a Metal Oxide?
A metal oxide is a compound containing a metal chemically combined with oxygen.
Examples include:
- magnesium oxide — MgO
- copper(II) oxide — CuO
- calcium oxide — CaO
- zinc oxide — ZnO
- iron(III) oxide — Fe₂O₃
Many metal oxides are basic oxides. This means they can react with acids and neutralize them.
For example, magnesium oxide reacts with hydrochloric acid:
magnesium oxide + hydrochloric acid → magnesium chloride + water
MgO + 2HCl → MgCl₂ + H₂O
The magnesium oxide acts as the base, while hydrochloric acid acts as the acid.
Why Are Metal Oxides Basic?
A base is a substance that can neutralize an acid.
Many metal oxides contain oxide ions, O²⁻. During a reaction with an acid, these oxide ions react with hydrogen ions, H⁺, from the acid.
The simplified ionic reaction is:
O²⁻ + 2H⁺ → H₂O
The hydrogen ions responsible for the acidic properties of the solution are consumed as water forms.
This is why the reaction is described as neutralization.
Base or alkali?
It is important not to confuse a base with an alkali.
A base neutralizes an acid.
An alkali is a base that dissolves in water.
Copper(II) oxide, for example, does not dissolve significantly in water, but it reacts with acids. Therefore:
Copper(II) oxide is a base, but it is not an alkali.
The General Reaction
The pattern to remember is:
acid + metal oxide → salt + water
The metal in the metal oxide becomes part of the salt.
The acid determines the other part of the salt.
For example:
hydrochloric acid + magnesium oxide → magnesium chloride + water
The magnesium comes from magnesium oxide.
The chloride comes from hydrochloric acid.
Together they form magnesium chloride.
The Acid Determines the Type of Salt
Three common acids produce predictable families of salts.
| Acid | Formula | Type of salt formed |
|---|---|---|
| Hydrochloric acid | HCl | Chloride |
| Sulfuric acid | H₂SO₄ | Sulfate |
| Nitric acid | HNO₃ | Nitrate |
This gives us a useful rule:
hydrochloric acid → chloride
sulfuric acid → sulfate
nitric acid → nitrate
Example
Suppose zinc oxide reacts with nitric acid.
The metal is zinc.
Nitric acid produces nitrate salts.
Therefore, the salt must be zinc nitrate.
nitric acid + zinc oxide → zinc nitrate + water
Copper(II) Oxide and Sulfuric Acid
One of the most common examples of an acid reacting with a metal oxide is the reaction between copper(II) oxide and sulfuric acid.
Copper(II) oxide is a black solid.
Dilute sulfuric acid is a colourless solution.
When they react, they form a blue copper(II) sulfate solution and water.
Word equation
copper(II) oxide + sulfuric acid → copper(II) sulfate + water
Symbol equation
CuO + H₂SO₄ → CuSO₄ + H₂O
During the reaction, the black copper(II) oxide gradually disappears as it reacts.
The formation of the blue solution provides evidence that copper(II) sulfate has formed.
What Happens to the Particles?
We can understand this reaction at the particle level.
Copper(II) oxide contains:
Cu²⁺ ions and O²⁻ ions
Sulfuric acid provides:
H⁺ ions and sulfate ions
The oxide ions react with the hydrogen ions:
O²⁻ + 2H⁺ → H₂O
The copper ions and sulfate ions remain in solution, forming copper(II) sulfate.
So:
CuO + H₂SO₄ → CuSO₄ + H₂O
The atoms have not disappeared. They have simply been rearranged into new substances.
More Examples
Magnesium Oxide and Hydrochloric Acid
Magnesium oxide reacts with hydrochloric acid to form magnesium chloride and water.
Word equation
magnesium oxide + hydrochloric acid → magnesium chloride + water
Symbol equation
MgO + 2HCl → MgCl₂ + H₂O
The reaction follows the same pattern:
acid + metal oxide → salt + water
Calcium Oxide and Nitric Acid
Calcium oxide reacts with nitric acid.
First identify the salt.
The metal oxide provides calcium.
Nitric acid produces a nitrate.
Therefore, the salt is calcium nitrate.
Word equation
calcium oxide + nitric acid → calcium nitrate + water
Symbol equation
CaO + 2HNO₃ → Ca(NO₃)₂ + H₂O
Zinc Oxide and Sulfuric Acid
Zinc oxide reacts with sulfuric acid.
Sulfuric acid produces sulfate salts, so the salt is zinc sulfate.
Word equation
zinc oxide + sulfuric acid → zinc sulfate + water
Symbol equation
ZnO + H₂SO₄ → ZnSO₄ + H₂O
Predicting the Products
Once you recognize the pattern, you can predict the products of many acid-metal oxide reactions.
Consider:
magnesium oxide + nitric acid → ?
Step 1: Identify the metal
Magnesium oxide contains magnesium.
Step 2: Identify the acid
The acid is nitric acid.
Step 3: Determine the salt family
Nitric acid produces nitrates.
Step 4: Name the salt
Magnesium + nitrate gives:
magnesium nitrate
Step 5: Add water
The completed word equation is:
magnesium oxide + nitric acid → magnesium nitrate + water
The balanced symbol equation is:
MgO + 2HNO₃ → Mg(NO₃)₂ + H₂O
Why Is This a Neutralization Reaction?
Neutralization occurs when an acid reacts with a base and the acidic properties of the acid are reduced or removed.
The general pattern is:
acid + base → salt + water
Because many metal oxides are bases, their reaction with an acid fits this pattern exactly:
acid + metal oxide → salt + water
At the particle level, H⁺ ions from the acid are consumed as water forms.
This is the key reason the reaction is considered neutralization.
What Would We Observe?
Suppose copper(II) oxide is gradually added to warm dilute sulfuric acid.
Initially, the sulfuric acid is colourless and the copper(II) oxide is a black solid.
As the reaction occurs:
- some of the black copper(II) oxide disappears
- a blue copper(II) sulfate solution forms
- the temperature may increase
- no hydrogen gas is produced
If copper(II) oxide continues to be added, eventually some black solid will remain.
Why?
All of the acid has been used up.
There is no longer enough acid available to react with the additional copper(II) oxide.
The remaining copper(II) oxide is therefore described as being in excess.
Making a Salt from an Acid and Metal Oxide
Acid-metal oxide reactions can be used in the laboratory to prepare soluble salts.
For example, copper(II) sulfate crystals can be prepared using copper(II) oxide and sulfuric acid.
A typical procedure is:
- Place dilute sulfuric acid in a beaker.
- Warm the acid gently.
- Add a small amount of copper(II) oxide.
- Stir the mixture.
- Continue adding copper(II) oxide until some remains unreacted.
- Filter the mixture to remove the excess copper(II) oxide.
- Collect the copper(II) sulfate solution.
- Gently evaporate some of the water.
- Allow the concentrated solution to cool.
- Copper(II) sulfate crystals form.
This method works particularly well when the metal oxide is an insoluble base.
Why Add Excess Metal Oxide?
Adding excess metal oxide makes sure that all of the acid has reacted.
Once all the acid has been neutralized, additional metal oxide cannot react.
The excess solid can then be removed by filtration.
This leaves a salt solution without excess acid.
Why Filter the Mixture?
The salt is dissolved in the water, but the excess metal oxide is an insoluble solid.
Filtration separates them.
The solid left on the filter paper is the residue.
The salt solution that passes through the filter paper is the filtrate.
For the copper(II) oxide example:
Residue: excess copper(II) oxide
Filtrate: copper(II) sulfate solution
Forming Crystals
The filtrate contains the salt dissolved in water.
Some of the water can be removed by evaporation, making the solution more concentrated.
The concentrated solution is then allowed to cool.
As it cools, salt begins to leave the solution and form solid crystals. The crystals can then be separated and dried.
In the copper(II) sulfate experiment, the final product is characteristic blue copper(II) sulfate crystals.
Comparing Acid Reactions
Several types of substances react with acids, but they do not all produce the same products.
| Reaction | Products |
|---|---|
| acid + metal oxide | salt + water |
| acid + metal hydroxide | salt + water |
| acid + metal | salt + hydrogen |
| acid + metal carbonate | salt + water + carbon dioxide |
These reaction patterns are very useful when predicting products.
Example
hydrochloric acid + magnesium → magnesium chloride + hydrogen
but
hydrochloric acid + magnesium oxide → magnesium chloride + water
The presence of oxide changes the products.
Worked Example: Identifying an Unknown Reaction
A student adds a black metal oxide to dilute sulfuric acid.
The black solid gradually disappears and a coloured salt solution forms. No bubbles are observed.
Which general reaction has occurred?
We know:
- an acid was present
- a metal oxide was added
- no gas was produced
Therefore:
acid + metal oxide → salt + water
This is a neutralization reaction.
If the black solid was copper(II) oxide:
CuO + H₂SO₄ → CuSO₄ + H₂O
Worked Example: Predicting a Salt
Predict the salt formed when calcium oxide reacts with hydrochloric acid.
Identify the metal
Calcium oxide contains calcium.
Identify the salt family
Hydrochloric acid produces chlorides.
Combine the names
Calcium + chloride gives:
calcium chloride
Therefore:
calcium oxide + hydrochloric acid → calcium chloride + water
Real-World Connection: Controlling Acidity
The ability of basic oxides to neutralize acids has practical importance.
Calcium oxide, for example, is strongly basic. Materials containing calcium compounds can be used in processes where acidity needs to be reduced.
The underlying chemistry follows the same principle:
acid + base → salt + water
This same idea connects laboratory neutralization reactions with environmental and industrial chemistry.
Common Mistakes
Thinking an acid and metal oxide produce hydrogen
They do not.
acid + metal → salt + hydrogen
but:
acid + metal oxide → salt + water
Thinking every base is an alkali
An alkali must be soluble in water.
Copper(II) oxide is an insoluble base, so it is not an alkali.
Forgetting water
Water is one of the products of an acid-metal oxide reaction.
acid + metal oxide → salt + water
Naming the salt from the metal oxide alone
Both reactants determine the salt.
The metal oxide determines the metal, while the acid determines the negative part of the salt.
For example:
magnesium oxide + sulfuric acid
Magnesium oxide supplies magnesium.
Sulfuric acid supplies sulfate.
The salt is therefore:
magnesium sulfate
Confusing sulfate and sulfide
Sulfuric acid produces sulfate salts, not sulfides.
For example:
H₂SO₄ + MgO → MgSO₄ + H₂O
The product is magnesium sulfate.
Assuming neutralization must always produce a solution with pH 7
Neutralization means that an acid and base react; it does not guarantee that every resulting salt solution will have a pH of exactly 7. The final pH depends on the substances involved and their amounts. For example, copper sulfate solutions can remain somewhat acidic.
Key Terms
Acid — A substance that produces hydrogen ions, H⁺, in aqueous solution.
Metal oxide — A compound containing a metal chemically combined with oxygen.
Base — A substance that reacts with and neutralizes an acid.
Basic oxide — An oxide that behaves as a base and reacts with acids.
Alkali — A soluble base that produces hydroxide ions in aqueous solution.
Neutralization — A reaction in which an acid reacts with a base.
Salt — An ionic compound produced when the hydrogen ions of an acid are replaced by positive ions such as metal ions.
Chloride — A salt formed from hydrochloric acid.
Sulfate — A salt formed from sulfuric acid.
Nitrate — A salt formed from nitric acid.
Oxide ion — The O²⁻ ion found in many ionic metal oxides.
Hydrogen ion — H⁺; the ion associated with acidic behaviour in aqueous solutions.
Soluble — Able to dissolve in a particular solvent.
Insoluble — Unable, or only slightly able, to dissolve in a particular solvent.
Excess reactant — A reactant that is present in more than the amount required to react completely with another reactant.
Residue — The solid remaining on filter paper after filtration.
Filtrate — The liquid that passes through filter paper during filtration.
Evaporation — The process by which liquid particles escape and become a gas.
Crystallization — The formation of solid crystals from a solution.
Key Takeaways
- Metal oxides are compounds containing a metal and oxygen.
- Many metal oxides behave as bases.
- A base can neutralize an acid.
- A base does not have to dissolve in water.
- A soluble base is called an alkali.
- The general reaction is:
acid + metal oxide → salt + water
- Acid-metal oxide reactions are neutralization reactions.
- Hydrogen ions from the acid ultimately combine with oxide ions to form water.
- The metal oxide provides the metal ion in the salt.
- The acid determines the salt family.
- Hydrochloric acid produces chlorides.
- Sulfuric acid produces sulfates.
- Nitric acid produces nitrates.
- Acid-metal oxide reactions do not normally produce hydrogen gas.
- An insoluble metal oxide can be added in excess to ensure that all of an acid has reacted.
- Excess insoluble metal oxide can then be removed by filtration.
- The resulting salt solution can be concentrated and cooled to produce crystals.
- Copper(II) oxide and sulfuric acid provide a classic example:
CuO + H₂SO₄ → CuSO₄ + H₂O
- The black copper(II) oxide is consumed and a blue copper(II) sulfate solution forms.
- Understanding this reaction pattern makes it possible to predict the products of many acid-base reactions.
Check Your Understanding
1. What is a metal oxide?
2. Explain why many metal oxides are described as bases.
3. Complete the general equation:
acid + metal oxide → ______ + ______
4. What is the difference between a base and an alkali?
5. Name the salt produced when magnesium oxide reacts with hydrochloric acid.
6. Complete:
copper(II) oxide + sulfuric acid → ______ + ______
7. What type of salt is produced when nitric acid reacts with a metal oxide?
8. Predict the products:
zinc oxide + hydrochloric acid → ______ + ______
9. Explain why hydrogen gas is not expected during an acid-metal oxide reaction.
10. Copper(II) oxide is added to sulfuric acid until some black solid remains. Explain why the remaining solid no longer reacts.
11. Why is the mixture filtered after excess copper(II) oxide has been added?
12. What is the difference between the residue and the filtrate?
13. Explain how copper(II) sulfate crystals could be obtained from a copper(II) sulfate solution.
14. Write a word equation for the reaction between calcium oxide and nitric acid.
15. Challenge: Explain at the ion level why the reaction between an acid and a metal oxide can be classified as neutralization.