Writing Acid-Base Equations

Chemical equations are a concise way of describing what happens during a chemical reaction. Instead of describing the reaction in a paragraph, we can show the substances that react, the substances that form, and eventually the relative amounts of each substance involved.

For acid-base chemistry, equations are especially useful because many reactions follow predictable patterns. Once you identify the acid and the type of substance reacting with it, you can often predict the products before carrying out the reaction.

For a typical neutralization:

acid + base → salt + water

For example:

hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl + NaOH → NaCl + H₂O

Reactants and Products

Every chemical equation has two sides.

The substances present before the reaction are called the reactants.

The substances formed by the reaction are called the products.

For example:

hydrochloric acid + sodium hydroxide → sodium chloride + water

The reactants are:

• hydrochloric acid
• sodium hydroxide

The products are:

• sodium chloride
• water

The arrow means "reacts to form" or "produces."

So we can read the equation as:

Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.

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Word Equations

A word equation uses the names of substances rather than their chemical formulas.

For example:

hydrochloric acid + sodium hydroxide → sodium chloride + water

Word equations are useful because they clearly show:

reactants → products

Before attempting a symbol equation, it is often helpful to write the word equation first.

Another example

Sulfuric acid reacts with magnesium oxide.

The word equation is:

sulfuric acid + magnesium oxide → magnesium sulfate + water

Once the substances have been identified correctly, their formulas can be substituted:

H₂SO₄ + MgO → MgSO₄ + H₂O

This equation is already balanced.

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Recognizing Different Acid Reactions

Not every substance that reacts with an acid produces the same products.

Several important reaction patterns should be recognized.

The first three can all involve neutralization of an acid, although the carbonate reaction also produces carbon dioxide. Acid-metal reactions are normally classified separately as redox reactions rather than acid-base neutralizations.

Recognizing the type of reactant therefore helps us predict the products.

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Acid + Hydroxide

Metal hydroxides are bases.

When an acid reacts with a metal hydroxide, the general reaction is:

acid + metal hydroxide → salt + water

This is a classic neutralization reaction.

Example

hydrochloric acid + sodium hydroxide → sodium chloride + water

Symbol equation:

HCl + NaOH → NaCl + H₂O

At the particle level, the important reaction is:

H⁺ + OH⁻ → H₂O

Hydrogen ions from the acid combine with hydroxide ions from the base to form water.

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Acid + Metal Oxide

Many metal oxides are basic and react with acids.

The general reaction is:

acid + metal oxide → salt + water

For example:

sulfuric acid + copper(II) oxide → copper(II) sulfate + water

Symbol equation:

H₂SO₄ + CuO → CuSO₄ + H₂O

Another example is:

hydrochloric acid + magnesium oxide → magnesium chloride + water

Symbol equation:

2HCl + MgO → MgCl₂ + H₂O

Notice that the second equation requires a 2 in front of HCl. We will examine why when we look at balancing equations.

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Acid + Carbonate

Carbonates also react with acids.

The general equation is:

acid + carbonate → salt + water + carbon dioxide

The production of carbon dioxide distinguishes this reaction from a simple acid-hydroxide or acid-metal oxide reaction.

Example

hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide

Symbol equation:

2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

The carbon dioxide normally produces visible bubbling or fizzing.

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Acid + Metal

Acids can also react with certain metals.

This reaction follows a different pattern:

acid + metal → salt + hydrogen

For example:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

Symbol equation:

2HCl + Mg → MgCl₂ + H₂

This is an important reaction to distinguish from acid-base reactions.

Compare:

acid + metal oxide → salt + water

but:

acid + metal → salt + hydrogen

The word oxide completely changes the products.

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Predicting the Salt

A major skill in writing acid-base equations is predicting the salt that will form.

Two pieces of information are needed:

1. The acid determines the negative ion in the salt.
2. The base, metal oxide, carbonate, or metal usually determines the positive metal ion.

Three common acids are especially important:

So remember:

hydrochloric acid → chloride

sulfuric acid → sulfate

nitric acid → nitrate

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Worked Example: Predicting a Salt

Predict the products when nitric acid reacts with magnesium oxide.

Identify the reaction type

Magnesium oxide is a metal oxide.

Therefore:

acid + metal oxide → salt + water

Identify the metal

The metal is magnesium.

Identify the acid

The acid is nitric acid.

Nitric acid produces nitrate salts.

Name the salt

Magnesium + nitrate gives:

magnesium nitrate

Therefore, the word equation is:

nitric acid + magnesium oxide → magnesium nitrate + water

The formulas are:

HNO₃ + MgO → Mg(NO₃)₂ + H₂O

However, this equation is not yet balanced.

The balanced equation is:

2HNO₃ + MgO → Mg(NO₃)₂ + H₂O

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From Word Equations to Symbol Equations

Consider:

hydrochloric acid + calcium hydroxide → calcium chloride + water

First replace each name with its correct formula:

hydrochloric acid = HCl

calcium hydroxide = Ca(OH)₂

calcium chloride = CaCl₂

water = H₂O

This gives:

HCl + Ca(OH)₂ → CaCl₂ + H₂O

But the equation is not balanced.

The balanced equation is:

2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O

A correct symbol equation therefore requires two skills:

correct formulas + correct balancing

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Why Equations Must Be Balanced

Chemical reactions obey the law of conservation of mass.

Atoms are not created or destroyed during an ordinary chemical reaction. Instead, existing atoms are rearranged to form new substances.

Therefore, a chemical equation must contain the same number of atoms of each element on both sides.

Consider:

HCl + MgO → MgCl₂ + H₂O

Count the chlorine atoms.

Left side:

1 Cl

Right side:

2 Cl

The equation cannot be correct as written.

Place a coefficient of 2 before HCl:

2HCl + MgO → MgCl₂ + H₂O

Now count the atoms:

The equation is balanced.

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Coefficients and Subscripts

This distinction is extremely important.

Consider:

2HCl

The 2 in front is a coefficient. It tells us that two units or moles of HCl are involved.

Now consider:

H₂O

The small 2 after H is a subscript. It is part of the chemical formula and tells us that each water molecule contains two hydrogen atoms.

When balancing an equation:

change coefficients, not subscripts.

You may write:

2HCl

You must not change HCl into HCl₂ simply to make the chlorine numbers match.

Changing a subscript changes the identity of the substance.

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Worked Example: Balancing an Equation

Start with:

H₂SO₄ + NaOH → Na₂SO₄ + H₂O

Count sodium

Left:

1 Na

Right:

2 Na

Place a 2 before NaOH:

H₂SO₄ + 2NaOH → Na₂SO₄ + H₂O

Now count hydrogen.

Left:

2 from H₂SO₄ + 2 from 2NaOH = 4 H

Right:

H₂O contains only 2 H.

Place a 2 before H₂O:

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Now check everything:

The equation is balanced.

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Another Worked Example

Balance:

HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

Calcium is already balanced.

The product contains two nitrate groups, so we need two HNO₃:

2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

Now count hydrogen.

There are:

• 2 H from 2HNO₃
• 2 H from Ca(OH)₂

Total = 4 H

We therefore need two water molecules:

2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

Now the equation is balanced.

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A Useful Balancing Strategy

When balancing a chemical equation:

First: Write the correct formulas.

Second: Count the atoms on each side.

Third: Choose an element that is unbalanced.

Fourth: Add a coefficient in front of a formula.

Fifth: Recount the atoms.

Sixth: Continue until every element balances.

Finally: Check the entire equation again.

Never change the chemical formulas themselves merely to balance the equation.

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What Is Actually Happening During Neutralization?

For a typical reaction between a strong acid and a soluble hydroxide:

HCl + NaOH → NaCl + H₂O

In solution, HCl provides H⁺ ions and NaOH provides OH⁻ ions.

The key reaction is:

H⁺ + OH⁻ → H₂O

The sodium and chloride ions remain and form the salt solution. This H⁺ + OH⁻ process is the core of many aqueous neutralization reactions.

This explains why many apparently different reactions have the same overall pattern:

acid + base → salt + water

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Classifying Reactions from Equations

You can often classify a reaction simply by examining its reactants and products.

Example A

HCl + NaOH → NaCl + H₂O

Products:

salt + water

This is an acid-hydroxide neutralization.

Example B

H₂SO₄ + MgO → MgSO₄ + H₂O

Products:

salt + water

The reactants are an acid and metal oxide.

This is an acid-metal oxide neutralization.

Example C

2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

Carbon dioxide is produced.

This is an acid-carbonate reaction.

Example D

2HCl + Mg → MgCl₂ + H₂

Hydrogen is produced.

This is an acid-metal reaction, not a standard acid-base neutralization.

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Using Products to Identify a Reaction

Sometimes you may be given only an equation and asked to identify the type of reaction.

Look for characteristic products.

Salt + water

Likely acid + hydroxide or acid + metal oxide.

Salt + water + CO₂

Acid + carbonate.

Salt + H₂

Acid + metal.

This makes the products useful clues for identifying an unknown reaction.

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Worked Example: Predicting an Entire Equation

Predict the reaction between sulfuric acid and potassium hydroxide.

Identify the reaction

Potassium hydroxide is a base.

Therefore:

acid + hydroxide → salt + water

Determine the salt

Sulfuric acid produces sulfate.

Potassium hydroxide provides potassium.

The salt is:

potassium sulfate

Write the word equation

sulfuric acid + potassium hydroxide → potassium sulfate + water

Write the formulas

H₂SO₄ + KOH → K₂SO₄ + H₂O

Balance the equation

H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

The final balanced equation is:

H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

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Worked Example: Acid and Carbonate

Predict the reaction between nitric acid and calcium carbonate.

Reaction pattern

acid + carbonate → salt + water + carbon dioxide

Determine the salt

Nitric acid produces nitrate.

Calcium carbonate provides calcium.

Therefore:

calcium nitrate

Word equation

nitric acid + calcium carbonate → calcium nitrate + water + carbon dioxide

Formula equation before balancing

HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂

Balanced equation

2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂

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Why Word Equations Matter

It may be tempting to skip directly to formulas, but word equations are extremely useful.

They allow you to determine the chemistry first.

For example, suppose you are asked about hydrochloric acid and magnesium carbonate.

Before worrying about formulas, recognize:

acid + carbonate → salt + water + carbon dioxide

Then identify the salt:

hydrochloric acid → chloride

magnesium carbonate → magnesium

Therefore:

hydrochloric acid + magnesium carbonate → magnesium chloride + water + carbon dioxide

Only then convert it to:

2HCl + MgCO₃ → MgCl₂ + H₂O + CO₂

This approach reduces errors.

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Common Mistakes

Putting products on the wrong side

Remember:

reactants → products

Reactants are on the left.

Products are on the right.

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Guessing the salt

Use the acid to determine the salt family:

HCl → chloride

H₂SO₄ → sulfate

HNO₃ → nitrate

Then combine this with the metal ion.

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Forgetting carbon dioxide

For a carbonate:

acid + carbonate → salt + water + carbon dioxide

CO₂ must be included.

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Producing hydrogen from a metal oxide

Compare carefully:

acid + metal → salt + hydrogen

but:

acid + metal oxide → salt + water

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Changing subscripts to balance an equation

Do not change:

H₂O

into something such as:

H₂O₂

just to make the atoms balance.

That creates a different substance.

Use coefficients instead.

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Balancing before writing correct formulas

Balancing cannot fix an incorrect formula.

For example, calcium chloride is:

CaCl₂

not:

CaCl

Always determine the correct formulas first.

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Assuming every neutralization finishes at pH 7

The term neutralization describes an acid-base reaction; it does not mean every resulting solution must have exactly pH 7. Strong acid–strong base reactions at the correct stoichiometric amounts can give pH 7, while other acid-base combinations can have different equivalence-point pH values.

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Key Terms

Chemical equation — A representation of a chemical reaction using words or chemical formulas.

Word equation — An equation that uses substance names rather than formulas.

Symbol equation — A chemical equation written using chemical formulas and symbols.

Reactant — A starting substance in a chemical reaction.

Product — A substance formed during a chemical reaction.

Reaction arrow (→) — Indicates that the reactants form the products.

Acid — A substance that provides H⁺ ions in aqueous solution in the Arrhenius model.

Base — A substance that neutralizes an acid.

Alkali — A soluble base.

Neutralization — A reaction involving an acid and a base.

Salt — An ionic compound produced in many acid reactions.

Hydroxide — A compound containing OH⁻ ions.

Metal oxide — A compound containing a metal and oxygen; many metal oxides are basic.

Carbonate — A compound containing CO₃²⁻ ions.

Chloride — The salt family associated with hydrochloric acid.

Sulfate — The salt family associated with sulfuric acid.

Nitrate — The salt family associated with nitric acid.

Coefficient — A number placed before a chemical formula to show the relative amount of that substance.

Subscript — A small number within a chemical formula showing the number of atoms or ions present.

Balanced equation — An equation containing the same number of atoms of each element on both sides.

Conservation of mass — The principle that matter is not created or destroyed during an ordinary chemical reaction.

Carbon dioxide — CO₂; a gas produced in acid-carbonate reactions.

Hydrogen — H₂; a gas produced when suitable metals react with many dilute acids.

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Key Takeaways

• Chemical equations describe reactants changing into products.
• Reactants appear on the left of the reaction arrow.
• Products appear on the right.
• Word equations use substance names.
• Symbol equations use chemical formulas.
• A typical acid-base neutralization follows:

acid + base → salt + water

• For a hydroxide:

acid + hydroxide → salt + water

• For a metal oxide:

acid + metal oxide → salt + water

• For a carbonate:

acid + carbonate → salt + water + carbon dioxide

• For a suitable metal:

acid + metal → salt + hydrogen

• Hydrochloric acid normally produces chloride salts.
• Sulfuric acid normally produces sulfate salts.
• Nitric acid normally produces nitrate salts.
• The identity of the reactants can therefore be used to predict the products.
• Equations must obey the law of conservation of mass.
• The number of atoms of every element must be equal on both sides of a balanced equation.
• Use coefficients to balance equations.
• Never change a subscript simply to make an equation balance.
• A useful approach is:

identify reaction → predict products → write word equation → write formulas → balance → check

• For many aqueous acid-hydroxide neutralizations, the key ionic change is:

H⁺ + OH⁻ → H₂O

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Check Your Understanding

1. What is the difference between a reactant and a product?

2. What does the arrow in a chemical equation represent?

3. Write a word equation for hydrochloric acid reacting with potassium hydroxide.

4. Predict the products:

sulfuric acid + magnesium oxide → ?

5. Predict the products:

nitric acid + calcium carbonate → ?

6. What type of salt is normally produced by hydrochloric acid?

7. What type of salt is normally produced by nitric acid?

8. Complete and classify:

acid + carbonate → ______ + ______ + ______

9. Balance:

HCl + MgO → MgCl₂ + H₂O

10. Balance:

HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

11. Balance:

H₂SO₄ + KOH → K₂SO₄ + H₂O

12. Explain why changing a subscript is not an acceptable way to balance a chemical equation.

13. A reaction produces a salt, water and carbon dioxide. What type of substance probably reacted with the acid?

14. A reaction produces a salt and hydrogen gas. What type of substance probably reacted with the acid?

15. Write a balanced symbol equation for hydrochloric acid reacting with calcium hydroxide.

16. Challenge: A student is told that sulfuric acid reacts with aluminium hydroxide to form aluminium sulfate and water. Write the word equation, determine the correct formulas, and then write the balanced symbol equation.