Chemical Equation Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Chemical Equation.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A written representation of a chemical reaction using chemical formulas for reactants and products, coefficients to indicate mole ratios, state symbols to show physical states.

A recipe that shows what goes in, what comes out, and in what amounts.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Chemical Equation starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to chemical equation but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?

Common Mistakes to Watch For

Before you work through the examples, skim the mistake guide so you know which shortcuts and sign errors to avoid.

Balancing Equations Mistakes

Worked Examples

Example 1

easy
What information does a balanced chemical equation convey? Explain using 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} as an example.

Answer

Shows reactants, products, mole ratios, and conserved atoms\text{Shows reactants, products, mole ratios, and conserved atoms}

First step

1
A chemical equation shows: (1) the reactants (left side: H2\text{H}_2, O2\text{O}_2) and products (right side: H2O\text{H}_2\text{O}).

Full solution

  1. 2
    (2) The coefficients show mole ratios: 2 mol H2\text{H}_2 reacts with 1 mol O2\text{O}_2 to produce 2 mol H2O\text{H}_2\text{O}.
  2. 3
    (3) The equation is balanced — same number of each type of atom on both sides (H: 4=4, O: 2=2), satisfying conservation of mass.
A balanced chemical equation is a quantitative recipe for a chemical reaction. It tells us what reacts, what is produced, and in what proportions, all while obeying the law of conservation of mass.

Example 2

medium
Interpret the following chemical equation including state symbols: Na2CO3(s)+2HCl(aq)→2NaCl(aq)+H2O(l)+CO2(g)\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}. What does each state symbol mean?

Example 3

medium
Write the balanced equation with states for aqueous lead(II) nitrate reacting with aqueous potassium iodide to give solid lead(II) iodide and aqueous potassium nitrate.

Example 4

hard
Write and balance: aqueous barium chloride + aqueous sodium sulfate →\to solid barium sulfate + aqueous sodium chloride.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
Write a balanced chemical equation with state symbols for the reaction of solid iron with oxygen gas to form solid iron(III) oxide.

Example 2

hard
A student writes the following unbalanced equation for the reaction of aluminum with sulfuric acid: Al+H2SO4→Al2(SO4)3+H2\text{Al} + \text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + \text{H}_2. Balance this equation and verify by counting all atoms on each side.

Example 3

easy
What does the arrow (→\rightarrow) mean in a chemical equation?

Example 4

easy
In 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O, what does the coefficient 2 in front of H2H_2 mean?

Example 5

easy
What does the state symbol (l)(l) represent?

Example 6

easy
To balance an equation, do you change coefficients or subscripts?

Example 7

easy
Write the reactant side of: hydrogen gas reacts with oxygen gas.

Example 8

easy
In an equation, where do reactants and products go relative to the arrow?

Example 9

easy
What symbol shows a reversible reaction?

Example 10

easy
Is 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O balanced? Count hydrogen atoms on each side.

Example 11

medium
Write the balanced equation for hydrogen plus oxygen forming water, with coefficients.

Example 12

medium
Add state symbols: solid zinc plus aqueous hydrochloric acid gives aqueous zinc chloride plus hydrogen gas.

Example 13

medium
Balance: Fe+O2→Fe2O3\text{Fe} + O_2 \rightarrow Fe_2O_3.

Example 14

medium
Why does writing the arrow as '=' misrepresent a chemical equation?

Example 15

medium
Balance: C3H8+O2→CO2+H2OC_3H_8 + O_2 \rightarrow CO_2 + H_2O (propane combustion).

Example 16

medium
Translate to a word equation: CaCO3(s)→CaO(s)+CO2(g)CaCO_3(s) \rightarrow CaO(s) + CO_2(g).

Example 17

medium
Balance the polyatomic equation: Al+H2SO4→Al2(SO4)3+H2\text{Al} + H_2SO_4 \rightarrow Al_2(SO_4)_3 + H_2.

Example 18

medium
Given 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O, what mole ratio of H2H_2 to H2OH_2O does the equation specify?

Example 19

medium
Balance: H2+N2→NH3H_2 + N_2 \rightarrow NH_3 and state the coefficient on H2H_2.

Example 20

challenge
A student balances Mg+O2→MgO\text{Mg} + O_2 \rightarrow MgO as Mg+O2→MgO2Mg + O_2 \rightarrow MgO_2. Identify the error and give the correct balanced equation.

Example 21

challenge
Write and balance the full equation with states for the reaction of aqueous silver nitrate with aqueous sodium chloride forming silver chloride precipitate and aqueous sodium nitrate.

Example 22

challenge
Balance: C2H6+O2→CO2+H2OC_2H_6 + O_2 \rightarrow CO_2 + H_2O, then explain why the coefficient on O2O_2 is fractional before doubling.

Example 23

easy
Write the unbalanced equation for solid magnesium reacting with hydrochloric acid to form aqueous magnesium chloride and hydrogen gas.

Example 24

easy
What symbol indicates that heat is supplied to a reaction?

Example 25

easy
Identify the error: H2+O→H2OH_2 + O \rightarrow H_2O.

Example 26

medium
Translate 2KClO3(s)→Δ2KCl(s)+3O2(g)2\text{KClO}_3\text{(s)} \xrightarrow{\Delta} 2\text{KCl(s)} + 3\text{O}_2\text{(g)} into a word equation.

Example 27

medium
Write the balanced equation for the combustion of methane in oxygen.

Example 28

medium
In N2+3H2⇌2NH3\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3, what does the ⇌\rightleftharpoons symbol communicate?

Example 29

medium
Write the balanced equation for neutralization of NaOH(aq)\text{NaOH(aq)} by H2SO4(aq)\text{H}_2\text{SO}_4\text{(aq)}.

Example 30

medium
Write the balanced equation with state symbols for solid calcium carbonate reacting with aqueous HCl to give aqueous calcium chloride, water, and carbon dioxide gas.

Example 31

medium
For C4H10(g)+132O2(g)→4CO2(g)+5H2O(l)\text{C}_4\text{H}_{10}\text{(g)} + \dfrac{13}{2}\text{O}_2\text{(g)} \rightarrow 4\text{CO}_2\text{(g)} + 5\text{H}_2\text{O(l)}, write the equivalent equation with smallest whole-number coefficients.

Example 32

medium
How does a balanced chemical equation embody conservation of mass?

Example 33

hard
Write the balanced ionic equation for AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)\text{AgNO}_3\text{(aq)} + \text{NaCl(aq)} \rightarrow \text{AgCl(s)} + \text{NaNO}_3\text{(aq)}.

Example 34

hard
Add state symbols to: Fe+CuSO4→FeSO4+CuFe + CuSO_4 \rightarrow FeSO_4 + Cu given iron is metal, copper sulfate is dissolved, iron(II) sulfate dissolves, copper precipitates.

Example 35

hard
For C3H8+5O2→3CO2+4H2OC_3H_8 + 5O_2 \to 3CO_2 + 4H_2O, if 2 mol of propane burn, how many moles of H2OH_2O form?

Example 36

hard
Why is writing HCl++NaOH−\text{HCl}^+ + \text{NaOH}^- as reactants wrong even though HCl ionizes in water?

Example 37

hard
Write the balanced equation for the thermal decomposition of Cu(NO3)2\text{Cu(NO}_3\text{)}_2 to give CuO, NO2NO_2, and O2O_2.

Example 38

challenge
Write the balanced equation for the reaction of solid sodium with chlorine gas to form solid sodium chloride.

Example 39

challenge
A student writes H2O→H+OHH_2O \to H + OH for water 'decomposition.' Identify two errors in this equation.
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Background Knowledge

These ideas may be useful before you work through the harder examples.

reactantproductmolecular formula