Reactant Examples in Chemistry

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

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 starting substance that is consumed and chemically transformed during a chemical reaction, appearing on the left side of a chemical equation before the reaction.

What you start with — the ingredients that get used up to make something new.

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: Reactant starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to reactant 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?

Worked Examples

Example 1

easy
Define the term 'reactant' in a chemical reaction. In the reaction 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}, identify all reactants.

Answer

Reactants: H2 and O2 (left side of the arrow)\text{Reactants: H}_2\text{ and O}_2\text{ (left side of the arrow)}

First step

1
A reactant is a substance that is consumed (used up) during a chemical reaction. Reactants appear on the left side of the chemical equation.

Full solution

  1. 2
    In 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}, the reactants are H2\text{H}_2 (hydrogen gas) and O2\text{O}_2 (oxygen gas).
  2. 3
    The coefficients (2 and 1) tell us the molar ratio: 2 moles of H2\text{H}_2 react with 1 mole of O2\text{O}_2.
Reactants are the starting materials in a chemical reaction. They are transformed into products through bond breaking and bond forming. The arrow in a chemical equation points from reactants to products.

Example 2

medium
In the reaction CaCO3+2HCl→CaCl2+H2O+CO2\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2, identify the reactants and determine the mole ratio in which they react.

Example 3

medium
For 4NH3+5O2→4NO+6H2O4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}, list the reactants and give the mole ratio in which they react.

Example 4

medium
In AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq), name the reactants and the role of water.

Example 5

medium
Given Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq) \rightarrow \text{PbI}_2(s) + 2\text{KNO}_3(aq), list reactants and how to verify mass conservation.

Example 6

hard
In 2C4H10+13O2→8CO2+10H2O2\text{C}_4\text{H}_{10} + 13\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O}, if 0.400.40 mol butane burns completely, how many moles of each reactant are consumed?

Practice Problems

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

Example 1

medium
In the combustion of propane, C3H8+5O2→3CO2+4H2O\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}, how many moles of O2\text{O}_2 are needed to react completely with 22 moles of C3H8\text{C}_3\text{H}_8?

Example 2

hard
A student mixes 5.05.0 g of sodium (Na\text{Na}) with excess water. The reaction is: 2Na+2H2O→2NaOH+H22\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2. Calculate the mass of the reactant sodium consumed and the moles of H2\text{H}_2 produced. (Na = 23.0 g/mol23.0\,\text{g/mol})

Example 3

easy
In 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O, name the reactants.

Example 4

easy
Which side of the arrow do reactants appear on?

Example 5

easy
In CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O, is oxygen a reactant or product?

Example 6

easy
Is a catalyst counted as a reactant in the net equation?

Example 7

easy
In an aqueous reaction, is the water solvent always a reactant?

Example 8

easy
What state symbol shows a reactant is an aqueous solution?

Example 9

easy
In Zn+2HCl→ZnCl2+H2Zn + 2HCl \rightarrow ZnCl_2 + H_2, list all reactants.

Example 10

easy
True or false: reactants are used up as the reaction proceeds.

Example 11

medium
In 2Na+Cl2→2NaCl2Na + Cl_2 \rightarrow 2NaCl, how many moles of each reactant are needed to make 2 mol NaClNaCl?

Example 12

medium
Mixing H2SO4H_2SO_4 and NaOHNaOH in water, identify the reactants and explain why water is not one.

Example 13

medium
A reaction has reactants AA and BB plus a catalyst KK. After reacting, KK is fully recovered. Which are reactants?

Example 14

medium
In 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O, if you start with 4 mol H2H_2 and 1 mol O2O_2, which reactant runs out first?

Example 15

medium
Why must you know the reactants before you can balance an equation?

Example 16

medium
In CaCO3→CaO+CO2CaCO_3 \rightarrow CaO + CO_2, identify the single reactant and explain decomposition.

Example 17

medium
If 8 g of a reactant fully reacts and the products total 8 g, what does this confirm about the reactants?

Example 18

medium
A flask contains N2N_2 and H2H_2. Which is the limiting reactant in N2+3H2→2NH3N_2 + 3H_2 \rightarrow 2NH_3 if there are 1 mol N2N_2 and 2 mol H2H_2?

Example 19

medium
In CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O, how many mol O2O_2 react with 3 mol CH4CH_4?

Example 20

challenge
A student claims that since water appears in 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O and also dissolves the products, water is both a reactant and a solvent. Critique this.

Example 21

challenge
Given 2C8H18+25O2→16CO2+18H2O2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O, if 5 mol octane reacts, how many mol O2O_2 are consumed, and why is O2O_2 a reactant?

Example 22

challenge
Two reactions share the same catalyst but different reactants. Explain how you would experimentally tell reactants from the catalyst.

Example 23

easy
In the reaction Mg+2HCl→MgCl2+H2\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2, name the reactants.

Example 24

easy
In S+O2→SO2\text{S} + \text{O}_2 \rightarrow \text{SO}_2, how many reactants are present?

Example 25

easy
In Fe+S→FeS\text{Fe} + \text{S} \rightarrow \text{FeS}, identify the reactants and their states (assume both solids).

Example 26

easy
In H2CO3→H2O+CO2\text{H}_2\text{CO}_3 \rightarrow \text{H}_2\text{O} + \text{CO}_2, what is the lone reactant?

Example 27

medium
In 2KClO3→MnO22KCl+3O22\text{KClO}_3 \xrightarrow{\text{MnO}_2} 2\text{KCl} + 3\text{O}_2, is MnO2_2 a reactant?

Example 28

medium
For C2H5OH+3O2→2CO2+3H2O\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}, how many moles of O2_2 react with 0.500.50 mol ethanol?

Example 29

medium
A student mixes 3.03.0 mol H2_2 and 1.01.0 mol N2_2 for N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3. Which reactant runs out first?

Example 30

medium
In photosynthesis 6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2, is water a reactant here?

Example 31

medium
In CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}, how many grams of O2_2 (M=32 g/molM=32\,\text{g/mol}) react with 0.250.25 mol CH4_4?

Example 32

medium
A flask shows HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l). Which reactant is the acid?

Example 33

medium
How many distinct reactants are in CaO+H2O→Ca(OH)2\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2?

Example 34

medium
In 2H2O2→2H2O+O22\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2, identify the reactant and reaction type.

Example 35

medium
In the Haber process N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3, if 1414 g of N2_2 (M=28M=28) reacts, how many moles of H2_2 are consumed?

Example 36

hard
In Mg+2HCl→MgCl2+H2\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2, 0.100.10 mol Mg is mixed with 0.150.15 mol HCl. Which reactant is limiting?

Example 37

hard
For 2Al+3CuSO4→Al2(SO4)3+3Cu2\text{Al} + 3\text{CuSO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{Cu}, 0.200.20 mol Al and 0.200.20 mol CuSO4_4 are mixed. Which reactant limits product formation?

Example 38

hard
A student writes 'water is a reactant in the dissolution of NaCl in water'. Critique this.

Example 39

hard
Excess Mg ribbon is burned in 0.0800.080 mol O2_2: 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}. Which reactant is limiting and how many moles of MgO form?

Example 40

hard
In Fe2O3+3CO→2Fe+3CO2\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2, 160160 g Fe2_2O3_3 (M=160M=160) reacts with excess CO. How many moles of CO are consumed?

Example 41

challenge
In a lab, 4.0 g of H2_2 (M=2) and 32 g of O2_2 (M=32) are sealed and ignited: 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}. Identify the limiting reactant and grams of water (M=18) produced.
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Background Knowledge

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

chemical reaction