Wize University Chemistry Textbook > Stoichiometry

Introduction to Chemical Equations and Balancing

Balancing Reactions

Reactions must be balanced before the stoichiometry can be analyzed.

What makes a reaction balanced?

The number of ________ of each type on the ________ must equal the number of atoms of each type on the________ of the reaction.

Example:

\textrm{Unbalanced:} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ C_3H_{8(g)}+O_{2(g)}\rightarrow CO_{2(g)}+H_2O_{(g)}

\textrm{Balanced:} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ C_3H_{8(g)}+5\ O_{2(g)}\rightarrow 3\ CO_{2(g)}+4\ H_2O_{(g)}

Balancing Chemical Reactions

Unbalanced Reaction

$\large\bf CH_4\ +\ O_2\ \longrightarrow\ CO_2\ +\ H_2O$

The subscripts tell us the ratio of different atoms in one molecule.

Example: One CH4 molecule has 4 hydrogen atoms and 1 carbon atom

Steps to Balance Chemical Equations

Balance elements that only appear once on the reactant side and once on the product side (typically elements not including C, N, H, O)
Group polyatomic ions and balance them as a group (not as separate elements, e.g. balance NO3 as 1 NO3 group, not 1 N and 3 O)
Balance all other elements, starting with the least common elements to the most common elements
Make sure all coefficients are whole numbers AND are represented in the simplest integer ratio possible
Check to make sure all elements are balanced on each side of the chemical equation

Balanced Reaction

Image illustrating Reaction Stoichiometry

The stoichiometric coefficients of the balanced equation tell us that we need ________ oxygen molecules for each methane (CH4) molecule and we produce ________ waters for every methane consumed.
It also tells us that for every 1 mole of methane used we produce ________ mole of CO2.

Example: Balancing a Chemical Reaction

Balance the following equation:

$NO_{\left(aq\right)}\ \ \ +\ \ \ O_{2\left(g\right)}\ \ \ +\ \ \ H_2O\ _{\left(l\right)}\ \ \ \rightarrow\ \ \ \ HNO_{2\left(aq\right)}$

Balanced chemical equation: $4NO\ \left(aq\right)\ +\ \ \ O_2\ \left(g\right)\ \ \ +\ \ \ 2H_2O\ \left(l\right)\ \ \ \ \rightarrow\ \ \ \ 4HNO_2\ \left(aq\right)$

Polyatomic Ions

Note: There is no need to memorize the following table. It is just included here so you have an idea about what a polyatomic ion is!

Example: Balance a Chemical Equation with Polyatomic Ions

$\left(NH_4\right)_2SO_4\ \left(aq\right)\ \ \ +\ \ \ Ba\left(NO_3\right)_2\ \left(aq\right)\ \ \ \rightarrow\ \ \ \ NH_4NO_3\ \left(aq\right)\ \ +\ \ \ BaSO_4\ \left(s\right)$

$\left(NH_4\right)_2SO_4\ \left(aq\right)\ \ \ +\ \ \ Ba\left(NO_3\right)_2\ \left(aq\right)\ \ \ \ \rightarrow\ \ \ \ 2NH_4NO_3\ \left(aq\right)\ \ \ +\ \ \ BaSO_4\ \left(s\right)$

Practice: Balancing Combustion of Furan

Choose the answer that balances the reaction shown below:

\text{A}\ C_4H_4O(l)+\text{B}\ O_2(g)\rightarrow\text{C}\ CO_2(g)+\text{D}\ H_2O(g)

A = 2; B = 9 ;C = 8; D = 4

A = 1; B = 3 ;C = 5; D = 2

A = 1; B = 1 ;C = 1; D = 1

A = 4; B = 25 ;C = 20; D = 10

I don't know

Extra Practice

Chemical Reactions: Balancing

Balance the following reaction:

$\text{C}_5H_{10}\text{O}_{\left(l\right)}+\text{O}_{2\left(g\right)}\rightarrow\ \text{CO}_{2\left(g\right)}+\text{H}_2\text{O}_{\left(l\right)}$

What coefficient is in front of oxygen gas in the balanced reaction?

Chemical Reactions: Balancing the Combustion of Pyridine

Choose the answer that balances the reaction shown below:

\text{A}\ C_4H_4O(l)+\text{B}\ O_2(g)\rightarrow\text{C}\ CO_2(g)+\text{D}\ H_2O(g)

Chemical Reactions: Balancing Combustion Reactions

Balance the reaction below. Make sure that the reaction has the lowest possible stoichiometric coefficients.

$C_{12}H_{14}O\ \ \ \ +\ \ \ \ O_2\ \ \ \ \rightarrow\ \ \ \ CO_2\ \ \ \ +\ \ \ \ H_2O$

Empirical Formulae

4. An organic compound, C3H8O2, is combusted in air to produce carbon dioxide and water vapour. Balance the chemical reaction that occurs.

Empirical Formulae

An organic compound, C5H12O2, is combusted in air to produce carbon dioxide and water vapour. Balance the chemical reaction that occurs.

Combustion

Select the correct balanced equation for the complete combustion reaction expected to occur between C3H7OH and O2.

A. C3H7OH + O2 → 3 CO2 + 4 H2O

B. 2 C3H7OH + 9 CO2 → 6 CO2 + 8 H2O + 9 C

Balancing Redox Equation

What is the coefficient of KI when the unbalanced expression below is balanced using the smallest whole number coefficients?

K2S2O3 + I2 → K2S4O6 + KI

Net Ionic and Complete Molecular Equation

Consider the following unbalanced reaction:

KMnO4 (aq) + FeSO4 (aq) + H2SO4 (aq) → Fe2(SO4)3 (aq) + MnSO4 (aq) + K2SO4 (aq) + H2O (l)

Write the balanced net ionic and complete molecular equation

Consider the following unbalanced equation:

Zn (s) + NO3− (aq) + H+ (aq) → Zn2+ (aq) + NH4+ (aq) + H2O (l)

Write the net ionic and complete molecular reaction

Which one of the following is not a correct formation equation?

Stoichiometry: Limiting reagents

17g of P4 reacts with 64g of oxygen, O2 as described below in the UNBALANCED reaction. What is the maximum amount of P4O6 that can be formed?

P_{4(s)}+O_{2(g)}\rightarrow P_4O_{6(s)}

Sodium metal reacts with water to give sodium hydroxide and hydrogen gas as per the following unbalanced equation:

$Na(s)+H_2O(l)\rightarrow NaOH(aq)+H_2(g)$

If 1.00 g of sodium were reacted with 0.786 g of water which species would be the limiting reagent?

Chemical Reactions: Limiting Reagents

Consider the following unbalanced reaction

Which reagent is in excess?

Writing Molecular and Net Ionic Equations

Write the balanced net ionic equations (NIE) and molecular equations (ME) for the following. Show all your work. Be sure to show the states in the final equations as well as the charges on any ions.

A. NaCl (aq) + Pb(NO3)2 (aq) →

ME:

Balancing equations and naming compounds

Balance these chemical reactions and answer the following questions. Write a 1 in the blank if the coefficient is 1.

KF (aq) + HI (aq) → KI (aq) + HF (aq)

The full name of the acid that is produced in this reaction is

Chemical reactions: Combustion

Select the correct balanced equation for the complete combustion reaction expected to occur between C4H10 and O2.

Introduction to Chemical Equations and Balancing

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Balancing Reactions

Example:

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Balancing Chemical Reactions

Unbalanced Reaction

Balanced Reaction

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Example: Balancing a Chemical Reaction

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Polyatomic Ions

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Example: Balance a Chemical Equation with Polyatomic Ions

Practice: Balancing Combustion of Furan

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Extra Practice