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Wize University Chemistry Textbook > Stoichiometry

Introduction to Chemical Equations and Balancing

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Balancing Reactions

Reactions must be balanced before the stoichiometry can be analyzed.
What makes a reaction balanced?

The number of
atoms
of each type on the
reactants side
must equal the number of atoms of each type on the
products side
of the reaction.

Example:

Unbalanced:                   C3H8(g)+O2(g)→CO2(g)+H2O(g)\textrm{Unbalanced:} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ C_3H_{8(g)}+O_{2(g)}\rightarrow CO_{2(g)}+H_2O_{(g)}



Balanced:                C3H8(g)+5 O2(g)→3 CO2(g)+4 H2O(g)\textrm{Balanced:} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ C_3H_{8(g)}+5\ O_{2(g)}\rightarrow 3\ CO_{2(g)}+4\ H_2O_{(g)}

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Balancing Chemical Reactions

Unbalanced Reaction

CH4 + O2 ⟶ CO2 + H2O\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

Watch Out!
Sometimes you could be given unbalanced equations and you are expected to balance them!
We need to use the balanced equation for calculations in order to get the correct answer!
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Steps to Balance Chemical Equations
  1. Balance elements that only appear once on the reactant side and once on the product side (typically elements not including C, N, H, O)
  2. 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)
  3. Balance all other elements, starting with the least common elements to the most common elements
  4. Make sure all coefficients are whole numbers AND are represented in the simplest integer ratio possible
  5. Check to make sure all elements are balanced on each side of the chemical equation

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Balanced Reaction



  • The stoichiometric coefficients of the balanced equation tell us that we need
    2
    oxygen molecules for each methane (CH4) molecule and we produce
    2
    waters for every methane consumed.
  • It also tells us that for every 1 mole of methane used we produce
    1
    mole of CO2.

Wize Concept
Multipy coefficients by subscripts to see exactly how much of an atom we have on each side.

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Watch Out!
We want the lowest possible coefficients.

Example:
If I balanced an equation and got:

2MgCl2(aq)+4NaOH(aq) →2Mg(OH)2(s) + 4NaCl(aq)2MgCl_2\left(aq\right)+4NaOH\left(aq\right)\ \rightarrow2Mg\left(OH\right)_2\left(s\right)\ +\ 4NaCl\left(aq\right)

The greatest common factor that can go into each coefficient is 2 so we need to divide each coefficient by 2 to get the lowest possible numbers!
MgCl2(aq)+2NaOH(aq) →Mg(OH)2(s) + 2NaCl(aq)MgCl_2\left(aq\right)+2NaOH\left(aq\right)\ \rightarrow Mg\left(OH\right)_2\left(s\right)\ +\ 2NaCl\left(aq\right)


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Example: Balancing a Chemical Reaction

Balance the following equation:

NO(aq)   +   O2(g)   +   H2O (l)   →    HNO2(aq)NO_{\left(aq\right)}\ \ \ +\ \ \ O_{2\left(g\right)}\ \ \ +\ \ \ H_2O\ _{\left(l\right)}\ \ \ \rightarrow\ \ \ \ HNO_{2\left(aq\right)}








Balanced chemical equation: 4NO (aq) +   O2 (g)   +   2H2O (l)    →    4HNO2 (aq)4NO\ \left(aq\right)\ +\ \ \ O_2\ \left(g\right)\ \ \ +\ \ \ 2H_2O\ \left(l\right)\ \ \ \ \rightarrow\ \ \ \ 4HNO_2\ \left(aq\right)


Wize Concept
The physical state of each chemical compound is often included in chemical equations:

(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous

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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!


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Example: Balance a Chemical Equation with Polyatomic Ions


(NH4)2SO4 (aq)   +   Ba(NO3)2 (aq)   →    NH4NO3 (aq)  +   BaSO4 (s)\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)





(NH4)2SO4 (aq)   +   Ba(NO3)2 (aq)    →    2NH4NO3 (aq)   +   BaSO4 (s)\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 the Combustion of Furan

Choose the answer that balances the reaction shown below:
A C4H4O(l)+B O2(g)→C CO2(g)+D H2O(g)\text{A}\ C_4H_4O(l)+\text{B}\ O_2(g)\rightarrow\text{C}\ CO_2(g)+\text{D}\ H_2O(g)

Extra Practice
Chemical reactions: Combustion
Select the correct balanced equation for the complete combustion reaction expected to occur between C4H10 and O2.
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.
1
KF (aq) +
1
HI (aq) →
1
KI (aq) +
1
HF (aq)
The full name of the acid that is produced in this reaction is
hydrofluoric acid
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:
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?
P4(s)+O2(g)→P4O6(s)P_{4(s)}+O_{2(g)}\rightarrow P_4O_{6(s)}
Chemical Reactions: Limiting Reagents
Consider the following unbalanced reaction
Which reagent is in excess?
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
Sodium metal reacts with water to give sodium hydroxide and hydrogen gas as per the following unbalanced equation:
Na(s)+H2O(l)→NaOH(aq)+H2(g)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?
Which one of the following is not a correct formation 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
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
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
Empirical Formulae
An organic compound, C5H12O2, is combusted in air to produce carbon dioxide and water vapour. Balance the chemical reaction that occurs.
Empirical Formulae
4. An organic compound, C3H8O2, is combusted in air to produce carbon dioxide and water vapour. Balance the chemical reaction that occurs.
Chemical Reactions: Balancing Combustion Reactions
Balance the reaction below. Make sure that the reaction has the lowest possible stoichiometric coefficients.
C12H14O    +    O2    →    CO2    +    H2OC_{12}H_{14}O\ \ \ \ +\ \ \ \ O_2\ \ \ \ \rightarrow\ \ \ \ CO_2\ \ \ \ +\ \ \ \ H_2O
Chemical Reactions: Balancing the Combustion of Pyridine
Choose the answer that balances the reaction shown below:
A C4H4O(l)+B O2(g)→C CO2(g)+D H2O(g)\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
Balance the following reaction:
C5H10O(l)+O2(g)→ CO2(g)+H2O(l)\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?