Chemical Reactions and Chemical Quantities: Equations and Stoichiometry

Chemical Reactions and Chemical Quantities

Chemical Reaction

A chemical reaction is a process in which one or more substances are converted into one or more different substances. These processes involve chemical changes that result in new chemical substances.

• Combustion reaction: A specific type of chemical reaction where a substance combines with oxygen to form one or more oxygen-containing compounds, usually releasing heat.

Chemical Equations

Chemical equations are a shorthand way of describing chemical reactions. They provide essential information about the reaction, including:

• Formulas of reactants and products

• States of reactants and products

• Relative numbers of reactant and product molecules (stoichiometry), which can be used to determine the weights of reactants used and products formed

The general form of a chemical equation is:

Reactants → Products

States of Reactants and Products in Chemical Equations

Abbreviation	State
(g)	Gas
(l)	Liquid
(s)	Solid
(aq)	Aqueous (water solution)

Combustion of Methane: An Example

The combustion of methane is a classic example of a chemical reaction:

$\mathrm{CH_4(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)}$

• When methane burns, it reacts with oxygen to produce carbon dioxide and water vapor.

• To ensure the equation obeys the Law of Conservation of Mass, it must be balanced so that the number of atoms of each element is the same on both sides.

Unbalanced equation:

$\mathrm{CH_4(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)}$

Counting atoms reveals imbalance (e.g., 4 H on left, 2 H on right).

Balanced equation:

$\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}$

• Now, both sides have 1 C, 4 H, and 4 O atoms.

Balancing Chemical Equations

Balancing chemical equations ensures the Law of Conservation of Mass is obeyed. The process involves adjusting coefficients (not subscripts) so that the number of atoms of each element is equal on both sides of the equation.

1. Write the unbalanced equation with correct formulas for all reactants and products.

2. Balance atoms for elements that appear only once on each side first (often metals or nonmetals other than H and O).

3. Balance polyatomic ions as a unit if they appear unchanged on both sides.

4. Balance H and O atoms last.

5. Check to ensure all atoms are balanced.

Example:

Balance the equation for the reaction between iron(III) carbonate and oxygen to produce iron(III) oxide and carbon dioxide:

$4\,\mathrm{FeCO_3(s)} + O_2(g) \rightarrow 2\,\mathrm{Fe_2O_3(s)} + 4\,\mathrm{CO_2(g)}$

• There are 4 Fe, 4 C, and 12 O atoms on both sides.

Balancing Equations with Polyatomic Ions

• If a polyatomic ion appears unchanged on both sides, balance it as a unit to simplify the process.

Reaction Stoichiometry

Stoichiometry is the study of the numerical relationships between chemical quantities in a chemical reaction. The coefficients in a balanced chemical equation specify the relative amounts in moles of each substance involved.

Example:

$2\,\mathrm{C_8H_{18}(l)} + 25\,\mathrm{O_2(g)} \rightarrow 16\,\mathrm{CO_2(g)} + 18\,\mathrm{H_2O(g)}$

• 2 moles of octane react with 25 moles of oxygen to form 16 moles of carbon dioxide and 18 moles of water.

• This ratio can be used to calculate the amount of any reactant or product if the amount of another is known.

Analogy: Making Pizza

• Just as a recipe specifies the ratio of ingredients needed to make a certain number of pizzas, a balanced chemical equation specifies the ratio of reactants and products.

• For example, if 1 crust + 5 oz tomato sauce + 2 cups cheese = 1 pizza, then 10 cups cheese can make 5 pizzas (if other ingredients are sufficient).

Mole-to-Mole Conversions

Stoichiometric ratios from balanced equations are used as conversion factors between amounts of reactants and products in moles.

Example:

Given the reaction:

$2\,\mathrm{C_8H_{18}(l)} + 25\,\mathrm{O_2(g)} \rightarrow 16\,\mathrm{CO_2(g)} + 18\,\mathrm{H_2O(g)}$

If 22.0 moles of $\mathrm{C_8H_{18}}$ are burned, how many moles of $\mathrm{CO_2}$ are formed?

$22.0\,\mathrm{mol\,C_8H_{18}} \times \frac{16\,\mathrm{mol\,CO_2}}{2\,\mathrm{mol\,C_8H_{18}}} = 176\,\mathrm{mol\,CO_2}$

Key Terms and Concepts

• Reactant: A starting substance in a chemical reaction.

• Product: A substance formed as a result of a chemical reaction.

• Coefficient: The number in front of a chemical formula in an equation, indicating the number of molecules or moles.

• Stoichiometry: The calculation of quantities of reactants and products in chemical reactions.

• Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction; the mass of reactants equals the mass of products.

Summary Table: States of Matter in Chemical Equations

Abbreviation	Meaning
(g)	Gas
(l)	Liquid
(s)	Solid
(aq)	Aqueous (dissolved in water)

Additional info:

• Balancing equations is a foundational skill for all stoichiometric calculations in chemistry.

• Always check your final equation to ensure the number of atoms for each element is equal on both sides.