Chapter 4: Chemical Reactions and Chemical Quantities – General Chemistry 1

Chemical Reactions and Equations

Introduction to Chemical Reactions

Chemical reactions are processes in which substances undergo chemical changes, resulting in the formation of new substances. These changes involve the rearrangement and possible exchange of atoms to produce new molecules. Importantly, elements are not transmuted during a chemical reaction; the identity of each element remains unchanged.

• Reactants are the starting substances in a chemical reaction.

• Products are the substances formed as a result of the reaction.

• The number of atoms of each element must be the same on both sides of the chemical equation, reflecting the law of conservation of mass.

Chemical Equations

Chemical equations provide a symbolic representation of chemical reactions. They convey essential information about the reaction, including the formulas and states of reactants and products, and the relative numbers of molecules involved.

• Formulas of reactants and products (e.g., H2, O2, H2O).

• States of reactants and products: (g) for gas, (l) for liquid, (s) for solid, (aq) for aqueous solution.

• Stoichiometric coefficients indicate the relative numbers of molecules or moles required or produced.

• Equations can be used to determine the masses of reactants and products.

Example:

$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

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

Balancing Chemical Equations

Principles of Balancing

Balancing chemical equations ensures that the number of atoms for each element is the same on both sides of the equation. This is achieved by adjusting the stoichiometric coefficients in front of each formula.

• Balance atoms in compounds first, then atoms in pure elements last.

• Use multipliers to equate the number of atoms on both sides.

• Check the final equation to ensure all elements are balanced.

Stepwise Balancing Example: Cobalt(III) Oxide and Carbon

Write a balanced equation for the reaction between solid cobalt(III) oxide and solid carbon to produce solid cobalt and carbon dioxide gas.

Skeletal equation: $Co_2O_3(s) + C(s) \rightarrow Co(s) + CO_2(g)$

• Balance O: $Co_2O_3(s) + C(s) \rightarrow Co(s) + CO_2(g)$ To balance O, put a 2 before $Co_2O_3$ and a 3 before $CO_2$: $2Co_2O_3(s) + C(s) \rightarrow Co(s) + 3CO_2(g)$

• Balance Co: $2Co_2O_3(s) + C(s) \rightarrow Co(s) + 3CO_2(g)$ To balance Co, put a 4 before $Co(s)$: $2Co_2O_3(s) + C(s) \rightarrow 4Co(s) + 3CO_2(g)$

• Balance C: $2Co_2O_3(s) + C(s) \rightarrow 4Co(s) + 3CO_2(g)$ To balance C, put a 3 before $C(s)$: $2Co_2O_3(s) + 3C(s) \rightarrow 4Co(s) + 3CO_2(g)$

Check the balance:

Left	Right
4 Co atoms	4 Co atoms
6 O atoms	6 O atoms
3 C atoms	3 C atoms

The equation is balanced.

Stepwise Balancing Example: Combustion of Butane

Write a balanced equation for the combustion of gaseous butane ($C_4H_{10}$), a fuel used in portable stoves and grills, in which it combines with gaseous oxygen to form gaseous carbon dioxide and water.

Skeletal equation: $C_4H_{10}(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)$

• Balance C: $C_4H_{10}(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)$ To balance C, put a 4 before $CO_2$: $C_4H_{10}(g) + O_2(g) \rightarrow 4CO_2(g) + H_2O(g)$

• Balance H: $C_4H_{10}(g) + O_2(g) \rightarrow 4CO_2(g) + H_2O(g)$ To balance H, put a 5 before $H_2O$: $C_4H_{10}(g) + O_2(g) \rightarrow 4CO_2(g) + 5H_2O(g)$

Further steps (not shown in the images) would include balancing O by adjusting the coefficient for $O_2$ and checking the final balance.

Key Terms and Concepts

• Reactant: A substance that undergoes change during a chemical reaction.

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

• Stoichiometric coefficient: The number placed in front of a chemical formula in an equation to indicate the relative number of molecules or moles.

• Conservation of mass: The principle that the total mass of reactants equals the total mass of products in a chemical reaction.

Summary Table: States of Matter in Chemical Equations

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

Example Equations

• $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

• $2Co_2O_3(s) + 3C(s) \rightarrow 4Co(s) + 3CO_2(g)$

• $C_4H_{10}(g) + O_2(g) \rightarrow 4CO_2(g) + 5H_2O(g)$

Additional info: Later sections (not shown in images) likely cover more advanced balancing, stoichiometry, limiting reactants, and yield calculations, as suggested by the text transcript.