Balancing Chemical Equations

Chemical Reactions

• A chemical reaction converts one or more substances into different substances.
• A chemical equation uses chemical symbols to represent a chemical reaction.
  • It shows the formulas of reactants (starting substances) and products (resulting substances).
  • Reactants are on the left, products are on the right, separated by an arrow.
  • $reactants \longrightarrow products$
  • Balanced chemical equations have the same number of each type of atom on both sides.

Writing Balanced Chemical Equations

• Write the unbalanced equation using correct chemical formulas for reactants/products.
  • Example: $CH_4(g) + O_2(g) \longrightarrow CO_2(g) + H_2O(g)$
• Find coefficients to equalize atoms of each element on both sides of the arrow.
  • Balance elements that appear in only one reactant and one product, starting with elements other than H and O.
  • Balance polyatomic ions as a unit.
  • Balance H atoms, then O atoms.
  • Example: $CH_4(g) + 2O_2(g) \longrightarrow CO_2(g) + 2H_2O(g)$
• Check that the number of atoms of each element is balanced.
• Ensure coefficients are the smallest possible whole numbers.

Additional Information in Chemical Equations

• States: $s$ (solid), $l$ (liquid), $g$ (gas), $aq$ (aqueous)
• $\triangle$ indicates addition of heat
• Chemical equations should specify the physical states of all reactants and products.
  • Example: $CH_4(g) + 2O_2(g) \longrightarrow CO_2(g) + 2H_2O(g)$

Stoichiometry

• Is the quantitative study of reactants and products in a chemical reaction.
• Balanced chemical equations are used to determine the amounts of reactants and products.

Limiting Reactants

• The limiting reactant is completely used up in a reaction.
• The excess reactant is present in excess.
• The amount of product formed is determined by the limiting reactant.
• Example: $N_2(g) + 3H_2(g) \longrightarrow 2NH_3(g)$
  • If 1 mol $N_2$ and 3 mol $H_2$ are present, both are completely used up
  • If 1 mol $N_2$ and 2 mol $H_2$ are present, $H_2$ is limiting reactant

Reaction Yield

• Theoretical yield is the product amount if all limiting reactant reacts.
• Actual yield is the amount of product obtained from a reaction.
• Percent yield indicates the percentage of theoretical yield actually obtained.
  • Percent yield = $\frac{Actual,yield}{Theoretical,yield} \times 100$%

Aqueous Solutions

• A solution is a homogeneous mixture of two or more substances.
• The solute is the substance present in a smaller amount.
• The solvent is the substance present in a larger amount.
• An aqueous solution has water as the solvent.

Electrolytes

• An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity.
  • Example: $NaCl(s) \overset{H_2O}{\longrightarrow} Na^+(aq) + Cl^-(aq)$
• A nonelectrolyte results in a solution that does not conduct electricity.
  • Example: $C_6H_{12}O_6(s) \overset{H_2O}{\longrightarrow} C_6H_{12}O_6(aq)$

Acids and Bases

• An acid produces $H^+$ ions when dissolved in water.
  • Example: $HCl(g) \overset{H_2O}{\longrightarrow} H^+(aq) + Cl^-(aq)$
• A base produces $OH^-$ ions when dissolved in water.
  • Example: $NaOH(s) \overset{H_2O}{\longrightarrow} Na^+(aq) + OH^-(aq)$

Types of Reactions

• Precipitation reaction, resulting in formation of an insoluble product
  • Example: $AgNO_3(aq) + NaCl(aq) \longrightarrow AgCl(s) + NaNO_3(aq)$
• Acid-base neutralization reaction, involving reaction of an acid with a base
  • Example: $HCl(aq) + NaOH(aq) \longrightarrow H_2O(l) + NaCl(aq)$
• Redox (oxidation-reduction) reaction, involving transfer of electrons
  • Example: $2Mg(s) + O_2(g) \longrightarrow 2MgO(s)$

Concentration

• Is the amount of solute in a given quantity of solvent or solution.

Molarity (M)

• Expresses concentration as moles of solute per liter of solution.
  • $M = \frac{moles,of,solute}{liters,of,solution}$

Dilution

• Is the process of preparing a less concentrated solution from a more concentrated one.
  • $M_1V_1 = M_2V_2$
    • $M_1$ and $V_1$ are molarity and volume of the concentrated solution.
    • $M_2$ and $V_2$ are molarity and volume of the diluted solution.