Chemistry Unit 7

Balancing Equations

  1. Law of Conservation of Mass:

    • Atoms are neither created nor destroyed in a chemical reaction.

    • The number of atoms of each element must be the same on both sides of the equation.

  2. Steps for Balancing Chemical Equations:

    1. Write the unbalanced equation with correct chemical formulas.

    2. Count the number of atoms of each element on both sides of the equation.

    3. Balance the elements one at a time by adjusting coefficients. Start with elements that appear in only one reactant and one product. Avoid balancing hydrogen and oxygen until last.

    4. Check that the equation is balanced: the number of each type of atom is the same on both sides.

    5. Make sure the coefficients are in the simplest whole-number ratio. If necessary, divide all coefficients by their greatest common divisor.

  3. Example:

    • Unbalanced equation: H2 + O2 \rightarrow H_2O

    • Balanced equation: 2H2 + O2 \rightarrow 2H_2O

Reaction Types

  1. Synthesis (Combination):

    • Two or more reactants combine to form a single product.

    • General form: A + B \rightarrow AB

    • Example: 2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)

  2. Decomposition:

    • A single reactant breaks down into two or more products.

    • General form: AB \rightarrow A + B

    • Example: CaCO3(s) \rightarrow CaO(s) + CO2(g)

  3. Single Replacement (Displacement):

    • One element replaces another in a compound.

    • General form: A + BC \rightarrow AC + B

    • Example: Zn(s) + CuSO4(aq) \rightarrow ZnSO4(aq) + Cu(s)

  4. Double Replacement (Metathesis):

    • Ions in two compounds exchange places

    • General form: AB + CD \rightarrow AD + CB

    • Example: AgNO3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO3(aq)

  5. Combustion:

    • A substance rapidly combines with oxygen, usually producing heat and light.

    • General form: Fuel + O2 \rightarrow CO2 + H_2O

    • Example: CH4(g) + 2O2(g) \rightarrow CO2(g) + 2H2O(g)

Classifying Equations

  1. Acid-Base Reactions:

    • Involve the transfer of protons (H^+ ions).

    • Acid + Base → Salt + Water (Neutralization).

    • Example: HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)

  2. Redox Reactions (Oxidation-Reduction):

    • Involve the transfer of electrons.

    • Oxidation: Loss of electrons.

    • Reduction: Gain of electrons.

    • Example: 2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)

Chemical Reactions

  1. Evidence of a Chemical Reaction:

    • Color change

    • Formation of a precipitate (solid)

    • Gas production

    • Heat absorption or release (temperature change)

  2. Exothermic Reactions:

    • Release heat to the surroundings.

    • The temperature of the surroundings increases.

    • Example: Combustion reactions.

  3. Endothermic Reactions:

    • Abs