Notes on Chemical Reactions and Energy Calculations

Chemical Reactions

• Conversion of substances changes a compound's composition.

• Key processes:

  • Breaking bonds in reactants.

  • Forming new bonds in products.

Writing Chemical Reactions

• Reactants on the left, products on the right.

• Example: $CH<em>4 + O</em>2 \rightarrow CO<em>2 + H</em>2O$

• The law of conservation of mass: Atoms cannot be created or destroyed.

• Balancing uses coefficients.

Balancing Equations

• Start with complex compounds; balance simpler ones last.

• Trial and error often needed.

Solubility of Ionic Compounds

• Ionic solids held by electrostatic forces.

• Insoluble compounds do not dissociate in water when separation energy is too high.

Precipitation Reactions

• Form insoluble products (precipitates).

• Use solubility rules to predict outcomes.

• Reaction example: $KI(aq) + Pb(NO<em>3)</em>2(aq) \rightarrow PbI_2(s)$

Redox Reactions

• Involves electron transfer:

  • Oxidation: Loss of electrons.

  • Reduction: Gain of electrons.

• Agents defined: reducing agent (oxidized) & oxidizing agent (reduced).

The Mole and Avogadro's Number

• $1 \text{ mole} = 6.02 \times 10^{23} \text{ particles}$

• Molar mass (g) equals formula weight (amu).

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Converting Grams to Moles

• Use molar mass as a conversion factor.

Theoretical and Actual Yields

• Theoretical yield: expected from reaction; actual yield: what is isolated.

• Percent yield formula: $\text{Percent Yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$

Energy Calculations

• Energy units: $1 ext{ cal} = 4.184 ext{ J}$, $1 ext{ kJ} = 1000 ext{ J}$.

• Specific heat equation: $Q = SH \cdot m \cdot \Delta T$

• Exothermic/Endothermic reactions based on heat absorption or release concerning enthalpy (ΔH).

• ΔH = (+) means endothermic; ΔH = (-) means exothermic.