Chemistry 20 Unit D: Gravimetric Stoichiometry Study Notes

Fundamentals of Chemical Changes

• Reaction Components: Reactants are substances that react; products are the new substances created.

• Energy Changes:

  • Exothermic: Energy is released to the environment (e.g., combustion).

  • Endothermic: Energy is absorbed from the environment (e.g., cold packs).

• Evidence of Change: Color change, light emission, heat flux, smell, and formation of a precipitate or gas.

• Law of Conservation of Mass: The mass of the reactants must equal the mass of the products.

Chemical Equations and Balancing

• Skeleton Equations: Unbalanced formula equations.

• Balancing Rules:

  • Use coefficients to balance atoms on both sides.

  • Balance elements in the largest compound first.

  • Leave oxygen and single elements for last.

  • Treat identical polyatomic ions as single units if they remain intact.

  • Fractions or decimals should not be used as final coefficients.

States of Matter and Solubility

• Metals: $(s)$ at room temperature except for $Hg(l)$.

• Diatomics: $H_2(g)$, $N_2(g)$, $O_2(g)$, $F_2(g)$, $Cl_2(g)$, $Br_2(l)$, and $I_2(s)$.

• Others: $S_8(s)$, $P_4(s)$, and $C(s)$.

• Solubility Rules:

  • Group 1 ions ($Na^+$, $K^+$) and $NH_4^+$ or $NO_3^{-}$ are always $(aq)$.

  • Precipitate: A solid formed from mixing two aqueous solutions.

  • Molecular compounds: Small are $(g)$, medium are $(l)$, and large are $(s)$.

Types of Chemical Reactions

• Formation (Synthesis): $A + B \rightarrow AB$.

• Decomposition: $AB \rightarrow A + B$.

• Combustion: Reaction with $O_2(g)$. Hydrocarbon combustion always produces $CO_2(g)$ and $H_2O(g)$.

• Single Replacement: $A + BC \rightarrow B + AC$. One element replaces another in a compound.

• Double Replacement: $AB + CD \rightarrow AD + CB$. Two compounds exchange ions, often forming a precipitate.

Calculations and The Mole

• Significant Digits: Calculations must reflect the precision of the tools used. In multiplication/division, use the least number of significant digits from the given values.

• Molar Mass ($M$): The sum of atomic masses in a compound (unit: $g/mol$).

• The Mole ($n$): A chemical amount representing $6.02 \times 10^{23}$ particles (Avogadro's Number).

• Unit Analysis: A method to convert units using ratios where the desired unit is on top ($\frac{\text{UNKNOWN}}{\text{KNOWN}}$).

Stoichiometry and Gravimetric Analysis

• Stoichiometry: Dealing with relative quantities of reactants and products based on the Law of Conservation of Mass.

• Mole Ratio: Uses coefficients from the balanced equation to convert moles of a given substance to a required substance ($\frac{\text{coefficient of unknown}}{\text{coefficient of known}}$).

• Gravimetric Analysis: Determining values based on the mass of a precipitate.

• Yield Calculations:

  • Predicted (Theoretical) Yield: Mass calculated via stoichiometry.

  • Experimental Yield: Mass actually obtained in a lab.

  • $\text{Percent Yield} = \frac{\text{Experimental}}{\text{Predicted}} \times 100\%$

  • $\text{\% Difference (Error)} = \frac{|\text{Predicted} - \text{Experimental}|}{\text{Predicted}} \times 100\%$

Limiting and Excess Reagents

• Limiting Reagent ($LR$): The reactant completely consumed in a reaction; it determines the maximum amount of product formed.

• Excess Reagent: The reactant present in a greater amount than required; some remains after the reaction stops.

• Identification: Calculate the moles of a product that could be formed from each reactant; the reactant producing the smallest amount is the $LR$.

• Remaining Excess: $\text{Initial mass} - \text{Mass actually consumed during reaction}$.