Chemistry and Chemical Reactivity - Chapter 4: Stoichiometry

Chemistry and Chemical Reactivity - Chapter 4: Stoichiometry

4-1 Mass Relationships in Chemical Reactions: Stoichiometry

  • Stoichiometry is the study of mass relationships in chemical reactions, providing quantitative information about reactions.

  • The principle of mass conservation states that the total mass of reactants must equal the total mass of products:

    Total mass of reactants=Total mass of products\text{Total mass of reactants} = \text{Total mass of products}

  • Chemical equations must be balanced, meaning that the number of atoms in reactants equals the number of atoms in products.

4-2 Reactions in Which One Reactant is Present in Limited Supply

  • In chemical reactions involving multiple reactants, sometimes one reactant is in shortage, limiting the amount of products formed.
  • This reactant is referred to as the "Limiting Reactant."
  • The theoretical yield of products is determined by the limiting reactant; it is the maximum amount that can be formed based on the masses of reactants and their stoichiometry.

4-3 Percent Yield

  • The percent yield is a measure of the efficiency of a reaction, calculated as:

    % Yield=(Actual Yield (g or moles)Theoretical Yield (g or moles))×100\%\text{ Yield} = \left( \frac{\text{Actual Yield (g or moles)}}{\text{Theoretical Yield (g or moles)}} \right) \times 100

4-4 Chemical Equations and Chemical Analysis

  • Stoichiometric coefficients in a balanced equation represent the molar ratio of reactants and products, which can be used to convert between quantities.

  • Example: In the reaction

    2C<em>2H</em>6(l)+7O<em>2(g)→4CO</em>2(g)+6H2O(l)2C<em>2H</em>6(l) + 7O<em>2(g) \rightarrow 4CO</em>2(g) + 6H_2O(l)

  • For every 7 moles of oxygen, 4 moles of water are produced.

4-5 Measuring Concentrations of Compounds in Solutions

  • The concentration of a solution can be expressed in terms of molarity (M), defined as:

    M=moles of solutevolume of solution (L)M = \frac{\text{moles of solute}}{\text{volume of solution (L)}}

4-6 pH, a Concentration Scale for Acids and Bases

  • The pH scale measures the acidity or basicity of a solution and is calculated as:

    pH=−log⁡[H+]pH = -\log[H^+]

  • pH 7 represents a neutral solution. As [H+] increases, pH decreases, indicating higher acidity.

4-7 Stoichiometry of Reactions in Aqueous Solution

  • Aqueous reactions typically involve the transfer of ions in solution; thus, stoichiometric calculations involve moles, concentrations, and balanced equations.

4-8 Spectrometry

  • Spectrometry is a technique used to measure the concentration of substances in a solution based on the interaction of light and matter.

Molarity and Dilutions

  • Molarity (M) expresses the concentration of a solution as moles of solute per liter of solution.

  • Molarity can be affected by altering the volume of the solution. When solvent is added, the concentration (M) decreases:

    M<em>1V</em>1=M<em>2V</em>2M<em>1 V</em>1 = M<em>2 V</em>2

  • Where:

    • $M_1$: initial molarity
    • $V_1$: initial volume
    • $M_2$: final molarity
    • $V_2$: final volume

Preparation of Solutions

  • To prepare a solution of known concentration:
    1. Measure the desired amount of solute.
    2. Add less solvent than the final volume to dissolve the solute.
    3. Add solvent up to the mark on the volumetric flask.
    4. Mix the solution well.

Chemical Analysis Techniques

  • When analyzing a sample containing a sulfate ion,
    1. Add barium chloride to produce insoluble barium sulfate.
    2. Precipitated barium sulfate is collected by filtration and weighed.
  • This allows for the calculation of the sulfate ion concentration in the sample based on the mass of barium sulfate produced.

Combustion Analysis

  • In combustion analysis, hydrocarbons are burned to produce CO₂ and H₂O.
  • The amounts of CO₂ and H₂O produced can be used to deduce the empirical formula of the hydrocarbon.

Spectrophotometry Overview

  • The Beer-Lambert Law relates absorbance ($A$) to solute concentration ( extit{c}):

    A=ϵlcA = \epsilon l c

  • Where:

    • $\epsilon$: molar absorptivity
    • $l$: path length of the sample
    • $c$: concentration of the solute

  • This law shows a linear relationship between absorbance and concentration, allowing for concentration determination through calibration.