Reaction Stoichiometry and Conservation of Mass

Unit 8B: Reaction Stoichiometry

• Conservation of Mass
  • Mass is conserved in chemical reactions; it is neither created nor destroyed.
  • Mass-to-mass conversions are based on balanced chemical equations.

Hydrocarbon Combustion Example

• Example Reaction: CxHy + O2 → CO2 + H2O
• If 1.50 grams of CO2 and 0.817 grams of H2O are produced:
  • a) Find the empirical formula of the hydrocarbon (CxHy).
  • b) Balance the reaction using coefficients.
  • Hint: Calculate the mole ratio of carbon to hydrogen relevant to the empirical formula.

Balanced Chemical Equations

• Knowing the amount of one substance allows predicting the amounts of other substances in a reaction.

Key Concepts

• Mole: Essential counting unit in chemistry; represents 6.022 x 10²³ particles.

  • Not measurable directly; based on conversions using molar mass.

• Molar Mass:

  • Each chemical has a unique molar mass (g/mol), found on the periodic table.

• Balancing Chemical Equations:

  • Required to uphold the law of conservation of matter.
  • Achieved through coefficients in the equations.

Example Balancing Exercise

• Balance the following equation:
  • Mg + __ HCl → MgCl2 + ___ H2
  • In this case, the balanced equation translates to:
    • Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)

Measuring Moles in Reactions

• Cannot measure moles directly by weight.

• Example reaction: Mg + 2 HCl → H2 + MgCl2

  • Mg: 24.31 g
  • HCl: 2(36.46) g = 72.92 g
  • H2: 2.02 g
  • MgCl2: 95.21 g

• Total mass remains constant:

  • Both sides add up to 97.23 grams.

Mole Ratios

• Compare the relative amounts of reactants and/or products.
  • Derived from balanced equations, serving as a 'recipe' for reactions, and solving scaling issues.

Reaction Stoichiometry

• Utilize information about one chemical to deduce information about others in the reaction.
• Predictive capabilities help determine success of a reaction.
  • Example: Given 12.0 grams of magnesium, calculate grams of magnesium chloride produced:
  • Reaction: Mg + 2 HCl → H2 + MgCl2
  • 1:1 mole ratio: 1 mole Mg yields 1 mole MgCl2.

Solving Stoichiometry Problems

• Determine mass and mole relationships.
  • Example: If 12.0 g of Mg is known:
  • Molar mass of Mg: 24.31 g/mol (12.0 g Mg = 0.494 mol Mg).
  • Resulting moles of MgCl2 produced can be calculated (same as Mg, 0.494 mol).
  • Mass of MgCl2 = moles × 95.21 g/mol = 47.0 g.

Substance Relationships in Stoichiometry

• Mass Relationships:
  • Known mass of a reactant allows calculation of unknown product mass via ratios.
  • Use coefficients from balanced equations to simplify calculations.
• Mole Relationships:
  • 1 mole = 22.4 L @ STP, or 6.022 x 10²³ particles.

Example Reaction Calculation

• Given: N2 + 3 H2 → 2 NH3
  • If 14.58 grams of nitrogen reacts, calculate expected mass of ammonia produced, considering significant figures.

Conclusion

• Understanding reaction stoichiometry is imperative for predicting outcomes in chemical reactions, confirming the conservation of mass, and utilizing mole relationships effectively.