How Much? The Amount of Chemical Change

R2.1.1 Chemical Equations :

  • Chemical equations show the ratio in which chemical species each with each other and the relationship to the amount of product formed
  • Mass is conserved in all chemical reactions – total number of atoms on both sides must be same – balancing equations
  • State symbols are added to chemical equations to provide information about the state of the reactants and products

R2.1.2 Using Mole Ratios in Equations :

  • The mole ratio of an equation can be used to determine the masses of reactants and products
  • Keep these formulas in mind
    • n = m/M
    • n = v//molar volume
    • c = n/v
  • Most problems involve identifying mole ratio, converting to moles, and then using information from the question to solve for mass/concentration/etc
  • Avogadro’s law : equal volumes of all gases measured under the same conditions of pressure and temperature contain equal number of moles
  • Molar volume – volume occupied by 1 mole of gas (at STP) = 22.7 dm3 mol-1
  • Increase in temperature, increase in molar volume
  • Increase in pressure, decrease in molar volume

🧠 Paper 2 Tip : STP refers to a temperature of 273K and a pressure of 100 kPa.

  • A standard solution is a solution of known concentration
  • Titration is a technique of volumetric analysis used to find the unknown concentration of a solution by reacting it with a solution of known concentration and volume
  • C1V1/ N1 = C2V2/N2
    • C = concentration
    • V = volume
    • N = coefficient of substance in balance eq

🧠 Paper 2 Tips (for Data Based Responses) : Steps of titration

  • A pipette is used to measure known volume of one solution into a conical flask
  • The other solution is poured using a funnel into a burette
  • The point at which the two solutions have reacted fully is called the equivalence point
  • The equivalence point is determined using an indicator, which changes color at the end point
  • Back titration – unknown excess of A added to X
  • Excess A titrated with B – to find how much of A reacted with X – find n(X)
  • Works when X = solid

R2.1.3 The limiting reactant and theoretical yield :

  • Reaction finishes when limiting reactant runs out
  • Limiting reactant determines quantity of product
  • Theoretical yield = maximum amount of product produced assuming 100% of reactants are converted to products
  • For calculations of actual and theoretical yield – ensure to used limiting reactant in the mole ratios

⭐️ Limiting reactant is reactant with smallest n.

R2.1.4 Percentage Yield :

  • % yield = experimental yield/theoretical yield * 100
  • If experimental yield < theoretical yield
    • Losses during transfer of substances
    • Side reactions
    • Product decomposing
    • Incomplete reactions
  • If experimental yield > theoretical yield
    • Residual solvent
    • Impurities

R2.1.5 Atom Economy :

  • Atom economy is a measure of how efficient a reaction is in converting as much of reactants into useful product
    • Molar mass of product/ Total molar mass of all reactants * 100
  • Principles of green chemistry
    • Reduce waste
    • Using readily available and safe materials
    • Using little solvent
    • Using little energy
    • Safe disposal