Chemical Change: Stoichiometry
Quantities in chemical reactions
- coefficients in chemical equation give relative number of molecules,
formula units or moles
Mole to mole calculations
- stoichiometric ratio shows molar amount of any two substances
used/produced in a reaction
- stoichiometric relationship expressed by general chemical reaction
Calculations involving mass and moles
- can find the amount of moles from mass
- can then be used to find molar amounts of other substances in reaction
Measuring gases in chemical change
- amount of gas used/produced measured by volume (L)
- ideal gas equation:
PV= nRT
n= PV/ RT
P= pressure of gas in kPa
V= volume of gas in litres
n= moles of gas
R= universal gas constant 8.3145 JK-1 mol-1
T= temperature of gas in Kelvin
- if gas pressure is given in atm or millimitres of mercury convert it to kPa
o P(kPa) = P(atm) x 101.3
o P(kPa)= P(mm Hg) x 101.3 / 760
- If temp given in Celsius convert it to Kelvin
o T(K) = T(C) + 273
Stoichiometry with gas volumes
- use ideal gas equation to calculate moles of gas in a reaction
- can be used to find molar amounts of other reagents in equation
Stoichiometry with solutions
- n= cV
Stoichiometry with percentage purity
- many reagents are less than 100% pure
- percentage purity gives percentage by mass of element/compound in
impure sample
- % purity= m(pure compound) x 100 / m(impure sample)
Percentage yield of a chemical reaction
- theoretical/maximum yield of a reaction gives amount of product that
would be formed when limiting reagent is fully consumed
- actual yield is usually less because
o reaction may not go to completion
o side reactions may give different products
o may not be possible to fully extract product from final reaction
mixture
- % yield= actual yield x 100 / theoretical yield
Limiting reagent
- in a reaction one reagent is in excess, other is fully consumed (limiting
reagent)
- found by comparing actual mole ratio and stoichiometric mole ratio
- actual ratio uses amounts given
- stoichiometric ratio uses equation coefficients
- if actual ratio < stoichiometric ratio then numerator is limiting reagent
- if actual ratio > stoichiometric ratio then denominator is limiting reagent
- ratio= n(reagent) / n(other reagent)
- limiting reagent can then be used to calculate amount of any other
substance in reaction