FLOW CHART FOR LEARNING REACTION STIOCHIOMETRY
VOLUME
OF GAS
VOLUME
OF GAS
The mole ratio, and the reaction enthalpy (∆Hr)
are given by the balanced thermochemical equation.
n=
PV / RT
GRAMS OF
A SOLID
molar
mass
V=
nRT / P
MOLES OF
“A”
mole
ratio
MOLES OF
“B”
GRAMS OF
A SOLID
molar
mass
(or liquid or gas)
(or liquid or gas)
concentration
(molarity)
∆Hr
q = ∆Hr x moles
VOLUME OF
SOLUTION
‘Given’ quantities
are here at the left
moles = q / ∆Hr
volume
of solution
∆Hr
HEAT OF REACTION (q)
q, ∆H positive: heat absorbed, endothermic rxn
q, ∆H negative: heat produced, exothermic rxn
CONCENTRATION
OF A SOLUTION
‘Desired’ quantities
are here at the right
The Big Picture: In essence, all reaction stoichiometry problems are the same. You are given a measured quantity (an amount
of substance or heat), and this dictates the amounts of all the other substances consumed (reactants) or produced (products), as well as the amount of heat (either produced or consumed) . Sometimes you are given quantities of two or more
reactants, and you must identify the limiting reagent before you proceed. This becomes MOLES "A" in the chart above.
Clarifying points are in hexagonal boxes. USE THIS AS A LEARNING TOOL - DON’T RELY ON IT INDEFINITELY.
Using this chart:
1) The quantities you are given in the problem are in the boxes at the left - start there. Conversion factors needed to get to
moles are in the ovals. Mole ratios come from the coefficients in the balanced equation. Then (again) use the factors in the
ovals to get from moles to the desired result (in the boxes on the right). Watch your units, and you’ll know whether you
should be multiplying or dividing when converting to and from moles.
2) The chart works from left to right regardless of whether your are given a reactant quantity or product quantity.
3) Heat is usually an answer to the problem (e.g. ‘How much heat is produced when 5 g react ?’), but can also be a given
(‘5 kJ of heat was produced, how many grams reacted?’). You can start or end at the “Heat of Reaction” box, as appropriate. In any event, heed the arrows in the chart – they may seem backwards at first glance.
Examples: See if you can "trace" these through the chart.
• Theoretical Yield: Usually the standard "grams to moles, moles to moles, moles to grams" problem.
• Gas Laws Lab: How many grams reacted when a given volume of gas was produced? Start at the upper left, VOLUME
OF GAS (even though the gas is a product in the reaction of interest).
• Volumetric Lab: What is the concentration of an unknown solution that react with a given volume of a "known" solution? Start at the lower left.
•Problem 6.45 b) : Start at GRAMS -> MOLES OF A -> HEAT OF REACTION.
•Problem 6.47 a) : Start at HEAT OF REACTION -> MOLES OF “B” -> GRAMS