Stoichiometry Level 2
Stoichiometry with Gases
• If the pressure and temperature are constant,
the ratio of moles in the balanced equation is
the ratio of liters in an all gas reaction.
• At STP 1 mole of gas occupies 22.4 L
– 1 mole = 22.4 L
Molar Volume
• In the following reaction, if 17 g of Mg react,
how many L of H2 forms?
Mg (s) + HCl (aq)  MgCl2 (aq) + H2 (g)
17.0 g
Mg
1 mol
Mg
1 mol
=
15.7
L
H2 22.4 L H2
24.31 g 1 mol
Mg
Mg
1 mol
H2
H2
Molar Volume Problems
• How many grams of KClO3 are req’d to
produce 9.00 L of O2 at STP?
2KClO3  2KCl + 3O2
?g
9.00 L
9.00 L
O2
1 mol
O2
2 mol 122.55
KClO3 g KClO3
22.4 L
O2
3 mol
O2
1 mol
KClO3
= 32.8 g
KClO3
Energy &
Stoichiometry
Exothermic and Endothermic
• Exothermic process – heat is released into the
surroundings
– Exo = Exit
HEAT
• Endothermic Process – heat is absorbed from
the surroundings
– Endo = Into
HEAT
Thermochemical Equations
• In a thermochemical equation, the energy of change
for the reaction can be written as either a reactant
or a product
• Enthalpy: the heat content of a system at constant
pressure (ΔH)
• Endothermic (positive ΔH)
2NaHCO3 + 129kJ
Na2CO3 + H2O + CO2
• Exothermic (negative ΔH)
CaO + H2O
Ca(OH)2 + 65.2kJ
Write the thermochemical equation for the oxidation of
Iron (III) if its ΔH= -1652 kJ
Exo
4Fe(s) + 3 O2(g)→ 2 Fe2O3(s) + 1652 kJ
How much heat is evolved when 10.00g of Iron is
reacted with excess oxygen?
10.00g Fe
1 mol
55.85g Fe
1652 kJ
4 mol Fe
=73.97 kJ of heat
Write the thermochemical equation for the
decomposition of sodium bicarbonate, with
a ΔH = + 129 kJ:
Endo
2 NaHCO3 + 129kJ → Na2CO3(s) + H2O + CO2
How much heat is required to break down 50.0g of
sodium bicarbonate?
50.0 g NaHCO3 1 mol NaHCO3 129 kJ
83.9 g NaHCO3 2 mol NaHCO3
=38.4 kJ of heat
Write the thermochemical equation for a single
replacement of calcium oxide and water with a ΔH= 65.2 kJ:
Exo
CaO + H2O → Ca(OH)2 + 65.2kJ
How much energy is released when 100 g of calcium
oxide reacts?
100 g CaO
1 mol CaO
65.2 kJ
56.07 g CaO
1 mol CaO
=116 kJ of heat
Write the thermochemical equation for the
decomposition of magnesium oxide with a
61.5 kJ:
Endo
ΔH= +
2 MgO + 61.5 → 2 Mg + O2
How many grams of oxygen are produced when
magnesium oxide is decomposed by adding 420 kJ of
Energy?
420 kJ
1 mol O2
61.5 kJ
31.98 g O2 =218 g of O
2
1 mol O2