Stoichiometry
Chemical Equations
Short hand way to represent chemical reactions
H2 + Cl
2 → HCl
Symbols
+ = reacts with
→ = produces, yields
Δ = adding energy (usually heat)
↔ = reversible
aq - aqueous
s - solid
l - liquid
g - gas
Balancing Reactions
Law of Conservation of mass Change coefficients not subscripts why? Start balancing with elements other than H and
O Next balance H and then O last Trial and Error
Balancing Practice
__Al +__HCl → __AlCl3 + __H
2
__H3PO
4 +__ HCl → __PCl
5 + __ H
2O
__ CO(g) + __H2(g) → __C
8H
18(l) + __H
2O
__C2H
6 + __O
2 → __H
2O + __CO
2
__(NH4)
3PO
4 + __Pb(NO
3)
4 → __Pb
3(PO
4)
4 +
__NH4NO
3
Types of Reactions
Combination – A + B → C
N2(g) + 3H2(g) → 2NH3(g)
Decomposition – C → A + B
2KClO3(s) → 2KCl(s) + 3O2(g)
Combustion – oxidation reaction that is exothermic
Most often involves O2
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
Single Replacement – AB + C → A + BC
Use activity series
2HCl + F2 → 2HF + Cl2 Double Replacement – AB + CD → AD + CB
3 Driving Factors
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Formula Weights
Definition – sum of the atomic weights of each atom
Molecular weight – formula weight of a molecule
Calculate the formula weight of: Ca(MnO
4)
2
KH2PO
4
Percent Compostion
Definition - percent of mass contributed by each element
% Comp = # of a particular atom x atomic mass
Total mass of the compound Calculate the percentage of phosphorus in
P4O
10.
Mole
Avogadro's # - 1 mole = 6.02x1023 objects Representative Particles
Formula Units Molecules Atoms
Calculate the number of oxygen atoms in 1.5 moles of sodium carbonate.
Mole
Molar mass – mass of one mole of a compound Check if you are working with an isotope Find the sum of the masses of the atoms
Unit is g/mol
Grams ← use g/mol → Moles ← use Avogadro's # → Representative particles
Mole Practice
What is the mass, in grams of 6.33 mol of NaHCO
3?
How many nitric acid molecules are in 4.20 grams of HNO
3? How many oxygen atoms?
Empirical and Molecular Formula
Finding Empirical Formula Assume 100 g Convert from grams to moles Divide by smallest # of moles Use results as subscripts
Convert Empirical to Molecular Divide molar mass by empirical mass Multiply empirical formula by result
Molecular Formula Example
What is the molecular formula of a compound that is 71.65% Cl, 24.27% C, 4.07% H and has a molar mass of 98.96g/mol?
Molecular Formula Practice
Caffeine contains 49.48% C, 5.15% H, 28.27% N, 16.49% by mass and has a molar mass of 194.2 g/mol. Determine the molecular formula of caffeine.
Molecular Formula Example
Isopropyl alcohol is made up of C,H, and O. Combustion of 0.255g of this alcohol produces 0.521g CO
2 and 0.306g H
2O. What is the
molecular formula if the molar mass is 60 g/mol.
Molecular Formula Practice
Caproic acid, which is responsible for the foul odor of dirty socks, is composed of C, H, and O atoms. Combustion of a 0.255g sample of this compound produces 0.512 g CO
2 and 0.209g
H2O. What is the molecular formula is the molar
mass is 116 g/mol?
Calculations from Balances Equations
First ensure that equation is balanced. Use mole ratios Grams A → Moles A → Moles B → Grams B When potassium chlorate is heated it
decomposes in potassium chloride and oxygen. How many grams of oxygen can be prepared from 4.50g of potassium chlorate.
Calculations practice
Milk of magnesia (Mg(OH)2) is often used as an
antacid. It neutralizes excess hydrochloric acid secreted by the stomach. How many grams of milk of magnesia are needed to remove 150 mg of hydrochloric acid?
Limiting Reagents
Limiting Reagents – reactant that runs out Excess Reagents – reactant that is left over Limiting reagent gives you the least amount of
product
Limiting Reagent Example
A strip of zinc metal weighing 2.00g is placed in an aqueous solution containing 2.50g of silver nitrate. Silver and zinc nitrate are produced. Which reactant is limiting? How many grams of each product will form? How grams of excess reagent will be left over?
Limiting Reagent Practice
Nitrogen gas can prepared by passing gaseous ammonia over solid copper (II) oxide at high temperatures. The other products of the reaction are solid coper and water vapor. If a sample containing 18.1g NH
3 is reacted with
90.4g CuO, which is the limiting reagent? How many grams of N
2 are made? How much
excess reagent is left?
Yield
Theoretical Yield – amount of product formed when all of the LR is used up
Actual Yield – amount of product formed during an experiment
Percent Yield – actual yield x 100%
theorectical yield
Yield Example
When iron (III) oxide react with carbon monoxide to form iron and carbon dioxide. If you start with 150g of iron (III) oxide as the limiting reagent, what is the theoretical yield? If the actual yield of iron was 87.9g, what was the percent yield?
Yield Practice
Methanol (CH3OH) is used as a fuel in race cars. It can be manufactured by a combination of gaseous carbon monoxide and hydrogen. If 68.5 kg of CO is reacted with 8.60 kg H2. Calculate the theoretical yield of methanol. If 3.57x104g of methanol is produced, what is the percent yield of methanol?
Homework
6, 8, 18, 30, 46, 48, 56, 58, 60, 70, 74, 76, 78
Homework
6, 8, 18, 30, 46, 48, 56, 58, 60, 74, 76, 78