introduction reaction stoichiometry involves the mass relationships between reactants and products...
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Introduction
Reaction stoichiometry involves the mass relationshipsbetween reactants and products in a chemical reaction.
It is based on chemical equations similar to the ones studied in the last section.
All reaction stoichiometry calculations start with a balanced equation.
You will need to be familiar with gram/mole relationships as studied earlier this year.
C3H8 + 5O2 3CO2 + 4H2O
The mole enables chemists to move from the microscopic world of atoms and molecules to the real world of grams .
Stoichiometry problems are classified between the information given in the problem and the information you are expected to find, the unknown.
The given and the unknown may be expressed in grams or moles.
The masses in the reaction are usually expressed in grams.
Definition of mole:
mole of a substance = grams of substance/MW of substance
You will need to use: i. molar ratios in a balanced equation.ii. molar masses of reactants and products.iii. balancing equations.iv. conversions between grams and moles.
Mole RatiosA mole ratio converts moles of one
compound in a balanced chemical equation into moles of another compound.
All stoichiometry problems use mole ratios.
Example
Reaction between magnesium and oxygen to form magnesium oxide. ( fireworks)
2 Mg(s) + O2(g) 2 MgO(s) (balanced)
Mole Ratios:2 : 1 : 2
Stoichiometry (working with ratios)
Ratios are found within a chemical equation.
2HCl + Ba(OH)2 2H2O + BaCl2 1 1
2 moles of HCl react with 1 mole of Ba(OH)2 to form 2 moles of H2O and 1 mole of BaCl2
coefficients give MOLAR RATIOS
Review: Molar Mass
A substance’s molar mass (molecular weight) is the mass in grams of one mole of the compound.
CO2 = 44.01 grams per mole
H2O = 18.02 grams per mole
Ca(OH)2 = 74.10 grams per mole
Review: Chemical Equations
C2H5OH + 3O2 ® 2CO2 + 3H2O
reactants products
1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water
Problem Type 1:
When you are given the amount of starting material in a reaction in moles and asked to calculate the amount of product in moles:
amount of reactant in moles amount of product in moles
Problem Type 2:
When you are given the amount of starting material in moles and asked to calculate the mass of product in grams:
amount of reactant in moles amount of product in moles mass of product in grams
Problem Type 3:
When you are given the mass of starting material in grams and asked to calculate the amount of product in moles.
amount of reactant in grams amount of reactant in moles amount of product in moles
Problem Type 4:
1. When you are given the mass of starting material in grams and asked to calculate the amount of product in grams.
amount of reactant amount of reactant in moles amount of product in moles amount of product
in grams in grams
Problem Type 1: Given and unknown quantities are in moles
Amount in molesof known substance
Amount in moles of unknown substance
CO2 + LiOH Li2CO3 + H2O
How many moles of lithium hydroxide are required to react with 20 moles of CO2
CO2 + 2 LiOH Li2CO3 + H2O balanced equation
Given: amount of CO2 = 20 moles
Unknown: amount of LiOH in moles
Amount of CO2 in moles Amount of LiOH in moles
mol CO2 x mol LiOH / mol CO2 = mol LiOH
20 mol CO2 x 2 mol LiOH / 1 mol CO2 = 40 mol LiOH
mole ratio
When N2O5 is heated, it decomposes:
2N2O5(g) 4NO2(g) + O2(g)
a. How many moles of NO2 can be produced from 4.3 moles of N2O5?
2N2O5(g) 4NO2(g) + O2(g)
4.3 mol ? mol
Problem Type 1
Units match
Mole – Mole Conversions
When N2O5 is heated, it decomposes:
2N2O5(g) 4NO2(g) + O2(g)
a. How many moles of NO2 can be produced from 4.3 moles of N2O5?
= moles NO2
4.3 mol N2O5
52
2
ON mol2
NO mol48.6
2N2O5(g) 4NO2(g) + O2(g)4.3 mol ? mol
Mole – Mole Conversions
Units match
b. How many moles of O2 can be produced from 4.3 moles of N2O5?
= mole O2
4.3 mol N2O5
52
2
ON 2mol
O mol12.2
2N2O5(g) 4NO2(g) + O2(g)
4.3 mol ? mol
Problem Type 1: Given and unknown quantities are in moles
Amount in molesof known substance
Amount in moles of unknown substance
How many moles of ammonia are produced when 6 moles of hydrogen gas react with an excess of nitrogen gas.
3 H2 + N2 2 NH3 balanced equation
H2 + N2 NH3 unbalanced equation
Given: amount of H2 = 6 moles
Unknown: amount of NH3 in moles
Amount of H2 in moles Amount of NH3 in moles
(mol H2) x (mol NH3 / mol H2) = mol NH3
(6 mol H2) x (2 mol NH3 / 3 mol H2) = 4 mol NH3
Mole ratio
Problem Type 2: Given amount is in moles and unknown quantity is in grams
Amount in molesof known substance
Amount in grams of unknown substance
Problem Type 3: Given amount is in grams and unknown quantities are in moles
Amount in gramsof known substance
Amount in moles of unknown substance
Problem Type 2
mole ↔ gram
In plants when carbon dioxide reacts with water it produces glucose and oxygen:
6CO2 + 6H2O(l) C6H12O6(s) + 6O2(g)
How many grams of C6H12O6 is produced when 3.0 mol of water react with carbon dioxide?
= g C6H12O6
3.0 mol H2O
6126
6126
OHC mol1
OHC g2.18090
OH mol6
OHC mol1
2
6126
3.0 mol ? gramsUnits match
6CO2 + 6H2O C6H12O6 + 6O2
Problem Type 2
mole ↔ gram
In plants when carbon dioxide reacts with water it produces glucose and oxygen:
6CO2 + 6H2O(l) C6H12O6(s) + 6O2(g)
How many grams of CO2 is needed to react with 3.0 mol of water?
= g CO2
3.0 mol H2O
2
2
CO mol1
CO 4g4132
OH mol6
CO mol6
2
2
3.0 mol? gramsUnits match
6CO2 + 6H2O C6H12O6 + 6O2
Problem Type 2
mole ↔ gram
When magnesium burns in air, it combines with oxygento form magnesium oxide according to the following equation:
2Mg + O2(g) 2MgO(s)
How many grams of MgO is produced from 2.0 mol of magnesium?
= g MgO2.0 mol Mg
MgO mol1
MgO 0g480
Mg mol2
MgO mol2
2.0 mol ? gramsUnits match
2Mg + O2 2MgO
Problem Type 3
When N2O5 is heated, it decomposes:2N2O5(g) 4NO2(g) + O2(g)
a. How many moles of N2O5 were used if 210g of NO2 were produced?
= moles N2O5
210 g NO2
2
52
NO mol4
ON mol22.28
2
2
NO g0.46
NO mol
gram ↔ mole
2N2O5(g) 4NO2(g) + O2(g)
210g? moles
Units match
Problem Type 3
Nitric acid is produced from the catalytic oxidation of ammonia
NH3(g) + O2(g) NO(g) + H2O(g)
a. How many moles of NO were produced from 824g of NH3?
= moles NO824 g NH3
3NH 4
NO mol4
mol48.4
3
3
NH 04.17
NH mol1
g
gram ↔ mole
824g ? moles
Units match
4NH3(g) + 5O2 4NO(g) + 6H2O(g)
Steps Involved in Solving Mass-Mass Stoichiometry Problems
• Balance the chemical equation correctly• Using the molar mass of the given substance,
convert the mass given to moles. • Determine the molar ratio.• Using the molar mass of the unknown
substance, convert the moles just calculated to mass.
Amount in gramsof known substance
Amount in molesof known substance
Amount in molesof unknown substance
Amount in gramsof unknown substance mw mwmolar
ratio
How many grams of N2O5 are needed to produce 75.0 grams of O2?
= grams N2O5
75.0 g O2
2
52
O 1mol
ON mol2506
2
2
O g 32.0
O mol
52
52
ON mol
ON g108
2N2O5(g) 4NO2(g) + O2(g)75.0 g? grams
grams ↔ grams
When N2O5 is heated, it decomposes:
Problem Type 4
Stoichiometry Problem- Type 4
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.
1. Identify reactants and products and write the balanced equation.
Al + O2 Al2O3
a. What are the reactants?
b. What are the products?
c. What are the balanced coefficients?
Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?
4 Al + 3 O2 2Al2O3
=6.50 g Al
? g Al2O3
1 mol Al
26.98 g Al 4 mol Al
2 mol Al2O3
1 mol Al2O3
101.96 g Al2O3
6.50 x 2 x 101.96 ÷ 26.98 ÷ 4 =
12.3 g Al2O3
How many grams of acetylene are produced by adding water to 5.0 grams of CaC2?
= grams C2H2
5.0 g CaC2
2
22
CaC 1mol
HC mol 12.0
2
2
CaC g 64.1
CaC mol 122
22
HC mol 1
HC 26g
CaC2 + 2H2O C2H2 + Ca(OH)2 Balanced?
5.0 g ? grams
Acetylene gas (C2H2) is produced by adding water to calcium carbide:
How many moles of CaC2 are needed to react completelywith 49.0 g H2O:
CaC2 + 2H20 C2H2(g) + Ca(OH)2
How many moles of CaC2 are needed to react 49.0 g of H20?
= moles CaC2
49 g H2O
OH mol 2
CaC mol 1
2
21.36
OH 0.18
OH mol 1
2
2
g
CaC2 + 2H20 C2H2(g) + Ca(OH)2
49.0g? moles
Units match
Suppose you want to figure out how many grams of ammonia you can produce if you react 60.0 g of hydrogen gas with excess nitrogen.
Remember the balanced chemical equation gives the mole relationship between reactants and products.
N2 + 3 H2 2 NH3
1) Take the grams of hydrogen gas (recall that it is diatomic) and convert it to moles:
2) Convert the moles of hydrogen gas to moles of ammonia, using the coefficients in the balanced chemical equation:
3) Convert from moles of ammonia to grams of ammonia using the molecular mass and solve the equation:
Stoichiometry Review Problem
Calculating the Percent Yield
The predicted amount of product – which we have been doing with our stoichiometry problems has been for 100% yield, or the theoretical yield of a reaction.
The theoretical yield is the maximum amount of product that can be produces in a given reaction.
When chemical reactions take place, they are almost never 100% complete. A reaction may not go to 100% due to not all the reactants becoming involved, impurities in the reactants, competing side reactions, loss of product due to filtering, or just not getting it all out of the vessel.
The actual yield is how much product can be collected – and measured. The percent yield is a ratio given by:
Percent Yield = Actual Yield x 100% Theoretical Yield
Calcium carbonate is decomposed by heating, as shown in the following equation:
CaCO3 CaO + CO2
1) What is the theoretical yield of this reaction if 24.8 g CaCO3 is heated to yield 13.1 g CaO?
Handle like a typical stoichiometry problem: grams to grams
2) What is the percent yield?
Ignore the 13.1 g CaO to solve. This is the actual yield.
Chlorobenzene is used in the production of many different chemicals, such as aspirin, dyes and disinfectants. One method of preparing chlorobenzene is to react benzene C6H6, with chlorine according to the following equation:
1) When 36.8 g of C6H6 react with excess of Cl2, the actual yield of C6H5Cl is 38.8g?
What is the theoretical yield?
Handle like a typical stoichiometry problem: grams to grams
2) What is the percent yield?
C6H6 + Cl2 C6H5Cl + HCl
Standard Molar VolumeEqual volumes of all gases at the same temperature and pressure contain the same number of molecules.
- Amedeo Avogadro
At STP (Standard Temperature and Pressure):
1 mole of a gas occupies 22.4 liters of volume
Gas Stoichiometry #1If reactants and products are at the same conditions of temperature and pressure, then mole ratios of gases are also volume ratios.
3 H2(g) + N2(g) 2NH3(g)
3 moles H2 + 1 mole N2 2 moles NH3
3 liters H2 + 1 liter N2 2 liters NH3
Gas Stoichiometry #2How many liters of ammonia can be produced when 12 liters of hydrogen react with an excess of nitrogen?
3 H2(g) + N2(g) 2NH3(g)
12 L H2
L H2
= L NH3 L NH3
3
28.0