Chemistry 103

Lecture 15

Outline

I. Chemical Reactions - balancing (review)II. Stoichiometry - Limiting ReactantIII. Classification of Reaction Types

Molar Mass from Periodic TableMolar mass • Is the atomic

mass expressed in grams

Avogadro’s Number

Solution = 6.022 x 1023

Avogadro’s number is equal to 1 mole

Makes working with large numbers easier

The Mole

Molar mass 1 mole 6.022 x 1023

Mass in grams Individual particles

Concept Review

Do 7.0g of CO and 7.5 g of C2H6 contain the same number of C atoms?

(Prove your answer mathematically)

A. YES B. NO

Chemical ReactionIn a chemical reaction, Old bonds are broken and

new bonds are formed Atoms in the reactants are

rearranged to form one or more different substances

Fe and O2 form rust (Fe2O3)

Chemical’s Shorthand - Chemical Rxn Fe (s) + O2(g) --------> Fe2O3 (s)

2Fe(s) + O2(g) --------> Fe2O3(s)

4Fe(s) + O2(g) --------> 2Fe2O3(s)

4Fe(s) + 3O2(g) --------> 2Fe2O3(s) BALANCED!!!!!

Balancing Practice

Li3N(s) -------> Li(s) + N2(g)

Mg + H3PO4 ------> Mg3(PO4)2 + H2

MgCl2 + AgNO3 ------> Mg(NO3)2 + AgCl

Stoichiometry Chemical Stoichiometry: using mass and quantity

relationships among reactants and products in a chemical reaction to make predictions about how much product will be made.

We can read the equation in “moles” by placing the word “mole” or “mol” between each coefficient and formula.

4Fe(s) + 3O2(g) 2Fe2O3(s)

4 mol Fe + 3 mol O2 2 mol Fe2O3

Moles in Equations

Mole to Mole problems

Using the balanced chemical reaction, how many moles of O2 will react with 4.28 moles of CS2 to produce CO2 and SO2?

CS2 + O2 ------> CO2 + SO2 (unbalanced)

Mole to Mole problems

When 4.28 moles of CS2 reacts according to the balanced chemical equation below, how many moles of CO2 will be produced?

CS2 + 3O2 ------> CO2 + 2SO2

Mole to Mole problems

When 4.28 moles of CS2 reacts according to the balanced chemical equation below, how many moles of SO2 will be produced?

CS2 + 3O2 ------> CO2 + 2SO2

Reaction Interpretation

Using the balanced chemical reaction, how many moles of O2 will react with 4.28 moles of CS2 to produce CO2 and SO2?

CS2 + 3O2 ------> CO2 + 2SO2BEFORE

AFTER

Moles-to-Grams or Grams-to-Mole

Chemical Stoichiometry2H2 + O2 ----> 2H2O

mole to mole comparisons: How many moles of O2 are needed to produce

1.39 moles of H2O?

Chemical Stoichiometry

2H2 + O2 ----> 2H2O

mole to gram or gram to mole comparisons:How many moles of O2 are needed to produce 7.4g

of H2O?

Chemical Stoichiometry

2H2 + O2 ----> 2H2O

mole to mole comparisons: How many moles of O2 are needed to exactly react

with 4.28 moles of H2?

Chemical Stoichiometry

2H2 + O2 ----> 2H2O

mole to gram or gram to mole comparisons:How many moles of O2 are needed to exactly react

with 12.4g of H2?

How many grams of O2 are needed to produce 0.400 mol Fe2O3?

4Fe(s) + 3O2(g) 2 Fe2O3(s)

A) 38.4 g O2

B) 19.2 g O2

C) 1.90 g O2

Grams-to-Mole

“gram to gram” calculations

The reaction between H2 and O2 produces 13.1 g water.

How many grams of O2 reacted?

2H2(g) + O2(g) 2H2O(g)

Plan: g H2O mole H2O mole O2 g O2

Calculating the Mass of a Reactant

The reaction between H2 and O2 produces 13.1 g water.

How many grams of O2 reacted?

2H2(g) + O2(g) 2H2O(g)

Before: x g

After: 13.1 g

Plan: g H2O mole H2O mole O2 g O2

Calculating the Mass of a Reactant

Mass calculations

How many grams of CS2 are needed to completely react with 6.0g of O2 according to the following reaction?

CS2 + 3O2 ------> CO2 + 2SO2

“gram to gram” continued….

One way to remove gaseous carbon dioxide from the air in a spacecraft is to let canisters of solid lithium hydroxide absorb it according to the following reaction:

2LiOH(s) + CO2(g) ------> Li2CO3(s) + H2O(l)

How many grams of LiOH must be used to produce 10.0g of Li2CO3?

Learning Check

Acetylene gas C2H2 burns in the oxyacetylene torch for welding. How many grams of C2H2 are burned if the reaction produces 75.0 g CO2?

2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g)

A) 88.6 g C2H2

B) 44.3 g C2H2

C) 22.2 g C2H2

Chemical Reactions

Limiting Reactant &

Percent Yield

Limiting Reactant

A limiting reactant in a chemical reaction is thesubstance that: • Is used up first• Stops the reaction• Limits the amount of product that can form

Reacting Amounts

In a table setting, there is 1plate, 1 fork, 1 knife, and 1 spoon.

How many table settings arepossible from 5 plates, 6 forks, 4spoons, and 7 knives?

What is the limiting item?

Reaction Equivalent

1plate + 1fork + 1knife + 1spoon ----> 1place setting

BEFORE 5 6 7 4 0

AFTER 1 2 3 0 4 LR

Reacting Amounts

In a table setting, there is 1plate, 2 forks, 1 knife, and

1spoon.

How many table settings arepossible from 5 plates, 6 forks, 4spoons, and 7 knives?

What is the limiting item?

Reaction Equivalent

1plate + 2fork + 1knife + 1spoon ----> 1place setting

BEFORE 5 6 7 4 0

AFTER 2 0 4 1 3 LR

Limiting Reactants

When 4.00 mol H2 is mixed with 2.00 mol Cl2,how many moles of HCl can form?

H2(g) + Cl2(g) 2HCl (g)

4.00 mol 2.00 mol ??? Mol

STEPS:

• Calculate the moles of product from each reactant, H2 and Cl2.

• The limiting reactant is the one that produces the smallest amount of product.

Checking Calculations

Initially H2

4.00 mol

Cl2

2.00 mol

2HCl

0 mol

Reacted/Formed

-2.00 mol -2.00 mol +4.00 mol

Left after reaction

2.00 mol Excess

0 mol Limiting

4.00 mol

Limiting Reactant

If 4.80 grams Ca mixed with 2.00 grams N2, which is the limiting reactant?

3Ca(s) + N2(g) Ca3N2(s)

Limiting Reactants Using MassCalculate the mass of water produced when 8.00 g H2

and 24.0 g O2 react?

2H2(g) + O2(g) 2H2O(l)