Chapter #3 Lectures Part Ii

Atoms,Molecules,

and Ions

g CO2 mol CO2 mol C g C

g H2O mol H2O mol H g H

g of O = g of sample – (g of C + g of H)

Combust 11.5 g ethanol

Collect 22.0 g CO2 and 13.5 g H2O

6.0 g C = 0.5 mol C

1.5 g H = 1.5 mol H

4.0 g O = 0.25 mol O

Empirical formula C0.5H1.5O0.25

Divide by smallest subscript (0.25)

Empirical formula C2H6O

Atoms,Molecules,

and Ions

A process in which one or more substances is changed into one or more new substances is a chemical reaction

A chemical equation uses chemical symbols to show what happens during a chemical reaction

Atoms,Molecules,

and Ions

How to “Read” Chemical Equations

2 Mg + O2 2 MgO

Atoms,Molecules,

and Ions

Balancing Chemical Equations

1. Write the correct formula(s) for the reactants on the left side and the correct formula(s) for the product(s) on the right side of the equation.

Ethane reacts with oxygen to form carbon dioxide and water

C2H6 + O2 CO2 + H2O

2. Change the numbers in front of the formulas (coefficients) to make the number of atoms of each element the same on both sides of the equation. Do not change the subscripts.

Atoms,Molecules,

and Ions

Balancing Chemical Equations

3. Start by balancing those elements that appear in only one reactant and one product.

Atoms,Molecules,

and Ions

Balancing Chemical Equations

4. Balance those elements that appear in two or more reactants or products.

Atoms,Molecules,

and Ions

Balancing Chemical Equations

5. Check to make sure that you have the same number of each type of atom on both sides of the equation.

Atoms,Molecules,

and Ions

1. Write balanced chemical equation

2. Convert quantities of known substances into moles

3. Use coefficients in balanced equation to calculate the number of moles of the sought quantity

4. Convert moles of sought quantity into desired units

Mass Changes in Chemical Reactions

Atoms,Molecules,

and Ions

Stoichiometric Calculations

From the mass of Substance A you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it’s a product) or used (if it’s a reactant)

Atoms,Molecules,

and Ions

Methanol burns in air according to the equation

2CH3OH + 3O2 2CO2 + 4H2O

If 209 g of methanol are used up in the combustion, what mass of water is produced?

Atoms,Molecules,

and Ions

Limiting Reagents

Atoms,Molecules,

and Ions

Limiting Reactants

The limiting reactant is the reactant present in the smallest stoichiometric amount

Atoms,Molecules,

and Ions

Theoretical Yield

• The theoretical yield is the amount of product that can be madeIn other words it’s the amount of product

possible as calculated through the stoichiometry problem

• This is different from the actual yield, the amount one actually produces and measures

Atoms,Molecules,

and Ions

In one process, 124 g of Al are reacted with 601 g of Fe2O3

2Al + Fe2O3 Al2O3 + 2Fe

Calculate the mass of Al2O3 formed.

Atoms,Molecules,

and Ions

Use limiting reagent (Al) to calculate amount of product thatcan be formed.

Atoms,Molecules,

and Ions

Theoretical Yield is the amount of product that wouldresult if all the limiting reagent reacted.

Actual Yield is the amount of product actually obtainedfrom a reaction.

Atoms,Molecules,

and Ions

Lecture Problem #1

A reaction mixture contains 23.5 grams of phosphorus trichloride, PCl3, and 14.56 grams of lead (II) fluoride, PbF2. What mass of PbCl2 can be obtained from the following reaction?

3 PbF2 + 2 PCl3 → 2 PF3 + 3 PbCl2

Atoms,Molecules,

and Ions

Lecture Problem #1 Solution

1 mole of PCl3 = 137.32 grams

23.5 grams PCl3 = 0.171 moles PCl3

1 mole PbF2 = 245.2 grams

14.56 grams PbF2 = 0.0594 moles

Atoms,Molecules,

and Ions

Lecture Problem #2

What is the percent yield if 108 milligrams of sulfur dioxide, SO2, is isolated from the combustion of 82.7 mg of carbon disulfide, CS2, according to the reaction:

CS2 + 3 O2 → CO2 + 2 SO2

Atoms,Molecules,

and Ions

Lecture Problem #2 Solution

1 mole CS2 = 76.15 grams

82.7 mg CS2 = 1.09 x 10-3 moles CS2

1 mole CS2 yields 2 moles of SO2

1 Mole SO2 = 64.07 grams

Atoms,Molecules,

and Ions

Lecture Problem #3

The percent yield of the following reaction is consistently 78%.

CH4(g) + 4 S(g) → CS2(g) + 2 H2Sg)

How many grams of sulfur would be required to obtain 56.0 grams of CS2?