unit 3 - quantities in chemical...

Unit 3 Quantities in Chemical Reactions.notebook December 14, 2012

Unit 3 Quantities in Chemical Reactions

• the mole and Avogadro's constant• converting moles to number of particles (and reverse)• calculating molar mass (the mass of one mole)• converting mass to moles, and moles to mass• converting mass to number of particles (and reverse)• chemical proportions and percent composition• determining the empirical formula• determining the molecular formula• determining the formula of a hydrate• understanding mole ratios• mass stoichiometry• limiting and excess reagents• reaction yields


CHEMICAL Proportions

CC

C

H

H

H

H

HH

H

H


Which form of water has the most hydrogen? oxygen?

Pull

Pull

Pull

Pull

Pull

Pull

Pull

Pull


Law of Definite Proportions: the elements in a chemical compound are always present in the same proportions by mass


Determining the Mass Percent of an Element in a Compound

Q. A 13.8g sample of a compound contains 8.80g of iron. Determine the mass percent of iron and oxygen in the sample


Determining the Percent Composition from Mass Data

Q. A sample of gasoline has a mass of 35.8g. The sample contains 30.10 g of carbon and 5.70g of hydrogen. What is the percent composition of the compound?


Determining the Percent Composition from a Chemical Formula

Q. The most common copper ores come from a mineral with the chemical formula CuFeS2. Determine the percent composition of this mineral.


Empirical and Molecular Formula

• "empirical" > Greek word 'emperikos' > 'by experiment'• Empirical formula shows the lowest whole number ratio of the elements that make up a molecule• Molecular Formula: describes the number of atoms of each element that make up a molecule


Compound Empirical Formula Molecular Formula

water H2O H2O

hydrogen peroxide HO H2O2

benzene CH C6H6

ethyne CH C2H2

Examples of Empirical/Molecular Formulas


Recall that percent composition involves taking the chemical formula and calculating percentages.

RECALL

Determining the empirical formula is the reverse process.

NEX

T ST

EP


Finding a compounds Empirical Formula from Percent Composition

Q. Calculate the empirical formula of a compound that is 85.6% carbon and 14.4% hydrogen

CH2


Finding a compounds Empirical Formula from Percent CompositionQ. A fuel has a composition of 81.7% carbon and the rest is hydrogen. Calculate the empirical formula of this fuel.

C3H8


Determining the Molecular Formula the difference between an empirical formula and the molecular formula is a whole number multiple.

the goal is to determine how the molar mass of the molecular formula relates to the molar mass of the empirical formula


Q. Ribose has the empirical formula of CH2O. During an experiment, the molar mass was determined to be 150 g/mol. What is the molecular formula of ribose

Determining the Molecular Formula

C5H10O5


Stoichiometry

From the Greek word "stoicheion" which means "to measure the elements".

Mass and amount relationships between the products and reactants in a chemical reaction.


Stoichiometry

Ingredients for making the perfect sandwich


StoichiometryInterpreting chemical Equations

N2 + 3H 2 2NH 3

Atoms:

Molecules:

Moles:


Using Molar Ratios

Billy has 7.5 mol H 2 and he wants to know how many mols of NH 3 he can make, assuming he has plenty N 2 gas.

Given:Wanted:Find balanced equation:

Determine appropriate molar ratio:

Solve:


Try it again.

Water is formed when hydrogen reacts with oxygen. How many mols of oxygen are needed to react with 2.2 mols of hydrogen to make water?



Solve:

1.1 mol O 2


Quiz

4NH 3 + 6NO 5N 2 + 6H 2O

Pull

Pull

7.5 mol H 2O 4 mol NH 36 mol H 2O

5 mol NH 3=

Pull

Pull

2.3 mol NH 3 5 mol N 24 mol NH 3

2.87 mol N 2=Pull Pull

Pull

Pull


mols to mass



Solve:

According to the balanced equation belowHow many grams of N 2 can be produced when 6.2 mols of NH 3 reacts with an excess of NO?

NH 3 NO N2 H2O+ +

217 g N 2


Let's try it again



Solve:

How many grams of Al 2O3 can be produced when 6.5 mol of O 2 reacts with an excess of Al?

442 g Al 2O3


mass to mols



Solve:

According to the balanced equation belowHow many mols of NO would be needed to produce 60 g of N 2?

NH 3 NO N2 H2O+ +4 6 5 6

2.57 mol NO


Let's try it again



Solve:

How many mols of Al are needed to produce 80 g of Al 2O3?

1.57 mol Al


Mass to Mass



Solve:

NH 3 NO N2 H2O+ +4 6 5 6Philbert needs to produce 1500 g of water. How many grams of NH3 will he need to react with an excess of NO to accomplish this seemingly impossible feat?


Mass to Mass



Solve:

Jack Canyon has 650 g of O2 at his disposal and he know what to do with it. How many grams of Aluminum would Jack need to completely react with the oxygen?


Cool Chart...actually it's a stoichiometric road map.


Stoichiometry Roadmap


Limiting Reactants

1 hot dog = 1 hot dog2 packet of ketchup2 packet of mustard1 bun

How many hot dogs can you make if you have: 550 hotdogs

20000 packets of catsup15700 packets of mustard250 buns

WHY?

Limiting Reactant: the chemical you run out of 1 st during a chemical reaction.

The L.R. determines how much product you can produce.

Excess reactant: The left over reactant.


How to solve L.R. problems.

• read the problem.• Notice you have 2 (two) givens both are reactants.• Use given 'a' to solve for the given 'b'.• Compare your answer from 3 to the actual amount of given you have.• If you have more given 'b' than the calculated amount of given 'b' then given 'a' is the L.R. If you have less given 'b' than the calculated amount of given 'b' then given 'b' is the L.R.• Use the L.R. to solve the problem.


Let's try just finding the L.R. (limiting reactant).

How many mols of Fe 2O3 are produced according to the equation below when 4.5 mols of Fe reacts with 5.4 mols of CO 2?

2Fe + 3CO 2 Fe 2O3 + 3COGiven: (actual amounts)Wanted:


Let's try that again but this time we have a a different amount of mols of CO 2 .

How many mols of Fe 2O3 are produced according to the equation below when 4.5 mols of Fe reacts with 7.4 mols of CO 2?

2Fe + 3CO 2 Fe 2O3 + 3CO

Given: (actual amounts)Wanted:

So what is the L.R.? WHY?If you have more than enough CO 2 then you will run out of Fe first. This makes Fe the L.R.

Pull

Pull


Now let's make it useful.

You have 100 grams of aluminum and 250 grams of chlorine gas. How many grams of aluminum chloride can you make according to the following reaction?

Al + Cl 2 AlCl 3100 g Pull Pull 250 g PullPull

Balance the equation PullPull

2nd step:

Given:

Wanted:

1st step:Find L.R. Pull Pull

Use L.R. to find Given PullPull


Percent Yield

Actual Yield: The given found during a live experiment.Theoretical Yield: The given you calculated.

Percent Yield = Actual YieldTheoretical Yield

x 100


Percent Yield problem steps:

Step 1: Read the problem

Step 2: Notice that there are 2 givens, one given is a reactant the other is a product.

Step 3: use the given reactant to solve for the theoretical yield.

Step 4: The given product is the actual yield.

Step 5: divide A.Y. by T.Y multiply by 100.


Let's try it.

When hydrogen reacts with oxygen water forms. You add 800 g of H 2 to an excess of O2 and produce 4300 g H 2O. What is the % yield?

Given:Wanted:A.Y:

800 g H 2 Pull

Pull grams H 2O Pull

Pull

4300 g H 2O Pull Pull

Write balanced Equation:

2H 2 + O 2 2H 2O PullPull

Use the Given reactant to find the theoretical yield:

4300 g7200 g

x 100 = 59.7 % yield

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unit 3 - quantities in chemical...

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