Conservation of Mass (again)Conservation of Mass (again)

I think it must be important?I think it must be important? Lavoisier- observed that the total mass of a Lavoisier- observed that the total mass of a

substance present after a chemical reaction is substance present after a chemical reaction is the same as the total mass before the reaction. the same as the total mass before the reaction.

We can conclude that atoms are not created or We can conclude that atoms are not created or destroyed during a chemical reaction, they are destroyed during a chemical reaction, they are simply rearranged. simply rearranged.

Chemical EquationsChemical Equations

Representations of chemical reactions.Representations of chemical reactions. Reactants to the left of the arrow Reactants to the left of the arrow Products to the rightProducts to the right

HH22 + O + O22 H H22OO

Where did the other oxygen go???Where did the other oxygen go???

We must balance!!We must balance!!

That is the purpose of stoichiometryThat is the purpose of stoichiometry Because atoms are neither created nor Because atoms are neither created nor

destroyed, a chemical equation must have destroyed, a chemical equation must have equal numbers of atoms on both sides of the equal numbers of atoms on both sides of the arrow.arrow.

Balance by changing coefficientsBalance by changing coefficients You never change subscripts to balance. You never change subscripts to balance.

Coefficients change the amount of a substance, Coefficients change the amount of a substance, changing subscripts would change the identity. changing subscripts would change the identity.

Balancing equationsBalancing equations

CH4 + O2 CO2 + H2OReactantsReactants ProductsProducts

C1 1

O2 3

H4 2

Balancing equationsBalancing equations

CH4 + O2 CO2 + 2 H2OReactantsReactants ProductsProducts

C1 1

O2 3

H4 2 4

Balancing equationsBalancing equations

CH4 + O2 CO2 + 2 H2OReactantsReactants ProductsProducts

C1 1

O2 3

H4 2 4

4

Balancing equationsBalancing equations

CH4 + 2O2 CO2 + 2 H2OReactantsReactants ProductsProducts

C1 1

O2 3

H4 2 4

44

AbbreviationsAbbreviations

((ss) ) ((gg) ) ((aqaq)) heatheat

catalystcatalyst

Patterns of ReactivityPatterns of Reactivity

We know that group 1 metals react with water We know that group 1 metals react with water to form metal hydroxides and hydrogen gas!to form metal hydroxides and hydrogen gas!

Sooo…… Sooo…… 2 Na + 2 HOH 2 Na + 2 HOH 2 NaOH + H 2 NaOH + H22

and..and.. 2 K + 2HOH 2 K + 2HOH 2 KOH + H 2 KOH + H22

Etc……Etc……

Combustion in AirCombustion in Air

Rapid reactions that produce a flame.Rapid reactions that produce a flame. Most combustion involves oxygen from the Most combustion involves oxygen from the

air.air. Usually involve burning hydrocarbons.Usually involve burning hydrocarbons. The products of burning a hydrocarbon are The products of burning a hydrocarbon are

always water and carbon dioxide.always water and carbon dioxide. CC33HH8 8 + 5O+ 5O2 2 3CO 3CO2 2 + 4H+ 4H22OO

Write the balanced equation:Write the balanced equation:

When methanol is burned in air:When methanol is burned in air:

CHCH33OH + OOH + O22 H H22O + COO + CO22

Combination & DecompositionCombination & Decomposition

What does the word combine mean?What does the word combine mean? What does the word decompose mean?What does the word decompose mean? If you know this, then you know what happens If you know this, then you know what happens

in a combination or decomposition reaction!in a combination or decomposition reaction! In a combination reaction, two or more In a combination reaction, two or more

substances react to form one product.substances react to form one product. Ex. Ca + OEx. Ca + O22 CaO CaO

More on combinationMore on combination

When reactants are nonmetal- metal, result is When reactants are nonmetal- metal, result is an ionic solid. Notice that the equation needs an ionic solid. Notice that the equation needs to be balanced.to be balanced.

2Ca + O2Ca + O22 2CaO 2CaO

DecompositionDecomposition

One substance breaks down to produce two or One substance breaks down to produce two or more different substances.more different substances.

Ex. Metal Carbonates break down when Ex. Metal Carbonates break down when heated to form their (basic) metal oxide and heated to form their (basic) metal oxide and carbon dioxide.carbon dioxide.

Try to write and balance the equation for the Try to write and balance the equation for the decomposition of lithium carbonate.decomposition of lithium carbonate.

Very Good!Very Good!

LiLi22COCO33 Li Li22O + COO + CO22

This works out to be already balanced!This works out to be already balanced!

PracticePractice

Ca(OH)Ca(OH)22 + H + H33POPO44 H H22O + CaO + Ca33(PO(PO44))22

Cr + SCr + S88 Cr Cr22SS33

Solid iron(III) sulfide reacts with gaseous Solid iron(III) sulfide reacts with gaseous hydrogen chloride to form solid iron(III) hydrogen chloride to form solid iron(III) chloride and hydrogen sulfide gas.chloride and hydrogen sulfide gas.

FeFe22OO33((ss) + Al() + Al(ss) ) Fe( Fe(ss) + Al) + Al22OO33((ss))

MeaningMeaning

A balanced equation can be used to describe a A balanced equation can be used to describe a reaction in molecules and atoms.reaction in molecules and atoms.

Not grams.Not grams. Chemical reactions happen molecules at a timeChemical reactions happen molecules at a time or dozens of molecules at a timeor dozens of molecules at a time or moles of molecules. or moles of molecules.

Atomic MassAtomic Mass

Atoms are so small, it is difficult to discuss Atoms are so small, it is difficult to discuss how much they weigh in grams.how much they weigh in grams.

Use atomic mass units.Use atomic mass units. an atomic mass unit (amu) is one twelth the an atomic mass unit (amu) is one twelth the

mass of a carbon-12 atom.mass of a carbon-12 atom. This gives us a basis for comparison.This gives us a basis for comparison. The decimal numbers on the table are atomic The decimal numbers on the table are atomic

masses in amu.masses in amu.

They are not whole numbers They are not whole numbers

Because they are based on averages of atoms Because they are based on averages of atoms and of isotopes.and of isotopes.

can figure out the average atomic mass from can figure out the average atomic mass from the mass of the isotopes and their relative the mass of the isotopes and their relative abundance.abundance.

add up the percent as decimals times the add up the percent as decimals times the masses of the isotopes.masses of the isotopes.

ExamplesExamples

There are two isotopes of carbon There are two isotopes of carbon 1212C with a C with a mass of 12.00000 amu(98.892%), and mass of 12.00000 amu(98.892%), and 1313C C with a mass of 13.00335 amu (1.108%).with a mass of 13.00335 amu (1.108%).

The MoleThe Mole

The mole is a number.The mole is a number. A very large number, but still, just a number.A very large number, but still, just a number. 6.022 x 106.022 x 102323 of anything is a mole of anything is a mole A large dozen.A large dozen. The number of atoms in exactly 12 grams of The number of atoms in exactly 12 grams of

carbon-12.carbon-12.

The MoleThe Mole

Makes the numbers on the table the mass of Makes the numbers on the table the mass of the average atom.the average atom.

More StoichiometryMore Stoichiometry

Molar massMolar mass

Mass of 1 mole of a substance.Mass of 1 mole of a substance. Often called molecular weight.Often called molecular weight. To determine the molar mass of an element, To determine the molar mass of an element,

look on the table.look on the table. To determine the molar mass of a compound, To determine the molar mass of a compound,

add up the molar masses of the elements that add up the molar masses of the elements that make it up.make it up.

Find the molar mass ofFind the molar mass of

CHCH44

MgMg33PP22

Ca(NOCa(NO33))22

AlAl22(Cr(Cr22OO77))33

CaSOCaSO44 · 2H · 2H22OO

Percent CompositionPercent Composition Percent of each element a compound is Percent of each element a compound is

composed of.composed of. Find the mass of each element, divide by the Find the mass of each element, divide by the

total mass, multiply by a 100.total mass, multiply by a 100. Easiest if you use a mole of the compound.Easiest if you use a mole of the compound. Find the percent composition of CHFind the percent composition of CH44

AlAl22(Cr(Cr22OO77))33

CaSOCaSO44 · 2H · 2H22OO

Working backwardsWorking backwards

From percent composition, you can determine From percent composition, you can determine the empirical formula.the empirical formula.

Empirical FormulaEmpirical Formula the lowest ratio of atoms in the lowest ratio of atoms in a molecule.a molecule.

Based on mole ratios.Based on mole ratios. A sample is 59.53% C, 5.38%H, 10.68%N, A sample is 59.53% C, 5.38%H, 10.68%N,

and 24.40%O what is its empirical formula.and 24.40%O what is its empirical formula.

Pure O2 in CO2 is absorbed

H2O is absorbed

Sample is burned completely to

form CO2 and

H2O

More StoichiometryMore Stoichiometry

Empirical To Molecular Empirical To Molecular FormulasFormulas

Empirical is lowest ratio.Empirical is lowest ratio. Molecular is actual molecule.Molecular is actual molecule. Need Molar mass.Need Molar mass. Ratio of empirical to molar mass will tell you Ratio of empirical to molar mass will tell you

the molecular formula.the molecular formula. Must be a whole number because...Must be a whole number because...

ExampleExample

A compound is made of only sulfur and A compound is made of only sulfur and oxygen. It is 69.6% S by mass. Its molar mass oxygen. It is 69.6% S by mass. Its molar mass is 192 g/mol. What is its formula?is 192 g/mol. What is its formula?

StoichiometryStoichiometry

Given an amount of either starting material or Given an amount of either starting material or product, determining the other quantities.product, determining the other quantities.

use conversion factors fromuse conversion factors from molar mass (g - mole)molar mass (g - mole) balanced equation (mole - mole)balanced equation (mole - mole)

keep track.keep track.

ExamplesExamples

One way of producing OOne way of producing O22((gg) involves the ) involves the

decomposition of potassium chlorate into decomposition of potassium chlorate into potassium chloride and oxygen gas. A 25.5 g potassium chloride and oxygen gas. A 25.5 g sample of Potassium chlorate is decomposed. sample of Potassium chlorate is decomposed. How many moles of OHow many moles of O22(g) are produced? (g) are produced?

How many grams of potassium chloride?How many grams of potassium chloride? How many grams of oxygen?How many grams of oxygen?

ExamplesExamples

A piece of aluminum foil 5.11 in x 3.23 in x A piece of aluminum foil 5.11 in x 3.23 in x 0.0381 in is dissolved in excess HCl(aq). How 0.0381 in is dissolved in excess HCl(aq). How many grams of Hmany grams of H22((gg) are produced?) are produced?

How many grams of each reactant are needed How many grams of each reactant are needed to produce 15 grams of iron form the to produce 15 grams of iron form the following reaction? following reaction? FeFe22OO33((ss) + Al() + Al(ss) ) Fe( Fe(ss) + Al) + Al22OO33((ss))

ExamplesExamples

KK22PtClPtCl44((aqaq) + NH) + NH33((aqaq) )

Pt(NH Pt(NH33))22ClCl22 ( (ss)+ KCl()+ KCl(aqaq))

what mass of Pt(NHwhat mass of Pt(NH33))22ClCl2 2 can be produced can be produced

from 65 g of Kfrom 65 g of K22PtClPtCl44 ? ?

How much KCl will be produced?How much KCl will be produced? How much from 65 grams of NHHow much from 65 grams of NH33??

YieldYield

How much you get from an chemical How much you get from an chemical reactionreaction

Limiting ReagentLimiting Reagent

Reactant that determines the amount of Reactant that determines the amount of product formed. product formed.

The one you run out of first.The one you run out of first. Makes the least product.Makes the least product. Book shows you a ratio method.Book shows you a ratio method. It works. It works. So does mineSo does mine

Limiting reagentLimiting reagent

To determine the limiting reagent requires that To determine the limiting reagent requires that you do two stoichiometry problems.you do two stoichiometry problems.

Figure out how much product each reactant Figure out how much product each reactant makes.makes.

The one that makes the least is the limiting The one that makes the least is the limiting reagent.reagent.

ExampleExample

Ammonia is produced by the following reactionAmmonia is produced by the following reaction

N N22 + H + H22 NH NH3 3

What mass of ammonia can be produced from What mass of ammonia can be produced from a mixture of 100. g Na mixture of 100. g N22 and 500. g H and 500. g H22 ? ?

How much unreacted material remains?How much unreacted material remains? First thing you must do to solve the problem is First thing you must do to solve the problem is

balance the equation!balance the equation!

Excess ReagentExcess Reagent

The reactant you don’t run out of.The reactant you don’t run out of. The amount of stuff you make is the yield.The amount of stuff you make is the yield. The The theoretical yield theoretical yield is the amount you would is the amount you would

make if everything went perfect.make if everything went perfect. The The actual yield actual yield is what you make in the lab.is what you make in the lab.

Percent YieldPercent Yield

% yield = Actual x 100%% yield = Actual x 100%

Theoretical Theoretical % yield = what you got x 100%% yield = what you got x 100%

what you could have gotwhat you could have got

ExamplesExamples

Aluminum burns in bromine producing Aluminum burns in bromine producing aluminum bromide. In a laboratory 6.0 g of aluminum bromide. In a laboratory 6.0 g of aluminum reacts with excess bromine. 50.3 g aluminum reacts with excess bromine. 50.3 g of aluminum bromide are produced. What are of aluminum bromide are produced. What are the three types of yield.the three types of yield.

ExamplesExamples

Years of experience have proven that the Years of experience have proven that the percent yield for the following reaction is percent yield for the following reaction is 74.3%74.3%

Hg + BrHg + Br22 HgBr HgBr22

If 10.0 g of Hg and 9.00 g of BrIf 10.0 g of Hg and 9.00 g of Br22 are reacted, are reacted,

how much HgBrhow much HgBr22 will be produced? will be produced?

If the reaction did go to completion, how much If the reaction did go to completion, how much excess reagent would be left?excess reagent would be left?