A Study of Chemical Reactions Equations, Mole Conversions, & Stoichiometry

A Study of Chemical A Study of Chemical ReactionsReactions

Equations, Mole Conversions, Equations, Mole Conversions, & Stoichiometry& Stoichiometry

Types of ReactionsTypes of Reactions

Many chemical Many chemical reactions have reactions have defining defining characteristics characteristics which allow which allow them to be them to be classified as to classified as to type.type.

Types of Chemical Types of Chemical ReactionsReactions

The five types of chemical reactions The five types of chemical reactions in this unit are:in this unit are: Combination/SynthesisCombination/Synthesis Decomposition/AnalysisDecomposition/Analysis Single Replacement/DisplacementSingle Replacement/Displacement Double Replacement/MetathesisDouble Replacement/Metathesis CombustionCombustion

Combination ReactionsCombination Reactions

Two or more substances combine to Two or more substances combine to form form one substanceone substance.. The general form is A + X The general form is A + X AXAX

Example:Example: Magnesium + oxygen Magnesium + oxygen

magnesium oxidemagnesium oxide 2Mg + O2Mg + O22 2MgO 2MgO

Magnesium + OxygenMagnesium + Oxygen


Combination reactions may also be Combination reactions may also be called called compositioncomposition or or synthesis synthesis reactionsreactions..

Some types of combination reactions:Some types of combination reactions: Combination of elementsCombination of elements

K + ClK + Cl22 One product will be formedOne product will be formed


K + ClK + Cl22

Write the ions: KWrite the ions: K++ Cl Cl--

Balance the charges: KClBalance the charges: KCl

Balance the equation: Balance the equation: 22K + ClK + Cl2 2 22KClKCl


Some types of combination Some types of combination reactions:reactions: Oxide + water Oxide + water

Nonmetal oxide + water Nonmetal oxide + water acid acidSOSO22 + H + H22O O H H22SOSO33

Metal oxide + water Metal oxide + water base baseBaO + HBaO + H22O O Ba(OH) Ba(OH)22


Some types of combination Some types of combination reactions:reactions: Metal oxides + nonmetal Metal oxides + nonmetal

oxidesoxidesNaNa22O + COO + CO22 Na Na22COCO33

CaO + SOCaO + SO2 2 CaSO CaSO33

Decomposition Decomposition ReactionsReactions

One substance reacts to form two or One substance reacts to form two or more substances. more substances. The general form is AX The general form is AX A + X A + X

Example:Example: Water can be decomposed by Water can be decomposed by

electrolysis.electrolysis. 2H2H22O O 2H 2H22 + O + O22

Electrolysis of WaterElectrolysis of Water


Types of Decomposition Reactions:Types of Decomposition Reactions: Decomposition of carbonatesDecomposition of carbonates

When heated, some When heated, some carbonatescarbonates break down to form an break down to form an oxideoxide and and carbon dioxidecarbon dioxide..

CaCOCaCO33 CaO + CO CaO + CO22

HH22COCO33 H H22O + COO + CO22


Types of decomposition reactions:Types of decomposition reactions: Some Some metal hydroxidesmetal hydroxides decompose decompose

into into oxidesoxides and and waterwater when heated. when heated.Ca(OH)Ca(OH)22 CaO + H CaO + H22OO

Note that this is the reverse of a Note that this is the reverse of a similar combination reaction.similar combination reaction.


Types of decomposition reactions:Types of decomposition reactions: Metal chloratesMetal chlorates decompose into decompose into

chlorideschlorides and and oxygenoxygen when heated. when heated.2KClO2KClO33 2KCl + 3O 2KCl + 3O22

Zn(ClOZn(ClO33))22 ZnCl ZnCl22 + 3O + 3O22

Some of these reactions are used in Some of these reactions are used in explosives.explosives.


Some substances can easily Some substances can easily decompose:decompose: Ammonium hydroxide is actually Ammonium hydroxide is actually

ammonia gas dissolved in water.ammonia gas dissolved in water.NHNH44OH OH NH NH33 + H + H22OO

Some acids decompose into water Some acids decompose into water and an oxide.and an oxide.HH22SOSO33 H H22O + SOO + SO22


Some decomposition reactions Some decomposition reactions are difficult to predict.are difficult to predict.

The decomposition of nitrogen The decomposition of nitrogen triiodide, NItriiodide, NI33, is an example of an , is an example of an interesting decomposition interesting decomposition reaction.reaction.

Nitrogen triiodideNitrogen triiodide

Single Replacement Single Replacement ReactionsReactions

CationicCationic: A metal will replace a : A metal will replace a metal ion in a compound.metal ion in a compound. The general form is A + BX The general form is A + BX AX + B AX + B

AnionicAnionic: A nonmetal will replace a : A nonmetal will replace a nonmetal ion in a compound.nonmetal ion in a compound. The general form is Y + BX The general form is Y + BX BY + X BY + X


Examples:Examples: Ni + AgNONi + AgNO33

Nickel replaces the metallic ion AgNickel replaces the metallic ion Ag++.. The silver becomes free silver and the The silver becomes free silver and the

nickel becomes the nickel(II) ion.nickel becomes the nickel(II) ion. Ni + AgNONi + AgNO33 Ag + Ni(NO Ag + Ni(NO33))22

Balance the equation:Balance the equation: Ni + 2AgNONi + 2AgNO33 2Ag + Ni(NO 2Ag + Ni(NO33))

Activity SeriesActivity Series


Not all single replacement reactions Not all single replacement reactions that can be written actually happen. that can be written actually happen.

The metal must be more active than The metal must be more active than the metal ion.the metal ion.

Aluminum is more active than iron in Aluminum is more active than iron in Al + FeAl + Fe22OO33 in the following reaction: in the following reaction:

Thermite ReactionThermite Reaction

Thermite ReactionThermite Reaction

Al + FeAl + Fe22OO33 Aluminum will replace iron(III)Aluminum will replace iron(III) Iron(III) becomes Fe and Iron(III) becomes Fe and

aluminum metal becomes Alaluminum metal becomes Al3+3+.. 22Al + FeAl + Fe22OO33 22Fe + AlFe + Al22OO33


An active nonmetal can replace a An active nonmetal can replace a less active nonmetal.less active nonmetal. The halogen (FThe halogen (F22, Cl, Cl22, Br, Br22, I, I22) )

reactions are good examples.reactions are good examples. FF22 is the most active and I is the most active and I22 is is

the least.the least.ClCl22 +2 NaI +2 NaI 2 NaCl + I 2 NaCl + I22

Double Replacement Double Replacement ReactionsReactions

Ions of two compounds exchange Ions of two compounds exchange places with each other.places with each other. The general form is AX + BY The general form is AX + BY

AY + BXAY + BX

MetathesisMetathesis is an alternate name is an alternate name for double replacement reactions.for double replacement reactions.

NaOH + CuSONaOH + CuSO44

Metathesis (sink or Metathesis (sink or float?)float?)

NaOH + CuSONaOH + CuSO44 The NaThe Na++ and Cu and Cu2+2+ switch places. switch places. NaNa++ combines with SO combines with SO44

2-2- to form Na to form Na22SOSO4.4.

CuCu2+2+ combines with OH combines with OH-- to form Cu(OH) to form Cu(OH)22

NaOH + CuSONaOH + CuSO44 Na Na22SOSO44 + Cu(OH) + Cu(OH)22

22NaOH + CuSONaOH + CuSO44 Na Na22SOSO44 + Cu(OH) + Cu(OH)22

CuSOCuSO44 + Na + Na22COCO33

Double ReplacementDouble Replacement

CuSOCuSO44 + Na + Na22COCO33 CuCu2+ 2+ combines with COcombines with CO33

2-2- to form to form CuCOCuCO33..

NaNa++ combines with SO combines with SO442-2- to form to form

NaNa22SOSO44.. CuSOCuSO44 + Na + Na22COCO33 CuCO CuCO33 + Na + Na22SOSO44

NaNa22COCO33 + HCl + HCl


NaNa22COCO33 + HCl + HCl Notice that gas bubbles were Notice that gas bubbles were

produced rather than a produced rather than a precipitate.precipitate.

What was the gas?What was the gas? Write the double replacement Write the double replacement

reaction first.reaction first.


NaNa22COCO33 + HCl + HCl NaNa++ combines with Cl combines with Cl-- to form NaCl. to form NaCl. HH++ combines with CO combines with CO33

2-2- to form H to form H22COCO33.. NaNa22COCO33 + 2HCl + 2HCl 2NaCl + H 2NaCl + H22COCO33

HH22COCO33 breaks up into H breaks up into H22O and COO and CO22..


The gas formed was carbon dioxide.The gas formed was carbon dioxide. The final balanced reaction is: The final balanced reaction is:

NaNa22COCO3 3 + HCl + HCl NaCl + H NaCl + H22O + COO + CO22.. Balance the equation.Balance the equation. NaNa22COCO33 + 2HCl + 2HCl 2NaCl + H 2NaCl + H22O + COO + CO22

Combustion ReactionCombustion Reaction When a substance combines with When a substance combines with

oxygen, a combustion reaction oxygen, a combustion reaction results.results.

The combustion reaction may The combustion reaction may also be an example of an earlier also be an example of an earlier type such as 2Mg + Otype such as 2Mg + O22 2MgO. 2MgO.

The combustion reaction may be The combustion reaction may be burning of a fuel.burning of a fuel.

Combustion ReactionCombustion Reaction

Methane, CHMethane, CH44, is natural gas., is natural gas. When hydrocarbon compounds are When hydrocarbon compounds are

burned in oxygen, the products burned in oxygen, the products are water and carbon dioxide.are water and carbon dioxide.

CHCH44 + O + O22 CO CO22 + H + H22OO CHCH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO

Combustion ReactionsCombustion Reactions

Combustion reactions involve light Combustion reactions involve light and heat energy released.and heat energy released.

Natural gas, propane, gasoline, etc. Natural gas, propane, gasoline, etc. are burned to produce heat energy.are burned to produce heat energy.

Most of these organic reactions Most of these organic reactions produce water and carbon dioxide.produce water and carbon dioxide.

PracticePractice Classify each of the following as to Classify each of the following as to

type:type:

HH22 + Cl + Cl22 2HCl 2HCl CombinationCombination

Ca + 2HCa + 2H22O O Ca(OH) Ca(OH)22 + H + H22

Single replacementSingle replacement

PracticePractice

2CO + O2CO + O22 2CO 2CO22

Combination and combustionCombination and combustion

2KClO2KClO33 2KCl + 3O 2KCl + 3O22

DecompositionDecomposition

PracticePractice

FeS + 2HCl FeS + 2HCl FeCl FeCl22 + H + H22SS Double replacementDouble replacement

Zn + HCl Zn + HCl ? ? Single replacementSingle replacement Zn + 2HCl Zn + 2HCl ZnCl ZnCl22 + H + H22

How molecules are symbolizedHow molecules are symbolizedClCl22 2Cl 2Cl 2Cl 2Cl22

• Molecules may also have brackets to indicate numbers of atoms. E.g. Ca(OH)2

O H

O H

Ca• Notice that the OH is a group• The 2 refers to both H and O• How many of each atom are in the following?

a) NaOHb) Ca(OH)2

c) 3Ca(OH)2

Na = 1, O = 1, H = 1Ca = 1, O = 2, H = 2Ca = 3, O = 6, H = 6

Balancing equations: MgOBalancing equations: MgO The law of conservation of mass states that The law of conservation of mass states that

matter can neither be created or destroyedmatter can neither be created or destroyed Thus, atoms are neither created or destroyed, Thus, atoms are neither created or destroyed,

only rearranged in a chemical reactiononly rearranged in a chemical reaction Thus, the number of a particular atom is the Thus, the number of a particular atom is the

same on both sides of a chemical equationsame on both sides of a chemical equation Example: Magnesium + Oxygen (from lab)Example: Magnesium + Oxygen (from lab) Mg + OMg + O22 MgO MgO O Mg O+ Mg O

• However, this is not balanced• Left: Mg = 1, O = 2• Right: Mg = 1, O = 1

Balance equations by “inspection”Balance equations by “inspection”

Hints: start with elements that occur in one Hints: start with elements that occur in one compound on each side. Treat polyatomic ions compound on each side. Treat polyatomic ions that repeat as if they were a single entity.that repeat as if they were a single entity.

5

2 33.52 7 4 6

2 222 63

C2H6 + O2 CO2 + H2O

a) P4 + O2 P4O10

b) Li + H2O H2 + LiOHc) Bi(NO3)3 + K2S Bi2S3 + KNO3

d) C2H6 + O2 CO2 + H2O

From Mg + O2 MgO 2Mg + O2 2MgO is correctMg + ½O2 MgO is incorrectMg2+ O2 2MgO is incorrect

4Mg + 2 O2 4MgO is incorrect

a)a) Mg + Mg + 22HCl HCl MgCl MgCl22 + H + H22

b)b) 33Ca + NCa + N22 Ca Ca33NN22

c)c) NHNH44NONO33 N N22O + O + 22HH22OO

d)d) 22BiClBiCl33 + + 33HH22S S Bi Bi22SS33 + + 66HClHCl

e)e) 22CC44HH1010 + + 1313OO22 88COCO22 + + 1010HH22OO

f)f) 66OO22 + C + C66HH1212OO66 66COCO22 + + 66HH22OO

g)g) 33NONO22 + H + H22O O 22HNOHNO33 + NO + NO

h)h) CrCr22(SO(SO44))33+ + 66NaOH NaOH 22Cr(OH)Cr(OH)33+ + 33NaNa22SOSO44

i)i) AlAl44CC33 + + 1212HH22O O 33CHCH44 + + 44Al(OH)Al(OH)33

Balance these skeleton equations:

The MoleThe Mole

Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second?

Background: atomic massesBackground: atomic masses Look at the “atomic masses” on the periodic Look at the “atomic masses” on the periodic

table. What do these represent?table. What do these represent? E.g. the atomic mass of C is 12 (atomic # is 6)E.g. the atomic mass of C is 12 (atomic # is 6) We know there are 6 protons and 6 neutronsWe know there are 6 protons and 6 neutrons Protons and neutrons have roughly the same Protons and neutrons have roughly the same

mass. So, C weighs 12 u (atomic mass units).mass. So, C weighs 12 u (atomic mass units). What is the actual mass of a C atom?What is the actual mass of a C atom? Answer: approx. 2 x 10Answer: approx. 2 x 10-23-23 grams (protons and grams (protons and

neutrons each weigh about 1.7 x10neutrons each weigh about 1.7 x10-24-24 grams) grams)Two problemsTwo problems1.1. Atomic masses do not convert easily to gramsAtomic masses do not convert easily to grams2.2. They can’t be weighed (they are too small)They can’t be weighed (they are too small)

The MoleThe MoleWithWith thesethese problems,problems, whywhy useuse atomicatomic massmass atat all?all?n Masses give information about # of pMasses give information about # of p++, n, n00, e, e––

n It is useful to know relative massIt is useful to know relative massE.g. E.g. Q - What ratio is needed to make Q - What ratio is needed to make

HH22O?O?A - 2A - 2::1 by atoms, but 21 by atoms, but 2::16 by mass16 by mass

It is useful to associate atomic mass with a It is useful to associate atomic mass with a mass in grams. It has been found thatmass in grams. It has been found that11 gg H,H, 1212 gg C,C, or 23or 23 gg NaNa havehave 6.026.02 xx 10102323 atomsatoms

6.02 x 106.02 x 102323 is a “mole” or “Avogadro’s number” is a “mole” or “Avogadro’s number” ““mol” is used in equations, “mole” is used in mol” is used in equations, “mole” is used in

writing; one gram = 1 g, one mole = 1 mol.writing; one gram = 1 g, one mole = 1 mol.

MollionaireMollionaireQ: how long would it take to spend a mole of Q: how long would it take to spend a mole of

$1 coins if they were being spent at a rate of $1 coins if they were being spent at a rate of 1 billion per second?1 billion per second?

A: $ 6.02 x 10A: $ 6.02 x 102323 / $1 000 000 000 / $1 000 000 000 = 6.02 x 10= 6.02 x 101414 payments = 6.02 x 10 payments = 6.02 x 101414 seconds seconds

6.02 x 106.02 x 101414 seconds / 60 = 1.003 x 10 seconds / 60 = 1.003 x 101313 minutes minutes1.003 x 101.003 x 101313 minutes / 60 = 1.672 x 10 minutes / 60 = 1.672 x 101111 hours hours1.672 x 101.672 x 101111 hours / 24 = 6.968 x 10 hours / 24 = 6.968 x 1099 days days6.968 x 106.968 x 1099 days / 365.25 = 1.908 x 10 days / 365.25 = 1.908 x 1077 years years

A: It would take 19 million yearsA: It would take 19 million years

Comparing sugar (CComparing sugar (C1212HH2222OO1111) & H) & H22OO

No, sugar has more (45:3 ratio)

Yes (6.02 x 1023 in each)

Yes.

No, molecules have dif. masses

No, molecules have dif. sizes.

1 mol each

Yes, that’s what grams are.

mass?

No, they have dif. molar masses# of moles?

No, they have dif. molar masses

# of molecules?

No# of atoms?

No, they have dif. densities.

volume?

1 gram eachSame

Molar massMolar mass The mass of one mole is called “molar mass”The mass of one mole is called “molar mass” E.g. 1 mol Li = 6.94 g LiE.g. 1 mol Li = 6.94 g Li This is expressed as 6.94 g/molThis is expressed as 6.94 g/mol What are the following molar masses?What are the following molar masses?

SS SOSO22

CuCu33(BO(BO33))22

32.06 g/mol 64.06 g/mol308.27 g/mol

Calculate molar masses (to 2 decimal places) CaCl2 (NH4)2CO3

O2

Pb3(PO4)2

C6H12O6

Cu x 3 = 63.55 x 3 = 190.65B x 2 = 10.81 x 2 = 21.62O x 6 = 16.00 x 6 = 96.00

308.27

Molar massMolar mass The mass of one mole is called “molar mass”The mass of one mole is called “molar mass” E.g. 1 mol Li = 6.94 g LiE.g. 1 mol Li = 6.94 g Li This is expressed as 6.94 g/molThis is expressed as 6.94 g/mol What are the following molar masses?What are the following molar masses?

SS SOSO22

CuCu33(BO(BO33))22

32.06 g/mol 64.06 g/mol308.27 g/mol

Calculate molar masses (to 2 decimal places) CaCl2 (NH4)2CO3

O2

Pb3(PO4)2

C6H12O6

110.98 g/mol (Ca x 1, Cl x 2)96.11 g/mol (N x 2, H x 8, C x 1, O x 3)32.00 g/mol (O x 2)

811.54 g/mol (Pb x 3, P x 2, O x 8)180.18 g/mol (C x 6, H x 12, O x 6)

Converting between grams and molesConverting between grams and moles If we are given the # of grams of a compound If we are given the # of grams of a compound

we can determine the # of moles, & vise-versawe can determine the # of moles, & vise-versa In order to convert from one to the other you In order to convert from one to the other you

must first calculate molar massmust first calculate molar mass

0.25HCl

53.15H2SO4

3.55NaCl1.27Cu

mol (n)gg/molFormula9.136.46

0.541998.0820758.44

0.020063.55

The MoleThe MoleConvert 36.0 grams of carbon into atoms.Convert 36.0 grams of carbon into atoms.

Convert 30 molecules of methane into Liters Convert 30 molecules of methane into Liters of gas.of gas.

Simplest and molecular formulaeSimplest and molecular formulaeConsider NaCl (ionic) vs. HConsider NaCl (ionic) vs. H22OO22 (covalent) (covalent)

Cl Na

Na Cl

Cl

Cl

Na

Na

• Chemical formulas are either “simplest” (a.k.a. “empirical”) or “molecular”. Ionic compounds are always expressed as simplest formulas.

• Covalent compounds can either be molecular formulas (I.e. H2O2) or simplest (e.g. HO)

Q - Write simplest formulas for propene (C3H6), C2H2, glucose (C6H12O6), octane (C8H14)

Q - Identify these as simplest formula, molecularformula, or both H2O, C4H10, CH, NaCl

HOO

H HOO H H

OO H

AnswersAnswersQ - Write simplest formulas for propene (CQ - Write simplest formulas for propene (C33HH66), ),

CC22HH22, glucose (C, glucose (C66HH1212OO66), octane (C), octane (C88HH1414))

Q - Identify these as simplest formula, molecularQ - Identify these as simplest formula, molecularformula, or both Hformula, or both H22O, CO, C44HH1010, CH, NaCl, CH, NaCl

A - CH2

A - H2O is both simplest and molecular

C4H10 is molecular (C2H5 would be simplest)

CH is simplest (not molecular since CH can’t form a molecule - recall Lewis diagrams)

NaCl is simplest (it’s ionic, thus it doesn’t form molecules; it has no molecular formula)

CH CH2O C4H7

Calculating percentage massCalculating percentage massIf you can work out Mr then this bit is easy…

Calculate the percentage mass of magnesium in magnesium oxide, MgO:

Ar for magnesium = 24 Ar for oxygen = 16

Mr for magnesium oxide = 24 + 16 = 40

Therefore percentage mass = 24/40 x 100% = 60%

Percentage mass (%) =

Mass of element Ar

Relative formula mass Mr

x100%

Calculate the percentage mass of the following:

1) Hydrogen in hydrochloric acid, HCl

2) Potassium in potassium chloride, KCl

3) Calcium in calcium chloride, CaCl2

4) Oxygen in water, H2O

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Empirical formulaeEmpirical formulaeEmpirical formulae is simply a way of showing how many atoms are in a molecule (like a chemical formula). For example, CaO, CaCO3, H20 and KMnO4 are all empirical formulae. Here’s how to work them out:

A classic exam question:

Find the simplest formula of 2.24g of iron reacting with 0.96g of oxygen.

Step 1: Divide both masses by the relative atomic mass:

For iron 2.24/56 = 0.04 For oxygen 0.96/16 = 0.06

Step 2: Write this as a ratio and simplify:

0.04:0.06 is equivalent to 2:3

Step 3: Write the formula:

2 iron atoms for 3 oxygen atoms means the formula is Fe2O3

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Example questionsExample questions1) Find the empirical formula of magnesium oxide which contains 48g of

magnesium and 32g of oxygen.

2) Find the empirical formula of a compound that contains 42g of nitrogen and 9g of hydrogen.

3) Find the empirical formula of a compound containing 20g of calcium, 6g of carbon and 24g of oxygen.

StoichiometryStoichiometry

Molar Mass of CompoundsMolar Mass of Compounds The molar mass (MM) of a compound is The molar mass (MM) of a compound is

determined the same way, except now determined the same way, except now you add up all the atomic masses for the you add up all the atomic masses for the molecule (or compound)molecule (or compound) Ex. Molar mass of CaClEx. Molar mass of CaCl22

Avg. Atomic mass of Calcium = 40.08gAvg. Atomic mass of Calcium = 40.08g Avg. Atomic mass of Chlorine = 35.45gAvg. Atomic mass of Chlorine = 35.45g Molar Mass of calcium chloride = Molar Mass of calcium chloride =

40.08 g/mol Ca + (2 X 35.45) g/mol Cl40.08 g/mol Ca + (2 X 35.45) g/mol Cl 110.98 g/mol CaCl 110.98 g/mol CaCl22

20

Ca 40.08 17

Cl 35.45

http://pearl1.lanl.gov/periodic/elements/20.html

http://pearl1.lanl.gov/periodic/elements/17.html

Mole Mole RoadmapRoadmap

Atoms or Molecules

Moles

Mass (grams)

6.02 X 1023

molar mass from periodic table

Liters of Gas

22.4

PracticePractice

Calculate the Molar Mass of Calculate the Molar Mass of calcium phosphatecalcium phosphate Formula = Formula = Masses elements:Masses elements:

Molar Mass = Molar Mass =

Ca3(PO4)2

molar mass Avogadro’s numbermolar mass Avogadro’s number Grams Grams MolesMoles particlesparticles

Everything must go Everything must go through Moles!!!through Moles!!!

CalculationsCalculations

Chocolate Chip Cookies!!Chocolate Chip Cookies!!1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chipsMakes 3 dozen

How many eggs are needed to make 3 dozen cookies?

How much butter is needed for the amount of chocolate chips used?

How many eggs would we need to make 9 dozen cookies?

How much brown sugar would I need if I had 1 ½ cups white sugar?

Cookies and Chemistry…Cookies and Chemistry…Huh!?!?Huh!?!?

Just like chocolate chip Just like chocolate chip cookies have recipes, cookies have recipes, chemists have recipes as chemists have recipes as wellwell

Instead of calling them Instead of calling them recipes, we call them recipes, we call them reaction equationsreaction equations

Furthermore, instead of Furthermore, instead of using cups and teaspoons, using cups and teaspoons, we use moleswe use moles

Lastly, instead of eggs, Lastly, instead of eggs, butter, sugar, etc. we use butter, sugar, etc. we use chemical compounds as chemical compounds as ingredientsingredients

Chemistry RecipesChemistry Recipes Looking at a reaction tells us how Looking at a reaction tells us how

much of something you need to react much of something you need to react with something else to get a product with something else to get a product (like the cookie recipe)(like the cookie recipe)

Be sure you have a balanced reaction Be sure you have a balanced reaction before you start!before you start!

Example: 2 Na + ClExample: 2 Na + Cl2 2 2 NaCl 2 NaCl This reaction tells us that by mixing 2 moles This reaction tells us that by mixing 2 moles

of sodium with 1 mole of chlorine we will get of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride2 moles of sodium chloride

What if we wanted 4 moles of NaCl? 10 What if we wanted 4 moles of NaCl? 10 moles? moles? 50 moles?50 moles?

PracticePractice

Write the balanced reaction for hydrogen gas Write the balanced reaction for hydrogen gas reacting with oxygen gas.reacting with oxygen gas.

2 H2 H22 + O + O22 2 H 2 H22OO How many moles of reactants are needed?How many moles of reactants are needed? What if we wanted 4 moles of water?What if we wanted 4 moles of water? What if we had 3 moles of oxygen, how much What if we had 3 moles of oxygen, how much

hydrogen would we need to react and how much hydrogen would we need to react and how much water would we get?water would we get?

What if we had 50 moles of hydrogen, how much What if we had 50 moles of hydrogen, how much oxygen would we need and how much water oxygen would we need and how much water produced? produced?

Mole RatiosMole Ratios

These mole ratios can be used to These mole ratios can be used to calculate the moles of one chemical calculate the moles of one chemical from the given amount of a different from the given amount of a different chemical chemical

Example: How many moles of chlorine Example: How many moles of chlorine is needed to react with 5 moles of is needed to react with 5 moles of sodium (without any sodium left over)?sodium (without any sodium left over)?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5 moles Na 1 mol Cl2

2 mol Na= 2.5 moles Cl2

Mole-Mole ConversionsMole-Mole Conversions How many moles of sodium chloride How many moles of sodium chloride

will be produced if you react 2.6 will be produced if you react 2.6 moles of chlorine gas with an excess moles of chlorine gas with an excess (more than you need) of sodium (more than you need) of sodium metal?metal?

Mole-Mass ConversionsMole-Mass Conversions Most of the time in chemistry, the amounts Most of the time in chemistry, the amounts

are given in grams instead of molesare given in grams instead of moles We still go through moles and use the mole We still go through moles and use the mole

ratio, but now we also use molar mass to ratio, but now we also use molar mass to get to gramsget to grams

Example: How many grams of chlorine are Example: How many grams of chlorine are required to react completely with 5.00 moles required to react completely with 5.00 moles of sodium to produce sodium chloride?of sodium to produce sodium chloride?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2

= 177g Cl2

PracticePractice

Calculate the mass in grams of Calculate the mass in grams of Iodine required to react completely Iodine required to react completely with 0.50 moles of aluminum.with 0.50 moles of aluminum.

Mass-MoleMass-Mole We can also start with mass and convert We can also start with mass and convert

to moles of product or another reactantto moles of product or another reactant We use molar mass and the mole ratio to We use molar mass and the mole ratio to

get to moles of the compound of interestget to moles of the compound of interest Calculate the number of moles of ethane Calculate the number of moles of ethane

(C(C22HH66) needed to produce 10.0 g of water) needed to produce 10.0 g of water 2 C2 C22HH66 + 7 O + 7 O22 4 CO 4 CO22 + 6 H + 6 H220 0

10.0 g H2O 1 mol H2O 2 mol C2H6

18.0 g H2O 6 mol H20

= 0.185 mol C2H6

PracticePractice Calculate how many moles of oxygen Calculate how many moles of oxygen

are required to make 10.0 g of are required to make 10.0 g of aluminum oxidealuminum oxide

Mass-Mass ConversionsMass-Mass Conversions Most often we are given a starting Most often we are given a starting

mass and want to find out the mass mass and want to find out the mass of a product we will get (called of a product we will get (called theoretical yield) or how much of theoretical yield) or how much of another reactant we need to another reactant we need to completely react with it (no leftover completely react with it (no leftover ingredients!)ingredients!)

Now we must go from grams to Now we must go from grams to moles, mole ratio, and back to grams moles, mole ratio, and back to grams of compound we are interested inof compound we are interested in

Mass-Mass ConversionMass-Mass Conversion

Ex. Calculate how many grams of Ex. Calculate how many grams of ammonia are produced when you ammonia are produced when you react 2.00g of nitrogen with excess react 2.00g of nitrogen with excess hydrogen.hydrogen.

NN2 2 + 3 H+ 3 H2 2 2 NH 2 NH332.00g N2 1 mol N2 2 mol NH3 17.06g NH3

28.02g N2 1 mol N2 1 mol NH3

= 2.4 g NH3

PracticePractice

How many grams of calcium nitride How many grams of calcium nitride are produced when 2.00 g of calcium are produced when 2.00 g of calcium reacts with an excess of nitrogen?reacts with an excess of nitrogen?

Limiting Reactant: CookiesLimiting Reactant: Cookies1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chipsMakes 3 dozen

If we had the specified amount of all ingredients listed, could we make 4 dozen cookies?

What if we had 6 eggs and twice as much of everything else, could we make 9 dozen cookies?

What if we only had one egg, could we make 3 dozen cookies?

Limiting ReactantLimiting Reactant Most of the time in chemistry we have Most of the time in chemistry we have

more of one reactant than we need to more of one reactant than we need to completely use up other reactant.completely use up other reactant.

That reactant is said to be in That reactant is said to be in excessexcess (there is too much).(there is too much).

The other reactant limits how much The other reactant limits how much product we get. Once it runs out, the product we get. Once it runs out, the reaction s. This is called the reaction s. This is called the limiting reactantlimiting reactant..

Limiting ReactantLimiting Reactant To find the correct answer, we have to try To find the correct answer, we have to try

allall of the reactants. We have to calculate of the reactants. We have to calculate how much of how much of aa product we can get from product we can get from eacheach of the reactants to determine which of the reactants to determine which reactant is the limiting one.reactant is the limiting one.

The The lowerlower amount of amount of aa product is the product is the correct answer.correct answer.

The reactant that makes the least amount The reactant that makes the least amount of product is the of product is the limiting reactantlimiting reactant. Once . Once you determine the limiting reactant, you you determine the limiting reactant, you should ALWAYS start with it!should ALWAYS start with it!

Be sure to pick Be sure to pick aa product! You can’t product! You can’t compare to see which is greater and which compare to see which is greater and which is lower unless the product is the same!is lower unless the product is the same!

Limiting Reactant: ExampleLimiting Reactant: Example 10.0g of aluminum reacts with 35.0 grams of 10.0g of aluminum reacts with 35.0 grams of

chlorine gas to produce aluminum chloride. chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, Which reactant is limiting, which is in excess, and how much product is produced?and how much product is produced?

2 Al + 3 Cl2 Al + 3 Cl22 2 AlCl 2 AlCl33 Start with Al:Start with Al:

Now ClNow Cl22::

10.0 g Al 1 mol Al 2 mol AlCl3 133.5 g AlCl3

27.0 g Al 2 mol Al 1 mol AlCl3

= 49.4g AlCl3

35.0g Cl2 1 mol Cl2 2 mol AlCl3 133.5 g AlCl3

71.0 g Cl2 3 mol Cl2 1 mol AlCl3

= 43.9g AlCl3

LimitingLimitingReactantReactant

LR Example ContinuedLR Example Continued We get We get 49.4g49.4g of aluminum chloride from the of aluminum chloride from the

given amount of aluminum, but only given amount of aluminum, but only 43.9g43.9g of aluminum chloride from the given of aluminum chloride from the given amount of chlorine. Therefore, chlorine is amount of chlorine. Therefore, chlorine is the limiting reactant. Once the 35.0g of the limiting reactant. Once the 35.0g of chlorine is used up, the reaction comes to a chlorine is used up, the reaction comes to a complete .complete .

Limiting Reactant PracticeLimiting Reactant Practice

15.0 g of potassium reacts with 15.0 15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant g of iodine. Calculate which reactant is limiting and how much product is is limiting and how much product is made.made.

Finding the Amount of Finding the Amount of ExcessExcess

By calculating the amount of the By calculating the amount of the excess reactant needed to excess reactant needed to completely react with the limiting completely react with the limiting reactant, we can subtract that reactant, we can subtract that amount from the given amount to amount from the given amount to find the amount of excess.find the amount of excess.

Can we find the amount of excess Can we find the amount of excess potassium in the previous problem?potassium in the previous problem?

Finding Excess PracticeFinding Excess Practice 15.0 g of potassium reacts with 15.0 g of 15.0 g of potassium reacts with 15.0 g of

iodine. iodine. 2 K + I2 K + I22 2 KI 2 KI

We found that Iodine is the limiting We found that Iodine is the limiting reactant, and 19.6 g of potassium iodide reactant, and 19.6 g of potassium iodide are produced.are produced.15.0 g I2 1 mol I2 2 mol K

39.1 g K

254 g I2 1 mol I2 1 mol K

= 4.62 g K

USED!15.0 g K – 4.62 g K = 10.38 g K EXCESS

Given amount of excess reactant

Amount of excess reactant actually used

Note that we started with the limiting reactant! Once you determine the LR, you should only start with it!

Limiting Reactant: RecapLimiting Reactant: Recap

1.1. You can recognize a limiting reactant problem You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT.because there is MORE THAN ONE GIVEN AMOUNT.

2.2. Convert ALL of the reactants to the SAME product Convert ALL of the reactants to the SAME product (pick any product you choose.)(pick any product you choose.)

3.3. The lowest answer is the correct answer.The lowest answer is the correct answer.4.4. The reactant that gave you the lowest answer is the The reactant that gave you the lowest answer is the

LIMITING REACTANT.LIMITING REACTANT.5.5. The other reactant(s) are in EXCESS.The other reactant(s) are in EXCESS.6.6. To find the amount of excess, subtract the amount To find the amount of excess, subtract the amount

used from the given amount.used from the given amount.7.7. If you have to find more than one product, be sure If you have to find more than one product, be sure

to start with the limiting reactant. You don’t have to start with the limiting reactant. You don’t have to determine which is the LR over and over again!to determine which is the LR over and over again!

Documents

A Study of Chemical Reactions Equations, Mole Conversions, & Stoichiometry