Chemical reactions: chemical changes that occur when ...ion.chem.usu.edu/~ensigns/chem1210/lectureoverheads...Chemical reactions: chemical changes that occur when substances react

Lecture outline: Section 3Chemical reactions: chemical changes that occur when substances react to that occur when substances react to form new substances

1. Chemical equations2. Atomic and molecular mass3. Chemical calculations

1S. Ensign, stoichiometry and calculations

Law of conservation of mass:atoms are not created or destroyed during chemicaldestroyed during chemical reactions. They simply y yrearrange.

Mass before = mass after


Stoichiometry: the quantitative nature of chemical formulas and reactionsreactions

Th t d f titi f - The study of quantities of materials consumed and produced in chemical reactions.


Chemical Equations

++reacts with

to produce

Left of arrow: starting substances, “reactants” right of arrow: substances produced, “products”products”

Conservation of mass: atoms, left side = atoms, right side


Chemical Equations

+ +

2H2 + O2 2H2O

Subscript after an element in a compound: number of atoms of that element in the compound

Stoichiometric coefficients: the number of compounds involved in the reaction


Note the difference in meaning for a ffi i t i f t f f lcoefficient in front of a formula vs. a

subscript to a formula


Balancing a chemical Equation•Identify the reactants and products

W it th b l d h i l ti•Write the unbalanced chemical equation

•Balance equation by addition of properBalance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on bothproducts so that the atoms balance on both sides of equation

B l fi t th l t( ) th t i lBalance first the element(s) that occur in only one substance on each side of the equation


•Sodium reacts with chlorine gas (Cl2) to produce sodium chloride

•Potassium reacts with water to produce potassium hydroxide and hydrogen gas (H2)

•Identify the reactants and products

•Write the unbalanced chemical equation

•Balance equation by addition of proper coefficients in front ofBalance equation by addition of proper coefficients in front ofreactants and products so that the atoms balance on both sides of equation


Write a balanced chemical equation for the reaction of methane gas (CH4) with molecular g ( 4)oxygen (O2) to produce water and carbon dioxide


•Write the unbalanced chemical equationWrite the unbalanced chemical equation

•Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation

Balance first the element(s) that occur in only one substance on each side of the equation


Additional information from b l d h i l ibalanced chemical equations

• Physical states of reactants and productsPhysical states of reactants and products

• Reactants or products that are dissolved in water


Common types of chemical reactionCommon types of chemical reaction

• CombustionCombustion• Combination

d iti• decomposition


Combustion: A chemical reaction where something burns in oxygen producing heatsomething burns in oxygen, producing heat

• Combustion of metals

• Combustion of diatomic molecules

• Combustion of nonmetal compoundsp

C b ti f h d b• Combustion of hydrocarbons

• Combustion of more complex organic molecules12

S. Ensign, stoichiometry and calculations

Hydrocarbon combustion• Reactants are a hydrocarbon (CxHy) and O2,

products are CO2 and H2O•


Write the balanced chemical equation for the combustion of butane (tetracarbon decahydride)combustion of butane (tetracarbon decahydride)


•Write the unbalanced chemical equation

•Start by balancing an element that only occurs in one compound on each side of the equation


C b tiCommon types of chemical reaction

• Combustion• Combination

decomposition• decompositionhttp://biochemistryportal.com/chemistry%201210%20preview/chemistry1210resources/media/CONTAC%7E1.MOV


Patterns of chemical reactivityPatterns of chemical reactivity

• Alkali metal + waterAlkali metal + water

• Alkali metal + halogen


Concept check• Chemical formulas

– subscriptssubscripts• Chemical equations

– coefficients– coefficients• Balancing equations• Common reactions• Common reactions

– CombustionCombination– Combination

– Decomposition


Chemical calculations

• Atomic massForm la mass• Formula mass

• The mole (mol)• Empirical and molecular formulas• Chemical equations and balancingChemical equations and balancing


Atomic Mass: The mass of an atom is proportional to the number of p+ n and e- the atom hasthe number of p , n, and e the atom has

subatomicparticle

mass in grams mass in amuparticleproton 1.673 x 10-24 g 1.0073 amuneutron 1.675 x 10-24 g 1.0087 amugelectron 9.11 x 10-28 g 5.49 x 10-4

1 amu = 1 66054 x 10-24 g1 amu = 1.66054 x 10-24 g


1 amu = 1 66054 x 10-24 g1 amu = 1.66054 x 10 g

1 1/12 th f 12C1 amu ≡ 1/12 the mass of 12C

th f 1 t f 12C 12the mass of 1 atom of 12C ≡ 12 amu


Sum the masses of the subatomic particles of 12C and

By definition, the mass of 1 atom of 12C ≡ 12 amu

Sum the masses of the subatomic particles of 12C and what mass do you get?

p+ nn

subatomicparticle

mass in grams mass in amu

p+n p+p+

n p+p+nn

proton 1.673 x 10-24 g 1.0073 amuneutron 1.675 x 10-24 g 1.0087 amuelectron 9.11 x 10-28 g 5.49 x 10-4


The masses of the elements are determined experimentally. They experimentally. They cannot be accurately predicted by the masses predicted by the masses of the constituent parts


The greater the mass defect, the more stable an isotope of anmore stable an isotope of an

element is


Average atomic masses•Most elements in nature are mixtures ofMost elements in nature are mixtures of more than 1 isotope•“Average atomic mass” incorporates the•“Average atomic mass” incorporates the relative abundance of each isotope in nature

p+

p+ nn p+

np+

n p+p+nn p+

p+ nn p+

np+

n p+p+nn n p+

p+ nn p+

np+

n p+p+nn n

n

6 6 66 protons6 neutrons

6 protons7 neutrons

6 protons8 neutrons

12C 13C98.892 % 1.108 % Trace amount (<0.001%)

14C(stable) (stable) (radioactive)


Average atomic mass of C found in naturep+

p+ nn p+

np+

nn p+

p+ nn p+

np+

nn n p+

p+ nn p+

np+

nn nn

pn p+p+

nn pn p+p+

nn n pn p+p+

nn n

6 protons6 neutrons

6 protons7 neutrons

6 protons8 neutrons

12C 13C 14C

6 neutrons 7 neutrons 8 neutrons

C C98.892 %12 amu

1.108 %13.00335 amu

Trace amount (<0.001%)C


Naturally occurring bromine is composed of two isotopes which have atomic masses of 78.918336 and 80.916289amu Isotope 1 is present at an abundance of 50 69%amu. Isotope 1 is present at an abundance of 50.69%while isotope 2 is present at an abundance of 49.31%. Use this information to determine the average atomic

f t ll i b imass of naturally occurring bromine


The atomic weights provided in a Periodic Table are the average atomic weights of the naturally occurring elements


Formula massFormula mass

• Atoms have atomic masses (atomicAtoms have atomic masses (atomic weights)

• Compounds have formula masses• Compounds have formula masses (formula weights)T l l t f l th• To calculate formula mass, sum the atomic masses of all atoms in the

l lmolecule


Molecular massRecall that the formulas for ionic compounds are• Recall that the formulas for ionic compounds are always written as “empirical formulas” because there is no discrete molecular formulathere is no discrete molecular formula

• The masses of molecular compounds with defined formulas may be referred to as either “formulaformulas may be referred to as either formula masses” or “molecular masses” (molecular weights)

• The masses of empirical compounds are always p p yreferred to as “formula masses” (formula weights)

NaCl, FW = 58.44 C6H6, FW = 78.11 C2H2, FW = 26.03629


What is the formula weight of ammonium sulfate?

8(NH4)2SO46N

1H

1 008

8O

16.00

(NH4)2SO4

N14.01

1.008

16S

32 0632.06


1 8

Determine the molecular mass of ethanol, C2H6O

6C

12 01

1H

1.008

O16.00

12.01

A.About 29 amu

B About 40 amuB.About 40 amu

C About 58 amuC.About 58 amu

D.About 46 amu


Percent composition: the percentage, by t ib t d b h l t imass, contributed by each element in a

substance

( )( ) 100AWElementofAtomsElement % ×= 00CompoundofFW

e e%


What are the mass percentages of each element in ammonium sulfate?

(NH4)2SO4 61 8 16

( )( ) 100Compound of FW

AWElementofAtomsElement % ×=

( 4)2 4 6N

14.01

1H

1.008

8O

16.00

16S

32.06FW = 132.10


How do we bridge the How do we bridge the microscopic world of amu p f

with the macroscopic ld f ?world of grams?


This week in chemistry• Chapters 1 and 2 graded quizzes Chapter 3 graded• Chapters 1 and 2 graded quizzes,Chapter 3 graded

quiz available Wednesday• Recitations meet graded bring iclickersRecitations meet, graded, bring iclickers.

– Grading: two points participation, two points maximum for correct answers. I wrote this week’s questions

• Office hours: Mon. and Wed., 2:30-4 PM, other times by appt.

• Supplemental instruction: M, T, W, R• Read materials for chapters 3 and 4- overheads and

text• work problems in chapters 1-3 self tests, watch self

35

test tutorials to help in developing problem-solving skills. Work old exam problems. 3535


The mole (mol)• The S.I unit of “amount of a substance”• A collection of a very large number of• A collection of a very large number of

particles• “The amount of a substance that

contains as many objects (atoms, y j (molecules, or other particles) as the number of atoms in exactly 12 grams ofnumber of atoms in exactly 12 grams of the 12C isotope”


1 mol = 6.022136736 x 1023

particles

“Avogadro’s number”

602,200,000,000,000,000,000,000

How big a quantity is the mol????


• One 12C atom weighs 12 amu• 6 02 x 1023 12C atoms weigh 12 grams• 6.02 x 10 C atoms weigh 12 grams• One mol of 12C atoms weigh 12 grams


Molar mass: the mass, in grams, of 1 mol (6.02 x 1023 particles) of a substancea substanceThe more a particular particle weighs, the p p g ,higher it’s molar mass

O l f i h thOne mol of pennies has more mass than one mol of carbon atoms


1 molecule of H2O: 1 dozen molecules of H2O: 1 mole of H2O molecules: 2mass = 18.0 amu = 2.99 x 10-23 g

2mass = 216 amu = 3.59 x 10-22 g

o e o 2O o ecu esmass = 1.08 x 1025 amu = 18.0 g

40

Mass = 18.0 amu/molecule Mass = 18.0 g/mole


Review some terms• Atomic mass: mass of a single atom of a single

isotope amu• Average atomic mass: average mass of a

collection of isotopes (as found in nature) amu• Formula mass: the sum of the average atomic

masses of the elements in a compound amu• Molecular mass: the same as formula weight,

applied to covalent (molecular) compounds amuM l th f l f ti l / l• Molar mass: the mass of a mol of particles g/mol


Interconversion of mass mols and Interconversion of mass, mols, and number of particles

Molar mass

Avogadro’snumbermass moles moleculesmass number


Using molar mass and Avogadro’s number as conversion factors

12.01 g C atoms 18.02 g H2O moleculesg1 mol C atoms

g 2

1 mol H2O molecules

1 mol C atoms 1 mol H O molecules12.01 g C atoms 18.02 g H2O molecules

1 mol H2O molecules

1 mol C atoms

12.01 g C atoms

1 mol C atoms6.02 x 1023 C atoms

6.02 x 1023 H2O molecules1 mol H2O molecules

12.01 g C atoms6.02 x 1023 C atoms

6 02 x 1023 C atoms

6.02 x 1023 H2O molecules

1 mol H2O molecules

12.01 g C atoms6.02 x 1023 C atoms 2


An experiment calls for 0.36 mol of Al. What mass of Al is needed?? 13What mass of Al is needed?? 13

Al26.99

26.99 amu or 26.99 g/mol


How many moles of silicon correspond to 454 g Si??14

28 09 28 09 / l Si28.09

28.09 amu or 28.09 g/mol


80You have a graduated cylinder containing 25.4 ml Hg. How many moles of Hg are in the cylinder?

Hg200.6

How many atoms of Hg are in the cylinder?

dHg = 13.534 g/ml


Stoichiometric relations of atoms in a moleculeSucrose is C12H22O1112 22 11

•1 molecule of sucrose contains 12 atoms of C

1 l l f i 22 f H•1 molecule of sucrose contains 22 atoms of H

•1 molecule of sucrose contains 11 atoms of O•1 mol of sucrose contains 12 mols of C

•1 mol of sucrose contains 22 mols of H1 mol of sucrose contains 22 mols of H

•1 mol of sucrose contains 11 mols of OW it th l ti i f tWrite these relations as conversion factors


Which conversion factor is incorrect for sucrose, C12H22O11 :

A. Catoms 12sucrose molecule 1

l lB.Hatoms 22

sucrose 1molecule

C. sucrose 1moleculeHmol 22

sucrose1molD.O mol 11

sucrose 1mol48S. Ensign, stoichiometry and calculations

Interconversion of mass mols and Interconversion of mass, mols, and number of particles

l l lMolar mass

Avog.#l

Stoich.relationmass molesa moleculesmass #molesbrelation


How many C atoms are present in 46.0 grams of sucrose, C12H22O11?grams of sucrose, C12H22O11?

6C

1H

1 008 12.011.008

8OO

16.00


You have 23.6 grams of ethanol, C2H6O.•How many moles of ethanol do you have?

•How many molecules of ethanol do youHow many molecules of ethanol do you have?

•How many atoms of carbon do you have?•How many atoms of carbon do you have?

•What is the mass of one molecule of th l i ?ethanol in grams?


You have 23.6 grams of ethanol, C2H6O.

•How many moles of ethanol do you have?mass molesa

moleculesor atoms

Molar mass

Avog.#

molesb

Stoich.relation

•How many molecules of ethanol do you have?

•How many atoms of carbon do you have?

•What is the mass of one molecule of ethanol in grams?


Problem solving strategy for stoichiometric calculationsMolar AvogStoich Stoich

M l i i f t ( / l)

mass molesamoleculesor atoms

Molar mass

Avog.#molesb

Stoich.relation

Stoich.relation molecules

or atoms

• Molar mass is a conversion factor (g/mol)• For a compound, the relationship between atoms

and molecules is a conversion factorand molecules is a conversion factor

• For a compound the relation between numbersFor a compound, the relation between numbers of mols is a conversion factor

• Avogradro’s number can be used as a converion foactor to convert mols and molecules (or atoms)


Example to illustrate problem solving strategy for stoichiometric calculations

B j t b i thi bik i f t b• By just observing this bike, no conversion factors can be deduced equating the mass of the bike, the bike frame, the wheels and pedalsp

• By just observing the bike, we can deduce conversion factors relating the numbers of the components of the bike

• Scale these relationships up to a collection of parts equal to a• Scale these relationships up to a collection of parts equal to a mol


Empirical formula from experimental analysisp p y

1. Base calculation on 100 grams of compound.

2. Determine moles of each element in 100 grams of compound.

3. Divide each value of moles by the3. Divide each value of moles by the smallest of the values.

4 Multiply each number by an integer to4. Multiply each number by an integer to obtain all whole numbers.


The compound hydrazine (FW = 32) consists f N d H Th t f N dof N and H. The mass percentages of N and

H in hydrazine are 87.42% and 12.58%, respecti elrespectively.

What is the empirical formula for hydrazine?hydrazine?What is the molecular formula for hydrazine?hydrazine?


The compound hydrazine (FW = 32) consists of N and H. The mass percentages of N and H in hydrazine are 87.42% and 12.58%, respectively.

What is the empirical formula for hydrazine?What is the empirical formula for hydrazine?What is the molecular formula for hydrazine?

1. Base calculation on 100 grams of compound.2. Determine moles of each element in 100 grams of compound.3. Divide each value of moles by the smallest of the values.4. Multiply each number by an integer to obtain all whole numbers.


What is the molecular formula for hydrazine?•Determine true molar mass of hydrazine experimentallyy p y

•Compare true molar mass to the molar mass based on empirical formula

•The true molar mass will be the empirical mass multiplied by some number “n”

•Multiply “n” by the empirical formula to get the molecular formula


Mothballs are made of napthalene, a C and H containing compound. The mass % of C in naphthalene is 93.71%, while the mass % of H is 6.29%. Naphthalene has a true molar mass of 128 amu.What is the empirical formula for naphthalene?What is the empirical formula for naphthalene?What is the molecular formula for naphthalene?


Mothballs are made of napthalene, a C and H containing compound. The mass % of C in naphthalene is 93.71%, while the mass % of H is 6.29%. Naphthalene has a true molar mass of 128 amu.What is the empirical formula for naphthalene?What is the molecular formula for naphthalene?What is the molecular formula for naphthalene?


Quantitative information from b l d h i l tibalanced chemical equations

2H2 + O2 2H2O2 2 2


Quantitative information from balanced chemical equations

2H2 + O2 2H2Obalanced chemical equations

Stoichiometricall eq i alent q antities are sed• Stoichiometrically equivalent quantities are used as conversion factors

• Molecules relate to molecules as mols relate to• Molecules relate to molecules as mols relate to mols

• How many mols of H O are produced from the• How many mols of H2O are produced from the reaction of excess H2 with 3.5 mols of O2?


A camping propane cylinder contains one pound of propane (C H ) What mass ofpound of propane (C3H8). What mass of CO2 is produced from the complete

b ti f ll th i thcombustion of all the propane in the cylinder? 1 lb = 454 gg

Write a chemical equation for this reactionBalance the equationBalance the equationUse the proper stoichiometric relationships

from the balanced equation

massa molesa massb

Molar mass

Molar massmolesb

Stoich.relation

63

massa molesa massbmolesb


A camping propane cylinder contains one pound of propane (C3H8). What mass of CO2 is produced from the complete combustion of all the propane in the cylinder? 1 lb = 454 gcombustion of all the propane in the cylinder? 1 lb 454 g

mass moles massb

Molar mass

Molar massmolesb

Stoich.relationmassa molesa massbmolesb


A camping propane cylinder contains one pound of propane (C3H8). What mass of CO2 is produced from the complete combustion of all the propane in the cylinder? 1 lb = 454 gcombustion of all the propane in the cylinder? 1 lb 454 g

mass moles massb

Molar mass

Molar massmolesb

Stoich.relationmassa molesa massbmolesb


Limiting reactants2H + O 2H O2H2 + O2 2H2O

66

No limiting reactant66S. Ensign, stoichiometry and calculations


67

No limiting reactant67S. Ensign, stoichiometry and calculations


68

O2 is the limiting reactant (H2 is in excess)68S. Ensign, stoichiometry and calculations


69

H2 is the limiting reactant (O2 is in excess)69S. Ensign, stoichiometry and calculations

Silicon is used in computer chips and solar ll Th fi l t i th f t i fcells. The final step in the manufacturing of

silicon “metal” is SiCl + 2M Si + 2M ClSiCl4(l) + 2Mg(s) Si(s) + 2MgCl2(s)

225 g of SiCl4 is mixed with 225 g Mg.Is one reagent present in a limiting

amount, and if so, which one?Wh t f Si( ) i f d if thWhat mass of Si(s) is formed if the

reaction goes to completion?What mass of the excess reagent is leftWhat mass of the excess reagent is left

over after the reaction goes to completion?Molar MolarStoich.

70massa molesa massb

Molar mass

Molar massmolesb

Stoich.relation


SiCl4(l) + 2Mg(s) Si(s) + 2MgCl2(s)225 g of SiCl4 is mixed with 225 g Mg.I t t i li iti t d if hi h ?

massa molesa massb

Molar mass

Molar mass

molesb

Stoich.relation

Is one reagent present in a limiting amount, and if so, which one?What mass of Si(s) is formed if the reaction goes to completion?What mass of the excess reagent is left over after the reaction

goes to completion?g p


SiCl4(l) + 2Mg(s) Si(s) + 2MgCl2(s)225 g of SiCl4 is mixed with 225 g Mg.I t t i li iti t d if hi h ?

massa molesa massb

Molar mass

Molar mass

molesb

Stoich.relation

Is one reagent present in a limiting amount, and if so, which one?What mass of Si(s) is formed if the reaction goes to completion?What mass of the excess reagent is left over after the reaction

goes to completion?g p


Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO(g) + 2H2(g) CH3OH(l)365 g of CO are mixed with 65 0 g H365 g of CO are mixed with 65.0 g H2.

Which is the limiting reagent?What is the maximal mass of methanol that can be formed??What mass of the excess reagent is left over after the reaction goes to completion?What mass of the excess reagent is left over after the reaction goes to completion?


Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO(g) + 2H2(g) CH3OH(l)365 g of CO are mixed with 65 0 g H365 g of CO are mixed with 65.0 g H2.

Which is the limiting reagent?What is the maximal mass of methanol that can be formed??What mass of the excess reagent is left over after the reaction goes to completion?What mass of the excess reagent is left over after the reaction goes to completion?


Theoretical vs. actual yield• “Theoretical yield” is the quantity of

product calculated to form when all of theproduct calculated to form when all of the limiting reagent reacts

• The “actual yield” is the amount of product• The actual yield is the amount of product actually obtained experimentallyA t l i ld ≤ th ti l i ld• Actual yield ≤ theoretical yield

001yieldactual% i ld 001yield ltheoretica

y%yield x=


Theoretical vs. actual yield

001 i ldlth ti

yield actual%yield x=yield ltheoretica


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Chemical reactions: chemical changes that occur when ...ion.chem.usu.edu/~ensigns/chem1210/lectureoverheads...Chemical reactions: chemical changes that occur when substances react