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Chapter 11

“Chemical Reactions”

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Section 11.1Section 11.1p. 321p. 321

Describing Chemical Describing Chemical ReactionsReactions

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All chemical reactions… have two parts:

1. Reactants = substances you start with

2. Products = end with reactants turn into products Reactants Products

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- Page 321

Reactants

Products

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In a chem rxn Atoms not created or destroyed (Law of

Conservation of Mass) rxn described in a:

#1. sentence every item is a word Copper reacts with chlorine to form copper (II)

chloride.

#2. word equation some symbols used

Copper + chlorine copper (II) chloride

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Symbols in equations? – Text page 323

arrow (→) separates reactants from products (points to products)

–Read as: “reacts to form” or yields plus sign = “and” (s) after formula = solid: Fe(s)

(g) = gas: CO2(g)

(l) = liquid: H2O(l)

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Symbols used in equations(aq) after formula = dissolved in

water, aqueous solution: NaCl(aq) is salt water solution

used after product - indicates gas produced: H2↑

used after product - indicates solid produced: PbI2↓

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Symbols used in equations■ double arrow indicates a

reversible reaction (more later)■ shows that

heat supplied to rxn■ indicates catalyst

supplied (here, platinum is catalyst)

heat ,

Pt

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What is a catalyst? substance that speeds up

rxn, w/o being changed or used up in rxn

Enzymes - biological or protein catalysts in your body

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#3. The Skeleton EquationUses formulas and symbols to

describe rxn

–but doesn’t indicate how many; means they’re NOT balanced

All chem equations are description of rxn

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Write a skeleton equation for:1. Solid iron (III) sulfide reacts with

gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.

2. Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

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Now, read these equations:

Fe(s) + O2(g) Fe2O3(s)

Cu(s) + AgNO3(aq) Ag(s) + Cu(NO3)2(aq)

NO2(g) N2(g) + O2(g)

Pt

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#4. Balanced Chemical EquationsAtoms can’t be created or destroyed

in an ordinary reaction:

–All atoms we start with we must end up with (balanced!)

balanced equation has same # of each element on both sides of equation

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Rules for balancing:1) Assemble correct formulas for all

reactants and products, using “+” and “→”2) Count # of atoms of each type on both sides3) Balance elements one at a time by adding

coefficients (the numbers in front) where you need more - save balancing the H and O until LAST!

(hint: I prefer to save O until the very last)4) Double-Check make sure balanced

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Never change subscript (only change coefficients)

– changing subscript (formula) describes different chemical

– H2O different than H2O2

Never put coefficient in middle of formula; only in front

2NaCl is okay, but Na2Cl is not.

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Practice Balancing Examples

_AgNO3 + _Cu _Cu(NO3)2 + _Ag

_Mg + _N2 _Mg3N2

_P + _O2 _P4O10

_Na + _H2O _H2 + _NaOH

_CH4 + _O2 _CO2 + _H2O

2 2

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4 5

2 22

2 2

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Balancing Equations

Balancing Chemical ReactionsMark Rosengarten – 8:21

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Section 11.2Section 11.2p. 330p. 330

Types of Chemical Types of Chemical ReactionsReactions

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Types of Reactions5 major types. predict the products predict whether or not they will happen at allHow? We recognize them by their reactants

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#1 - Combination Reactions Combine = put together 2 substances combine to make one

cmpd (also called “synthesis”) Ca + O2 CaO

SO3 + H2O H2SO4

predict products, especially if reactants are 2 elements

Mg + N2 Mg3N2 (symbols, charges, cross)

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Complete and balance: Ca + Cl2 Fe + O2 (assume iron (II) oxide is the product)

Al + O2 Remember first step…write correct

formulas – you can still change subscripts at this point, but not while balancing!

Then balance by changing just coefficients only

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#1 – Combination ReactionsAdditional Notes:

a) Some nonmetal oxides react with H2O - produces acid:

SO2 + H2O H2SO3

b) Some metallic oxides react with H2O - produces base:

CaO + H2O Ca(OH)2

(how “acid rain” forms)

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#2 - Decomposition Reactionsdecompose = fall apartone reactant breaks apart into 2 or

more elements or cmpdsNaCl Na + Cl2

CaCO3 CaO + CO2

Note: energy (heat, sunlight, electricity, etc.) usually required

electricity

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#2 - Decomposition Reactionspredict products if binary cmpd

(made of 2 elements)

–It breaks apart into the elements:H2OHgO

electricity

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#2 - Decomposition Reactions If cmpd has > 2 elements you must

be given one of products

–other product from the missing pieces

NiCO3 CO2 + ___

H2CO3(aq) CO2 + ___

heat

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#3 - Single Replacement ReactionsOne element replaces another (new

dance partner)

Reactants must be an element & cmpd

Products will be a different element and different cmpd

Na + KCl K + NaCl F2 + LiCl LiF + Cl2

(Cations switched)

(Anions switched)

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#3 Single Replacement Reactions Metals replace other metals (they can

also replace H) K + AlN Zn + HCl Think of water as: HOH

–Metals replace first H, then combines w/ hydroxide (OH).

Na + HOH

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#3 Single Replacement Reactions can even tell whether or not single

replacement rxn will happen:–b/c some chemicals more “active” than

others–More active replaces less active

list – p. 333 - Activity Series of Metals

Higher on list replaces lower

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The “Activity Series” of Metals Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead HydrogenHydrogen Bismuth Copper Mercury Silver Platinum Gold

1) Metals can replace other metals, if they are above metal trying to replace (i.e. Zn will replace Pb)

2) Metals above H can replace H in acids.

3) Metals from Na upward can replace hydrogen in H2O

Higher activity

Lower activity

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The “Activity Series” of Halogens

Fluorine Chlorine Bromine Iodine

Halogens can replace other halogens in compounds, if they are above halogen they are replacing

2NaCl(s) + F2(g) 2NaF(s) + Cl2(g)

MgCl2(s) + Br2(g) ???No Reaction!

???

Higher ActivityHigher Activity

Lower ActivityLower Activity

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#3 Single Replacement Reactions Practice:

Fe + CuSO4

Pb + KCl

Al + HCl

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#4 - Double Replacement Reactions Two things replace each other.

–Reactants must be two ionic compounds, in aqueous solution

NaOH + FeCl3 –positive ions change place (dance partners)

NaOH + FeCl3 Fe+3 OH- + Na+1 Cl-1

= NaOH + FeCl3 Fe(OH)3 + NaCl

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#4 - Double Replacement Reactions Have certain “driving forces”, or reasons

–only happens if one product:

a) doesn’t dissolve in water & forms solid (a “precipitate”), or

b) is gas that bubbles out, or

c) is molecular compound (usually water)

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Complete and balance:assume all of the following

reactions actually take place:

CaCl2 + NaOH

CuCl2 + K2S

KOH + Fe(NO3)3

(NH4)2SO4 + BaF2

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How to recognize which type?Look at the reactants:

E + E =Combination

C = Decomposition

E + C = Single replacement

C + C = Double replacement

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Practice Examples: H2 + O2

H2O Zn + H2SO4 HgO KBr + Cl2

AgNO3 + NaCl

Mg(OH)2 + H2SO3

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#5 – Combustion Reactions Combustion means “add oxygen” Normally, a cmpd composed of only

C, H, (and maybe O) is reacted with oxygen – called “burning”

Complete combustion, products are CO2 and H2O

If incomplete, products are CO (or possibly just C) and H2O

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Combustion Reaction Examples:

C4H10 + O2 (assume complete)

C4H10 + O2 (incomplete)

C6H12O6 + O2 (complete)

C8H8 + O2 (incomplete)

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SUMMARY: An equation... Describes a rxn Must be balanced (follows the Law of

Conservation of Mass) only balance by changing coefficients special symbols to indicate physical

state, catalyst or energy required, etc.

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Reactions 5 major types We can tell what type they are by

looking at reactants Single Replacement happens based on

the Activity Series Double Replacement happens if one

product is: 1) a precipitate (an insoluble solid), 2) water (a molecular compound), or 3) a gas

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Section 11.3Section 11.3p. 342p. 342

Reactions in Aqueous SolutionReactions in Aqueous Solution

Co(NOCo(NO33))22

KK22CrCr22OO77 KK22CrOCrO44

NiClNiCl22 CuSOCuSO44

KMnOKMnO44

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Net Ionic EquationsMany reactions occur in water- that

is, in aqueous solutionWhen dissolved in water, many

ionic cmpds “dissociate”, or separate, into cations & anions

Now write ionic equation

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Net Ionic Equations Example (needs to be a double replacement reaction)

AgNO3 + NaCl AgCl + NaNO3

1. this is the full balanced equation

2. next, write it as ionic equation by splitting the cmpds into their ions:

Ag1+ + NO31- + Na1+ + Cl1-

AgCl + Na1+ + NO31-

Note that the AgCl did not ionize, because it is a “precipitate” “precipitate” (Table 11.3 p. 344)(Table 11.3 p. 344)

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Net Ionic Equations3. simplify by crossing out ions not

directly involved (called spectator ions)

Ag1+ + Cl1- AgCl

This is called the net ionic equation

Let’s talk about precipitates before we do some other examples

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Predicting the Precipitate Insoluble salt is a precipitate

[note Figure 11.11, p.342 (AgCl)] General solubility rules are found:

a) Table 11.3, p. 344 in textbook

b) Reference section - page R54 (Table B.9)

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Let’s do some examples together of net ionic equations, starting with these reactants:

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BaCl2 + AgNO3 →

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NaCl + Ba(NO3)2 →

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Pb(NO3)2(aq) + H2SO4(aq)