Chemical Reactions Chapter 11. What Is a Reaction? It is only a reaction if a new substance has...

Chemical ReactionsChemical ReactionsChapter 11Chapter 11

What Is a Reaction?What Is a Reaction? It is only a reaction if a new substance has It is only a reaction if a new substance has

been produced.been produced. Four clues a reaction has happened:Four clues a reaction has happened:

1.1. Formation of a gas (bubbles)Formation of a gas (bubbles)2.2. Change of colorChange of color3.3. Heat change (hotter or colder)Heat change (hotter or colder)4.4. Precipitate (solid formed when two liquids Precipitate (solid formed when two liquids

meet)meet)

Reaction EquationsReaction Equations

Reactants (before rxn) Reactants (before rxn) →→ Products(after rxn) Products(after rxn)

CHCH44 + O + O22 → CO→ CO22 + H + H22OO Methane reacts with oxygen to produce carbon dioxide and waterMethane reacts with oxygen to produce carbon dioxide and water

NN22 + H + H22 → NH → NH33

Reaction EquationsReaction Equations If a catalyst was used in the reaction, the If a catalyst was used in the reaction, the

equation will show that: equation will show that:

NN22 + H + H22 → NH → NH33 If heat was required to make the reaction If heat was required to make the reaction

happen, the equation will show that:happen, the equation will show that:

NN22 + H + H22 → NH → NH33

Phases in EquationsPhases in Equations Solids-denoted by a subscript (s)Solids-denoted by a subscript (s) Liquids-denoted by a subscript (l)Liquids-denoted by a subscript (l) Gases-denoted by a subscript (g)Gases-denoted by a subscript (g) Substance in solution-denoted by subscript (aq)Substance in solution-denoted by subscript (aq) Example:Example:

ZnZn(s)(s) + HCl + HCl(aq)(aq) → ZnCl→ ZnCl(aq)(aq) + H + H2(g)2(g) (aq) is necessary because ionic compounds (aq) is necessary because ionic compounds

IONIZE in water. Explanation:IONIZE in water. Explanation:

Diatomic MoleculesDiatomic Molecules All elements exist in nature as single atoms, All elements exist in nature as single atoms,

except seven: except seven: bromine, iodine, nitrogen, chlorine, hydrogen, bromine, iodine, nitrogen, chlorine, hydrogen,

oxygen, and fluorine.oxygen, and fluorine. These exist as diatomic molecules, or two-These exist as diatomic molecules, or two-

atom molecules (Bratom molecules (Br22, H, H22, O, O22, I, I22, Cl, Cl22, N, N22, F, F22)) How can I possibly remember them???How can I possibly remember them???

BrINClHOFBrINClHOF

Balancing EquationsBalancing Equations The Law of Conservation of Mass The Law of Conservation of Mass

forces chemical equations to be forces chemical equations to be balanced. You can not have atoms balanced. You can not have atoms disappear, and you can’t form new disappear, and you can’t form new substances out of thin air.substances out of thin air.

The total mass of reactants The total mass of reactants MUSTMUST equal the total mass of products, even equal the total mass of products, even though they are not the same though they are not the same substances.substances.

NN22 + H + H22 → NH → NH33

Types of ReactionsTypes of Reactions Some kinds of reactions happen often Some kinds of reactions happen often

enough that we have noticed categories.enough that we have noticed categories.

DecompositionDecomposition

AB AB → A + B→ A + BHH22OO2(aq)2(aq) → →

CC66HH1212OO6(s)6(s) → →

Single ReplacementSingle ReplacementA + BX A + BX → AX + B→ AX + B

AlAl(s)(s) + CuCl + CuCl2(aq)2(aq) → →

CuCu(s)(s) + AgNO + AgNO3(aq)3(aq) → →

Double ReplacementDouble ReplacementAX + BY AX + BY → AY + BX→ AY + BX

Pb(NOPb(NO33))2(aq)2(aq) + KI + KI(aq) (aq) →→

AgNOAgNO3(aq)3(aq) + NaCl + NaCl(aq)(aq) → →

Direct Direct Combination/SynthesisCombination/SynthesisA + B A + B → AB→ AB

MgMg(s)(s) + O + O2(g)2(g) → →

HH2(g)2(g) + O + O2(g)2(g) → →

CombustionCombustion Just a fancy word for burning a Just a fancy word for burning a

substance. If you are burning substance. If you are burning something, what substance do you something, what substance do you need besides the one being burnt?need besides the one being burnt?

_____ + O_____ + O2(g)2(g) → CO→ CO2(g)2(g) + H + H22OO(g)(g)

How to Write an Eqn. “from scratch”How to Write an Eqn. “from scratch” Magnesium reacts with hydrochloric acid in a Magnesium reacts with hydrochloric acid in a

single replacement rxn.single replacement rxn.

1. Write the formulas for your reactants.(ion list)1. Write the formulas for your reactants.(ion list) 2. Determine what your products are, and write 2. Determine what your products are, and write

the formulas for those (ion list again).the formulas for those (ion list again). 3. Balance the equation LAST.3. Balance the equation LAST.

Predicting ProductsPredicting Products

1.1. Determine the formulas for your reactants Determine the formulas for your reactants (use ion list, if necessary).(use ion list, if necessary).

2.2. Based on the type of reaction, determine the Based on the type of reaction, determine the namename of your product(s). of your product(s).

3.3. From those names, determine the From those names, determine the formulaformula for for your product(s) (use ion list, if necessary).your product(s) (use ion list, if necessary).

4.4. Balance the equation.Balance the equation.

Ionic EquationsIonic Equations Reminder: the (aq) designation on a formula Reminder: the (aq) designation on a formula

says that not only does the substance dissolve, says that not only does the substance dissolve, it IONIZES.it IONIZES.

NaINaI(s)(s) → Na → Na++(aq)(aq) + I + I--

(aq)(aq)

Or: 2NaIOr: 2NaI(s)(s) → 2Na → 2Na++(aq)(aq) + 2I + 2I--

(aq)(aq)

Or: Pb(NOOr: Pb(NO33))2(s) 2(s) → Pb→ Pb2+2+(aq)(aq) + 2NO + 2NO33

--(aq)(aq)

Or: AlOr: Al22(SO(SO44))3(s)3(s) → 2Al → 2Al3+3+(aq)(aq) + 3SO + 3SO44

2-2-(aq)(aq)

2NaI2NaI(aq)(aq) + Pb(NO + Pb(NO33))2(aq)2(aq) → 2NaNO → 2NaNO3(aq)3(aq) + PbI + PbI2(s)2(s)

Let’s backtrack and put the reactants in water:Let’s backtrack and put the reactants in water: 2NaI2NaI(s)(s) → 2Na → 2Na++

(aq)(aq) + 2I + 2I--(aq)(aq)

Pb(NOPb(NO33))2(s) 2(s) → Pb→ Pb2+2+(aq)(aq) + 2NO + 2NO33

--(aq)(aq)

Then what your reactants actually look like is this:Then what your reactants actually look like is this:2Na2Na++

(aq)(aq) + 2I + 2I--(aq) (aq) ++ PbPb2+2+

(aq)(aq) + 2NO + 2NO33--(aq)(aq)

……and what your products actually look like is and what your products actually look like is this:this:

→ → 2Na2Na++(aq)(aq) + 2NO + 2NO33

--(aq)(aq) + PbI + PbI2(s)2(s)

So, in terms of ions, the reaction actually goes So, in terms of ions, the reaction actually goes like this:like this:

2Na2Na++(aq)(aq)+ 2I+ 2I--

(aq) (aq) ++ PbPb2+2+(aq)(aq) + 2NO + 2NO33

--(aq)(aq)→2Na→2Na++

(aq)(aq)+ 2NO+ 2NO33--(aq)(aq)+ PbI+ PbI2(s)2(s)

Cancel the items that appear identical on both sidesCancel the items that appear identical on both sides2Na2Na++

(aq)(aq)+ 2I+ 2I--(aq) (aq) ++ PbPb2+2+

(aq)(aq) + 2NO + 2NO33--(aq)(aq)→2Na→2Na++

(aq)(aq)+ 2NO+ 2NO33--(aq)(aq)+ PbI+ PbI2(s)2(s)

So, in actuality, what happens is only this:So, in actuality, what happens is only this: PbPb2+2+

(aq)(aq) + 2I + 2I--(aq) (aq) → PbI→ PbI2(s)2(s)

Ionic EquationsIonic Equations Here’s another one:Here’s another one: 2NaCl2NaCl(aq)(aq) + Ba(NO + Ba(NO33))2(aq)2(aq) → 2NaNO→ 2NaNO3(aq)3(aq) + BaCl + BaCl2(aq)2(aq)

Well, in terms of ions, it looks like this:Well, in terms of ions, it looks like this: 2Na2Na++

(aq)(aq) + 2Cl + 2Cl--(aq)(aq) + Ba + Ba2+2+

(aq)(aq) + 2NO + 2NO33--(aq)(aq) →2Na →2Na++

(aq)(aq) 2NO 2NO33--(aq)(aq) + Ba + Ba2+2+

(aq)(aq) + 2Cl + 2Cl--(aq)(aq)

Well, if you cancel what appears identical on both Well, if you cancel what appears identical on both sides, what are you left with?sides, what are you left with?

This reaction has no net ionic equation, because, in This reaction has no net ionic equation, because, in effect, nothing has happened. effect, nothing has happened.

The items that are your reactants have remained The items that are your reactants have remained unchanged after the reaction, so no reaction has unchanged after the reaction, so no reaction has actually occurred.actually occurred.