Predicting Chemical Reactions

Part 1: Basic Reactions

AP ChemistryLast Update: 9/24/2013

Why should I care?

Previous AP Chemistry exams… consistently had several multiple choice

problems requiring you to predict the products of a reaction.

always had a free response problem on chemical reactions

The new AP Chemistry exam (spring 2014) will probably still emphasize chemical

reactions (no one is really sure what’s going to be on the exam)

How to Predict Products

Nomenclature

Formula of Reactants

Reaction Conditions

Type of Reaction

A solution of copper (II) sulfate is spilled onto a sheet of freshly polished aluminum metal.

CuSO4 + Al ?

Element + Ionic Compound = Single Replacement

A + BC AC + B

CuSO4 + Al Cu + Al2(SO4)3

3 CuSO4 +2 Al 3 Cu + Al2(SO4)3

Description

Balanced Chemical Reaction

Products

Reaction Pattern

How to Identify the Type of Reaction

Certain key words and chemicals allow you to identify a reaction. For each type of reaction, these key

words/chemicals will be listed as “You just might have a _____ reaction if…”

This is blatant piracy from Jeff Foxworthy’s skit “You just might be a redneck if…”▪ This was the equivalent of Chuck Norris

facts in the early 1990’s.

Play external video clip now http://www.youtube.com/watch?v=_

WAQGDa9pJ8

Precipitation/Double Replacement/Metathesis

You just might have a double replacement reaction if… both reactants are soluble ionic compounds or the reactants are a solube ionic compound and an

acid. Pattern: AB + CD AD + CB Examples:

A solution of copper (II) chloride is added to a solution of sodium sulfide.▪ CuCl2+ Na2S CuS + 2 NaCl

Solutions of strontium nitrate and sodium sulfate are mixed▪ Sr(NO3)2 + Na2SO4 SrSO4 + 2 NaNO3

Combustion

Fire at the Caribbean Company Oil Storage Depot (2009)

Source: news.bbc.co.uk Week in Pictures (downloaded 10/23/2009)

Combustion

You just might have a combustion reaction if… the problem includes words like “burned” or “heated”; and the reactant is an organic compound, H2, a metal, P4, or S8.

General Pattern: AB + O2 AO + BO In general, the products are all oxides (H2O, CO2, Al2O3, SO2, SO3,

P4O10) Examples:

Methane is burned in air: CH4 + 2 O2 CO2 + 2 H2O

Butane is burned in air: 2 C4H10 + 13 O2 8 CO2 + 10 H2O

White phosphorous is heated in air: P4 + 5 O2 P4 O10

Magnesium ribbon is heated in air: 2 Mg + O2 2 MgO note: ionic compounds do not burn when heated

If ionic compounds are heated, then it is most likely a decomposition reaction

Single Replacement/Simple Redox

You might have a single replacement reaction if… the reactants are an element and a compound.

General Patterns: if the element is a metal (M): M + BC MC + B if the element is a non-metal (X): X + BC BX + C

Examples: Liquid bromine is shaken with a 0.5 M sodium iodide solution.▪ Br2 + 2 NaI 2 NaBr + I2

A strip of magnesium metal is added to a solution of silver nitrate.▪ Mg + 2 AgNO3 Ag + Mg(NO3)2

Zinc metal powder is poured into a beaker of hydrochloric acid.▪ Zn + 2 HCl H2 + ZnCl2

Acid-Base

You just might have an acid-base reaction if… the reactants are an acid and a base. acids: H+, NH4

+, or H2O bases (B): OH-, O2-, CO3

2-, HCO3-, S2-, NH3, or H2O

General Pattern: HA + MB MA + HB Often, the reaction produces an ionic compound called a salt there are a lot of exceptions, depending on the base▪ Acid + OH- H2O + salt

▪ Acid + O2- salt with OH- as the anion▪ Acid + CO3

2- or HCO3- CO2 + H2O + salt

▪ Acid + S2- H2S (smells like rotten eggs) + salt

▪ Acid + NH3 salt with NH4+ as the cation

When H2O acts as an acid it produces OH-

When H2O acts as a base, it produces H+

Acid-Base (cont.)

Chemistry Joke: What does this mean?

NaOH

NaCl

Acid-Base (cont.)

Solutions of potassium hydroxide and acetic acid are mixed. KOH + HC2H3O2 H2O + KC2H3O2

Calcium oxide powder is added to distilled water. CaO + H2O Ca(OH)2

Solid lead (II) carbonate is added to a 0.5 M solution of sulfuric acid PbCO3 + H2SO4 CO2 + H2O + PbSO4

An excess of hydrochloric acid is added to solid zinc sulfide. 2 HCl + ZnS ZnCl2 + H2S

Ammonia is added to a solution of phosphoric acid. 3 NH3 + H3PO4 (NH4)3PO4

Combination Reactions

Play external video clips now http://

www.youtube.com/watch?v=AkRvn0pXTiw

http://www.youtube.com/watch?v=tZZv5Z2Iz_s

Combination Reactions (cont.)

You just might have a combination reaction if… both reactants are elements, or two molecules are mixed, or carbon dioxide is reacted with an oxide.

General Pattern: A + B AB multiple reactants one product molecular oxide + water often an acid ionic oxide + CO2 CO3

2-

Combination Reactions (cont.) Examples:

Sulfur trioxide gas is bubbled through water.▪ SO3 + H2O H2SO4

▪ This reaction is responsible for acid rain Solid dinitrogen pentoxide is added to water.▪ N2O5 + H2O 2 HNO3

Carbon dioxide is passed over hot, solid sodium oxide:▪ CO2 + Na2O Na2CO3

▪ This reaction has been considered as a means of carbon sequestration.

Decomposition

Source: Wikimedia Commons (downloaded 9/24/2013)

Decomposition (cont.)

You just might have a decomposition reaction if… there is only one reactant and energy is added (light, electricity, or heat); or a solid ionic compound is heated. Note: heat can also mean a combustion reaction. The

reactants in a decomposition reaction must not be flammable.

General Pattern: A B + C One reactant multiple products CO3

2- CO2 + O2-

HCO32- CO2 + H2O + CO3

2-

2 H2O2 2 H2O + O2

ClO3-, BrO3

-, or IO3- O2 + Cl-, Br-, or I-

Decomposition (cont.)

Examples Solid sodium hydrogen carbonate is

strongly heated:▪ 2 NaHCO3 CO2 + Na2CO3 + H2O

Solid sodium carbonate is strongly heated:▪ Na2CO3 CO2 + Na2O

A vial of ammonium sulfide is smashed, producing a foul smell of rotting organic matter:▪ (NH4)2S 2 NH3 + H2S

Practice

For each of the following lab situations, write the balanced molecular equation for the resulting reaction.1. A strip of zinc is placed in a solution of

nickel (II) nitrate.2. Ethyne is burned in air.3. Solid calcium carbonate is added to a

solution of acetic acid.4. Lithium metal is strongly heated in

nitrogen gas.

Homework

Due next class: Download this PowerPoint from

NetClassroom Read/review Mr. Tom’s Guide to Chemical

Reactions. Predicting Chemical Reactions

Worksheet 1 Due on Tuesday

Predicting Chemical Reactions Worksheet 2

Predicting Chemical ReactionsPart 2: Coordination and Non-Trivial Redox

AP ChemistryLast Update: 9/24/2013

Previously on AP Chemistry…

Nomenclature

Formula of Reactants

Reaction Conditions

Type of Reaction

Reaction Pattern

A solution of copper (II) sulfate is spilled onto a sheet of freshly polished aluminum metal.

CuSO4 + Al ?

Element + Ionic Compound = Single Replacement

A + BC AC + B

CuSO4 + Al Cu + Al2(SO4)3

3 CuSO4 +2 Al 3 Cu + Al2(SO4)3

Description

Balanced Chemical Reaction

Products

Previously on AP Chemistry…

Identifying Factors

Reaction Type and Pattern

2 elements or2 molecular comp. orCO + O2-

CombinationA + B AB

Identifying Factors

Reaction Type and Pattern

2 ionic compounds orIonic + acid

Double Repl.AB + CD AD + CB

Organic compound or element is burned/heated

AB + O2 AO + BO

Element + Compound

Single Repl.M + BC B + MCX + BC C + BX

Acids: H+, NH4+,

H2OBases (B): OH-, O2-, CO3

2-, HCO3-, S2-,

NH3, H2O

Acid-BaseH+ is transferred from acid to baseHA + MBMA + HC

1 reactant + energyIonic comp. + energy

DecompositionA B + C

Synthesis of a Coordination Compound

Coordination compounds/coordination complexes have covalent bonds between a metal ion and a ligand. The metal ion must…▪ want more electrons and ▪ be able to pull electrons into a bond (high electronegativity).▪ Transition metal cations (i.e. Cu2+, Co2+, Ni2+) and main group metals (i.e.

Pb2+, Sn4+) have these properties.

The transition metal is bonded to a ligand.▪ Any species that has an unshared pair of electrons (NH3, Cl-, H2O)

▪ In order to drive the formation of the coordination bond, the ligand must be present in large quantities.▪ The only exception is thiocyanate (SCN-). Even small quantities of thiocyanate can form

a coordination compound.

Usually several ligands will bond to the metal ion▪ The number of ligands per metal ion is called the coordination number.

Synthesis of a Coordination Compound (cont.)

Different colored coordination compounds of Nickel (II)

[Ni(NH3)6]2+ [Ni(en)3]2+ [NiCl4]2- [Ni(H2O)6]2+

Source: Wikimedia Commons (downloaded 9/24/2013)

Synthesis of a Coordination Compound (cont.)

You just might have a coordination compound if… You have a metal ion and…▪ It must be either a main group metal ion or a transition

metal ion. You have a ligand and…▪ Ligands must have a pair of unshaired electrons.▪ Ligands must either be SCN- or be present in large

quantities.▪ Often words like “concentrated” or “excess” are used.

The metal and the ligand cannot react in any other way (i.e. double exchange won’t work).

Synthesis of a Coordination Compound (cont.)

Skip straight to the net ionic equation.

General pattern: Mx+ + n Ly- [M(L)n]x-y

Mx+ = metal ion Ly- = ligand n = coordination number (usually 2, 4,

or 6)▪ usually you just arbitrarily pick a either 2, 4,

or 6

[M(L)n]x-y = the coordination complex

Synthesis of a Coordination Compound (cont.)

Excess concentrated ammonia solution is added to a solution of nickel (II) sulfate. metal = Ni2+

ligand = NH3

coordination number: 4 (just pick either 2, 4, or 6) reaction: 4 NH3 + Ni2+ [Ni(NH3)4]2+

Potassium thiocyanate is added to a solution of iron (III) nitrate. metal = Fe3+

ligand = SCN-

coordination number: 6 reaction: 6 SCN- + Fe3+ [Fe(SCN)6]3-

Synthesis of a Coordination Compound (cont.)

Excess concentrated hydrochloric acid is added to a solution of 1.0 M cobalt (II) chloride. try a double exchange rxn: 2 HCl + CoCl2 CoCl2 + 2 HCl metal = Co2+ ligand = Cl-

coordination number: 2 4 reaction: 2 Cl- + Co2+ [Co(Cl)2] 4 Cl- + Co2+ [Co(Cl)4]2-

Excess concentrated sodium nitrate is added to a solution of iron (III) nitrate. try a double exchange rxn: 3 NaNO3 + Fe(NO3)3 Fe(NO3)3 + 3

NaNO3 metal = Fe3+ ligand = NO3

-

coordination number: 2 6 reaction: 2 NO3

- + Fe3+ [Fe(NO3)2]+ 6 NO3- + Fe3+

[Fe(NO3)6]3-

Decomposition of a Coordination Compound

+ Energy

Decomposition of a Coordination Compound

Play external video clip now (heating copper (II) sulfate.

http://www.youtube.com/watch?v=M6IIbZx7xvQ

Decomposition of a Coordination Compound

Almost always a dehydration reaction. In the solid state, many transition metals

form a hydrated crystal. These crystals have water covalently

bonded into their structure. These bonds are weak and can be broken

with a small input of energy.▪ The coordination compound decomposes▪ This is sometimes called a dehydration reaction▪ This usually occurs at such high temperature that

the water is released as water vapor.

Decomposition of a Coordination Compound

General Pattern Hydrated ionic solid anhydrous ionic

solid + water MA • n H2O (s) MA (s) + n H2O (g)

Examples: Solid copper (II) sulfate pentahydrate is

heated.▪ CuSO4 • 5 H2O CuSO4 + 5 H2O

Solid nickel (II) chloride hexahydrate heated.▪ NiCl2 • 6 H2O NiCl2 + 6 H2O

Non-Trivial Redox Reactions You just might have a non-trivial redox reaction

if… One of your reactants is a multivalent metal such as

Sn, Mn, Fe, or Cu; or One of your reactants is Ag+, Cr2O7

2-, MnO4-, CO, or

O22-; or

The reaction happens in solution and the problem specifies whether the solution is acidic or basic.

Often, with non-trivial redox reactions, you will be given some of the products. The challenge is balancing the reaction. Skip straight to the net ionic equation.

Non-Trivial Redox ReactionsBalancing non-trivial redox reactions1. Separate the overall reaction into two half-reactions.

In each half-reaction, pair up the elements (i.e. the reactant with Cr is in the same half-reaction as the product with Cr).

▪ 4 substances 1 reactant and 1 product per half reaction▪ If you do not have enough substances, use the same substance in both reactions.▪ Substances with just H and O can be alone

2. Balance all elements except the H and O3. Balance O using H2O.4. Balance H using H+

5. Balance charge using e-.6. (basic solutions only) For every H+, add an OH- to each side.

Combine H+ + OH- H2O. Cancel out H2O.7. Combine the half-reactions.

1. Multiply each half-reaction by a coefficient so that the e- cancel out.2. Add the two half-reactions to get a net ionic reaction.3. Cancel out anything which occurs on both sides.

Non-Trivial Redox Reactions

Examples (full procedure on board) Fe2+ + Cr2O7

2- Fe3+ + Cr3+ (in acid)

Cr2O72- + C2O4

2- Cr3+ + CO2 (in acid)

Cu + NO3- Cu2+ + NO2 (in acid)

Mn2+ + H2O2 MnO2 (in base)

One more note on half-reactions Simple redox reactions can be broke into

half-reactions as well. Molecular reaction: M + AB A + MB Net Ionic reaction: M + A+ M+ + A Half reactions: M M+ and A+ A

Example: Magnesium metal is added to a solution of copper (II) chloride. Molecular Reaction: Mg + CuCl2 MgCl2 + Cu Net Ionic Reaction: Mg + Cu2+ Mg2+ + Cu Half Reactions: Mg Mg2+ + 2 e- Cu2+ + 2 e- Cu

Overwhelmed yet?

We’re only half way through today’s presentation!

Summary (Just kidding, we’re almost done)

Identifying Factors

Reaction Type and Pattern

2 ionic compounds orIonic + acid

Double Repl.AB + CD AD + CB

Organic compound or element is burned/heated

AB + O2 AO + BO

Element + Compound

Single Repl.M + BC B + MCX + BC C + BX

Acids: H+, NH4+,

H2OBases (B): OH-, O2-, CO3

2-, HCO3-, S2-,

NH3, H2O

Acid-BaseH+ is transferred from acid to baseHA + MBMA + HC

1 reactant + energyIonic comp. + energy

DecompositionA B + C

Identifying Factors

Reaction Type and Pattern

2 elements or2 molecular comp. orCO + O2-

CombinationA + B AB

Transition/main grp. metal + excess ligand

CoordinationMx+ + n Ly- [M(L)n]x-y

Hydrate is heated

DehydrationMA • n H2O MA + n H2O (g)

Multivalent metal Ag+, Cr2O7

2-, MnO4

-, CO or O2

2- Acidic or basic soln.

Non-Trivial Redox

Practice

1. Solid tetraphosphorous decoxide is added to distilled water.

2. Equal volumes of 0.1 M solutions of lead (II) nitrate and magnesium iodide are combined.

3. Excess concentrated sodium chloride is added to a solution of tin (II) chloride.

4. What is the coefficient of H+ after the following reaction is balanced?

▪ Sn2+ + Cr2O72- + H+ Sn4+ + Cr3+ + H2O

5. Iron metal is burnt in a thermic lance.

Homework

Due next class: Predicting Chemical Reactions

Worksheet 2