CHEMISTRY OF MATCHES

P4S3 + KClO3 P2O5 + KCl + SO2

tetraphosphorustrisulfide

potassiumchlorate

diphosphoruspentoxide

potassiumchloride

sulfurdioxide

D

The substances P4S3 and KClO3 are both present on the tip of a strike anywhere match. When the match is struck on a rough surface, the two chemicals (reactants) ignite and produce a flame.

Charles H.Corwin, Introductory Chemistry 2005, page 182

Safety matches

The products from this reaction are P2O5, KCl, and SO2,the last of which is responsible for the characteristic sulfur smell.

Strike anywhere matchesThe substances P4S3 and KClO3 are separated. The P4S3 is onthe matchbox cover. Only when the chemicals combine do they react and produce a flame.

                                                   

CHAPTER 8

Chemical Equations and Reactions

Describing Chemical Reactions

SECTION 1

Chemical reaction process by which one or more substances are changed into one or more different substances

Represented by chemical equations represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction

(NH4)2Cr2O7 (s) N2(g) + Cr2O3(s) + 4H2O(g)

ORGANIZE YOUR THOUGHTSChemicalreactions

Chemicalequations

Chemicalequations

• Balancing equations

• Predicting products from reactants

• Synthesis• Decomposition• Single replacement• Double replacement• Combustion

Packard, Jacobs, Marshall, Chemistry Pearson AGS Globe, page 175

SIGNS OF CHEMICAL REACTIONSThere are five main signs that indicate a chemical reaction has taken place:

change in color change in odor production of newgases or vapor

input or releaseof energy

difficult to reverse

releaseinput

CHEMICAL EQUATIONSDepict the kind of reactants and products and their relative amounts in a reaction.

4 Al(s) + 3 O2(g) 2 Al2O3(s)

The numbers in the front are called stoichiometric coefficients.

The letters (s), (g), and (l) are the physical states of compounds.

reactants product

aluminum oxide

CHEMICAL EQUATIONS

This equation means:

4 Al(s) + 3 O2(g) 2 Al2O3(s)

4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3

4 Al moles + 3 O2 moles yield 2 moles of Al2O3

or

4 g Al + 3 g O2 yield 2 g Al2O3

4 mol Al@27g/mol 3 mol O2@32g/mol 2 mol Al2O3@102g/mol108 g + 96 g = 204 g

aluminum oxide sandpaper

FORMULA EQUATIONS Formula equation represents

reactants and products of chemical reaction by their symbols or formulas

CH4(g) + O2(g) CO2(g) + H2O(g)

Is the law of conservation of mass satisfied here?

CH4 + O2 CO2 + H2O Reactants – 7 atoms Products – 6 atoms

Hydrogen = 4 on left, = 2 on right Oxygen = 2 on left, = 3 on right

In order to satisfy law of conservation of mass, must balance the equation by adding coefficients

BALANCING A CHEMICAL EQUATION1. Write a formula equation by substituting

correct formulas for the names of the reactants and products (if you do not start with a formula equation).

1. H2O(l) H2(g) + O2(g)

2. Balance the formula equation according to the law of conservation of mass. There are a few simple guidelines to use for this…..

GUIDELINES FOR BALANCING EQUATIONS Balance the different types of atoms one at a time.

First balance the atoms of elements that are combined and that appear only once on each side of the equation.

Balance polyatomic ions that appear on both sides as single units.

Balance H and O atoms (or any other lone atoms) last.

H2O(l) H2(g) + O2(g)

2 oxygen on right and only 1 on the left Start with placing 2 in front of H2O

2H2O(l) H2(g) + O2(g)

Oxygen is now balance with 2 on left and 2 on right

Now balance hydrogen – 4 on left, 2 on right Add coefficient 2 to H2

2H2O(l) 2H2(g) + O2(g) ---- balanced!

DO NOT!..... DO NOT WRITE INCORRECT FORMULAS,

THIS WILL MESS UP YOUR BALANCING

DO NOT CHANGE SUBSCRIPTS IN FORMULAS TO BALANCE THE EQUATION!

H2O(l) H2(g) + O2(g) H2O(l) H2(g) + O(g)

DO!!!!..... When you think you have balanced the

equation, COUNT THE NUMBERS OF EACH TYPE OF ATOM ON EITHER SIDE OF THE EQUATION

2H2O(l) 2H2(g) + O2(g)Reactant

sideProduct

sideH 4 4O 2 2

SAMPLE PROBLEM The reaction of zinc with aqueous

hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write a balanced equation for the reaction.

1. Write the word equation

Zinc + hydrochloric acid zinc chloride + hydrogen

2. Write the formula equation

Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)

ADJUST THE COEFFICIENTSZn(s) + HCl(aq) ZnCl2(aq) + H2(g)

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Count atoms:Zn – 1 on left, 1 on rightH – 2 on left, 2 on rightCl – 2 on left, 2 on right

PRACTICE PROBLEM 1A Write word, formula, and balanced

chemical equations for magnesium and hydrochloric acid react to produce magnesium chloride and hydrogen.

Magnesium + hydrochloric acid magnesium chloride + hydrogen

Mg + HCl MgCl2 + H2 Mg + 2HCl MgCl2 + H2

PRACTICE PROBLEM 1B Write word, formula, and balanced

chemical equations for silicon dioxide and hydrofluoric acid reacting to produce silicon tetrafluoride and water.

Silicon dioxide+ hydrofluoric acid silicon tetrafluoride + water

SiO2+ HF SiF4 + H2O SiO2+ 4HF SiF4 + 2H2O

PRACTICE PROBLEM 2 Write word, formula and balanced equations

for aqueous nitric acid reacts with solid magnesium hydroxide to produce aqueous magnesium nitrate and water.

Nitric acid + magnesium hydroxide magnesium nitrate + water

HNO3(aq) + Mg(OH)2(s) Mg(NO3)2(aq) + H2O(l) 2HNO3(aq) + Mg(OH)2(s) Mg(NO3)2(aq) + 2H2O(l)

PRACTICE PROBLEM 3 Ammonium sulfate crystals are

made by treating ammonia gas, often a by-product from coke-ovens, with aqueous sulfuric acid:

2 NH3(g) + H2SO4(aq) → (NH4)2SO4(s)

PRACTICE PROBLEM 4 Aluminum sulfate and calcium

hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce 2 insoluble products, aluminum hydroxide and calcium sulfate. Write a balanced equation for this reaction.

Al2(SO4)3(aq) + 3Ca(OH)2(aq) 2Al(OH)3(s) + 3CaSO4(s)

PRACTICE PROBLEM 5 Write balanced chemical equations

for the following reaction: Solid sodium combines with chlorine gas to produce solid sodium chloride.

2Na(s) + Cl2(g) → 2NaCl(s)

PRACTICE PROBLEM 6 When solid copper reacts with

aqueous silver nitrate, the products are aqueous copper(II) nitrate and solid silver.

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

PRACTICE PROBLEM 7 In a blast furnace, the reaction

between solid iron(III) oxide and carbon monoxide gas produces solid iron and carbon dioxide gas.

Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)

Types of Chemical ReactionsSECTION 2

1. DECOMPOSITION REACTIONS Decomposition reaction a single

compound has a reaction that makes two or more simpler substances

General equation AX A + X

Most happen when energy (light/heat) is added

2. SYNTHESIS REACTIONS Synthesis (composition) reaction

two or more substances combine to form a new compound

General equation A + X AX

3. SINGLE-REPLACEMENT REACTIONS

Single-replacement reaction one element replaces a similar element in a compound

General equation A + BX AX + B

Y + BX BY + X

4. DOUBLE REPLACEMENT REACTIONS Double-replacement reaction ions

of two compounds exchange places in an aqueous solution to form two new compounds

General equation AX + BY AY + BX

5. COMBUSTION REACTIONS Combustion reaction substance

combines with oxygen, releasing large amount of energy as heat and light

2H2(g) + O2(g) → 2H2O(g)

REACTIONS OF ELEMENTS WITH OXYGEN AND SULFUR

One simple type of synthesis reaction is combination of element with oxygen to form an oxide of the element

Almost all metals react with oxygen to form oxides

Ex. Magnesium burned magnesium oxide

2Mg(s) + O2(g) 2MgO(s)

GROUP 2 ELEMENTS Group 2 elements react to form oxides

with general formula MO M represents metal

Group 1 metals form oxides with general formula M2O

Li + O2 Li2O

REACTIONS WITH SULFUR Groups 1 and 2 react with sulfur to make

sulfides of the element

Group 1 M2S Group 2 MS

16Rb(s) + S8(s) → 8Rb2S(s) 8Ba(s) + S8(s) → 8BaS(s)

Some metals (usually transition metals) combine with O2 to make two different oxides

Ex. Fe – can be +2 or +3

2Fe(s) + O2(g) → 2FeO(s) 4Fe(s) + 3O2(g) → 2Fe2O3(s)

NONMETALS Nonmetals also react with oxygen to

make oxides Sulfur reacts with oxygen to make

sulfur dioxide When carbon is burned, it makes

carbon dioxide S8(s) + 8O2(g) → 8SO2(g) C(s) + O2(g) → CO2(g)

Hydrogen reacts with oxygen to make dihydrogen monoxide

2H2(g) + O2(g) → 2H2O(g)

REACTIONS OF METALS & HALOGENS

Most metals react with halogens to make either ionic or covalent compounds

Ex. Group 1 reacts with halogens to form ionic compounds with formula MX

M = metal, X = halogen

2Na(s) + Cl2(g) → 2NaCl(s) 2K(s) + I2(g) → 2KI(s)

GROUP 2 METALS & HALOGENS Formula MX2

Mg(s) + F2(g) → MgF2(s) Sr(s) + Br2(l) → SrBr2(s)

SYNTHESIS REACTIONS WITH OXIDES

Active metals highly reactive metals Oxides of active metals react with water to

make metal hydroxides

CaO(s) + H2O(l) → Ca(OH)2(s)

CaO = lime

Ca(OH)2 is important in setting cement

OXYACIDS Many oxides of nonmetals react with

water to make oxyacids

SO2(g) + H2O(l) → H2SO3(aq) In air polluted with SO2, reacts with

oxygen in air to form sulfuric acid (acid rain)

2H2SO3(aq) + O2(g) → 2H2SO4(aq)

PRACTICE PROBLEMS Identify the products in each of the

following reactions: a. hydrogen burned in oxygen H2O b. H2(g) + N2(g) NH3 c. CaO(s) + H2O(l) Ca(OH)2(aq)

DECOMPOSITION OF BINARY COMPOUNDS

Simplest kind of decomposition reaction is binary compound into its elements

Ex. Passing electricity through water

2H2O(l) 2H2(g) + O2(g)electricity

Electrolysis decomposition of a substance by electricity

Oxides of less-active metals (lower center of PT) decompose into elements when heated

Δ 2HgO(s) 2Hg(l) + O2(g)

DECOMPOSITION OF METAL CARBONATES

When heated, metal carbonates break down to make a metal oxide and CO2

Δ

CaCO3(s) CaO(s) + CO2(g)

DECOMPOSITION OF METAL HYDROXIDES

All except with Group 1 metals decompose when heated to make metal oxides and water

Δ Ca(OH)2(s) CaO(s) + H2O(g)

DECOMPOSITION OF METAL CHLORATES

When heated, metal chlorates decompose to make metal chloride and oxygen

Δ 2KClO3(s) 2KCl(s) + 3O2(g)

MnO2(s)

DECOMPOSITION OF ACIDS Certain acids decompose into nonmetal

oxides and water

H2CO3(aq) → CO2(g) + H2O(l)

Δ H2SO4(aq) SO3(g) + H2O(l)

PRACTICE PROBLEMS Predict the products for these

decomposition reactions a. sodium chlorate Sodium chloride + oxygen b. calcium carbonate Calcium oxide + carbon dioxide c. potassium bromide Potassium + bromine

REPLACEMENT OF A METAL IN A COMPOUND BY ANOTHER METAL

Aluminum is more active than lead When solid aluminum is placed in

aqueous lead(II) nitrate, the aluminum replaces the lead

2Al(s) + 3Pb(NO3)2(aq) → 3Pb(s) + 2Al(NO3)3(aq)

Based on activity series of metals

REPLACEMENT OF HYDROGEN IN WATER BY METAL

Most-active metals (Group 1) react strongly with water to make metal hydroxides and hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Less-active metals react with steam or other form of energy

REPLACEMENT OF HYDROGEN IN ACID BY METAL

More-active metals react with certain acidic solutions and replace hydrogen

Reaction products are metal compound (salt) and hydrogen gas

Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)

REPLACEMENT OF HALOGENS One halogen replaces another

Fluorine is most reactive Can replace any other halogen

Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l) F2(g) + 2NaCl(aq) → 2NaF(aq) + Cl2(g) Br2(l) + KCl(aq) → no reaction

PRACTICE PROBLEMS For the following equations, predict what

the products will be: a. Ag + KNO3 →No reaction b. Zn + AgNO3 →Zn(NO3)2 + Ag c. Cl2 + KI →I2 + 2KCl

d. Cu + FeSO4 →No reaction e. Fe + Pb(NO3)2 →Pb + Fe(NO3)2 f. Cu + Al2(SO4)3 →No reaction g. Al + Pb(NO3)2 →Pb + Al(NO3)3 h. Cl2 + NaI →I2 + NaCl

i. Fe + AgC2H3O2 →Fe(C2H3O2)2 + Ag j. Al + CuCl2 →Cu + Al2Cl3 k. Br2 + CaI2 →I2 + CaBr2 l. Fe + CuSO4 →FeSO4 + Cu m. Cl2 + MgI2 →I2 + MgCl2

FORMATION OF A PRECIPITATE Occurs when cations of one reactant

combine with anions of another to form insoluble (or slightly soluble) compound

2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq)

FORMATION OF A GAS In some D-R reactions, one product is

insoluble gas that bubbles out of mixture

FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq)

FORMATION OF WATER In some D-R reactions, water is one

of the products

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

PRACTICE PROBLEMS Classify each of the following reactions as

synthesis, decomposition, single-replacement, double-replacement, or combustion:

a. N2(g) + 3H2(g) → 2NH3(g) synthesis b. 2Li(s) + 2H2O(l ) → 2LiOH(aq) + H2(g) single-replacement c. 2NaNO3(s) → 2NaNO2(s) + O2(g) decomposition

d. 2C6H14(l ) + 19O2(g) →12CO2(g) + 14H2O(l )

combustion e. NH4Cl(s) → NH3(g) + HCl(g) decomposition f. BaO(s) + H2O(l ) → Ba(OH)2(aq) synthesis g. AgNO3(aq) + NaCl(aq) →AgCl(s) + NaNO3(aq) double-replacement

PRACTICE PROBLEM For each of the following reactions,

identify the missing substances, then balance the final equation. Each slot may be one OR MORE substances.

a. synthesis: _____ → Li2O 4Li + O2 2Li2O b. decomposition: Mg(ClO3)2 → _____ Mg(ClO3)2 MgCl2 + 3O2

c. single-replacement: Na + H2O → _____

2Na + 2H2O 2NaOH + H2 d. double-replacement: HNO3 + Ca(OH)2 →

_____ 2HNO3 + Ca(OH)2 Ca(NO3)2 + 2H2O e. combustion: C5H12 + O2 → _____ C5H12 + 8O2 5CO2 + 6H2O