Ch. 9 Chemical Reactions & Equations

Zn + I2Reactants

Zn I2Product

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–Chemical reactions occur when bonds between atoms are formed or broken

–Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

–Symbols represent elements, –Formulas represent compounds, –Chemical equations represent chemical

reactions

Introduction

Combustion of MethaneCH4 + 2O2 CO2 + 2H2O

Atoms are rearranged!

Chemical Equations

Their Job: Depict 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 of formulas are called COEFFICIENTS

The letters (s), (g), and (l) are the physical states of compounds.Starting Materials What’s created

Yields or produces!

• The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide.

carbon + oxygen carbon dioxideThis is the “Word Equation” for that reaction

The skeleton equation for that reaction is:

C + O2 CO2Chemical Equations must be balanced in order to conform to the Law of Conservation of Mass - same # & type of atoms on each side of the yield arrow.

A skeleton equation does NOT indicate the relative amounts of reactants and products(no coefficients!)

Word Equations: show the names of the reactants and the products

Lavoisier, 1788

Word Equation:Methane + Oxygen gas carbon dioxide + waterSkeleton equation: CH4 + O2 CO2 + H2O

• Solid (s)• Liquid (l)• Gas (g)• Aqueous solution (aq) (dissolved in water)• Catalyst H2SO4

• Escaping gas ()• Change of temperature ()• Precipitate ( )

Symbols Used in Equations

–When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but

you may not change the subscripts.• Changing the subscripts changes the

compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Balancing Equations

Never put a coefficient in the middle of a formula 2 NaCl is ok Na2Cl is not.

Subscripts vs. Coefficients

• The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

There are a number of ways to interpret balanced

equations2 H2(s) + O2(g) ---> 2 H2O(s)

This equation means:2 molecules H2 + 1 molecules O2 ---->2 molecules H2O(g)

There are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if:– The numbers of atoms on both sides of the

equation are equal.– The coefficients are in the lowest possible whole

number ratios. (reduced)

Steps to Balancing Equations

Some Suggestions to Help You

Helpful Hints for balancing equations:• Take one element at a time. • Save pure elements for last• IF everything balances except for that last

pure element, and there is no way to balance it with a whole number, double all the coefficients and try again.

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

That is, don’t separate them into individual atoms!

Example

H2 + O2 H2O

R P

2 H 2

2 O 1

Need another O on the product side

Make a table to keep track of atoms

Example

R P

2 H 2

2 O 1

H2 + O2 2H2O

Changes the O

Place a coefficient of 2 in front of H2O

Example

R P

2 H 2

2 O 1

H2 + O2 2H2O

This also changes the H

2

Example

R P

2 H 2

2 O 1

H2 + O2 2H2O

Now we need twice as much H in the reactant

2

4

Example

R P

2 H 2

2 O 1

2H2 + O2 2H2O

Add a coefficient of 2 in front of H2

2

4

Example

R P

2 H 2

2 O 1

2H2 + O2 2H2O

Recount to check

2

44

Your answer

Balancing EquationsSodium phosphate + iron (III) oxide

sodium oxide + iron (III) phosphate

Na3PO4 + Fe2O3 ---> Na2O + FePO4 22 3R L

Na

PO4

Fe

O

3

1

2

3

2

1

1

1

6

2

2

3

6

2

Balancing Equations

__C3H8 + __O2 ----> __CO2 + __ H2O3 45

__B4H10 + ___O2 ---->___ B2O3 + ___H2O2 11 4 102 5

EXAMPLE:Leave oxygen for last!Balance B first.Now balance HHow many O’s are now on the right?Can you get that many on the left?

11

When this happens, try doubling everything!How many O’s on the right, now?Can we get that on the left?

22

NO

YES!!!

Types of Reactions• There are millions of chemical reactions.

The only way to be sure what your products will be is to carry them out in the lab!

• Not very practical – or cost effective, however…

• There are five types of chemical reactions we can make some predictions for:

1. Combination reactions (Synthesis)2. Decomposition reactions3. Single Replacement reactions4. Double Replacement reactions5. Combustion reactions

• You need to be able to identify the type of reaction and predict the product(s) given the reactants

1. Combination Reactions• Combination reactions occur when

two or more substances combine to form a single compound.

• (Sometimes these are called synthesis reactions.)

reactant + reactant 1 product• Basically: A + B AB

• Example: 2H2 + O2 2H2O • Example: C + O2 CO2

Combination Reactions(Synthesis)

General form: A + B ABelement or element or compound compound compound

Mg2+O2-Mg

O

O

Mg2+ O2-

2 Mg + O2 2 MgO

Mg

Synthesis Reactions• example

Balancing Equations

___ Al(s) + ___ Br2(l) ---> 2AlBr3(s)2 3

Practice

• Predict the products. Write and balance the following synthesis reaction equations.

• Sodium metal reacts with chlorine gas Na(s) + Cl2(g) • Solid Magnesium reacts with fluorine gas Mg(s) + F2(g) • Aluminum metal reacts with fluorine gas Al(s) + F2(g)

NaCl2 2

MgF2

AlF332 2

2. Decomposition Reactions• Decomposition reactions occur

when a single compound breaks up into two or more simpler substances

• 1 Reactant Product + Product • In general: AB A + B• Example: 2 H2O 2H2 + O2

• Example: 2 HgO 2Hg + O2

Decomposition Reactions• Another view of a decomposition reaction:

Example: Decomposition of hydrogen peroxide.

• Word equation:

hydrogen peroxide water + oxygen

Example: Decomposition of hydrogen peroxide.

• Skeleton equation:

H2O2 H2O + O2

Example: Decomposition of hydrogen peroxide.

• Balanced Equation:

2H2O2 2H2O + O2

Example: Decomposition of hydrogen peroxide in the presence

of a catalyst.Balanced equation showing • the catalyst (MnO2) • the state of the reactants and products:

MnO2

2H2O2(l) 2H2O (l) + O2 (g)

Decomposition Types• These are hardest to predict products for!• There are many special cases, but we will not

explore those in Chemistry. For Ex.:• Carbonates and chlorates are special case

decomposition reactions that do not go to the elements.• Carbonates (CO3

2-) decompose to carbon dioxide and a metal oxide• Example: CaCO3 CO2 + CaO

• Chlorates (ClO3-) decompose to oxygen gas and a

metal chloride• Example: 2 Al(ClO3)3 2 AlCl3 + 9 O2

• I

Practice

• Predict the products. Then, write and balance the following decomposition reaction equations:

• Solid Lead (IV) oxide decomposes PbO2

• Aluminum nitride decomposes AlN

Pb + O2

Al + N22 2

3. Single Replacement Reactions• Single Replacement Reactions occur when

one element replaces another in a compound.• A metal can replace a metal (+) OR

a nonmetal can replace a nonmetal (-).• element + compound element + compound

A + BC AC + B (if A is a metal) ORA + BC BA + C (if A is a nonmetal)

*(remember the cation always goes first!)

*Use Activity Series of Metals to see if reaction works.

Single Replacement Reactions• Another view:

Single Replacement Reactions

• Single Replacement Reaction

Mg + CuSO4 MgSO4 + Cu

General form: A + BC AC + B

Here is a mixture of aluminum powder and iron (III) oxide powder. Once ignited the aluminum rips the oxygen off of the iron oxide resulting in molten iron, which falls down to thepot of sand below. One time someone used wet sand, which was a mistake because the molten iron turned the water to steam instantly. This sprayed molten iron all over the front two rows of students.

Write the balanced equation for this reaction.

2 Al + Fe2O3 2 Fe + Al2O3

We have looked at several reactions:Such experiments reveal trends. The Activity Series of Metals ranks the relative reactivity of metals.Used predict if single displacement reactions will occur: metals near the top are most reactive and will displacing metals near the bottom.Q: Which of these will react?

Fe + CuSO4 Ni + NaCl Li + ZnCO3 Al + CuCl2

Cu + Fe2(SO4)3Yes, Fe is above CuNR (no reaction)

No, Ni is below Na

Zn + Li2CO3Cu + AlCl3

Yes, Al is above Cu

LiKCaNaMgAlZnFeNiSnPbH

CuHgAgAu

Yes, Li is above Zn

p. 295

Ca

Activity Series

Foiled again – Aluminum loses to Calcium

Metal ReactivityLi Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2

Cu Hg Ag Pt Au

Halogen Reactivity

F2

Cl2 Br2 I2

H is the only nonmetal listed. H2 may be displaced from acids or can be given off when a metal reacts with H2O.

Single Replacement Reactions

• Write and balance the following single replacement reaction equation:

• Zinc metal reacts with aqueous hydrochloric acid

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

Note: Zinc replaces the hydrogen ion in the reaction

2

LiKCaNaMgAlZnFeNiSnPbH

CuHgAgAu

Single Replacement Reactions• Sodium chloride solid reacts with fluorine gas NaCl(s) + F2(g) NaF(s) + Cl2(g)

Note that fluorine replaces chlorine in the compound

• Aluminum metal reacts with aqueous copper (II) nitrate

Al(s)+ Cu(NO3)2(aq)

2 2

Li

KCaNaMgAlZnFeNiSnPbH

CuHgAg

NR

Complete these reactions:

Cu + Sn2S Ca + Cu2SO4 Na + PbO

2 Cu + CaSO4Pb + Na2O2

4. Double Replacement Reactions• Double Replacement Reactions occur

when a metal in one compound replaces a metal in another compound.

compound + compound compound + compound• AB + CD AD + CB

THEY ARE USUALLY DRIVEN BY THE FORMATION OF EITHER A GAS, A PRECIPITATE (INSOLUBLE SOLID), OR A MOLECULAR COMPOUND (LIKE WATER).

Double replacement reactions usually occur between 2 ionic compounds in aqueous solution.

CaCO3 + 2 HCl CaCl2 + H2CO3

Double Replacement ReactionsDouble-replacement reaction

General form: AB + CD AD + CB

Double Replacement Reactions

• Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together

• Example: AgNO3(aq) + NaCl(s) AgCl(s) + NaNO3(aq)

• Another example:K2SO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaSO4(s) 2

MnO2 + CO --> Mn2O3 + CO2

Carbon monoxide is commonly used to strip off oxygen atoms from metals.This reaction shows the first step. If more CO is present, eventually all oxygen atoms will be grabbed by CO and manganese (Mn) metal will be left. This is how metal ores get converted to metals.

Balance this equation!2 MnO2 + CO --> Mn2O3 + CO2

Practice

• Predict the products. Balance the equation1. HCl(aq) + AgNO3(aq) 2. CaCl2(aq) + Na3PO4(aq) 3. Pb(NO3)2(aq) + BaCl2(aq) 4. FeCl3(aq) + NaOH(aq) 5. H2SO4(aq) + NaOH(aq) 6. KOH(aq) + CuSO4(aq)

5. Combustion Reactions• Combustion reactions occur

whenever something reacts with oxygen gas – often producing noticeable heat & light.

• This is also called burning!!!

Combustion is used to heat homes, cook food, and power automobiles. -propane is C3H8 -octane, as in gasoline, is C8H18

-ethane, as in ‘natural gas’, is C2H6

Hydrocarbons are compounds made of hydrogen & carbon. Surprise!There are *gobs of energy stored in the bonds between carbon atoms!When we burn them (react them with O2)we release that stored energy to do work for us!

Combustion of Hydrocarbons

*Important scientific term!

Complete Combustion of Hydrocarbons• Meaning – enough oxygen to

react with all of the carbon!

• Products are ALWAYS carbon dioxide and water.

• In general:

CxHy + O2 CO2 + H2O

• Incomplete Combustion does cause some by-products like carbon monoxide & elemental carbon. (CO & C)It occurs when oxygen can’t get to all the carbon atoms fast enough.*Hint: If you see CO and/or C as products it is incomplete combustion!

Incomplete Combustion of Hydrocarbons

Combustion Reactions

Edgar Allen Poe’s drooping eyes and mouth are potential signs of CO poisoning.

Combustion of Hydrocarbons

• *Balance C’s, then H’s. Save O’s for last!• Example:

• C5H12 + O2 CO2 + H2O

• Write the products and balance the following combustion reaction:• C10H22 + O2

5 68

• Some steps for Writing Reactions1. Identify the type of reaction2. Predict the product(s) using the type of

reaction as a modelRemember! If you write a formula – You

MUST balance it!3. Balance the equation (using coefficients)Don’t forget about the diatomic elements!

(BrINClHOF or HON 7) For example, Oxygen is O2 as an element.

In a compound, it can’t be a diatomic element because it’s not an element anymore, it’s part of a compound!

PracticeIdentify the type of reaction for each of the

following synthesis or decomposition reactions, and write the balanced equation:

N2(g) + O2(g) BaCO3(s) Co(s)+ S(s) NH3(g) + H2CO3(aq) NI3(s)

(make Co be +3)

Nitrogen monoxide

Mixed Practice

• State the type, predict the products, and balance the following reactions:

1. BaCl2 + H2SO4 2. C6H12 + O2 3. Zn + CuSO4 4. Cs + Br2 5. FeCO3