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Chapter 7 Chemical Reactions

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Section 7.1Describing Chemical Change

OBJECTIVES:

–Write equations describing chemical reactions, using appropriate symbols

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Section 7.1Describing Chemical Change

OBJECTIVES:

–Write balanced chemical equations, when given the names or formulas of the reactants and products in a chemical reaction.

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All chemical reactions have two parts:

–Reactants - the substances you start with

–Products- the substances you end up with

The reactants turn into the products. Reactants Products

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In a chemical reaction The way atoms are joined is changed Atoms aren’t created of destroyed. Can be described several ways:

1. In a sentence

Copper reacts with silver nitrate to form silver and copper (II) nitrate .

2. In a word equation

Copper + silver nitrate silver + copper (II) nitrate

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

reactants products

Or a skeleton equation

Or a balanced equation

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Symbols in equations-p.144 the arrow separates the reactants from

the products Read “reacts to form” The plus sign = “and” (s) after the formula = solid (g) after the formula = gas (l) after the formula = liquid

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Symbols used in equations (aq) after the formula - dissolved in

water, an aqueous solution. used after a product indicates a gas

(same as (g)) used after a product indicates a solid

(same as (s))

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Symbols used in equations indicates a reversible reaction

(more later) shows that heat is

supplied to the reaction is used to indicate a catalyst

is supplied, in this case, platinum.

heat ,

Pt

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What is a catalyst? A substance that speeds up a reaction,

without being changed or used up by the reaction.

Enzymes are biological or protein catalysts.

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Skeleton Equation Uses formulas and symbols to describe

a reaction doesn’t indicate how many. All chemical equations are sentences

that describe reactions.

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Convert these to equations Solid iron (III) sulfide reacts with gaseous

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

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:

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

Cu(s) + AgNO3(aq)

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

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

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Balancing Chemical Equations

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Balanced Equation Atoms can’t be created or destroyed All the atoms we start with we must end

up with A balanced equation has the same

number of each element on both sides of the equation.

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C + O2 CO2

This equation is already balanced What if it isn’t?

C + OO COO

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C + O2 CO We need one more oxygen in the products. Can’t change the formula, because it

describes what it is (carbon monoxide in this example)

C + O COO

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Must be used to make another CO But where did the other C come from?

C +O

C

OO

OC

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Must have started with two C 2 C + O2 2 CO

C

+O

C

OO

OC

C

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Rules for balancing: Assemble, write the correct formulas for all the

reactants and products Count the number of atoms of each type appearing

on both sides Balance the elements one at a time by adding

coefficients (the numbers in front) - save H and O until LAST!

Element Carbon Hydrogen Oxygen Check to make sure it is balanced.

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Never change a subscript to balance an equation.– If you change the formula (subscripts) you are

describing a different reaction.

–H2O is a different compound than H2O2 Never put a coefficient in the middle of a formula–2 NaCl is okay, Na2Cl is not.

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Example

H2 + H2OO2

Make a table to keep track of where you are at

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Example

H2 + H2OO2

Need twice as much O in the product

R PH

O

2

2

2

1

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Example

H2 + H2OO2

Changes the O

R PH

O

2

2

2

1

2

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Example

H2 + H2OO2

Also changes the H

R PH

O

2

2

2

1

2

2

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Example

H2 + H2OO2

Need twice as much H in the reactant

R PH

O

2

2

2

1

2

2

4

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Example

H2 + H2OO2

Recount

R PH

O

2

2

2

1

2

2

4

2

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Example

H2 + H2OO2

The equation is balanced, has the same number of each kind of atom on both sides

R PH

O

2

2

2

1

2

2

4

2

4

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Example

H2 + H2OO2

This is the answer

R PH

O

2

2

2

1

2

2

4

2

4

Not this

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Balancing Examples _AgNO3 + _Cu _Cu(NO3)2 + _Ag

_Mg + _N2 _Mg3N2

_P + _O2 _P4O10

_Na + _H2O _H2 + _NaOH

_CH4 + _O2 _CO2 + _H2O

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Section 7.2Types of Chemical Reactions

OBJECTIVES:

–Identify a reaction as combination, decomposition, single-replacement, double-replacement, or combustion

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Section 7.2Types of Chemical Reactions

OBJECTIVES:

–Predict the products of combination, decomposition, single-replacement, double-replacement, and combustion reactions.

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Types of Reactions There are millions of reactions. Can’t remember them all Fall into several categories. We will learn 5 major types. Will be able to predict the products. For some, we will be able to predict whether

they will happen at all. Will recognize them by the reactants

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#1 - Combination Reactions Combine - put together (synthesis) 2 substances combine to make one

compound. Ca +O2 CaO

SO3 + H2O H2SO4 We can predict the products if they are two

elements. Mg + N2

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Write and balance Ca + Cl2 Fe + O2 iron (II) oxide

Al + O2 Remember that the first step is to write

the correct formulas Then balance by using coefficients only

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#2 - Decomposition Reactions decompose = fall apart one reactant falls apart into two or more

elements or compounds. NaCl Na + Cl2

CaCO3 CaO + CO2

Note that energy is usually required to decompose

electricity

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#2 - Decomposition Reactions Can predict the products if it is a binary

compound Made up of only two elements Falls apart into its elements H2O HgO

electricity

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#2 - Decomposition Reactions If the compound has more than two

elements you must be given one of the products

The other product will be from the missing pieces

NiCO3 CO2 + ?

H2CO3(aq) CO2 + ?

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The Activity Series of the Metals

   Lithium    Potassium    Calcium    Sodium    Magnesium    Aluminum    Zinc    Chromium    Iron    Nickel    Lead    Hydrogen Hydrogen    Bismuth    Copper    Mercury    Silver    Platinum    Gold

Metals can replace other metals provided that they are above the metal that they are trying to replace.

Ex. Metals above hydrogen can replace hydrogen in acids.

Cu + AgNO3

Zn + NaCl

Copy list to your periodic table

CuNO3 + Ag

No reaction

???

???

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The Activity Series of the Halogens

  Fluorine   Chlorine   Bromine   Iodine

Halogens can replace other halogens in compounds, provided that they are above the halogen that they are trying to replace.

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

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

???

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#3 - Single Replacement One element replaces another Reactants must be an element and a

compound. Products will be a different element and a

different compound. Na + KCl K + NaCl F2 + LiCl LiF + Cl2

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#3 Single Replacement Metals replace other metals (and hydrogen) K + AlN Zn + HCl Think of water as HOH Metals replace one of the H, combine with

hydroxide. Na + HOH

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#3 Single Replacement We can tell whether a reaction will happen Some chemicals are more “active” than others More active replaces less active There is a list on page 155 - called the Activity

Series of Metals Higher on the list replaces lower.

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#3 Single Replacement Note the * concerning Hydrogen H can be replaced in acids by everything higher Li, K, Ba, Ca, & Na replace H from acids and

water Fe + CuSO4 Pb + KCl Al + HCl

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#3 - Single Replacement What does it mean that Hg and Ag are on the

bottom of the list? Nonmetals can replace other nonmetals Limited to F2 , Cl2 , Br2 , I2 (halogens) Higher replaces lower. F2 + HCl

Br2 + KCl

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#4 - Double Replacement Two things replace each other. Reactants must be two ionic compounds or acids. Usually in aqueous solution NaOH + FeCl3 The positive ions change place.

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

NaOH + FeCl3 Fe(OH)3 + NaCl

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#4 - Double Replacement Has certain “driving forces”

–Will only happen if one of the products:

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

– is a gas that bubbles out, or

– is a covalent compound (usually water).

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Complete and balance assume all of the following reactions

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:

Combination

Element + Element Compound (s,l or g) (s,l or g) (s,l or g)

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Decomposition

Compound Element + Element

(s,l or g) (s,l or g) (s,l or g)

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Single Displacement

Element + Compound Element + Compound

(s,l or g) (aq) (s,l or g) (aq or s)

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Compound + Compound Compound + Compound

(aq) (aq) (aq or s) (aq or s)

Double Displacement

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Combustion

CxHy + O2 (g) CO2(g) + H2O(g)

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Examples H2 + O2

H2O Zn + H2SO4 HgO KBr +Cl2

AgNO3 + NaCl

Mg(OH)2 + H2SO3

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#5 - Combustion Means “add oxygen” A compound composed of only C, H, and

maybe O is reacted with oxygen If the combustion is complete, the products

will be CO2 and H2O.

If the combustion is incomplete, the products will be CO (possibly just C) and H2O.

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Examples C4H10 + O2 (assume complete)

C4H10 + O2 (incomplete)

C6H12O6 + O2 (complete)

C8H8 +O2 (incomplete)

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An equation... Describes a reaction Must be balanced in order to follow the Law

of Conservation of Mass Can only be balanced by changing the

coefficients. Has special symbols to indicate physical

state, and if a catalyst or energy is required.

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Reactions Come in 5 major types. Can tell what type they are by the reactants. Single Replacement happens based on the

activity series Double Replacement happens if the product

is a solid, water, or a gas.

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Section 7.3Reactions in Aqueous Solution

OBJECTIVES:

–Write and balance net ionic equations.

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Section 7.3Reactions in Aqueous Solution

OBJECTIVES:

–Use solubility rules to predict the precipitate formed in double-replacement reactions.

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Net Ionic Equations Many reactions occur in water- that is, in

aqueous solution Many ionic compounds “dissociate”, or

separate, into cations and anions when dissolved in water

Now we can write a complete ionic equation

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Net Ionic Equations Example:– AgNO3(aq) + NaCl(aq) AgCl(s)+ NaNO3(aq)

1. this is the full equation

2. now write it as an ionic equation

3. can be simplified by eliminating ions not directly involved (spectator ions) = net ionic equation

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Predicting the Precipitate Insoluble salt = a precipitate - note Figure 7.13,

p.156 General rules: Table 7.3, p. 161, Reference

p.708 (back of textbook) Sample problem 7-9, p.156