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Unit 9Chemical Equations

CP Chemistry

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Describing Chemical Change

OBJECTIVES:

–Write equations describing chemical reactions, using appropriate symbols

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Describing 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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Symbols in equations the arrow separates the

reactants from the products Read “reacts to form” or “yields” 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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Exothermic and Endothermic Exothermic Reactions give off heat

– Ex: explosions

Endothermic Reactions absorb heat and feel cold

– Ex: ice packs from the nurse

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

1. In a sentence

Copper reacts with chlorine to form copper (II) chloride.

2. In a word equation

Copper + chlorine copper (II) chloride

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DO NOT FORGET DIATOMIC MOLECULES!!!

BrINClHOF

Never travel alone!!

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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, write these: Fe(s) + O2(g) Fe2O3(s)

Cu(s) + AgNO3(aq)

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

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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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!

Check to make sure it is balanced.

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Never change a subscript to balance an equation.– If you change the formula 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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Balancing Examples 2AgNO3 + 2Cu Cu(NO3)2 + 2Ag

3Mg +N2 Mg3N2

4P + 5O2 P4O10

2Na + 2H2O H2 + 2NaOH

CH4 + 2O2 CO2 + 2H2O

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Types of Chemical Reactions

OBJECTIVES:

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

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Types 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 (SYNTHESIS)

Combine - put together 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 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 will only be asked to balance them

__ NiCO3 iO + ___ CO2

__KClO3(aq) __ KCl +__ O2

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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. K + NaCl Na + KCl 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 the last page of your packet -

called the Activity Series of Metals Higher on the list replaces lower. If something isn’t on the activity series you

have, assume the one with the higher atomic number is higher on the list

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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 Al + HCl Fe + CuSO4 Pb + KCl

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#3 - Single Replacement 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:

E + E = Combination

C = Decomposition

E + C = Single replacement

C + C = Double replacement

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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

C6H12O6 + O2

C8H8 +O2

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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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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 + NaCl AgCl + NaNO3

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 8.13, p.227 General rules: Table 8.3, p. 227,

Reference p.7 (back of textbook), and in Lab manual p.338

Sample problem 8-11, p.228