I II III IV V

Unit 5 – Chemical Reactions

Chapter 9

A.Signs of a Chemical Reaction Evolution of heat and light Formation of a gas Formation of a precipitate Color change

B.Law of Conservation of Mass mass is neither created nor destroyed in a

chemical reaction

4 H

2 O

4 H

2 O4 g 32 g

36 g

total mass stays the same atoms can only rearrange

C. Chemical Equations

A+B C+DREACTANTS PRODUCTS

p. 283C. Chemical Equations

D. Writing Equations

Identify the substances involved. Use symbols to show:

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

How many? - coefficient

Of what? - chemical formula

In what state? - physical state Remember the diatomic elements.

D. Writing EquationsTwo atoms of aluminum react with three units of aqueous copper(II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride.

• How many?• Of what?• In what state?

Al 2 (s) + 3CuCl2(aq) 3 Cu(s) + 2AlCl3(aq)

E. Describing Equations Describing Coefficients:

individual atom = “atom”

covalent substance = “molecule”

ionic substance = “unit”

3 molecules of carbon dioxide

2 atoms of magnesium

4 units of magnesium oxide

3CO2

2Mg

4MgO

E. Describing Equations

to produce

• How many?• Of what?• In what state?

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

One atom of solid zinc reacts withtwo molecules of aqueous hydrochloric acid one unitof aqueous zinc chloride and onemolecule of hydrogen gas.

F. Chemical vs. Physical(revisited) Chemical changes – make something

newEx: sodium and chlorine react to make table

salt

Physical changes – just change the physical nature of the substance (the chemical formula remains the same)Ex: Ice melts into liquid water (formula

remains H2O)

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9.1 - continued

A. Balancing Steps1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Coefficient subscript = # of atoms

4. Reduce coefficients to lowest possible ratio, if necessary.

5. Double check atom balance!!!

B. Helpful Tips Balance one element at a time. Update ALL atom counts after adding a

coefficient. If an element appears more than once per

side, balance it last. Balance polyatomic ions as single units.

“1 SO4” instead of “1 S” and “4 O”

Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

1 1

1 1

2 3

2

3

6

3

33 2

C. Balancing ExampleAluminum and copper(II) chloride react to form copper and aluminum chloride.

2

2

6

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9.2 – Classifying Chemical Reactions

A. Combustion the burning of any substance in O2 to

produce heat

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

A + O2 B

Na(s)+ O2(g)

C3H8(g)+ O2(g) 5 3 4

A. Combustion Products:

contain oxygenhydrocarbons form CO2 + H2O

CO2(g)+ H2O(g)

Na2O(s) 4 2

B. Synthesis the combination of 2 or more substances

to form a compound

only one product

A + B AB

B. Synthesis

H2(g) + Cl2(g) 2 HCl(g)

Al(s)+ Cl2(g) AlCl3(s)2 3 2

B. Synthesis Product:

ionic – cross-over chargescovalent - hard to tell

C. Decomposition a compound breaks down into 2 or more

simpler substances

only ONE REACTANT

AB A + B

C. Decomposition

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

KBr(l) K(s) + Br2(l) 2 2

C. Decomposition Products:

binary - break into elementsDiatomics – N2, O2, F2, Cl2, Br2, I2

others - hard to tell

D. Single Replacement one element replaces another in a

compoundmetal replaces metal (+)nonmetal replaces nonmetal (-)

A + BC B + AC

D. Single Replacement

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

Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)

D. Single Replacement Products:

Free element kicks out nonmetal nonmetal (-)free element on reactants side must be more active

(check activity series)

Br2(l)+ NaCl(aq) N.R.

AB + CD AD + CB

E. Double Replacement ions in two compounds “change partners” cation of one compound combines with

anion of the other

E. Double ReplacementPb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)

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

E. Double Replacement Products:

switch negative ionsone product must be insoluble (check

solubility table)

NaNO3(aq)+ KI(aq) N.R.

2 2