IIIIIIIVV I.Intro to Reactions Unit 7 – Chemical Reactions.

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I. Intro to Reactions

Unit 7 – Chemical Reactions

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

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C. 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 Equations

Two 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.

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Unit 7 – Chemical Reactions

II. Types of Chemical Reactions

A. Combustion

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

the burning of any substance in O2 to produce heat

A + O2 B

Na(s)+ O2(g)

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

A. Combustion

Products: contain oxygen hydrocarbons 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

Products: ionic - cancel charges covalent - 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 elements others - hard to tell

D. Single Replacement one element replaces another in a

compound metal 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:

metal metal (+) nonmetal nonmetal (-) free element 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 Replacement

Pb(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 ions one product must be insoluble (check

solubility table)

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

2 2

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III. Balancing Equations

Unit 7 – Chemical Reactions

A. Balancing Steps

1. 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