iiiiiiivv i.intro to reactions ch.7– chemical reactions

I II III IV V

I. Intro to Reactions

Ch.7– Chemical Reactions

A.Signs of a Chemical Reaction

heat and light produced 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.

Example:

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

Example

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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II. Balancing Equations

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. Check yourself before you wreck yourself

B. Helpful Tips

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.

Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

1 1

1 1

2 3

2

3

6

3

33 2

C. Balancing Example

Aluminum and copper(II) chloride react to form copper and aluminum chloride.

2

2

6

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Chemical Reactions

III. 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

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

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

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

Products: ionic – cross charges covalent - hard to tell

C. Decomposition

a compound breaks down into 2 or more simpler substances

only one reactant

AB A + B

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

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

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

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

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

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

Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)

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

Products: switch negative ions one product must be insoluble (check

solubility table)

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

2 2