I II III IV V

I. Intro to Reactions

Ch. 8 – 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

C. Chemical Equations

Pt a catalyst is present (in this case, platinum)

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 formula units of aqueous copper(II) chloride to produce three atoms of copper and two formula 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 = “formula unit”

3 molecules of carbon dioxide

2 atoms of magnesium

4 formula units of magnesium

oxide

3CO2

2Mg

4MgO

Subscripts vs. Coefficients

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 f.u.of aqueous zinc chloride and onemolecule of hydrogen gas.

I II III IV V

II. Balancing Equations

A. Balancing Steps1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Note: you may not change a subscript

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 Example

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

2

2

6

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

____C____C33HH88(g) + _____ O(g) + _____ O22(g) ---->(g) ---->

_____CO_____CO22(g) + _____(g) + _____ H H22O(g)O(g)

________BB44HH1010(g(g) ) + _____ O+ _____ O22(g) ---->(g) ---->

___ B___ B22OO33(g) + _____ H(g) + _____ H22O(g)O(g)

Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

Sodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphatesodium oxide + iron (III) phosphate

NaNa33POPO44 + Fe + Fe22OO33 ----> ---->

NaNa22O + FePOO + FePO44