I II III IV V

Intro to Reactions

Chemical Reactions

Signs of a Chemical Reaction

Evolution of heat and light Formation of a gas Formation of a precipitate (solid) Color change

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

Chemical Equations

A+B C+DREACTANTS PRODUCTS

Chemical Equations

Subscripts

Subscripts are the numbers proceeding an element.

The number represents how many atoms are present of that specific element.

Examples: H2O = 2H, 1O

AgNO3 = 1Ag, 1N, 3O

Mg(NO3)2 = 1Mg, 2N, 6O

Coefficients

Coefficient – is the number in front of an element or compound.

The number represents how many of that specific compound is present.

Examples: 2H2O = 4H, 2O 3CO = 3C, 3O 2Mg(NO3)2= 2 Mg, 4N,12O

Writing Equations

Identify the substances involved. Use symbols to show:

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

How many? – coefficient & subscript

Of what? - chemical formula

In what state? - physical state

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)

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.

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!!!

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

Balancing Example

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

2

2

6