I II III IV V

If the 4 young ladies in the front row have a combined

weight of 2.15x1011micrograms. What is the average

weight of each? Convert their weight to Pounds!

World’s Most Dangerous Bellringer

I II III IV V

I. Intro to Reactions

Ch. 9 – Chemical Reactions

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

A. Chemical Equations

A+B C+DREACTANTS PRODUCTS

A. Chemical Equations

B. Writing Equations

Identify the substances involved. Use symbols to show:

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

How many? - coefficient Of what? - chemical formula

Remember the diatomic elements.

In what state? - physical state

B. 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)

C. 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

D. Balancing Equations

An equation is balanced when it has the same # and kind of atoms.

D. Balancing Equations Law of Conservation of Mass: mass is

neither created nor destroyed in a chemical reaction (Lavoisier)

4 H

2 O

4 H

2 O4 g 32 g

36 g

total mass stays the same; atoms can only rearrange.

D. Balancing Steps1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Coefficient subscript = # atoms

3MgCl2 4Ca(OH)2

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

5. Double check atom balance!!!

D. Helpful Tips

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

a coefficient. If elements appear more than once

per side, balance them last. Balance polyatomic ions as single

units. “1 SO4” instead of “1 S” and “4 O”