SPECIAL TOPICS IN AP CHEM

NET IONIC EQUATIONS

Molecular Equations are frequently used to show what chemicals are mixed in a reaction, not which actually react.

Net Ionic Equations are a simplified form of molecular equations that eliminate spectator ions and shows only those species that react.

NET IONIC EQUATIONS

Molecular Equation:

BaCl2 + Na2SO4 BaSO4 + 2NaCl

Why did Na+ and Cl- cancel???

Ionic Equation:Ba+2 + 2Cl- + 2Na+ + SO4

2- BaSO4 + 2Na+ + 2Cl-

Net Ionic Equation: Ba+2 + SO4

2- BaSO4

TIPS:

• For practice, write out the ionic form, breaking down only soluble aqueous substances. Then cancel and re-write the net ionic.

• Don’t worry about writing the phases, although it may help to write if a substance is a solid or gas, to remind you not to ionize it.

• The AP will always give reactions that have a net ionic reaction, but your exercises may not. If every substance ionizes, then the reaction does not occur.

THINK

• How will you know if a substance is solid, aqueous, liquid, or gas?

• Substances are only aqueous if they are soluble and water is present.

• All insoluble compounds that contain metals are solids. Also, if the compound has a metal but no water is present, it is a solid.

• Most compounds that do not contain metals are not soluble and will not be broken down into ions. They may be gaseous.

NOW TRY THESE…

1. Al(NO3)3 + NaOH

2. 2K3PO4 + 3Ca(NO3)2

3. PbCl2 + LiI

NOW TRY THESE…

1. KOH + HCl

2. HF + Li2S

The net ionic equations you will be studying can be broken into categories to help you get through the questions faster. Remember the AP exam is a speed test!

CONCENTRATION AND BEER’S LAW

BEER’S LAW

A=abcA= Absorbance (light absorbed by the solution)

a = molar absorptivity (given for the sample and

wavelength of light(M-1cm-1))

b = path length (thickness of the sample(cm))

c = concentration (M)

BEER’S LAW

• If A=2 then 99% of the light was absorbed by the solution. The solution has a high Molarity.

• Molarity and Absorbance have a direct relationship: a dilute solution will absorb less light. More light will pass through.

BEER’S LAW

BEER’S LAW

• The molar absorptivity constant of a particular chemical is 1.5/M·cm. What is the concentration of a solution made from this chemical that has an absorbance of 0.72 with a cell path length of 1.1cm?

• A=abc

• .72 = (1.5)(1.1)(x)

• x=.44M

BEER’S LAW

• A solution of 0.125M copper (II) nitrate solution records an absorptivity of 0.59. What is the concentration of a solution made from this chemical that has an absorbance of 0.72?

• A=abc but “ab” are constant (same curvet, same wavelength)

• A1/c1 = A2/c2

• .125/.59 = x/.72

• x= .15M