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 Net Ionic Equation: Ba+2 + SO42-  BaSO4 Molecular Equation: BaCl2 + Na2SO4  BaSO4 + 2NaCl Ionic Equation: Ba+2 + 2Cl- + 2Na+ + SO42-  BaSO4 + 2Na+ + 2Cl- Net Ionic Equation: Ba+2 + SO42-  BaSO4 Why did Na+ and Cl- cancel???

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… Al(NO3)3 + NaOH  2K3PO4 + 3Ca(NO3)2  PbCl2 + LiI 

Now try these… KOH + HCl  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=abc A= 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

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 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