Wednesday, Sept 29                                        Chapter 4

practice:25-52

homework:26, 40, 52

 

I.  Faraday - discovered that ions carried current in solution, but believed that the current itself tore solute particles apart to make the ions

 

Svante Arrhenius demonstrated that some substances dissociate into ions when they dissolve in water and these ions allow the flow of electricity.

            Theory of electrolytic dissociation

 

Demo:  Prep 0.1 M solutions of: glucose, ethanol, acetic acid, NaCl, HCl

 

            what are the chemical formulas of these molecules?

            what type of molecules are they?  Ionic, molecular, organic, acid, base etc.

            which type of compounds allow the strongest flow of electricity?

           

            note degrees of dissociation.

define nonelectrolyte, weak electrolyte and strong electrolyte by how much they

dissociate to produce ions in water.

 

            1.  Essentially all soluble ionic compounds are strong electrolytes.

            2.  Only a few molecular compounds are strong electrolytes (ie HCl)

            3.  Most molecular compounds are either non-electrolytes or weak electrolytes.

4.  Most organic compounds are molecular and nonelectrolytes:  carboxylic acids (such as acetic acid) and amines (such as ammonia) are weak electrolytes.

 

So when writing chemical equations denoting dissolution of a substance, it should be noted whether or not the compound dissociates.  If so, an ionic equation should be written that shows the ions in solution.

 

ex.  dissolution of sodium acetate:

 

            NaCH3COO (s) → Na+ (aq) + CH3COO- (aq)

 

ex.  dissoluction of sodium carbonate:

            Na2CO3 (s) → 2 Na+ (aq) + CO3 - (aq)

 

From these equations we can calculate ion concentrations.

 

If we have a 0.45 M solution of sodium carbonate, then we have 0.90 M sodium ions and 0.45 M carbonate ions.

 

 

II.  Acid Base reactions

 

Arrhenius definition of acids and bases:  an acid produces H+ when dissolved in water, a base produces OH- when dissolved in water.

 

Strong acids = strong electrolytes (not necessarily extremely corrosive)

weak acids = weak electrolytes

            HF a weak acid but extremely corrosive - etch glass, dissolve human tissue.

when the chemical equation for the dissolution of a weak acid is written, it needs to show partial dissociation - reversed arrows - ions on one side, molecular form on other.

            call this an equilibrium

 

ibid bases.

            mentions amines - weak bases.

 

Table 4.1 - list of acids and bases students should know.

 

polyprotic acids - some acids produce more than one acidic hydrogen

acidic hydrogens are either listed first in chemical formulas (H2SO4) or as acidic functional groups in structural formulas (CH3COOH).

 

first acidic hydrogen may be strong, second may be weak (H2SO4)

 

A reaction between acids and bases is called a neutralization reaction.  An acid and a base reaction to produce a salt ( an ionic compound) and water.

 

            NaOH (aq) + HCl (aq) → NaCl (aq)  + H2O (l)           (molecular equation)

 

            note phase labels.

 

This chemical equation can be written in ionic form:

           

            Na+ (aq) + OH- (aq) + H+(aq) + Cl+ (aq) → Na+ (aq) +Cl+ + H2O (l)

                        (complete ionic equation)

                        note that water is a molecular compound and is written as such.

 

We can cancel out the ions that are common to both sides - these are called spectator ions because they don’t really do anything.  What we are left with is called the net ionic equation.

 

            OH- (aq) + H+ (aq) → H2O (l) (net ionic equation)