Acid - Base Studies. Goals : Safety : HCl, NaOH, HC 2 H 3 O 2 and NH 3 are corrosive. If. Waste : Solutions may be flushed down the sink.

Transcription

1 Acid - Base Studies CH102 Lab 10 Goals : To use ph paper or a ph meter to measure the ph of various solution. Safety : HCl, NaOH, HC 2 H 3 O 2 and NH 3 are corrosive. If contact occurs with the eyes or skin, rinse with lots of water. HCl, HC 2 H 3 O 2 and NH 3 give off irritating vapors. Use the hood to avoid contact with vapors. Do not mix concentrated acid and base solutions. The reactions are exothermic and may splash. Waste : Solutions may be flushed down the sink.

2 Prelab Assistance Conjugate acid-base pairs differ by H 1+ acid HF / conjugate base F 1- remove H base H 2 O add H / F H 3 O make charge more negative by 1 conjugate acid H 3 O 1+ make charge more positive by 1

3 Prelab Assistance Acid strength is measured relative to water. HF (aq) + H 2 O (l) F 1- (aq) + H 3 O 1+ (aq) K a = [F 1- ][H 3 O 1+ ] [HF] Strong acids, such as HCl, have K a values of >1 and will 100% react with water. HCl (aq) + H 2 O (l) Cl 1- (aq) + H 3 O 1+ (aq) (~ none) (all) (all) Weak acids, such as HC 2 H 3 O 2, have K a values of <1 and will only react a little with water. HC 2 H 3 O 2 (aq) + H 2 O (l) C 2 H 3 O 2 1- (aq) + H 3 O 1+ (aq) (lots) (a bit) (a bit)

4 Prelab Assistance Base strength is also measured relative to water. F 1- (aq) + H 2 O (l) HF (aq) + OH 1- (aq) K b = [HF] [OH1- ] [F 1- ] Strong bases, such as NaOH, are soluble ionic salts and will 100% dissolve in water. NaCl (s) Na 1+ (aq) + OH 1- (aq) (~ none) (all) (all) Weak base, such as NH 3, have K b values of <1 and will only react a little with water. NH 3 (aq) + H 2 O (l) NH 4 1+ (aq) + OH 1- (aq) (lots) (a bit) (a bit)

5 Prelab Assistance Even water is a tiny bit acidic and basic: H 2 O (l) + H 2 O (l) H 3 O 1+ (aq) + OH 1- (aq) (lots) (a tiny bit) (a tiny bit) K w = [H 3 O 1+ ][OH 1- ] = In pure water: [H 3 O 1+ ] = [OH 1- ] = M ph = - log [H 3 O 1+ ] = - log ( ) = 7.00

6 Prelab and Postlab Assistance Since H 3 O 1+ concentrations are generally very low in most acid and base solutions, the ph scale is used to express a solution s hydronium ion concentration. ph = - log [H 3 O 1+ ] [H 3 O 1+ ] = 10 -ph Example 1 : What is the H 3 O 1+ concentration of a solution of ph 8.30? [H 3 O 1+ ] = = 5.0 x 10-9 Example 2 : What is the ph of a solution with [H 3 O 1+ ] = 4.3 x 10-4 M? ph = -log ( ) = 3.37

7 Prelab Assistance Acids :! produce H 3 O 1+! have a ph < 7! have a [H 3 O 1+ ] > ! have [H 3 O 1+ ] > [OH 1- ] Bases :! produce OH 1-! have a ph > 7! have a [H 3 O 1+ ] < ! have [H 3 O 1+ ] < [OH 1- ] Strong acids produce 100% H 3 O 1+ Weak acids produce some H 3 O 1+ Strong bases produce 100% OH 1- Weak bases produce some OH 1-

8 Part A. Experimental Overview Students will measure the ph of various solutions. Fill each test tube ½ to ¼ full of acidic and basic solutions and measure the ph. 7.2 Note : Please be careful with the tip as it does contain a glass blub. Remember to take the rubber cap off the ph meter before attempting to measure the ph.

9 Part B. Experimental Overview Students will use ph paper to determine the ph of various household chemicals. " Place 2 drops of each solution on the ph paper. " Compare the observed color to the ph color chart. Note : Record the first color change observed when testing bleach, as the color fades quickly. Note : Test ammonia last as the vapors will react with other paper in close proximity.

10 Part C. Experimental Overview Student will add NaOH solution to HCl and record the ph 0.1 M HCl 0.1 M NaOH H 3 O 1+ + OH 1- # H 2 O + H 2 O

11 Writing Acid Base Reactions Example : Step 1 : Write the acid-base reaction that occurs when carbonic acid reacts with sodium hydroxide. List the species in solution. Remember to separate ionic compounds and write strong acids (pk a > 1) as H 3 O 1+. H 2 CO 3, Na 1+, OH 1- Step 2 : Identify the acid and base. When in doubt, check the acid-base table on the front cover of the lab manual. H 2 CO 3 + OH 1- acid base

12 Writing Acid-Base Reactions Step 3 : On the product side, write the conjugate base of the acid, and the conjugate acid of the base. H 2 CO 3 + OH 1- HCO H 2 O acid base c. base c. acid WebAssign Input Example : (Subtract H 1+ from H 2 CO 3 ) H2CO3 + OH^1- -> HCO3^1- + H2O (Add H 1+ to OH 1- ) acid + base # c. base + c. acid

13 Writing Acid Base Reactions Example : Step 1 : Write the acid-base reaction that occurs when hydrobromic acid reacts with lithium sulfate. List the species in solution. Remember to separate ionic compounds and write strong acids (pk a > 1) as H 3 O 1+. H 3 O 1+, Li 1+, SO 4 2- Remember slide 3 strong acids with pk a > 1 react 100% with water to give H 3 O 1+. Step 2 : Identify the acid and base. When in doubt, check the acid-base table on the front cover of the lab manual. H 3 O 1+ + SO 4 2- acid base

14 Writing Acid-Base Reactions Step 3 : On the product side, write the conjugate base of the acid, and the conjugate acid of the base. 1+ H 3 O + SO 2-4 H 2 O + HSO 1-4 acid base c. base c. acid WebAssign Input Example : (Subtract H 1+ from H 3 O 1+ ) H3O^1+ + SO4^2- -> H2O + HSO4^1- (Add H 1+ to SO 4 2- ) acid + base # c. base + c. acid