Weak Acids/ICE Boxes

Evaluation/Assessment: Objective: Today I will be able to: Determine the concentrations of reactants and products intitially and during equilibrium for a reaction by using an ICE table Calculate the concentrations of weak acids and bases at equilbrium Evaluation/Assessment: Informal assessment: monitoring student questions as they complete the practice problems Formal assessment: analyzing responses to the practice and exit ticket Common Core Connection Build strong content knowledge Reason abstractly and quantitatively Make sense of problems and persevere in solving them

Lesson Sequence Evaluate: Warm – Up Explain: ICE Tables Elaborate: ICE Table Practice Evaluate: Exit Ticket

Warm - Up Complete #2 on your Equilibrium Practice (ICE Boxes) worksheet

Objective Today I will be able to: Determine the concentrations of reactants and products intitially and during equilibrium for a reaction by using an ICE table Calculate the concentrations of weak acids and bases at equilbrium

Homework Finish Practice Problems from Class

Agenda Warm – Up Intro to ICE Boxes Notes Ice Box Practice Weak Acid Notes Weak Acid Practice Problems Exit Ticket

Take out your homework for last night. Let’s review #3 & #4 ICE Boxes Take out your homework for last night. Let’s review #3 & #4

Weak Acids/Bases and ICE Boxes

Remember this next slide? …

Strong Acids/Bases vs. Weak Acids/Bases Ionize almost 100% in water (react to completion) WEAK: Do not ionize completely, achieve an equilibrium

Q: How can I tell the difference between strong and weak acids?

Two different ways: Study the list of strong acids and bases Look at the arrow in an acid/base balanced equation If arrow is one direction  it is STRONG If arrow is two directions   it is WEAK

Examples Strong Acid Example Weak Acid Example Strong Base Example: HNO3(aq) + H2O(l)  H3O+(aq) + NO3-(aq) Weak Acid Example CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO-(aq) Strong Base Example: NaOH(aq) + H2O(l)  Na+(aq) + OH-(aq) Weak Base Example: NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)

Instead of using Keq we use a different constant for weak acids and bases

Ionization Constant of a Weak Acid Ka = acid ionization constant Ka varies at different temperatures Generic example of a weak acid reaction:

Weak Acid Specific Example CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO-(aq) Weak acids have a Ka < 1 Leads to small [H3O+] pH of weak acids is calculated by solving the expression for [H3O+] at equilibrium and then taking the –log[H3O+]

Ionization Constant of a Weak Base Kb = base ionization constant Weak bases = Kb < 1 Leads to a small [OH-] To find pOH take -log[OH-]

Weak Acid/Base Worked Examples

1) Weak Acid Example HA(aq) + H2O(l)  H3O+(aq) + A-(aq) Q: Initially, you have 1.00 M HA. Calculate the equilibrium concentrations of HA, H3O+, A-, and calculate the pH. Ka = 1.80 x 10-5 Hint: Use an ICE table!

Weak Acid Example Cont. HA H3O+ A- I 1.00 C -x +x E 1.00 – x

Weak Acid Example Cont. Write Ka expression: Assume x is small because Ka is very small, therefore x can be ignored (5% Rule) Ka = 1.80 x 10-5 = [H3O+][A-] = x2 [HA] 1.00-x Ka = 1.80 x 10-5 = x2 1.00

Weak Acid Example Cont. Ka = 1.80 x 10-5 = x2 1.00 x = [H3O+] = [A-] = 4.20 x 10-3 M pH = - log [H3O+] = 2.37

2) Weak Base Example NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) Initially, you have a concentration of 0.010 M NH3. Find the concentrations at equilibrium. Calculate the pH. Kb = 1.80 x 10-5 Hint: Use an ICE table!

Weak Base Example Cont. NH3 NH4+ OH- I 0.010 C -x +x E 0.010 – x

Weak Base Example Cont. Write Kb expression: Assume x is small because Kb is very small, therefore x can be ignored Kb = 1.80 x 10-5 = [NH4][OH-] = x2 [NH3] 0.010-x Kb = 1.80 x 10-5 = x2 0.010

Weak Base Example Cont. Kb = 1.80 x 10-5 = x2 0.010 x = [NH4+] = [OH-] = 4.20 x 10-4 M pOH = - log [OH-] = 3.37 pH + pOH = 14 Therefore, 14 - 3.37 = 10.6 = pH

Weak Acid/Base Practice Complete the practice at your desk. Whatever you do not finish will become your homework.

Exit Ticket Which problems on the weak acid/base practice were the most challenging? One thing I am confused about is ___________________________.