12-3 Concentration vs. Strength for Acids and Bases (Section 16.2) And you

Review Review If needed Bozeman Neutralization reactions www.youtube.com/watch?v=HdmCagtasYg

Concentration refers to how many molecules are dissolved in a given amount of water (solvent). Strength refers to the degree of ionization, or how much ionization occurs. Strong acids ionize to a large degree Large K value

Concentration: Molarity = moles of solute Liters of solution High concentration is lots of molecules (moles) per volume: 1M NaCl = 58.5 g/L Low concentration is very few molecules (moles)/volume: 0.1M NaCl = 5.85 g/L

Strength: “Strong” acids completely ionize or dissociate: HCl → H+ + Cl-   “Weak” acids ionize only slightly due to the reverse reaction occurring and the establishment of an equilibrium:

Strong acid: HCl  H+ + Cl- (all dissociate into products) Ka (acid dissociation constant) = [H+] [Cl-] * K>>>1 Very product favored!!!!

Weak acid: HC2H3O2 ↔ H+ + C2H3O2 - Ka = [H+][C2H3O2-] = 1.8 x 10-5 [HC2H3O2] Notice how the equilibrium lies far to the left, meaning there is very little H+ in solution, especially compared to a strong acid! 0.001 M HCl is a low concentration of a strong acid, whereas 10 M HC2H3O2 is a high concentration of a weak acid.

12-4 Ionization of Water and Kw (Section 16.3) Water only slightly ionizes: H2O + H2O ↔ H3O+ + OH- The conjugate acid H3O+ is called the “hydronium” ion. This equilibrium is often simplified to : H2O ↔ H+ + OH- with an equilibrium expression:   Kw = [H+][OH-] = 10-14 at 25ºC This equation can be used to calculate the [ ] of H+ or OH- given this Kw value.

10-7, 10-7 equal when water is neutral [H3O+][OH-] = 10-14 = Kw 10-7, 10-7 equal when water is neutral   If [H3O+] > 10-7 the solution is acidic, If [H3O+] < 10-7 the solution is basic [H3O+] = 10-5 then ??? [OH-] = ? (10-5)( ???) = 10-14 That is correct!! 10-9 !!!!! 10-9

12-5 The pH Scale and Relevant Equations (Section 16.4) Background ~ Logarithms: large changes expressed in small scale A logarithm is an exponent. For example: log 10 = 1 log 100 = 2 (from 102) Log 1000 = 3 (from 103) In other words, the log of a number is the exponent n so that 10n equals the number.

pH = -log [H3O+] If [H3O+] = 10-3, pH = -log (10-3) = 3 if [OH-] = 10-6 Then [H3O+] = 10-14 / 10-6 = 10 -8 pH = -log (10-8) = 8 If [H3O+] = 1.38 x 10-3, pH = ???? 2.86 if pH = 4.67, then ? [H3O+] = - 4.67 10x (inverse log) 2.14 x 10-5 M

Relationships/Equations:   1) pH = -log[H+] 2) [H+] = 10-pH 3) pOH = -log[OH-] 4) [OH-] = 10-pOH 5) [H+][OH-] = 10-14

www.youtube.com/watch?v=LS67vS10O5Y

Recall the Arrhenius definition of acids and bases: Acids donate H+ ; Bases donate OH- Aqueous solutions can be described as acidic, basic, or neutral. Acidic solutions have high H+ and low OH- Or rather: [H+] > [OH-] Basic solutions have high OH- and low H+ Or rather [OH-] > [H+] Neutral solutions have [H+] = [OH-] where [ ] denotes “concentration” in moles/L 

Chemists have created a scale that relates acids and bases called the “pH Scale”, which allows them to express small numbers conveniently. This scale, along with others like the Richter scale (measures earthquake strength), are log based; that is, each unit on the scale represents a factor of 10 change in the strength.

For example, a change in pH from 1 to 3 represents a change in the [H+] of 100 just like an earthquake of 7 is 100x stronger than a quake of 5.

Please see supplemental handout to understand the math of the interrelationships. Thank you ever so much!

What is the pH of a solution with [H+] = 1.0 x 10-8?   What is the pH of a solution with [H+] = 1.9 x 10-2? What is the [H+] of a solution with pH of 6? What is the [H+] of a solution with pH of 4.5?    8 1.72 1 x10-6 3.16 x 10 -5

What is the [H+] of a solution that has [OH-] = 1.0 x 10–12? What is the pH of a solution that has a [OH-] = 3.9 x 10–3?  The pOH of a solution is 3.4. What is the [H+]? 1 x 10-2 4.76 x 10 -4 11.6 2.5 x 10 -11