Concept Attainment P H

Arrhenius Theory of Acids and Bases An acid is a substance that produces H+ ions in solution A base is a substance that produces OHˉ ions in solution Neutralization can be represented by: Arrhenius theory does not recognize the contribution of the solvent in the ionization of the solute It fails to explain weak bases such as carbonate (CO 3 2- ), and ammonia (NH 3 ) HCl (g) H + (aq) + Clˉ (aq) H 2 O NaOH (s) Na + (aq) + OHˉ (aq) H 2 O H + (aq) + OHˉ (aq) H 2 O (l)

Br ø nsted Lowry Acid/Base Theory NaOH(s) + H 2 O(l) Na + (aq) + OHˉ(aq) HBr(g) + H 2 O(l) H 3 O + (aq) + Brˉ(aq) For our purposes H 3 O + and H + can be considered equivalent terms Neutralization can be represented by: H 3 O + (aq) + OHˉ (aq) 2 H 2 O (l)

Br ø nsted Lowry Acid/Base Theory The main advantage of the Br ø nsted Lowry Theory is in its treatment of weak bases An acid is a proton (H + ) donor A base is a proton (H + ) acceptor Since NH 3 is a weak base, the reverse reaction should also be considered This reversible reaction may be written as follows: NH 3 + H 2 O NH 4 + + OHˉ conjugate acid conjugate base acid base NH 3 + H 2 O NH 4 + + OHˉ base acid NH 4 + + OHˉ NH 3 + H 2 O base acid

Review Question Identifying Br ø nsted-Lowry Acids and Bases and their Conjugates Question: For each of the following chemical equations, identify the acids and bases in both the forward and reverse reactions. i) HClO 2 + H 2 O ClO 2 ˉ + H 3 O + ii) OClˉ + H 2 O HOCl + OHˉ iii) NH 3 + H 2 PO 4 ˉ NH 4 + + HPO 4 2 ˉ conjugate acid conjugate base acid base conjugate acid conjugate base acid base conjugate acid conjugate base acid base

Last two things Concentration – how much of something is contained in a certain amount of solution Molarity – a chemical means of measuring concentration

Concentration is measured in M [C 6 H 12 O 6 ] = 5 M  “Five Molar glucose” [C 6 H 12 O 6 ] = 2 M  “Two Molar glucose” [C 6 H 12 O 6 ] = 0.5 M  “0.5 Molar glucose” [C 6 H 12 O 6 ] = 0.2 M  “0.2 Molar glucose” Which of these is the most concentrated? Which of these is the least concentrated?

What happens when the concentration gets really small? [C 6 H 12 O 6 ] = 0.00001 M [C 6 H 12 O 6 ] = 0.00000001 M [C 6 H 12 O 6 ] = 0.00000000000001 M We bring back SCIENTIFIC NOTATION!!! 0.00001 M = 1 x 10 -5 M 0.00000001 M = 1 x 10 -8 M 0.00000000000001 M = 1 x 10 -14 M

Let’s apply this concentration & scientific notation stuff to Acids/Bases [HCl] = 0.00001 M [CH 3 COOH] = 0.00000000000001 M [HCl] = 1 x 10 -5 M [CH 3 COOH] = 1 x 10 -14 M

Let’s apply this concentration & scientific notation stuff to Acids/Bases [HCl] & [CH 3 COOH] are acids Acids  generate H + So, [HCl] = 1 x 10 -5 M  [H + ] = 1 x 10 -5 M [CH 3 COOH] = 1 x 10 -14 M  [H + ] = 1 x 10 -14 M

Now, let’s use the scientific notation… Rank the following from least to greatest concentration of Hydrogen ions [HCl] = 1 x 10 -6 M [CH 3 COOH] = 1 x 10 -14 M [HNO 3 ] = 1 x 10 -2 M [HBr] = 1 x 10 -12 M [HClO 3 ] = 1 x 10 -3 M [HF] = 1 x 10 -13 M

[CH 3 COOH] = 1 x 10 -14 M [HF] = 1 x 10 -13 M [HBr] = 1 x 10 -12 M [HCl] = 1 x 10 -6 M [HClO 3 ] = 1 x 10 -3 M [HNO 3 ] = 1 x 10 -2 M Regardless of the acid’s identity, the concentrations of H + ions rank as follows:

[H + ] = 1 x 10 -14 M [H + ] = 1 x 10 -13 M [H + ] = 1 x 10 -12 M [H + ] = 1 x 10 -6 M [H + ] = 1 x 10 -3 M [H + ] = 1 x 10 -2 M It means the same thing!! So, we might as well just write…

Now, for the NEW idea: pH See if you can figure out what is going on in the next slides…

Part 1: Define a logarithm When the [H + ] = 1 x 10 -14 M -14 1 x 10 -13 M -13 1 x 10 -12 M -12 1 x 10 -11 M -11 1 x 10 -10 M -10 1 x 10 -9 M -9 1 x 10 -8 M -8 1 x 10 -7 M -7 1 x 10 -6 M -6 1 x 10 -5 M -5 1 x 10 -4 M -4 1 x 10 -3 M -3 1 x 10 -2 M -2 1 x 10 -1 M -1 The logarithm of [H + ] =

Part 2: How do we change the log result into a positive number? Multiply the log by -1 to change it to positive When [H + ] = Log [H + ] = - Log [H + ] = 1 x 10 -14 M -14 14 1 x 10 -13 M -13 13 1 x 10 -12 M -12 12 1 x 10 -11 M -11 11 1 x 10 -10 M -10 10 1 x 10 -9 M -9 9 1 x 10 -8 M -8 8 1 x 10 -7 M -7 “times -1” 7 1 x 10 -6 M -6 6 1 x 10 -5 M -5 5 1 x 10 -4 M -4 4 1 x 10 -3 M -3 3 1 x 10 -2 M -2 2 1 x 10 -1 M -1 1

It’s LOG!!!! So, what does a log do? It changes a number that is written in scientific notation into the EXPONENT of the scientific notation What does -log do? It changes a number that is written in scientific notation with a NEGATIVE EXPONENT into a POSITIVE NUMBER ALONE

Part 3: pH When the [H + ] = 1 x 10 -14 M 14 1 x 10 -13 M 13 1 x 10 -12 M 12 1 x 10 -11 M 11 1 x 10 -10 M 10 1 x 10 -9 M 9 1 x 10 -8 M 8 1 x 10 -7 M 7 1 x 10 -6 M 6 1 x 10 -5 M 5 1 x 10 -4 M 4 1 x 10 -3 M 3 1 x 10 -2 M 2 1 x 10 -1 M 1 The pH =

So, what is pH ? A mathematical operation Just like Sum, Difference, Product, etc. It does the same thing that taking the –log of a number does!!! The “p” means ‘take the negative log of’ The H stands for [H + ] Thus, pH = -log [H + ]

Find the pH of each of the following Concentration [HCl] = 1 x 10 -6 M [CH 3 COOH] = 1 x 10 -14 M [HNO 3 ] = 1 x 10 -2 M [HBr] = 1 x 10 -12 M [HClO 3 ] = 1 x 10 -3 M [HF] = 1 x 10 -13 M pH 6 14 2 12 3 13

What did you notice about the pH scale compared to concentration? The smaller the pH the higher the concentration of the [H + ]

Let’s try our using pH to rank acid strength

Now, let’s use the scientific notation… Rank the following from least to greatest concentration of Hydrogen ions pH of HCl = 6 pH of CH 3 COOH = 2 pH of HNO 3 = 3 pH of HBr = 11 pH of HClO 3 = 8 pH of HF = 10

The following are ranked from LEAST to GREATEST [H + ] pH of HBr = 11 pH of HF = 10 pH of HClO 3 = 8 pH of HCl = 6 pH of HNO 3 = 3 pH of CH 3 COOH = 2 The smaller the pH the higher the concentration of the [H + ]

Why use pH? It’s much more convenient way of measuring [H + ] than using scientific notation It transforms the negative exponents into positive numbers We can use pH to talk about acid strength

The pH scale 1 2 3 4 5 6 7 8 9 10 11 12 13 14

The Acid Concentration Scale When [H + ] = 1 x 10 to the… -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14

Let’s see if the pH concept makes sense to you What does the “H” part of pH mean? What does the “p” part of pH mean? What does taking the log of a number do? What does taking the –log of a number do? Could pH work for ions other than [H + ]? Could we take a pOH perhaps???

pOH: the flip-side of pH Works JUST LIKE pH, except we are talking now about the [OH - ] instead of [H + ] What is the pOH of a NaOH solution that is 1 x 10 -3 M? pOH = 3

The pOH scale 1 2 3 4 5 6 7 8 9 10 11 12 13 14

The Base Concentration Scale When [OH - ] = 1 x 10 to the… -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14

Find the pOH of each of the following Concentration [NaOH] = 1 x 10 -6 M [KOH] = 1 x 10 -14 M [LiOH] = 1 x 10 -2 M pOH 6 14 2

What’s the relationship between pH and pOH? pH + pOH = 14 ALWAYS!!!

pH and pOH The potential of the hydrogen ion was defined in 1909 as the negative of the logarithm of [H + ]. pH = -log[H 3 O + ] pOH = -log[OH - ] [H 3 O + ][OH - ]= 1.0x10 -14 pH + pOH = 14

Sample Question Calculate the pH and pOH of 0.10 M HBr HBr + H 2 O H 3 O + + Brˉ For our purposes H 3 O + and H + can be considered equivalent terms pH = -log [H + ] pH = -log (0.1) pH = 1.0 Since pH + pOH = 14 1.0 + pOH = 14 pOH = 13

Another Sample Question Calculate the pH of 0.10 M KOH KOH is a strong base and will fully dissociate; [OHˉ] = 0.10M Method # 2 Calculate pOH pOH = -log [OH ˉ ] pOH = -log (0.1) pOH = 1.0 pH + pOH = 14 pH + 1 = 14 pH = 13 Calculate pH

Why pH/pOH?? Autoprotolysis of Water H 2 O + H 2 O H 3 O + + OH - base acid c. acid c. base K c = [H 2 O][H 2 O] [H 3 O + ][OH - ] K W = K c [H 2 O][H 2 O] = = 1.0x10 -14 [H 3 O + ][OH - ] K W = = 1.0x10 -14 [H 3 O + ][OH - ]

Concept Attainment P H

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Concept Attainment P H