What are they?There are different definitions for acids and bases dependent on the circumstances.Operational Definition - (Properties in which they differ)

Acids 1. react with carbonates to produce

carbon dioxide gas.2. change pink phenolphthalein to

colourless3. make litmus paper red4. make bromothymol blue turn yellow5. taste sour6. have a pH below 77. react with most metals and produce

hydrogen gas

Bases

1. don’t react with carbonates to produce carbon dioxide gas.

2. change colourless phenolphthalein to pink

3. make red litmus paper turn blue4. make bromthymol blue turn blue5. taste bitter6. have a pH above 7

1. ____________ react with carbonates to produce carbon dioxide gas.

Acid

NaCO3

CO2 bubbles

Acids

2. ___________ change colourless phenolphthalein to pink

Phenolphthalein

Base

Litmus paper

Phenolphthalein

3. _________ make blue litmus paper turn redAcids

4. __________make bromthymol blue turn yellow

Acid or base?

Bromthymol blue

Acids

A lemon tastes sour. Is it an acid or a base?

Acid

7.01.8Is this solution an acid or a base?

AcidpH meter

Zn

Is this solution an acid or a base?

H2

Acid

Conceptual Definitions of Acids and Bases

A chemist named Arrhenius recognized acids were molecular compounds and as such didn't conduct electricity as liquids since they didn't release ions. But he observed that when acids were combined with water they did conduct electricity.He also observed bases conducted electricity as liquids and when they were combined with water.Bases were ionic but acids weren't.

Arrhenius’s Definition of Acids and Bases

Acids are substances which react in water and produce hydronium ions.

HCl(g) + H20 -------> H301+(aq) + Cl1-(aq)

Bases are substances which react with water and produce hydroxide ions.

NH3(g) + H20 ------> NH41+(aq)+ OH1-(aq)

Homework from NelsonPg. 367 #1-7,

This concept has its limitations however. Can’t substances be classified as acids or bases without the involvement of water?

Bronstead's and Lowry's Definition of Acids and Bases

Acids are substances which donate protons and bases are substances which accept protons. In the examples above HCl(g) is an acid because it donates protons to H2O molecules and NH3 is a base because it accepts protons from H2O molecules.

Conjugate Acid - Base Pairs - When using the Bronsted concept for acids and bases it is convenient to consider all acid - base reactions as reversible equilibria. For instance when sulfurous acid, H2SO3 reacts with water the following equilibrium is established:

acid base acid baseH2SO3 + H2O H301+ + HSO3

1-

conjugate pair

conjugate pair

In the forward direction the H2SO3 is the proton donor so it’s the acid and the H2O is the proton acceptor so it’s the base. In the reverse direction the H301+ is the proton donor so it’s the acid and the HSO3

1- is the proton acceptor so it’s a base.

H2SO3 + H2O H301+ + HSO31-

conjugate pair

conjugate pair

acid base acid base

When looking at both forward and reverse reactions it is easy to pick out a pair of molecules which differ by a single proton (H atom without its electron). These pairs are called conjugate acid-base pairs.

H2SO3 + H2O H301+ + HSO31-

conjugate pair

conjugate pair

acid base acid base

Homework from NelsonPg. 389 # 17-20Pg. 392 # 8-11

Why do acids of equal concentration have different levels of conductivity?Some acids are stronger than others.Why?Let's look at a container of water.

Water molecules

HCl molecules

Cl1

-

Strong acids and basesTotally ionize

Water molecules

HF molecules

Only 1/5 ionized20% ionizationWeak acid

Strength of Acids and Bases is determined by the degree to which a substance produces ions in solution. A strong acid or base is a substance which completely ionizes. In other words if 100 molecules of a strong acid like HCl are placed in water all 100 of them will react with H2O producing 100 H3O1+ ions and 100 Cl1- ions. Weak acids and bases only partially ionize. Strong Acid - the reaction below goes to completion.HCl(g) + H20 --------> H301+(aq) + Cl1-(aq)

Weak Acid - the reaction occurs to a limited extent. In the example below if 100 acetic acid molecules are placed in water only a few of them will successfully react with water molecules producing hydronium ions. Most CH3COOH molecules remain intact.

CH3COOH + H20 H301+(aq)+ CH3COO1-

(aq)

Strong Acids in order of decreasing strength are HClO4, HI, HBr, H2SO4, HCl, HNO3

Acid strength has to do with the ease with which an acid can lose a proton. If the binary acid strengths (HI, HBr, HCl) are compared it can be seen that HI is the strongest acid of this group because its iodide ion is the largest of the group so the force between the hydrogen ion and the iodide ion is the weakest so it loses its proton most easily.

I1-

Cl1-

Br1-

H1+

H1+

H1+

Force is strongest since the ions are closest

Force is weakest since the ions are furthest

Remember the weaker the force the stronger the acid

Strong Bases include hydroxides of group 1A and Ca2+, Ba2+, and Sr2+. A table with the remaining moderate and weak bases can be found on page 615. As with acids the weaker the bonds, the stronger the base since liberation of OH1- ions is easiest when the bonds are weakest.

Polyprotic Acids donate protons in steps. For instance carbonic acid, H2CO3 has two protons to donate and it does this in two steps:step 1H2CO3 + H20 HCO3

1- + H301+

step 2HCO3

1- + H20 CO32- + H301+

note: The arrows are constructed in this manner to show the reverse reaction has a greater tendency than the forward reaction.

Amphoteric (Amphiprotic) Substances can behave as both acids or bases dependent on the circumstances. Water molecules, for instance, can sometimes except protons and behave as bases or donate protons and behave as acids.

HBr(g) + H2O H301+(aq) + Br1-(aq)

base

NH3(g) + H2O 0H1-(aq) + NH41+(aq)

acid

Homework - Pg. 386 # 15,16Pg. 379 # 1-5

Aqueous solutions can be classified as acidic, basic, or neutral. This classification scheme is based on the quantities of 2 ions, hydronium ion, H301+ and hydroxide ion, OH1-. Where do these ions come from in solutions of pure water? Water molecules in motion will randomly collide with one another. When this happens occasionally a hydrogen nucleus from one molecule will be transferred from one molecule to the other. This can be illustrated.

Notice the nucleus of one hydrogen atom, a proton, was transferred, but the electron pair was left behind. This produces the H301+ ion (hydronium) and the OH1- ion (hydroxide)

H2O + H20 H3O1+(aq) + OH1-(aq)

Hydronium ion (H3O1+)

Hydroxide ion (OH1-)

H20 H1+(aq) + OH1-(aq)

Which is usually shortened to:

In mathematical termspH = -log[H1+]so if in an aqueous solution the [H1+] = 2.4 x 10-8, the pH is7.62Remember the whole number portion of a pH doesn’t count as a significant digit (SD), just like in the number 2.4 x 10-8 the exponent -8 doesn’t count as a SD.

If the pH of a solution is 1.45 find the [H1+].

[H1+] = 10-pH. [H1+] = 10-1.45

3.5 x 10-2 mol/L

If the [H1+] of a solution is 6.2 x 10-2 mol/L find the pH.

pH = -log[H1+]pH = -log 6.2 x 10-2 pH = 1.21

Increasing basicityIncreasing acidity

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Neutral

Since this is a logarithmic scale pH 9 is 10x’s more basic than pH 8, pH 12 is 1000 x’s more basic than pH 9

How much more acidic is pH 1 than pH 5?10 000 x’sWhat pH is 1000x’s more acidic than pH 2? pH of -1

HomeworkPg. 371 # 1-6Pg. 375 # 1-6

Acids and bases, when combined in equal quantities, neutralize each other forming salt and water.

HCl + NaOH HOH + NaCl1+ 1- 1+ 1- 1+ 1- 1+ 1-

H2SO4 + Al(OH)3 HOH + Al(SO4)1+ 2- 3+ 1- 1+ 1- 3+ 2-

2 323 6

This neutralization can be used to determine the concentrations or molar masses of unknowns.If the right indicator is placed in an acid or a base it will turn colour at the instant of neutralization.

Standardizing a Base - (Determining its concentration)

When a solution of NaOH or KOH is prepared from its solid reagent it will react with water in the air as it is being massed. This means the mass of base measured is less than the recorded quantity.If a massed quantity of stable acid (one which doesn’t react with the atmosphere) is placed in a flask and dissolved in distilled water the concentration of the basic solution can be determined by measuring the volume of base needed to neutralize it.

Magnetic stirrer

Erlenmeyer flask

buret

0.10 - 0.30 g of potassiumhydrogen phthalatedistilled water few drops phenolphthalein

Titrant - NaOH solution

22.52 mL

18.31 mL4.21 mL

m = 0.21 gM = 204.22 g/mol

When the solution turns pink the number# of mol of acid = # of mol of baseequivalence pt.

HX + NaOH HOH + NaX1+ 1- 1+ 1- 1+ 1- 1+ 1-

Given - m HX = 0.21gM HX = 204.22 g/molV NaOH = 4.21 mLC NaOH = ?

Cb = 0.21 g/(204.22 g/mol )(0.00421 L)Cb = 0.24 mol/L

When the solution turns pinkn HX = n NaOHma/Ma = CbVb

Cb = ma / Ma / Vb

HomeworkPg 399 # 1-9Pg 401 # 1-3

2.00

1.00

0.00mL

0.80

0.70

0.78

0.76

0.74

0.72

Estimate - 0.77

2.00

1.00

0.00mL

Measurements with burets must have 2 decimal places. Don’t record 1.1 instead record1.10

The extra zero tells us the measuring instrument measures to the nearest tenth of a mL.The last digit of any measurement is an estimated value.

Measuring the Concentration of a Vinegar Solution

A measured volume of vinegar is placed in an Erlenmeyer Flask using a pipette.

The standardized base from the 1st part of the experiment is placed in the buret

Magnetic stirrer

Erlenmeyer flask

buret

Measured vinegar solutionHC2H3O2 few drops phenolphthalein

Titrant - NaOH solution

22.52 mL

18.31 mL4.21 mL

When the solution turns pink the number# of mol of acid = # of mol of baseequivalence pt.

Determining the Molar Mass of an Unknown Acid

Mass out from 0.10 - 0.30 g of the unknown acid in an Erlenmeyer flask.Dissolve it in some distilled water then add a few drops of phenolphthalein.Place the standardized base in the burette.Measure the volume of base needed to reach the equivalence point. (End point is signaled by the solution turning and remaining pink)

Magnetic stirrer

Erlenmeyer flask

buret

0.10 - 0.30 g of unknown aciddistilled water few drops phenolphthalein

Titrant - NaOH solution of known concentration

m = 0.21 gM = ?

When the solution turns pink the number# of mol of acid = # of mol of baseequivalence pt.