acids and bases part i - ricksobers.com

Acids and Bases Part IDr. Sobers’ Lecture Notes

Robert Boyle

Robert Boyle 1661Acid Properties:

Sour taste Corrosive

Cause change in color in some vegetable dyes.

Lose acid properties when combined with bases (alkalies)

(makes blue litmus red)

Robert Boyle 1661Base Properties:

Feel slippery

Cause change in color in some vegetable dyes.

Lose acid properties when combined with acids

(makes red litmus blue)

Another property is that they taste bitter

Arrhenius Definition of Acids and Bases

Arrhenius DefinitionsSvante Arrhenius

Arrhenius acids are substances that ionizes in water to give H+ (H3O+) ions.

1884 Definitions

Arrhenius bases are substances that produce hydroxide ions in water.

H2SO4 + H2O → H3O+ + HSO4-

NaOH → Na+ + OH- H2O


Ammonia is often described as forming hydroxide in solution and so fits this definition:

NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH-(aq)

At the time ammonia was thought to form this in solution but the equilibrium actually favors the reactants.


This theory doesn’t explain acid-base reactions that are not in aqueous solution:

NH3(g) + HCl(g) ⇄ NH4Cl(s)

Brønsted-Lowry Definitions

Brønsted-Lowry DefinitionsJohannes Nicolaus Brønsted

Thomas Martin Lowry

Independently derived theory in 1923

Brønsted-Lowry acids are any species that donate a proton, H+.

Brønsted-Lowry bases are any species that accept a proton, H+.

The definition is not limited to aqueous solutions.


NH3(g) + HCl(g) ⇄ NH4Cl(s)

Ammonia accepts a proton (a base)

Hydrogen chloride donates a proton (an acid)

Conjugate Acids and BasesConjugate acid: when a base accepts a proton, it becomes an acid capable of returning that proton.

Conjugate base: when an acid donates its proton, it becomes capable of accepting that proton back.

Provide the conjugate acid or base of each formula:

Conjugate Acid Conjugate Base

HF

H2O

HCO3-

F-

OH-

CO32-

H2OH3O+

NH3 NH2-

NH3NH4+

CH3COOH CH3COO-

They differ by H+

Review: Behavior of Acids and Bases in Water

Strong and Weak Acids and Bases in Water

HCl(aq) + H2O(l) → H3O+ (aq) + Cl- Strong acids ionize in water 100%:

HNO2(aq) + H2O(l) ⇄ H3O+ (aq) + NO2-(aq) Weak acids ionize in water less than 100%:

An equilibrium exists. The conjugate base is a weak base.

The conjugate base is very weak; a negligible base.An acid considered strong, may not be in non-aqueous solvents.


NaOH(s) → Na+ (aq) + OH- (aq)Strong bases dissociate in water:

Strong bases are not Brønsted-Lowry bases but the hydroxide ion produced is. (It can accept a proton)

Dissociation is used to describe this process. The pure sodium hydroxide is ionic already.

Ionization described a reaction that produces new ions. Pure HCl is a molecular gas for instance. But it produces ions in water.


NaOH(s) → Na+ (aq) + OH- (aq)Strong bases dissociate in water:

NH3(aq) + H2O(l) ⇄ OH- (aq) + NH4+(aq) Weak bases ionize in water less than 100%:

An equilibrium exists. The conjugate acid is a weak acid.

Negligible bases do not react. The conjugate acid is very strong.

Cl-(aq) + H2O(l) → no reaction

The Auto-ionization of Water and the pH Scale

Amphiprotic/AmphotericAmphoteric - it may act as an acid or a base.

Amphiprotic - it may accept or donate a proton, H+.

Water and ammonia are examples.

Other examples: hydrogen sulfate ion, hydrogen carbonate ion

The Water Ion ProductWater auto-ionization: 2H2O ⇄ H3O+ + OH-

Equilibrium expression:

Kw ≈ [H3O+][OH-]

Kw = 1x10-14

Neutral water: [H3O+] = [OH-] = 1x10-7M

Kw = [H3O+][OH-]/[H2O]

The water ion product:

The Water Ion ProductExample: Calculate the concentration of hydronium and hydroxide ions in a 0.100M HCl solution.

HCl(aq) + H2O(l) → H3O+ (aq) + Cl- HCl is a strong acid. It ionizes 100%

The hydronium ion is the concentration of HCl itself:

[H3O+] = 0.100MUse the water-ion product equation to get [OH-]

1x10-14 = [H3O+][OH-]

The Water Ion ProductExample: Calculate the concentration of hydronium and hydroxide ions in a 0.100M HCl solution.

Use the water-ion product equation to get [OH-]

1x10-14 = [H3O+][OH-]

1x10-14 = (0.100)[OH-]

[OH-] = 1x10-13 M

The Water Ion ProductFor strong acids and bases (assuming no solubility issues), the concentration of hydronium and hydroxide are easy to find.

1x10-14 = [H3O+][OH-]

The hydronium ion concentration in a 1.000M HBr solution is 1.000M.

The hydroxide ion concentration in a 1.000M NaOH solution is 1.000M.If the hydronium ion or hydroxide ion concentration is known, then they are both known:

For weak acids and bases, it is not so simple.

pH and pOH

The function “-log( )” is the called the “p” function.

pH = -log([H3O+])

pOH = -log([OH-])

The pH is the cologarithm of the hydronium ion concentration:

The pOH is defined the same way:

How are pH and pOH mathematically calculated from one another?

pH and pOH

Apply “-log( )” to both sides:Kw = [H3O+][OH-]

-log(Kw) = -log([H3O+][OH-])

-log(1x10-14) = -log([H3O+]) + -log([OH-])

14 = pH + pOH

(pH) (pOH)(14)

Kw = [H3O+][OH-]

14 = pH + pOH

pH =

-log

([H3O

+ ])

pOH

= -l

og([O

H- ])

[H3O+] = 10-pH[OH-] = 10-pOH

[H3O+] [OH-]

pH pOH

The pH Scale

Acidic Solutions: < 7 > 1x10-7M

pH pOH[H3O+] [OH-]

> 7 < 1x10-7M

Basic Solutions: > 7 < 1x10-7M < 7 > 1x10-7M

7 1x10-7M 7 1x10-7MNeutral Solutions:

4 1x10-4M

pH pOH[H3O+] [OH-]

101x10-10M

7 1x10-7M 7 1x10-7M

-1 10M 151x10-15M

11 1x10-11M 31x10-3M

3.2x10-5M9.5 3.1x10-10M 4.5

0 1M 141x10-14M

Given the pH, pOH, hydronium or hydroxide concentration, calculate the rest for each solution.

acids and bases part i - ricksobers.com

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