Acids & Bases

Chemistry.Ms. Siddall.

Standard 5a: Properties

Properties Acid Base

Taste Sour (lemon) Bitter (soap)

Touch Like water Like soap

Reaction with metal

Vigorous No reaction

Conductivity

Conducts electricity

Litmus Paper

Blue red Red blue

Neutralization

Acid + base salt + watere.x. HCl + NaOH NaCl +

H2O

e.x. HCl + Mg H2(g) + MgCl2

Summary 1

1. Write the balanced equation for the reaction of aluminum with HCl to form hydrogen and aluminum chloride.

2. A substance turns red litmus blue and does not react with metal. What is it?

Standard 5b: Brønsted-Lowry Definition

• Acids donate hydrogen ions(Hydrogen ion = H+ = proton)

e.x. HBr + H2O H3O+ + Br-

• Bases accept hydrogen ionse.x. NH3 + H2O NH4

+ + OH-

Summary 2

• Write the balanced equation for the reaction of HF (hydrofluoric acid) with water.

WeakAcidEquilibrium.exe

Strong acid ionization

weak acid ionization

Standard 5c: Dissociation

• Acids:• Strong acid = strong electrolyte• Strong electrolyte = 100% dissociation

e.x. HNO3 + H2O H3O+ + NO3-

• in solution(H3O+ = hydronium ion)

• You MUST know these strong acids: HCl, HBr, HI, HNO3, H2SO4

0% 100%

Summary 3

• Which of the following are strong acids? H2SO4, HSO3, HClO2, HBr, HNO3, HNO2

Standard 5c: continued

• Weak acids = weak electrolytes• Weak electrolyte = Partial dissociation

(= equilibrium)

e.x. HCN + H2O H3O+ + CN-

• In solution• All other acids are weak acids

• Examples: acetic acid HC2H3O2, carbonic acid H2CO3, HCN, H3PO4

~97%

~3%

Summary 4

• At equilibrium does a weak acid solution contain mostly products or mostly reactants?

• At equilibrium does a strong acid solution contain mostly products or mostly reactants?

Standard 5c: continued

Conjugate acids & bases:e.x. HF + H2O H3O+ + F-

• acid– Donates hydrogen ion

• Conjugate base– Different from acid by a proton

• Base– Accepts hydrogen ion

• Conjugate acid – Different from base by a proton

A CBB CA

Summary 5

• Label the acid (A), base (B), conjugate acid (ca), and conjugate base (cb) in the following reaction:

HNO3 + H2O H3O+ + NO3-

Conjugate acids & base pairs:e.x. HF + H2O H3O+ + F-

• Acid and Conjugate base pair = HF & F-

• A strong acid has a weak conjugate base• A weak acid has a strong conjugate base

• Base and conjugate acid pair = H2O & H3O+

• A strong base has a weak conjugate acid• A weak base has a strong conjugate acid

Summary 6

HF + H2O H3O+ + F-

1. Identify the acid and conjugate base. Label each as ‘weak’ or ‘strong’

2. Identify the base and conjugate acid. Label each as ‘weak’ or ‘strong’

Standard 5d: pH scale

Definition:pH = -log[H3O+]

• pH measures the concentration (amount) of H3O+ ions in solution

Summary 7

1. Write the equation for HCl reacting with water to make a hydronium ion and a chloride ion

2. Is the concentration of hydronium ions in this solution high or low?

acidic

neutral

basic

pH0pH1pH2pH3pH4pH5pH6pH7pH8pH9pH10pH11pH12pH13pH14

Summary 8

Provide an example of:1. An acidic compound2. A basic compound3. A neutral compound

Add a minimum of 3 acids and 3 bases to your pH scale

Self ionization of pure waterH2O + H2O H3O+ + OH-

= Equilibrium system• For pure water: [H3O+] = [OH-] = 1x10-7M

• pH = -log[H3O+] = 7

• pH 7 = neutral• [H3O+] > [OH-] = pH < 7= acidic solution

• [H3O+] < [OH-] = pH > 7 = basic solution

Summary 9

Identify the following solutions as acidic, basic, or neutral:

1. pH 32. pH 53. pH 104. pH 7

Acids ‘produce’ H3O+ in solution

e.x. HCl + H2O H3O+ + Cl-

• pH measurement:[H3O+] = 0.1M = 1x10-1M

pH = -log [H3O+] = -log [1x10-1]

pH = 1

• High [H3O+] = ‘Low’ pH reading

= acidic solution

Summary 10

1. pH measures the concentration of ___________________ ions in solution.

2. A solution with pH = 2:a) has a high / low concentration of

these ionsb) is acidic / basic

Bases ‘decrease’ H3O+ in solution

e.x. NaOH + H3O+ Na+ + 2H2O

• pH measurement:[H3O+] = 1x10-14M

pH = -log [H3O+] = -log[1x10-14]

pH = 14

• Low [H3O+] = ‘High’ pH reading

= basic solution (Alkaline)

Summary 11

1. What is the hydronium ion concentration for a solution with pH = 12?

2. Is the solution acidic or basic?

5f: calculate pH

pH = -log[H3O+]

Example: [H3O+] = 4.5 x10-5M

• [H3O+] > 1x10-5M

• pH should be between 4 - 5• pH = -log[4.5x10-5] = 4.35

Summary 13

• Estimate then calculate the pH for a solution with:

•[H3O+] = 7 x 10-2 M

•[H3O+] = 2.2 x 10-9 M

pOH = -log[OH-] pOH + pH = 14

Example:• [OH-] = 1x10-2M• pOH = -log[1x10-2] = 2• pH = 14 - 2 = 12

Relationship between [OH-] and [H3O+]

Summary 14

Complete the following table:

[H3O+] [OH-] pH pOH

10-5M

10-5M

1

5e: acid/base definitions

Arrhenius: • acids are hydrogen containing

compounds that ionize to yield H+ ions in aqueous solution.

e.x. HCl(aq) H+(aq) + Cl-(aq)

• Bases ionize to yield OH- ions in aqueous solutions.

e.x. NaOH(aq) Na+(aq) + OH-

(aq)

• Problems: NH3 is not a base according to Arrhenius

Summary 15

• Explain why NH3 is not a base according to the Arrhenius definition of a base.

BrØnsted-Lowry:• Acid = hydrogen donor• Base = hydrogen acceptor

e.x. NH3(aq) +H2O NH4+ + OH-

Lewis:• An acid accepts a pair of electrons

(accepts a negative charge)• Acid = proton donor = electron acceptor• A base donates a pair of electrons• Base = proton acceptor = electron

donoro e.x. HCl + H2O H3O+ + Cl-

Electron acceptor

Electron donor

Summary 16

• Explain the difference between the Bronsted-Lowry definition and the Lewis definition of acids and bases

5g: buffers

Buffer = a solution whose pH does not change (much) when acid or base are added.

• A buffer is a solution made from an acid and its conjugate base

• Example: human blood must have a pH between 7.35 – 7.45 (or we will die)

• There are 2 buffer systems in human blood: H2PO4

-/HPO42- & H2CO3/HCO3

-

B- + H2PO4- HPO4

2- + HB

B- + H2CO3 HCO3- + HB

HB = acid (H+ donor)B- = base (H+ acceptor)

Summary 17

•Write a general equation showing the buffering ability of HSO4

- and its conjugate base. (use B- as the base that reacts with HSO4

-)