Acid-Base Chemistry
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Acid-Base Chemistry Arrhenius acid: Substance that dissolves in water and provides H + ions Arrhenius base: Substance that dissolves in water and provides OH - ions Examples: HCl  H + and Cl - Acid NaOH  Na + + OH - Base.

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Acid-Base Chemistry Arrhenius acid: Substance that dissolves in water and provides H + ions

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Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Acid-Base Chemistry

Arrhenius acid: Substance that dissolves in water and provides H+ ions

Arrhenius base: Substance that dissolves in water and provides OH- ions

Examples: HCl  H+ and Cl-Acid

NaOH  Na+ + OH- Base


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Bronsted Acid: Substance that donates proton to another substance

Bronsted base: Substance that accepts proton from another substance

Example: HCl + H2O  H3O+ + Cl-

HCl acts as acid; H2O acts as base

In the Reverse Reaction,

H3O+ acts as an acid; Cl- acts as a base

Note: (H3O+ = hydronium ion = H+ = proton)


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • A monoprotic acid contains one acidic proton.

HCl

  • A diprotic acid contains two acidic protons.

H2SO4

  • A triprotic acid contains three acidic protons.

H3PO4

  • A Brønsted–Lowry acid may be neutral or it may

  • carry a net positive or negative charge.

HCl, H3O+, HSO4−


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • Conjugate acid: Species formed after base accepts a proton

  • Conjugate base: Species remaining after an acid donates its proton

  • Conjugate acid-base pair: an acid and base on opposite sides of the equation that correspond to each other

  • Examples: HNO3 + H2O H3O+ + NO3-

  • acid base acid base

    • Conjugate pairs: HNO3 and NO3-

  • H2O and H3O+


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Example: HS- + H2O  H3O+ + S2-

Conjugate pairs: HS- and S2-

H2O and H3O+

Practice: HClO4 + H2O  H3O+ + ClO4-

What are the conjugate pairs?

HClO4 and ClO4-

H2O and H3O+


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

gain of H+

H2O

H

Br

+

Br−

H3O+

+

acid

acid

base

base

loss of H+

  • HBr and Br− are a conjugate acid–base pair.

  • H2O and H3O+ are a conjugate acid–base pair.

Note: The net charge must be the same on both sides

of the equation.


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Water can act as both an acid and a base (amphiprotic)!

HClO4 + H2O  H3O+ + ClO4 (base)

NH3 + H2O OH- + NH4+ (acid)

Strengths of Acids and Bases

Strong acids/bases: dissociate completely when dissolved in solution

Weak acids/bases: dissociate only partially when dissolved in solution


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Examples:

Strong Acid: HCl  H+ + Cl- (100% dissociation)

Strong Base: NaOH Na+ + OH (100% dissociation)

Weak Acid: CH3COOH  H+ + CH3COO- (1.3% dissociation)

Weak Base: NH3 + H+ NH4+


Table 9 1

Table 9.1


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Naming Acids

Binary Acids: hydo + root of anion + ic + “acid”

ex. HCl hydrochloric acid, HBr hydrobromic acid

HI

Polyatomic-based Acids: root of polyatomic ion + ic + “acid”

ex. H2SO4 sulfuric acid, H3PO4 phosphoric acid

H2CO3

HNO3

Hydroiodic acid

carbonic acid

nitric acid


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

The Self-Ionization of Water

H2O + H2O  H3O+ + OH-

Pure water: [H3O+]=[OH- = 10-7 M (at 250C)

Neutral Solution: Any solution in which the concentrations of H3O+ and OH- ions are equal (10-7 M)

Acidic Solution: Solutions having a greater concentration ofH3O+ thanOH- ions ([H3O+] greater than 10-7 M)

Example: A solution with [H3O+] = 10-5 M

Basic Solution: solution having a greater concentration of OH- thanH3O+ions ([H3O+] less than 10-7 M)

Example: A solution with [H3O+] = 10-12 M


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • The pH Scale

    • pH is a measure of acidity

    • Scale ranges from 0-14

  • pH = 7Neutral

  • pH < 7 Acidic

  • pH > 7 Basic

  • pH represents the concentration of H+ ions in solution

  • Pure water: 1 x 10-7 moles H+ per liter and1 x 10-7 moles OH- per liter


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Solutions with equal concentrations of and ions are called Neutral

Solutions with more than 1 x 10-7 moles H+ per liter are Acidic

Solutions with less than 1 x 10-7 moles H+ per liter are Basic

Note: [H+] x [OH-] = 10-14 (always!)


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • pH Scale Summary

    • pH scale refers to amount of H+ ions in solution

    • pH 7 is neutral, less than 7 is acidic, greater than 7 is basic

    • Lower pH = more acidic = more H+ ions

    • Higher pH = more basic = less H+ ions


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Each pH unit represents a 10-fold change in H+ ion concentration!

pH 4 has 10 times more H+ ions than pH 5

pH 9 has 10 times fewer H+ ions than pH 8

Mathematical equation for pH:

pH is the negative log of the H3O+ concentration

pH = -log [H3O+]


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

pH = -log [H3O+]

Any number can be expressed as 10 raised to some exponent: y = 10x

Examples: 100 = 10 2

1000 = 103

0.10 = 10 -1

The log is that exponent!

100 = 10 2; Log of 100 =2

1000 = 103; Log of 1000 = 3

0.10 = 10 –1; Log of 0.10 = -1


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • We can also take the log of non-whole numbers, but we use our calculators for this.

  • Example: Find the log of 2.4 x 10-3

  • Enter 2.4 x 10-3into calculator

  • Press the “log” key

  • 0.0024 “log” = -2.62

  • Therefore, 10-2.62 = 2.4 x 10-3


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • Calculating pH from [H3O+]

  • pH = -log [H3O+]

    • Enter [H3O+] into calculator

    • Press the “log” key

    • Change the sign

  • Example: [H3O+] = 1.0 x 10-7 M

  • pH = -log [H3O+]

  • pH = -log [1 x 10-7] = 7


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • Example: [H3O+] = 1 x 10-11M

  • pH = -log [H3O+]

    • pH = -log [1 x 10-11] = 11

  • Example: [H3O+] = 1 x 10-3 M

  • pH = -log [H3O+]

  • pH = -log [1 x 10-3] = 3


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • Example: [H3O+] = 4.2 x 10-5

  • pH = -log [H3O+]

    • Enter [H3O+] into calculator (4.2 x 10-5)

    • Press the “log” key (-4.3767507)

    • Change the sign (4.3767507)

      • pH = 4.3767507 = 4.4


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • Example: [H3O+] = 8.1 x 10-9

  • pH = -log [H3O+]

    • Enter [H3O+]into calculator (8.1 x 10-9)

    • Press the “log” key (-8.091515)

    • Change the sign (8.091515)

      • pH = 8.091515= 8.1


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Reactions Between Acids and Bases

Neutralization: reaction between an acid and a base; always produces salt and water

Example: Write a balanced equation for the reaction of hydrochloric acid with sodium hydroxide.

HCl(aq) + NaOH(aq)

H—OH(l) + NaCl(aq)


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Example: Write a balanced equation for the reaction of hydrochloric acid with magnesium hydroxide.

2

HCl + Mg(OH)2

2

H2O +

MgCl

2


Titration

Titration

Titration:a technique used to determine the concentration of an acid

or base in a solution

  • If we want to know the concentration of an acid

  • solution, a base of known concentration is

  • added slowly until the acid is neutralized.

  • When the acid is neutralized:

# of moles of acid = # of moles of base

  • This is called the end point of the titration.


Acid base titration

Acid-Base Titration

  • Titration is a laboratory procedure used to determine the molarity of an acid.

  • In a titration, a base such as NaOH is added to a specific volume of an acid.

Base (NaOH)

Acid

solution


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • A few drops of an indicator is added to the acid in the flask.

  • The indicator changes color when the base (NaOH) has neutralized the acid.


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • At the end point, the indicator has a permanent color.

  • The volume of the base used to reach the end point is measured.

  • The molarity of the acid is calculated using the neutralization equation for the reaction.


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Determining an unknown molarity from titration data

requires three operations:

mole–mole

conversion

factor

[2]

Moles of

base

Moles of

acid

M (mol/L)

conversion

factor

M (mol/L)

conversion

factor

[3]

[1]

Volume of

base solution

Molarity of

acid solution


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

HOW TO Determine the Molarity of an Acid Solution

from Titration

Example: What is the molarity of an HCl solution if 22.5 mL of a 0.100 M NaOH solution are needed to titrate a 25.0 mL sample of the acid?

volume of base (NaOH)

22.5 mL

volume of acid (HCl)

25.0 mL

conc. of acid (HCl)

?

conc. of base (NaOH)

0.100 M


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Determine the number of moles of base

used to neutralize the acid.

Step [1]

M (mol/L)

conversion factor

Volume of

base solution

1 L

1000 mL

0.100 mol NaOH

1 L

x

x

=

22.5 mL NaOH

mL–L

conversion factor

0.00225 mol NaOH


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Determine the number of moles of acid that react from the balanced chemical equation.

Step [2]

HCl(aq) + NaOH(aq)

H2O(l) + NaCl(aq)

1 mol HCl

1 mol NaOH

x

0.00225 mol NaOH

0.00225 mol HCl

=

mole–mole

conversion

factor


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

Determine the molarity of the acid from the number of moles and the known volume.

Step [3]

mol

L

1000 mL

1 L

0.00225 mol HCl

25.0 mL solution

M

=

=

x

mL–L

conversion factor

=

0.0900 M HCl

Answer


Buffers

Buffers

Buffer: a solution whose pH changes very little when acid or base is added.

Most buffers are solutions composed of roughly

equal amounts of

  • a weak acid

  • the salt of its conjugate base

The buffer resists change in pH because

  • added base, −OH, reacts with the weak acid

  • added acid, H3O+, reacts with the conjugate base


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

  • Buffers contain 2 compounds:

    • Compound with the ability to react with H+ ions

    • Compound with the ability to react with OH- ions

  • Example: HCO3- + H+ H2CO3

  • If acids (H+) are added, react with HCO3-

  • H2CO3 + OH-  HCO3- + H2O

  • If OH- ions are added, react with H2CO3

  • H2CO3 is unstable: H2CO3 H2O + CO2


More examples of a buffer

More Examples of a Buffer

If an acid is added to the following buffer equilibrium,

Adding more

product…

CH3COOH + H2O

H3O+ + CH3COO−

conjugate

base

weak acid

…drives the reaction to the left.

then the excess acid reacts with the conjugate base,

so the overall pH does not change much.


Acid base chemistry arrhenius acid substance that dissolves in water and provides h ions

If a base is added to the following buffer equilibrium,

Adding more

reactant…

H2O + CH3COO−

CH3COOH + −OH

conjugate

base

weak acid

…drives the reaction to the right.

then the excess base reacts with the weak acid, so

the overall pH does not change much.


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