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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 Æ H3O+ + Cl-
Arrhenius base: Substance that dissolves in water and provides OH- ions Examples:
HCl Æ H+ and Cl-
Acid
HCl acts as acid; H2O acts as base In the Reverse Reaction, H3O+ acts as an acid; Cl- acts as a base
NaOH Æ Na+ + OH- Base Note:
Conjugate acid: Species formed after base accepts a proton Conjugate base: Species remaining after an acid donates its proton
(H3O+ = hydronium ion = H+ = proton)
Example: HS- + H2O Æ H3O+ + S2Conjugate pairs:
HS- and S2H2O and H3O+
Conjugate acid-base pair: an acid and base on opposite sides of the equation that correspond to each other
Weak acids/bases: dissociate only partially when dissolved in solution
Weak Base: NH3 + H+ Æ NH4+
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Conjugate base
Naming Acids Binary Acids: hydo + root of anion + ic + “acid” ex. HCl hydrochloric acid, HBr hydrobromic acid HI
Hydroiodic acid
Polyatomic-based Acids: root of polyatomic ion + ic + “acid” ex. H2SO4 sulfuric acid, H3PO4 phosphoric acid H2CO3 carbonic acid HNO3
nitric acid
The Self-Ionization of Water
The pH Scale
H2O + H2O Æ H3O+ + OHPure 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 of H3O+ than OH- ions ([H3O+] greater than 10-7 M)
•pH is a measure of acidity •Scale ranges from 0-14 pH = 7 Neutral pH < 7 Acidic pH > 7 Basic
Example: A solution with [H3O+] = 10-5 M
pH represents the concentration of H+ ions in solution
Basic Solution: solution having a greater concentration of OH- than H3O+ions ([H3O+] less than 10-7 M)
Pure water: 1 x 10-7 moles H+ per liter and1 x 10-7 moles OH- per liter
Example: A solution with [H3O+] = 10-12 M
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pH Scale Summary
Solutions with equal concentrations of and ions are called Neutral
•pH scale refers to amount of H+ ions in solution Solutions with more than 1 x 10-7 moles H+ per liter are Acidic •pH 7 is neutral, less than 7 is acidic, greater than 7 is basic Solutions with less than 1 x 10-7 moles H+ per liter are Basic •Lower pH = more acidic = more H+ ions Note: [H+] x [OH-] = 10-14 (always!)
•Higher pH = more basic = less H+ ions
pH = -log [H3O+]
Each pH unit represents a 10-fold change in H+ ion concentration!
Any number can be expressed as 10 raised to some exponent: y = 10x pH 4 has 10 times more H+ ions than pH 5 Examples: pH 9 has 10 times fewer H+ ions than pH 8
100 = 10 2 1000 = 103 0.10 = 10 -1
Mathematical equation for pH: pH is the negative log of the H3O+ concentration
The log is that exponent! 100 = 10 2; Log of 100 =2
pH = -log [H3O+]
1000 = 103; Log of 1000 = 3 0.10 = 10 –1; Log of 0.10 = -1
Calculating pH from [H3O+] We can also take the log of non-whole numbers, but we use our calculators for this.
pH = -log [H3O+]
Example: Find the log of 2.4 x 10-3
•Enter [H3O+] into calculator
•Enter 2.4 x 10-3into calculator
•Press the “log” key
•Press the “log” key
•Change the sign
0.0024 “log” = -2.62 Therefore, 10-2.62 = 2.4 x 10-3
Example:
[H3O+] = 1.0 x 10-7 M pH = -log [H3O+] pH = -log [1 x 10-7] = 7
3
[H3O+] = 1 x 10-11M
Example:
pH = -log [H3O+] pH = -log [1 x 10-11] = 11
[H3O+] = 1 x 10-3 M
Example:
pH = -log [H3O+] pH = -log [1 x 10-3] = 3
Example:
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
[H3O+] = 8.1 x 10-9 Reactions Between Acids and Bases pH = -log [H3O+]
•Enter [H3O+] into calculator (8.1 x 10-9) •Press the “log” key (-8.091515)
Neutralization: reaction between an acid and a base; always produces salt and water Titration: Process by which acid or base of known concentration is used to neutralize a solution of unknown concentration, to determine its concentration
•Change the sign (8.091515) pH = 8.091515 = 8.1
Buffers contain 2 compounds:
Buffer: Solution that resists changing pH when acids or bases are added; solution that maintains constant pH
Acid-Base Titration
•Compound with the ability to react with H+ ions •Compound with the ability to react with OH- ions Example: HCO3- + H+ Æ H2CO3 If acids (H+) are added, react with HCO3H2CO3 + OH- Æ HCO3- + H2O If OH- ions are added, react with H2CO3 H2CO3 is unstable: H2CO3 Æ H2O + CO2
• Titration is a Base laboratory (NaOH) procedure used to determine the molarity of an acid. • In a titration, a base such as NaOH is added to Acid a specific volume solution of an acid.
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Indicator • 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.
End Point of Titration • 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.
