ACIDS AND BASES - College of DuPage · Acids turn litmus red . Common acids . Thinking about what...
Transcript of ACIDS AND BASES - College of DuPage · Acids turn litmus red . Common acids . Thinking about what...
ACIDS AND BASES
…for it cannot be
But I am pigeon-liver’d and lack gall
To make oppression bitter… Hamlet
Learning objectives
Describe properties of acids and bases
Define acid and base using Arrhenius and Brønsted definitions
Identify Brønsted acids and bases in solution equilibria
Distinguish between strength and concentration of acids and bases
Estimate pH of acids and bases
Describe phenomenon of acid rain
Thinking about what is an acid
Acids are sour
Acids attack metals
Acids react with
bases and form salts
Acids turn litmus red
Thinking about what makes a
base Bases taste bitter
Bases are slippery
Bases react with
acids to form salts
Bases turn litmus blue
Acids and bases cancel:
Neutralization Neutralization involves reaction of acid with base:
ACID + BASE = SALT + WATER
Example in nature: Reaction of carbonic acid (rainwater -
CO2 in H2O) with the ocean to give limestone:
H2CO3 + Ca(OH)2 = CaCO3 + 2H2O
Arrhenius: it’s about water
The meaning of acid and base has
changed over the years
Arrhenius acid is one that generates
protons when dissolved in water
Arrhenius base is one that generates
hydroxide ions when dissolved in water
Hydronium ion is active ingredient
of acid in water
Protons (H+) do not exist in solution
CH3CO2H + H2O = H3O+ + CH3CO2
-
Vinegar in water produces hydronium ions
Hydroxide ion is active ingredient of
base in water
NH3 + H2O = NH4+ + OH-
Ammonia, a base, dissolves in water and
produces hydroxide ions
The essence of neutralization
Elimination of the components of acid and
base by combination to give H2O
H+ + OH- H2O
ACID BASE
Brønsted and Lowry:
All about protons
Broader definition of acids and bases
Reaction NH3 + HCl = NH4Cl has all
elements of acid-base neutralization but
no H2O
Brønsted acid donates a proton
Brønsted base accepts a proton
Substances can be both acids and
bases – depends on environment
Note that in one instance H2O behaves
like a base – accepting protons, and in
another, behaves like an acid – donating
protons
HCl + H2O = H3O+ + Cl-
In presence of an acid H2O is a base
NH3 + H2O = NH4+ + OH-
In presence of a base H2O is an acid
Salts
Products of acid-base neutralization
Contain metal cation and nonmetal anion
Acid + base = salt + water
HCl + NaOH = NaCl + H2O
HCl + KOH = KCl + H2O
HNO3 + KOH = KNO3 + H2O
2HCl + Ca(OH)2 = CaCl2 + 2H2O
HCN + NaOH = NaCN + H2O
Strong coffee (or concentrated?)
Equilibrium: not all acids
completely donate protons
to water molecules
HA + H2O A- + H3O+
Strength: Degree of
ionization
Concentration: Number of
moles per unit volume
- +
2 3HA + H O = A + H O
Strong and weak
Strong acid (HCl) – Fully ionized
equilibrium to right
All H+ and Cl-
– Corrosive
Weak acid (Acetic) – Weakly ionized
Equilibrium to left
Mostly CH3COOH
– Edible
- +
2 3HA + H O = A + H O
- +
2 3HA + H O = A + H O
Changing concentration does not
change strength Strength refers to degree of ionization: – Strong is completely ionized (100 %)
– Weak is partly ionized (1 % - 1:106)
Concentration refers to number of moles per unit volume
An acid (or base) can be strong and concentrated, weak and concentrated, strong and dilute, weak and dilute
- +
2 3HA + H O = A + H O
Ionization of water
Even in pure water some molecules are
ionized
Concentrations of OH- and H3O+ are equal
[H3O+] = [OH-]
Concentration
- +
2 2 3H O + H O = OH + H O
Equilibrium constant: in all aqueous solutions,
product of concentrations is constant
[H3O+][OH-] = constant
Add acid or base alters balance of [H3O
+] and [OH-]
Increasing [H3O
+] decreases [OH-]
(acidic conditions)
Increasing [OH-] decreases [H3O+]
(basic conditions)
The pH scale – reduces large
range of numbers to small
In water [H3O+][OH-] = 10-14
pH = - log10[H3O+]
Range of [H3O+]:
– 10 M (conc acid) – 10-15 M (conc base)
Range of pH:
– -1 (conc acid) to +15 (conc base)
Low pH = acid; high pH = basic
pH = 7 = neutral
Relating pH to [H3O+]
For pH, take exponent of [H3O+], change sign
– Acid HCl(aq): [H3O+] = 1 x 10-1 M, pH = 1
– Pure H2O: [H3O+] = 1 x 10-7 M, pH = 7
– Base: NH3(aq): [H3O+] = 1 x 10-11 M, pH = 11
Note: change of 1 unit in pH is factor of ten in
[H3O+]
When [H3O+] is 1 x 10x, pH is whole number
Estimating pH
When [H3O+] is not 1 x 10x M pH is not
whole number
Estimating pH is often more useful than
doing exact calculations
Smaller pH value means larger H+
concentration
Estimating pH
Acidity and the environment
Rain is naturally weakly acidic because of CO2
Alkaline rocks – limestone – neutralize the acid
Acid Rain
Acid rain is polluted by acid
in the atmosphere. Two
common pollutants acidify
rain: sulphur dioxide (SO2)
and nitrogen oxides (NOX)
Following information from
The Green LaneTM,
Environment Canada's
World Wide Web site -
www.ec.gc.ca/regeng.html
What’s the big deal?
Damage to aquatic
life
Damage to buildings
Damage to forests
Damage to air quality
Source of the problem
Sulphur dioxide (SO2)
byproduct of industrial
processes and
burning fossil fuels.
– Ore smelting
– coal-fired power
generators
– natural gas processing
Where do NOX emissions come
from?
Main source of NOX is
combustion of fuels in
motor vehicles,
residential and
commercial furnaces,
industrial and electrical-
utility boilers and engines.
NOX emissions were 2.5
million tonnes in 2000.
U.S. NOX emissions for
2000 were 21 million
tonnes.
Legislative success with acid rain
Eastern Canada Acid Rain program committed Canada to cap SO2 emissions at 2.3 million tonnes by 1994 - 40% reduction from 1980 levels
Targets achieved or exceeded
By 2001, emissions were 63% reduction from 1980 levels.
Would acid rain remain a problem
without further controls?
Yes. That is why The Canada-Wide Acid Rain
Strategy for Post-2000 calls for further emission
reductions in both Canada and the United
States.
In total, without further controls, almost 800,000
km2 in southeastern Canada-an area the size of
France and the United Kingdom combined-
would receive harmful levels of acid rain; that is,
levels well above critical load limits for aquatic
systems.