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Acids and Bases. Acids Bases. Sour Bitter Also referred to as alkali H 2 O solutions feel slippery. Acid – Base Theories. Arrhenius Definition acid -- donates a hydrogen ion (H + ) in water base --donates a hydroxide ion in water (OH - )

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acids bases
AcidsBases

Sour Bitter

Also referred to as alkali

H2O solutions feel slippery

acid base theories
Acid – Base Theories

Arrhenius Definition

acid--donates a hydrogen ion (H+) in water

base--donates a hydroxide ion in water (OH-)

This theory was limited to

substances with those "parts";

ammonia is a MAJOR exception!

slide4
Bronsted-Lowry Definition

acid--donates a proton in water

base--accepts a proton in water

This theory is better; it explains

ammonia as a base! This is the main

theory that we will use for our

acid/base discussion.

slide5
Lewis Definition

acid--accepts an electron pair

base--donates an electron pair

This theory explains alltraditional

acids and bases + a host of

coordination compounds and is used

widely in organic chemistry. Uses

coordinate covalent bonds

conjugate acid base concept
Conjugate acid-base concept
  • Acid becomes the conjugate base
  • Base becomes the conjugate acid
acids donate a proton h
Acidsdonate a proton (H+)

HNO3 + H2O  H3O+ + NO3-

acid base CA CB

NH4+ + H2O  H3O+ + NH3

acid base CA CB

H2PO4- + H20  H3O+ + HPO42-

acid base CA CB

slide8
In each of the acid examples---notice

the formation of H3O+

This species is named the hydronium

ion.

It lets you know that the solution is

acidic!

bases accept a h
Basesaccept a H+

NH3 + H2O  NH4+ + OH-

base acid CA CB

CO32- + H2O  HCO3- + OH-

base acid CA CB

PO43- + H2O  HPO42- + OH-

base acid CA CB

slide10
In each of the basic examples--

notice the formation of OH- -- this

species is named the hydroxide

ion. It lets you know that the

solution is basic!

identify the acid base conjugate acid and conjugate base
Identify the acid, base, conjugate acid and conjugate base.

HBr + NH3 NH4+ + Br-

What is the conjugate base of H2S?

What is the conjugate acid of NO3-?

acids only donate one h at a time
Acids only donate one H+ at a time
  • monoprotic--acids donating one H+ (ex. HC2H3O2)
  • diprotic--acids donating two H+'s (ex. H2C2O4)
  • polyprotic--acids donating many H+'s (ex. H3PO4)
polyprotic bases
Polyprotic Bases

accept more than one H+

anions with -2 and -3 charges

(example: PO43- ; HPO42-)

amphiprotic or amphoteric
Amphiprotic or Amphoteric

molecules or ions that can behave as

EITHER acids or bases:

water, anions of weak acids

(look at the examples above—sometimes

water was an acid, sometimes it acted as

a base)

relative strengths of acids and bases
Relative Strengths of Acids and Bases

Strength is determined by the position of the "dissociation" equilibrium.

Do Not

confuse concentration

with strength!

strong acids strong bases
Strong acids/Strong bases

dissociate completely in water

have very large K values

weak acids weak bases
Weak acids/Weak bases

dissociate only to a slight extent in

water

dissociation constant, Ka, is very small

slide18

Strong

Weak

strong acids
Strong Acids

Hydrohalic acids:

HCl, HBr, HI (except HF)

Nitric: HNO3

Sulfuric: H2SO4

Perchloric: HClO4

strong bases
Strong Bases

Hydroxides OR oxides of IA and

IIA metals

Solubility plays a role (those that

are very soluble are strong!)

slide23
The equilibrium expression for acids

is known as the Ka (the acid

dissociation constant).

It is set up the same way as in

general equilibrium.

for weak acid reactions
For Weak Acid Reactions:

HA + H2O  H3O+ + A-

Ka = [H3O+][A-] Ka < 1

[HA]

for weak base reactions
For Weak Base Reactions:

B + H2O  HB+ + OH-

Kb = [H3O+][OH-] Kb < 1

[B]

slide26
Notice that Ka and Kb expressions

look very similar.

The difference is that a base

produces the hydroxide ion in

solution, while the acid produces the

hydronium ion in solution.

another note on this point
Another note on this point:

H+ and H3O+ are both equivalent

terms here. Often water is left

completely out of the equation since

it does not appear in the equilibrium.

This has become an accepted

practice.

(*However, water is very important

in causing the acid to dissociate.)

slide28
H2O(l) + H2O(l) <=> H3O+(aq) + OH-(aq)

Keq[H2O]2 = Kw = [H3O+][OH-]

Kw = 1.0 x 10-14

Kw = Ka x Kb (Another useful equation)

Knowing this value allows us to

calculate the OH- and H+

concentration for various situations.

slide29
[OH-] = [H+] : solution is neutral (in

pure water, each of these is 1.0 x 10-7)

[OH-] > [H+] : solution is basic

[OH-] < [H+] : solution is acidic

k w h oh 1 0 x 10 14 m
Kw = [H+] [OH-] = 1.0 X 10-14 M

Calculate [H+] or [OH-] as required for

each of the following solutions at

25°C, and state whether the solution

is neutral, acidic, or basic.

a. 1.0 X 10-5M OH-

b. 1.0 X 10-7M OH-

c. 10.0 M H+

slide32
Soluble salts dissociate in water to produce ions. Salts are basically ionic compounds that can be formed from the reaction from an acid and a base.
  • Not all aqueous salt solutions are neutral; their pH depends on how the cation and anion interact with water. This interaction is called hydrolysis.
neutral salts
Neutral salts

Salts from strong acids and strong bases are

NEUTRAL

Example:

NaCl from NaOH and HCl

LiCl from LiOH and HCl

basic salts
Basic salts

Cations from strong base and anions from weak acid have a strong conjugate base. This interacts with water by gaining a proton and generating hydroxide ions.

A- + H2O ↔ HA + OH-

Salts from STONG bases and WEAK acids are

BASIC

Example:

NaC2H3O2 from NaOH and HC2H3O2

C2H3O2- + H3O+↔ OH- + HC2H3O2

acidic salts
Acidic Salts

Cations from WEAK base and anions from STRONG acid have a strong conjugate acid. This interacts with water by gaining a proton and generating hydronium ions.

HB+ + H2O ↔ B + H3O+

Salts from WEAK base and STRONG acid are ACIDIC

Example:

NH4Cl from NH4OH and HCl

NH4+ + H2O ↔ NH3 + OH-

slide36
Cations from weak base and anions from weak acid have both strong conjugates. Since both can interact with water, the resulting pH depends upon the respective strength of the conjugate acid or base.

If Ka > Kb then salt is acid

If Ka < Kb then salt is base

If Ka =Kb then salt is neutral

would the following salts be acidic basic or neutral
Would the following salts be acidic, basic or neutral?

NaNO3 NaNO2

C5H5NHClO4 NH4NO2

KOCl KCl

CH3NH3Cl KF

NH4F CH3NH3CN