Acids and bases revision

8/7/2019 Acids and bases revision

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1. General Definitions:

yAcid: a substance which when added to water produces hydrogen ions[H

+].

y

Base: a substance which when added to water produces hydroxide ions[OH-].

2. Properties:

Acids:

y react with zinc, magnesium, or aluminum and form hydrogen (H2(g))

y react with compounds containing CO32- and form carbon dioxide and water

y turn litmus red

y taste sour (lemons contain citric acid, for example) DO NOT TASTEACIDS IN THE LABORATORY!!

Bases:

y feel soapy or slipperyy turn litmus bluey they react with most cations to precipitate hydroxidesy taste bitter (ever get soap in your mouth?) DO NOT TASTE BASES IN

THE LABORATORY!!

3. Water dissociation: H2O(l) H+(aq) + OH

-(aq)

equilibrium constant, KW = [H+][OH-] / [H2O]

Note: water is not involved in the equilibrium expression because it is a pure liquid,also, the amount of water not dissociated is so large compared to that dissociated that

we consider it a constant

Value for Kw = [H+][OH

-] = 1.0 x 10

-14

Note: The reverse reaction, H+

(aq) + OH-(aq) H2O(l)is not equal to 1 x 10

-14

[H+] for pure water = 1 x 10-7

[OH-] for pure water = 1 x 10 -7

Definitions of acidic, basic, and neutral solutions based on [H+]

y acidic: if [H+] is greaterthan 1 x 10

-7M

y basic: if [H+] is less than1 x 10

-7M


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y neutral: if [H+] ifequal to 1 x 10

-7M

Example 1: What is the [H+] of a sample of lake water with [OH-] of 4.0 x 10-9 M? Is the

lake acidic, basic, or neutral?

Solution: [H+] = 1 x 10-14 / 4 x 10-9 = 2.5 x 10-6 M

Therefore the lake is slightly acidic

Remember: the smaller the negative exponent, the larger the number is.

Therefore:

y acid solutions should have exponents of [H +] from 0 to -6.y basic solutions will have exponents of [H +] from -8 on.

Example 2: What is the [H+] of human saliva if its [OH-] is 4 x 10-8 M? Is human saliva

acidic, basic, or neutral?

Solution: [H+] = 1.0 x 10-14 / 4 x 10-8 = 2.5 x 10-7 M

The saliva is pretty neutral.

4. pH

relationship between [H+] and pH

pH = -log10[H+]

Definition of acidic, basic, and neutral solutions based on pH

acidic: if pH is less than 7basic: if pH is greaterthan 7neutral: if pH is equal to 7

The [H+] can be calculated from the pH by taking the antilog of the negative pH

Example 3: calculate the [OH-] of a solution of baking soda with a pH of 8.5.

Solution: First calculate the [H+]

if pH is 8.5, then the antilog of -8.5 is 3.2 x 10-9. Thus the [H+] is 3.2 x10-9 M

Next calculate the [OH-]

1.0 x 10-14 / 3.2 x 10-9 = 3.1 x 10-6 M

Example 4: Calculate the pH of a solution of household ammonia whose [OH -] is 7.93 x

10-3 M.


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Solution: This time you first calculate the [H+] from the [OH-]

7.93 x 10-3 M OH- = 1.26 x 10-12 M H+

Then find the pH

-log[1.26 x 10

-12

] = 11.9

Now you try a few by yourself. You can then check your answers using the Java applet

that follows, but remember, you won't learn how to do them if you don't try by yourselffirst.

Practice #1. What is the pH of a solution of NaOH that has a [OH -] of 3.5 x 10-3 M?

Practice #2. The H+ of vinegar that has a pH of 3.2 is what?

Practice #3. What is the pH of a 0.001 M HCl solution?

5. Strength ofAcids andBases:

Acids

1. StrongAcids:

y completely dissociate in water, forming H+ and an anion.

example: HN03 dissociates completely in water to form H+ and N03

1-.

The reaction is

HNO3(aq) H+

(aq) + N031-

(aq)

A 0.01 M solution of nitric acid contains 0.01 M of H + and 0.01 M N03- ions

and almost no HN03 molecules. The pH of the solution would be 2.0.

y There are only 6 strong acids: You must learn them. Theremainder of the acids therefore are considered weakacids.

1. HCl

2. H2SO43. HNO3

4. HClO4

5. HBr6. HI

Note: when a strong acid dissociates only one H+ ion is removed.H2S04 dissociates giving H

+ and HS04- ions.

H2SO4 H+ + HSO4

1-


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A 0.01 M solution of sulfuric acid would contain 0.01 M H + and 0.01 MHSO4

1- (bisulfate or hydrogen sulfate ion).

2. Weak acids:

y a weak acid only partially dissociates in water to give H+ and the anion

for example, HF dissociates in water to give H+ and F-. It is a weakacid. with a dissociation equation that is

HF(aq) H+

(aq) + F-(aq)

y Note the use of the double arrow with the weak acid. That is because an

equilibrium exists between the dissociated ions and the undissociatedmolecule. In the case of a strong acid dissociating, only one arrow ( ) is

required since the reaction goes virtually to completion.

y An equilibrium expression can be written for this system:

Ka = [ H+][F-] / [HF]

y Which are the weak acids? Anything that dissociates in water to produce H+

and is not one of the 6 strong acids.

1. Molecules containing an ionizable proton. (If the formula starts with H then it is a

prime candidate for being an acid.) Also: organic acids have at least one carboxyl group,

-COOH, with the H being ionizable.2. Anions that contain an ionizable proton. ( HSO 4

1- H+ + SO42- )

3. Cations: (transition metal cations and heavy metal cations with high charge)

also NH4+ dissociates into NH3 + H

+

Bases1. Strong Bases:

y They dissociate 100% into the cation and OH - (hydroxide ion).

example: NaOH(aq) Na+

(aq) + OH-(aq)

a. 0.010 M NaOH solution will contain 0.010 M OH - ions (as well as 0.010 M Na+ions) and have a pH of 12.

y Which are the strong bases?

The hydroxides of Groups I and II.

y Note: the hydroxides of Group II metals produce 2 mol of OH - ions for every

mole of base that dissociates. These hydroxides are not very soluble, but what

amount that does dissolve completely dissociates into ions.

exampIe: Ba(OH)2(aq) Ba2+

(aq) + 2OH-(aq)

a. 0.000100 M Ba(OH)2 solution will be 0.000200 M in OH - ions (as well as0.00100 M in Ba2+ ions) and will have a pH of 10.3.


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2. Weak Bases:

What compounds are considered to be weak bases?

1. Most weak bases are anions of weak acids.

2. Weak bases do not furnish OH- ions by dissociation. They react with water to

furnis

h the OH

-

ion

s.

Note that like weak acids, this reaction is shown to be at equilibrium,unlike the dissociation of a strong base which is shown to go to

completion.

3. When a weak base reacts with water the OH- comes from the water and theremaining H+ attaches itsef to the weak base, giving a weak acid as one of the

products. You may think of it as a two-step reaction similar to the hydrolysis of

water by cations to give acid solutions.

examples:

1. NH3(aq) + H2O(aq) NH4+

(aq) + OH-

(aq)

methylamine: CH3NH2(aq) + H20(l) CH3NH3+

(aq) + OH-(aq)

acetate ion: C2H3O2-(aq) + H2O(aq) HC2H302(aq) + OH

-(aq)

General reaction: weak base(aq) + H2O(aq) weak acid(aq)+ OH

-(aq)

Since the reaction does not go to completion relatively few OH -

ions are formed.

Acid-BaseProperties of Salt Solutions:definition of a salt:

y an ionic compound made of a cation and an anion, other than hydroxide.y the product besides water of a neutralization reaction

determining acidity or basicity of a salt solution:

1. split the salt into cation and anion2. add OH- to the cation

a. if you obtain a strong base. thecation is neutral

b. if you get a weak base, the cation is acidic

3. Add H+ to the anion

a. if you obtain a strong acid, the anion is neutral

b. if you obtain a weak acid. the anion is basic

4. Salt solutions are neutral if both ions are neutral5. Salt solutions are acidic if one ion is neutral and the other is acidic


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6. Salt solutions are basic is one of the ions is basic and the other is neutral.7. The acidity or basicity of a salt made of one acidic ion and one basic ion cannot

be determined without further information.

Examples: determine if the following solutions are acidic, basic, or neutral Click

on each one to find out the answer.

KC2H3O2 NaHPO4

Cu(NO3)2 LiHS

KClO4 NH4Cl

6. Acid-BaseReactions:

y Strong acid + strong base: HCl + NaOH NaCl + H2O

net ionic reaction: H

+

+ OH

-

H2Oy Strong acid + weak base:

example: write the net ionic equation for the reaction between hydrochloric acid, HCl,

and aqueous ammonia, NH3. What is the pH of the resulting solution?

y Strong base + weak acid:

example: write the net ionic equation for the reaction between citric acid (H 3C6H507)

and sodium hydroxide. What is the pH of the resulting solution?

7. Titrations

1. Nomenclature: these are terms that are used when talking about

titrating one substance with another. You need to learn these definitionswell enough to explain them to someone else.

y titrationy titranty indicator

y equivalence pointy end point

y titration cuve

2. Strong acid-strong base titration

example:

titration curve


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pH at equivalence pointspecies present

appropriate indicators

3. Strong acid-weak base titration

example

titration curve

pH at end pointspecies present

appropriate indicators

4. Weak acid-strong base titrations

example: titrationcurve for the titrationof vinegar with NaOH

pH at end point-

approximately 8.5

species present- H2O

and NaC2H3O2

appropiate indicator-phenolphthalein

Note: no matter what type of titration you do, at the equivalence (end) point the number of moles of H + is


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equivalent to the number of moles of OH -. This applies whether you have

weak or strong acids and/or bases.

Problems: l. Citric acid (C6H807) contains a mole of ionizable H+/mole of citric acid. A

sample containing citric acid has a mass of 1.286 g. The sample is dissolved in 100.0 mL

of water. The solution is titrated with 0.0150 M of NaOH. If 14.93 mL o f the base are

required to neutralize the acid.then what is the mass percent of citric acid in the sample?

2. A sample of solid calcium hydroxide is mixed with water at 30 oC and allowed tostand. A 100.0 mL sample of the solution is titrated with 59.4 mL of a 0.400 M solution

of hydrobromic acid.

a. What is the concentration of the calcium hydroxide solution?

b. What is the solubility of the calcium hydroxide in water at 30 oC? Express your answer

in grams of Ca(OH)2 / 100 mL water?

8. Three models of acids:

l. Arrhenius Model

Basis for the model--action in water

y acid definition: produces H+ in water solutiony base definition: produces OH - in water solution

Acids and bases revision

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