11 IB Topic 8: Acids & Bases 8.1 Theories of Acids & Bases 8.1.1Define acids and bases according to...

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IB Topic 8: Acids & Bases8.1 Theories of Acids & Bases

8.1.1 Define acids and bases according to the Brǿnsted-Lowry and Lewis theories.

8.1.2 Deduce whether or not a species could act as a Brǿnsted-Lowry and/or a Lewis acid or base.

8.1.3 Deduce the formula of the conjugate acid (or base) of any Brǿnsted-Lowry base (or acid)

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Terms to Know

• Brønsted-Lowry acid & base• Lewis acid & base• Strong acid & base• Weak acid & base• pH scale • Buffer • Conjugate acid & base• Indicator

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Historical Aspects

Acid: Comes from Latin word acidus, meaning sour or tart.

Base: Comes from an old English meaning of the word, “to bring low.” When bases are added to acids, they lower the amount of acids.

In the 1880’s Svante Arrhenius defined acids & bases:

Acids are substances that, when dissolved in water, increase the [H+].

Bases are substances that, when dissolved in water, increase the [OH-].


Arrhenius acid is a substance that produces H+ (H3O+) in water

Arrhenius base is a substance that produces OH- in water

4.3

Arrhenius definition

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Brǿnsted-Lowry Acids & BasesBrǿnsted-Lowry Acids & BasesIn 1923, Johannes Brǿnsted & Thomas Lowry

defined acids & bases:• Acids are substances that can transfer a proton to

another substance (proton donor)• Bases are substances that can accept a proton

(proton acceptor)

Lewis Acids & BasesGilbert Lewis defined acids and bases:• Lewis acid is an electron-pair acceptor• Lewis base is an electron-pair donor


A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acidbase

Lewis DefinitionLewis Definition

Lewis acid - a substance that Lewis acid - a substance that accepts an electron pairaccepts an electron pair

Lewis base - a substance Lewis base - a substance that donates an electron that donates an electron pairpair

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8.1.2 Deduce whether or not a species could act as a Brǿnsted-Lowry and/or a

Lewis acid or base.

HCl is an acid• HCl(g) H+(aq) + Cl-(aq) (Arrhenius)

– Forms H+ ions in aqueous solution

• HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) (Br-Lowry)

– H3O+(aq) is called the hydronium ion

– HCl donates a proton to water

• H+ accepts an electron pair from H2O (Lewis acid)

– H O H +

H


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8.1.2 Deduce whether or not a species could act as a Brǿnsted-Lowry and/or a

Lewis acid or base.

NaOH & NH3 are bases• NaOH(s) Na+(aq) + OH-(aq) (Arrhenius)

– Forms OH- ions in aqueous solution

• NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) (Br-

Lowry)

– NH3 accepts a proton from water

• NH3 donates an electron pair to H+ (Lewis base)

H– H N H +

H


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8.1.3 Deduce the formula of the conjugate acid (or base) of any Brǿnsted-

Lowry base (or acid)

In any acid-base equilibrium, the forward reaction and the reverse reaction involve proton transfer:

HX(g) + H2O(l) H3O+(aq) + X-(aq)

In the forward reaction, HX acts as an acid (donates a proton) and H2O acts as a base (accepts a proton).

In the reverse reaction, H3O+ acts as an acid (donates a proton) and X- acts as a base (accepts a proton).

HX & X- are an acid-conjugate base pair. H2O & H3O+ are a base-conjugate acid pair.

They differ only in the presence or absence of a proton.


Conjugate PairsConjugate Pairs

A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acidconjugate

basebase conjugate

acid

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HNO2(aq) + H2O(l) H3O+(aq) + NO2-(aq)

acid base conj acid conj base

A B-L acid loses a proton (H+) to form the conjugate base.

A B-L base gains a proton (H+) to form the conjugate acid.

remove H+

add H+


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H2O(l) + NH3(aq) NH4+(aq) + OH-(aq)

acid base conj acid conj base

A B-L acid loses a proton (H+) to form the conjugate base.

A B-L base gains a proton (H+) to form the conjugate acid.

remove H+

add H+


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Consider the reaction:

H2O(l) + HSO3-(aq) H3O+(aq) + SO3

2-(aq)

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


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Consider the reaction:

H2O(l) + HSO3-(aq) H3O+(aq) + SO3

2-(aq)

Identify the acid: HSO3

-(aq)

the base: H2O(l)

the conjugate acid: H3O+(aq)

the conjugate base: SO32-(aq)


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What is the conjugate base for each of the following acids:

HClO4; H2S; PH4+; HCO3

-?


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-?

• The conjugate base for HClO4 is ClO4- since a B-L

acid loses a proton (H+).

What is the conjugate acid of each of the following bases: CN-; SO4

2-; H2O; HCO3-?

• The conjugate acid for CN- is HCN since a B-L base gains a proton (H+).


19`19



What is the conjugate acid for each of the following bases:


-?


20`20





-?

• The conjugate base for HClO4 is ClO4- since a B-L

acid loses a proton (H+).

What is the conjugate acid of each of the following bases: CN-; SO4

2-; H2O; HCO3-?

• The conjugate acid for CN- is HCN since a B-L base gains a proton (H+).


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IB Topic 8: Acids and bases8.2: Properties of acids and bases

8.2.1 Outline the characteristic properties of acids and bases in aqueous solutions

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Acids

• Sour taste• Electrolytes• Neutralization reactions with bases (such as

metal hydroxides) to form a salt and water– HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

Since HCl and NaOH are strong, they exist as ions so the reaction is: H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) Na+(aq) + Cl-(aq) + H2O(l)Getting rid of “spectator” ions (ions that do not take part in the reaction), the equation can be written: H+(aq) + OH-(aq) H2O(l) and the ΔH = -57.3

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Acids

• Neutralization reactions with metal oxides to form a salt and water.– H2SO4(aq) + CaO(aq)

• Reactions with carbonates and hydrogen carbonates (bicarbonate) producing a salt, carbon dioxide and water.– H2SO4(aq) + CaCO3(aq) ?– HCl(aq) + NaHCO3(aq) ?

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Acids

• Reaction with metals above hydrogen in the reactivity series to form a salt and hydrogen gas.– H2SO4(aq) + Ca(s) ?or H+(aq) + Ca(s) ?

• Reactions with indicators. Indicators are substances that change color when the concentration of hydrogen ions changes. Ex. phenolpthalein, bromothymol blue, litmus – H+ + In- HIn

In- is one color, HIn is a different color. LeChatelier’s Principle– Universal indicator is a mixture of different indicators and

produces a range of colors.

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Bases

• Bitter taste• Feel slippery due to reaction with the oils on your

skin. This forms a soap.• Neutralization of acids (see acid properties).• Displacement of ammonia (NH3) from ammonium

salts.– NH4Cl(s) + NaOH(aq) ?

or NH4+(aq) + OH-(aq) ?

• Reactions with indicators. Bases do not react with indicators but the addition of a base changes the [H+] which affects the indicator.– H+(aq) + OH-(aq) H2O(l)

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IB Topic 8: Acids and bases8.3: Strong and weak acids and

bases8.3.1 Distinguish between strong and weak acids

and bases, in terms of the extent of dissociation, reaction with water and electrical conductivity.

8.3.2 State whether a given acid or base is strong or weak

8.3.3 Distinguish between strong and weak acids and bases, and determine the relative strengths of acids and bases, using experimental data.

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8.3.1 Distinguish between strong and weak acids and bases, in terms of the extent of dissociation, reaction with water and electrical conductivity.

Strong acids and bases dissociate completely in water

Acid: HX(aq) + H2O(aq) H3O+(aq) + X-(aq)

Base: YOH(aq) Y+(aq) + OH-(aq)

When strong acids and bases dissolve in water, the solution consists of almost entirely of ions with a negligible amount of molecules.

Write the dissociation reactions for H2SO4 & Ba(OH)2

STRONG ACID: HNOSTRONG ACID: HNO3 3 (aq) + H(aq) + H22O (l) --->O (l) --->

HH33OO+ + (aq) + NO(aq) + NO33- - (aq)(aq)

HCl, HNO33, H22SO44 and HClO44 are examples of strong acids that dissociate completely in water.

Strong and Weak Acids/BasesStrong and Weak Acids/Bases

• Strong Base:Strong Base: 100% dissociated in water. 100% dissociated in water.

NaOH (aq) ---> NaNaOH (aq) ---> Na+ + (aq) + OH(aq) + OH- - (aq)(aq)

Other common strong Other common strong bases include:bases include:

•Group 1 Group 1 hydroxideshydroxides (ex. (ex. KOH and LiOH) andKOH and LiOH) and

•Ba(OH)Ba(OH)22

CaOCaO


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Weak acids and bases only partly dissociate in water.

Acid: HX(aq) + H2O(aq) H3O+(aq) + X-

(aq)

Base: NH3(aq) + H2O(aq) NH4+(aq) + OH-

(aq)

When weak acids and bases dissolve in water, the solution consists of almost entirely of molecules with a negligible amount of ions.

Kc (Ka or Kb) << 1

Write the dissociation reactions for H2CO3 & C2H5NH2

• Weak acids are much less than 100% ionized in water. Weak acids are much less than 100% ionized in water.

• Examples include: Carboxylic acids, such as acetic acid Examples include: Carboxylic acids, such as acetic acid

aka ethanoic acid (CHaka ethanoic acid (CH33COOH) and carbonic acid (HCOOH) and carbonic acid (H22COCO33))


• Weak base:Weak base: less than 100% ionized in water less than 100% ionized in water

One of the best known weak bases is One of the best known weak bases is ammoniaammonia

NHNH3 3 (aq) + H(aq) + H22O (l) O (l) NH NH44+ + (aq) + OH(aq) + OH- - (aq)(aq)

As well as As well as aminesamines (nitrogen-containing compounds) (nitrogen-containing compounds)

Strong and Weak Acids/BasesStrong and Weak Acids/BasesStrong and Weak Acids/BasesStrong and Weak Acids/Bases

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Strong acids and bases and electrical conductivity

• Since strong acids and bases dissociate completely into ions, these solutions are excellent conductors of electricity.

• However, there can be weak solutions of strong acids and bases which lowers the ion concentration and lowers the conductivity.

Weak acids and bases and electrical conductivity

• Since weak acids and bases dissociate very little, few ions are present in solution so these are weak conductors of electricity.

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8.3.2 State whether a given acid or base is strong or weak

Strong acids and bases completely dissociate into their ions in aqueous solutions.

Weak acids and bases only slightly dissociate into their ions in aqueous solution

Strong acids Strong bases

Hydrochloric acid, HCl(aq) Group 1 hydroxides e.g. NaOH(aq) & KOH(aq)

Nitric acid, HNO3(aq) Barium hydroxide, Ba(OH)2(aq)

Sulfuric acid, H2SO4(aq)

Weak acids Weak bases

Carboxylic acids, e.g. ethanoic acid CH3COOH(aq) or HC2H3O2(aq)

Amines, e.g. ethylamine, C2H5NH2(aq)

Carbonic acid (CO2 in water) H2CO3(aq) Ammonia, NH3(aq)

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8.3.3 Distinguish between strong and weak acids and bases, and determine the relative strengths of

acids and bases, using experimental data.

Discussion of the lab: Properties of Acids & Bases

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IB Topic 8: Acids and bases8.4: The pH scale

8.4.1 Distinguish between aqueous solutions that are acidic, neutral or alkaline using the pH scale.

8.4.2 Identify which of two or more aqueous solutions is more acidic or alkaline using pH values.

8.4.3 State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+(aq)].

8.4.4 Deduce changes in [H+(aq)] when the pH of a solution changes by more than one pH unit.

The The pH scalepH scale is a way of is a way of expressing the strength of expressing the strength of acids and bases. Instead of acids and bases. Instead of using very small numbers, using very small numbers, we just use the NEGATIVE we just use the NEGATIVE power of 10 on the Molarity power of 10 on the Molarity of the Hof the H++ (or OH (or OH--) ion.) ion.

Under 7 = acidUnder 7 = acid7 = neutral7 = neutral

Over 7 = base Over 7 = base

pH of Common pH of Common SubstancesSubstances

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8.4.1 Distinguish between aqueous solutions that are acidic, neutral or

alkaline using the pH scale.

pH is a measure of the [H+(aq)] in a solution.

pH = -log [H+(aq)]

In a neutral solution the [H+(aq)] = 1 x 10-7

so the pH = -log(1 x 10-7) = 7

Addition of an acid to water increases the [H+(aq)] so the pH < 7. Low pH is acidic.

Addition of a base to water decreases the [H+(aq)] so the pH > 7 . High pH is basic.

Calculating the pHpH = - log [H+]

(Remember that the [ ] means Molarity aka mol dm-3)

Example: If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)

pH = 10

Example: If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)

pH = 4.74

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pH is a measure of the [H+(aq)] in a solution.pH = -log [H+(aq)]

pH < 7 is acidic pH = 7 is neutral pH > 7 is basic

Is a solution with a [H+(aq)] = 1 x 10-5 acidic, basic or neutral?

• Find the pH: pH = -log 1 x 10-5 = 5 so it is acidic

Is a solution with a [H+(aq)] = 1 x 10-10 acidic, basic or neutral?

• Find the pH: pH = -log 1 x 10-10 = 10 so it is basic

Try These!Try These!

Find the pH of these:Find the pH of these:

1)1) A 0.15 M solution of A 0.15 M solution of Hydrochloric acidHydrochloric acid

2) A 3.00 X 102) A 3.00 X 10-7-7 M M solution of Nitric solution of Nitric acidacid

pH = - log [HpH = - log [H++]]

pH = - log 0.15pH = - log 0.15

pH = - (- 0.82)pH = - (- 0.82)

pH = 0.82pH = 0.82

pH = - log 3 X 10pH = - log 3 X 10-7-7

pH = - (- 6.52)pH = - (- 6.52)

pH = 6.52pH = 6.52

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You can also calculate the [H+(aq)] if you know the pH

[H+(aq)] = inverse log(-pH)

In a solution with a pH = 6, what is the [H+(aq)]?• [H+(aq)] = inverse log(-6) = 1 x 10-6

Is a solution with a pH = 11, what is the [H+(aq)]?• [H+(aq)] = inverse log(-11) = 1 x 10-11

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Comparing pH of Strong and Weak Acids.

Strong acids dissociate completely. • A 0.010M HCl solution has a [H+(aq)] = 0.010M so

the pH = 2

Weak acids do not dissociate completely so their [H+(aq)] concentration will be less than that of an equal concentration of a strong acid.

• A 0.010M ethanoic acid (CH3COOH) solution has a [H+(aq)] = 0.00042M so the pH = 3.4

• A 0.01M carbonic acid (H2CO3) solution has a [H+

(aq)] = 6.6 x 10-5 so the pH = 4.2

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8.4.2 Identify which of two or more aqueous solutions is more acidic or

alkaline using pH values.

Acid/Alkaline pH

More basic 14

12

10

Less basic 8

Neutral 7

Less acidic 6

4

2

More acidic 0

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8.4.3 State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+(aq)].

• An increase in one pH unit decreases the [H+(aq)] by a factor of 10.

• A decrease in one pH unit increases the [H+

(aq)] by a factor of 10.

Acid/Alkaline pH [H+(aq)]

More basic 11 1 x 10-11

10 1 x 10-10

9 1 x 10-9

Less basic 8 1 x 10-8

Neutral 7 1 x 10-7

Less acidic 6 1 x 10-6

5 1 x 10-5

4 1 x 10-4

More acidic 3 1 x 10-3

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8.4.4 Deduce changes in [H+(aq)] when the pH of a solution changes by more

than one pH unit.

• An increase in three pH units decreases the [H+(aq)] by a factor of 1,000.

• A decrease in five pH units increases the [H+

(aq)] by a factor of 105.

Acid/Alkaline pH [H+(aq)]

More basic 14 1 x 10-14

12 1 x 10-12

10 1 x 10-10

Less basic 8 1 x 10-8

Neutral 7 1 x 10-7

Less acidic 6 1 x 10-6

4 1 x 10-4

2 1 x 10-2

More acidic 0 1 x 100

11 IB Topic 8: Acids & Bases 8.1 Theories of Acids & Bases 8.1.1Define acids and bases according to...

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