Acids, Bases and SaltsChapters 19

Acid - Base TheoriesSection 19.1

What are common examples of acids and bases?

What properties do you know about acids and bases?

Arrhenius acids● In 1887 A swedish Chemist, Svante Arrhenius defined

acids and bases in a new way● Acids are hydrogen containing compounds that

separate to yield hydrogen ions (H+) in aqueous solution

● Not all compounds that contain hydrogen are acids, and not all hydrogen in acids has to form hydrogen ions

● The hydrogen atom that can form a hydrogen ion is described as ionizable

● This means it must be bonded to a very electronegative element

● This bond is therefore very polar

What happens to the hydrogen ion?

The hydrogen does not simply float around in water - instead it joins with a water molecule to form a hydronium ion

A hydronium ion (H3O+) is the ion that forms when a water molecule gains a

hydrogen ion

Arrhenius acids and weak polar bondsIn some acids, they contain hydrogen atoms that do not ionize

To recognise which hydrogen is ionizable you need to look at the structural formula , and establish which hydrogen is bonded to an electronegative element

Arrhenius BasesIn an Arrhenius base, compounds ionize to yield hydroxide ions in aqueous solutions

Sodium hydroxide is a common example of a base

NaOH can be dangerous!It is easy to make a concentrated solution, because it is easy to dissolve in water

This solution is extremely caustic

Sodium hydroxide is made from reacting sodium with water

What do you use this for? Milk of magnesia is made of magnesium hydroxide

This is not very soluble in water, and their solutions are always very dilute

Although most bases are generally caustic, the low solubility make this base safe to consume

Bronsted-Lowry acids and basesThe Arrhenius definition of acids does not include some substances that have acidic or basic properties

Sodium carbonate (Na2CO3) and Ammonia (NH3) act as bases in aqueous solutions , but do not contain OH- ions.

In 1923 a new definition of bases was proposed by Johannes Bronsted and Thomas Lowry.

They proposed that an acid is a hydrogen ion donor and a base is a hydrogen ion acceptor

Example

Conjugate acids and basesThis reaction is reversible…

Conjugate acid and base pairs● A conjugate acid is the ion or molecule

formed when a base gains a hydrogen ion

○ It is an acid because it can donate the hydrogen ion in the reverse reaction

● A conjugate base is the ion or molecule that remains after an acid loses a hydrogen ion

○ It is a base because it can accept a hydrogen ion in the reverse reaction

A conjugate acid is always paired with a base, and a conjugate base is always paired with an acid

Amphoteric substancesSome substances can behave as both acids and bases

Water can BOTH donate hydrogen ion and accept hydorgen ions

An amphoteric substance is something that can act as either an acid or a base

How water behaves for example, depends on the other reactant

Lewis Acids and Bases ● Gilbert lewis proposed a third definition for acids and bases● An acid accepts a pair of electrons during a reaction, a base donates a pair of

electrons● A Lewis acid is a substance that can accept a pair of electrons to form a

covalent bond ● A lewis base is a substance that can donate a pair of electrons to form a

covalent bond ● This is a broader definition than the other two definitions, and includes all of

the acids and bases covered by the other definitions

Example

Example

Overview

Key questionsHow did Arrhenius define an acid and a base?

How do acids and bases differ in the Bronsted -Lowry theory?

How did lewis define an acid and a base?

Hydrogen Ions and AciditySection 19.2

Hydrogen ions from water? Two water molecules can react to produce hydrogen ions via a process called self - ionization

This occurs when the water molecules have enough energy for a reaction to occur

Self ionization waterThis is a reversible reaction, but the reverse reaction (formation of H2O is favored)

Self ionization is therefore not very common

The concentration of H+ and OH- ions in pure water at 25 C is 1 x 10-7.

Very small concentrations

Any aqueous solution with equal concentrations of hydrogen and hydroxide solutions are a neutral solution

Ion product constant for water The Kw value is the product of the concentration of H+ ions and OH- ions - the ion product constant

In an acidic solution, the conc. of H+ ions is great than that of OH-, meaning greater than 1 x 10-7

In a basic solution the conc. of H+ ions is less than that of OH- ions, meaning less than 1 x 10-7.

pH ● We use pH as a simple way of expressing

the strength of an acid; it is much simpler than discussing hydrogen ion molarity

● Proposed by Soren Sorensen - ranges from 0 to 14

● pH = -log[H+] ● In pure water [H+] = 1 x 10-7, and pH is 7

(neutral)● pH < 7 = acidic, [H+] > 1 x 10-7

● pH > 7 - basic [H+] < 1 x 10-7

●

Relationship between pH, [H+], and [OH-]

Why use scientific notation? ● It makes everything easier! ● When writing a concentration with a coefficient of 1.0, the pH is simply

the exponent● If the pH is not a whole number, it is not this simple, and you need to

use the antilog function of your calculator to get an accurate hydrogen ion concentration

● Example - milk of magnesia has pH 10.5 = 3.2 x 10-11

● You can also use the knowledge that [H+] x [OH-] = 1 x 10-14 to calculate [H+] and pH

How do you measure pH?● Acid- Base indicators● An indicator is an acid or a base that dissociates in a certain pH range● They work because they form different colors in solution● This change occurs in a narrow pH range● HIn: The acidic form of the indicator - found at low pHs● In-: The basic form of the indicator - found at high pHs

● Indicators do not give precise measurements - instead you can make a rough estimate

● This chart shows the position at which the color change occurs for indicators

● At lower pHs you would see the acid color, whereas at higher pHs you would see the basic color

Universal indicator solution

Limitations of an indicators usefulness● They only work at a specific temperature - 25 C

○ At any other temperatures the pH of a color change can vary

● Colored solution affect the usefulness of an indicator● Dissolved salts can also sometimes react with the indicator● These issues can be overcome with indicator strips● These are pieces of paper that have been soaked in indicator solution and then

dried● It is often important to test the pH of soil prior to planting - why?

Why are these plants different?

pH MeterspH meters are probes that can make fast, continuous measurements of pH

These are more accurate that measurements taken with an indicator solution

They can be accurate to within 0.01 pH of the true pH value

They are unaffected by color, or cloudiness

The only issues can occur with calibrating

Strengths of Acids and BasesSection 19.3

Why is not dangerous to drink lemonade, even though it contains citric acid? ● Acids can be classed as strong or

weak depending upon how much they ionize in water

● A strong acid ionizes completely in aqueous solution

○ HCl is a strong acid

● A weak acid only ionizes a little bit in water.

○ With ethanoic acid, less than 1% of ethanoic acid ionizes

Strengths of common acids and bases

Dissociation of a strong acid

Dissociation of a weak acid

The acid dissociation constantKa: The ratio of the concentration of the dissociated forms of an acid to the concentration of the undissociated form

This shows the fraction of the acid that ionizes

Weak acids have small Ka values

Strong acids have large Ka values

This is like an equilibrium constant, but water is removed in dilute aqueous solutions

The base dissociation constant● Just as there are strong and weak acids, there are strong and weak bases● A Strong Base dissociates fully to form metal ions and hydroxide ions in

aqueous solution○ Some, such as MgOH2 do not dissolve well, but those that do fully dissociate

● A Weak Base reacts with water to form the conjugate acid of the base and hydroxide ions

○ Ammonia is a common example of a weak base

NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)

Ammonia Water Ammonium ion

Hydroxide ion

Base dissociation constantThe concentration of water is constant in dilute solutions, so [H2O] can be removed when calculating the base dissociation constant (Kb)

Kb is the ratio of the conjugate acid times by the hydroxide, divided by concentration of base

Keq =[NH4

+] × [OH−][NH3] × [H2O]

Keq × [H2O] = Kb =

[NH4+] × [OH−][NH3]

Concentration vs Strength (they are different!!)Acids and Bases are strong or weak based on the degree to which they ionize in water

Comparing Concentration and Strength of Acids

Acidic solutionConcentration

StrengthQuantitative (or Molar) RelativeHydrochloric acid 12M HCl Concentrated StrongGastric juice 0.8M HCl Dilute StrongEthanoic acid 17M CH3COOH Concentrated WeakVinegar 0.2M CH3COOH Dilute Weak

Neutralization reactionsSection 19.4

What happens when you mix an acid and a base together?

● A neutralization reaction takes place● During a neutralization reaction, a salt and water are always produced

○ Remember - a salt is any ionic compound made of an anion and cation

Mole ratios and acid-base reactionsNeutralization is often used to describe the point at which a reaction has gone to completion

At completion, the acid and base solutions must have equal numbers of hydrogen and hydroxide ions

The mole ratio can be used to determine how much acid/base is required for neutralization to occur

How can you work out the concentration of an unknown acid or base?

● Titration ● A measured volume of a solution of known concentration is added to a known

volume of a solution of unknown concentration● If you want to find the concentration of an acid, you titrate into it a base of

known concentration ● If you want to find the concentration of a base, you titrate into it an acid of

known concentration● You want to add just enough acid/base to observe a color change - to the

nearest drop!

Titration curves

Interpreting titration curves

● Standard solution - the solution of known concentration in the biuret - can be either an acid or a base

● Equivalence point - the point in titration when neutralization occurs● End point - the point where the color change occurs● These are not necessarily the same thing!

○ The end point is the visible change in indicator color that occurs near the equivalence point

● Neutralization occurs when the number of moles of hydrogen ions equals the number of ions of hydroxide ions

Salts in SolutionSection 19.5

Are all salt solutions neutral?

Not all titration curves are the same

Why is there a difference when you titrate a strong acid or a weak acid with a strong base?

The equivalence point is at a higher pH with a weak acid because of salt hydrolysis

Salt hydrolysis is when the cations or anions of a dissociated salt remove hydrogen ions from water or donate hydrogen ions to water

The result is that a solution becomes either acidic or basic

Hydrolysis of the salt of a weak acid and strong base

● Sodium ethanoate is the salt produced from the combination of the weak acid ethanoic acid and the strong base sodium hydroxide

● Sodium ethanoate ionizes completely ● The ethanoate ion is a Bronsted-lowry base, so it can accept a hydrogen ion● This ion reacts with water to form an ethanoic acid and hydroxide ions● Because the concentration of OH- increases, the solution becomes basic

CH3COONa(aq) → CH3COO−(aq) + Na+(aq) Sodium ethanoate Ethanoate ion Sodium ion

CH3COO–(aq) + H2O(l) CH3COOH(aq) + OH–(aq) H+ donor

Brønsted-Lowry acidH+ acceptor

Brønsted-Lowry base(makes the

solution basic)

Hydrolysis of the salt of a strong acid and weak base

● Ammonium chloride is the salt of a strong acid (HCl) and the weak base Ammonia Ammonium chloride will ionize completely

○ NH4Cl(aq) → NH4+(aq) + Cl−(aq)

● The ammonium ion is a Bronsted - lowry acid ● In solution it donates a hydrogen ion to a water molecule ● The product is ammonia molecules and hydronium ions ● Due to the increase in H3O

+ ions, the solution becomes acidic

NH4+(aq) + H2O(l) NH3(aq) + H3O

+(aq) H+ donor

Brønsted-Lowry acidH+ acceptor

Brønsted-Lowry base(makes the

solution acidic)

General rules for hydrolysis

Strong acid + Strong base → Neutral solution

Strong acid + Weak base → Acidic solution

Weak acid + Strong base → Basic solution

How can you control the pH of a solution?● You can use a buffer solution● A buffer is a solution in which the pH remains fairly constant when small

amount of acid or base are added● They are made of a solution of a weak base and one of its salt● One component of the buffer acts as a hydrogen ion acceptor, whereas the

second component can react with the hydroxide ions ● Both components can work to “mop up” any hydrogen and hydroxide ions that

are added to the solution

What does all this mean?● Ethanoic acid and its salt sodium ethanoate works as a buffer solution● When you add H+ ions to this solution, the ethanoate ion from the salt sodium

ethanoate react with the H+. Ethanoic acid is formed, but this is a weak acid, only ionizing slightly in water, causing only a small change in pH

● Eventually all the H+ ion acceptors of H+ ion donors become used up, and the buffer can no longer control the pH of the solution

● The Buffer capacity is the amount of acid or base that can be added to a buffer before a large change in pH occurs.

Hydrogen ion

CH3COO–(aq) + H+(aq) CH3COOH(aq)Ethanoate ion Ethanoic acidHydrogen

ion

CH3COO–(aq) + H+(aq) CH3COOH(aq)Ethanoate ion Ethanoic acid