acids and bases

Acids and Bases

Acid/Base Reactions Common examples of acid/base

reactions are taking an antacid for an upset stomach, Using hair conditioner and spreading lime on soil

A study of acid/base reactions allows us understand what goes on in these everyday events

We can tell how acidic or basic a substance is by measuring something called pH

Some acids are dangerous and must be handled with care Eg. sulphuric acid

Others are harmless and can even form part of our diet Eg. Ethanoic acid in vinegar and citric acid in oranges and lemons

The word acid comes from the Latin word acidus meaning sour

Acids turn moist blue litmus paper red

Acids react with metals to release Hydrogen gas

Arrhenius’ Theory of Acids and Bases Svante Arrhenius was a Swedish

chemist who put forward a theory about acids and bases in 1884

Picture here

He suggested that some substances dissociate into ions when added to water

Defined an acid as: An acid is a substance that

dissociates in water to produce H+ ions

When Hydrogen chloride (HCl) is added to water it breaks up (dissociates) into H+ and Cl- ions

HCl H+ + Cl-

The dissociation of Nitric acid and Sulphuric acid may be presented as

HNO3 H +

+ NO3-

And

H2SO4 2H+ + SO42-

Acids like hydrochloric acid and Nitric acid which donate one H+ are called monobasic

Acids like Sulphuric acid which donate 2 H+ are called Dibasic

An acid like phosphoric acid H3PO4 which donates 3 H+ is called Tribasic

Strong and Weak acids Acids such as Hydrochloric acid, Sulphuric

acid and nitric acid are strong acids because they will fully dissociate in water – this means almost every molecule will break up to give H+

Acids such as ethanoic acids are called weak acids as they do not fully dissociate in water

Hydronium ion As chemists learned more about

atomic structure it was realised that H+ could not exist on its own as this is just a single proton

What is formed in reality is a Hydronium ion H3O+ which occurs when the H+ joins to a water molecule

The formation of a Hydronium Ion Fig 12.3

The hydronium ion is very common and is found in every acidic solution

It is more accurate to represent the dissociation of acids in water as

HA H3O+ + A-

Example

HCL H3O+ + Cl-

Importance of water If you add substances such as pure

ethanoic acid or pure hydrogen chloride to solvents such as benzene or methylbenzene then they can’t form an acid they will only form an acid when mixed with water

Bases A group of substances that behave

oppositely to acids are called bases

They turn red litmus paper blue

Sodium hydroxide NaOH is a common base found in the

laboratory Most bases have a soapy feel as they

turn the oil on your skin to soap Arrhenius defined a base as: A base is a substance that

dissociates in water to produce OH- ions

The OH- ion is commonly called the hydroxide ion

When NaOH is added to water it dissociates or breaks up into Na+ and OH- ions

NaOH Na+ and OH-

Strong and Weak bases A strong base dissociates fully Eg.

NaOH A weak base doesn’t dissociate fully

Eg. Calcium hydroxide Ca(OH)2 and Magnesium hydroxide Mg(OH)2

NB A base that dissolves in water is called an alkali

Problems with Arrhenius’ theory1. It is hydronium ions rather than bare

H+ ions that exist2. Definitions are only in aqueous

solutions and don’t take account of solvents such as ammonia, benzene etc.

3. Not all acid base reactions require liquid eg Ammonia gas and hydrogen chloride gas react

Bronsted/Lowry Theory of Acids and Bases 1923 Danish chemist Bronsted + English

chemist Lowry independently proposed new definitions for an acid and a base

These simply state that:

A base is a proton donor

An acid is a protons aceptor

Consider This When hydrogen chloride is added to

water the following reaction occurs

HCl + H2O H3O+ + Cl-

Consider This When ammonia gas dissolves in water

the following reaction occurs

NH3 + H2O NH4 + + OH-

Accepts a

proton B/L base

Donates a

proton B/L acid

Water can act as either an acid or a base and for this reason is known as amphoteric or amphiprotic

Bronsted Lowry theory does not only refer to reactions where water is the solvent

In the following reaction between Hydrogen chloride and Ammonia

HCl + NH3 NH4+ +

Cl-

Advantages of the Bronsted/Lowry Theory over Arrhenius’ Theory1. Arrhenius’ theory was confined to

aqueous solutions2. Bronsted/Lowry’s theory broadens

the range of species that can be defined as acids and bases

3. Substances that are not classified as acids or bases under Arrhenius’ theory are classified as acids or bases in the Bronsted/Lowry theory

Note An acid will only donate a proton

when there is something there to accept it

A base will only accept a proton if there is something there to donate it

Conjugate acid/base pairs Certain reactions are reversible and

can happen in both directions We use the symbol to show

this

Consider This• If the following reaction is written as CH3COOH + H2O CH3COO- + H3O+Ethanoic acid Water

This means the reactionCH3COOH + H2O CH3COO- + H3O+

And the reactionCH3COO- + H3O+ CH3COOH + H2OCan take place

Identify the B/L acid in the following Equations CH3COOH + H2O CH3COO- +

H3O+

CH3COO- + H3O+ CH3COOH + H2O

An acid changes to a conjugate base when it donates a proton

A base changes to a conjugate acid when it accepts a proton

• CH3COO- is the conjugate base of CH3COOH• CH3COOH is the conjugate acid of CH3COO-

Every base has a conjugate acid and every acid has a conjugate base

Since CH3COO- and CH3COOH only differ by a proton we refer to them as a conjugate acid base pair

Why can H2O and H3O+ be called a conjugate acid base pair?

NB Study examples 12.1 and 12.2 on pages 140 and 141 of your text book

Neutralisation• When acids and bases react with each

other in the right proportions they cancel each other out

• When this happens both the acid and the base lose their characteristic properties

• The solution formed is neutral and has no effect on litmus paper

• This type of reaction is known as neutralisation

Neutralisation is the reaction between an acid and a base to form salt and water

Reaction between HCl and NaOH HCl + NaOH H2O + NaCl Acid + Base Water + Salt The word salt is a general term

used to describe the substance formed when the hydrogen in an acid is replaced by a metal or an ammonium ion

Understanding what happens• HCl + NaOH H2O + NaCl

• In this equation the following ions are all present in solution

H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O

The spectator ions are crossed out as what is important is the reaction where the H+ (Or more correctly Hydronium ions )and OH- ions react to form water

H + + OH- H2O

Everyday examples of Neutralisation1. Medicine – Hydrochloric acid in

stomachs helps digestion. Over eating and stress can produce too much acid. Antacids such as Alka Seltzer, Bisodol and Milk of Magnesia may be used to try to neutralise excess acid

2. Agriculture – If soil is too acidic crop yields can be low, therefore farmers often spread lime (CaO) on soil to neutralise the acidity

3. Environmental Protection- In areas affected by acid rain limestone is often added to lakes to neutralise acidity

• Toothpaste is slightly basic to neutralise acids that cause tooth decay

• Baking soda may be used to neutralise the acid of bees

• Vinegar is used to neutralise the alkaline sting of wasps

• Shampoo is slightly basic and can cause scales on hair to stick out, conditioner is slightly acidic so neutralises the base flattening the scales and making hair more shiny and manageable