1 of 36© boardworks ltd 2009. 2 of 36© boardworks ltd 2009

1 of 36 © Boardworks Ltd 2009


What are acids and alkalis?


Naming acids and alkalis


What are acids?

Acids can be defined as substances that release H+ ions when they dissolve in water.

When ethanoic acid dissolves in water, it releases H+ ions and CH3COO– ions:

Since acids release H+ ions, we call them H+ donors.

When hydrogen chloride dissolves in water, it releases H+ ions and Cl– ions:

HCl H+ + Cl–

CH3COOH H+ + CH3COO–


What are H+ ions?

Hydrogen atoms contain one proton and one electron.

A H+ ion is a hydrogen atom that has lost its single electron. This leaves a lone, positively charged proton.

When dissolved in water, the H+ ion is hydrated. This is represented in formulae by adding a state symbol, H+ (aq).

Because acids donate protons, they are sometimes called proton donors.


What are bases and alkalis?

Bases are substances that react with acids.

The oxides, hydroxides and carbonates of metals, such as sodium carbonate, are bases.

They absorb hydrogen ions (H+). Bases are therefore H+ acceptors, or proton acceptors.

Some bases are soluble in water: these bases are called alkalis.

Alkalis release hydroxide ions (OH–) in solution. alkalis:

solublebases

bases: react with acids

All alkalis are bases, but not all bases are alkalis.


Examples of bases and alkalis

Sodium hydroxide is an alkali. It dissolves in water to release Na+ ions and OH– ions:

Ammonia is also an alkali. It dissolves in water to release NH4

+ ions and OH– ions:

NaOH Na+ + OH–

NH3 + H2O NH4+ + OH–

Ammonia is a base that does not contain a metal. It can accept a proton to form the NH4

+ ion.


What happens in neutralization?

In a neutralization reaction, an acid reacts with an alkali to produce a salt and water.

When the acid reacts with the alkali, the acid’s H+ ions react with the alkali’s OH– ions, forming water.

This process takes place in all neutralization reactions.

H+ + OH– H2O

+

acid alkali salt + water


The history of acids


Theories about acids and bases

When the 25-year-old Svante Arrhenius submitted his dissertation to Uppsala University in 1884, he was disappointed to receive only a fourth-class degree.

His theories relied on the idea that compounds disassociate in water to form ions. This was a new idea that had not been proposed before.

In contrast Brønsted and Lowry’s ideas were immediately accepted and embraced by other scientists.

Their theory was based on Arrhenius’s ideas. Both were among many respected scientists working on acids and bases at the time.

Why do you think the scientific community responded differently to the two theories?


Acids and alkalis – true or false?


Strong acids

Acids can be strong or weak.

Strong acids dissociate fully into solution, so that all their H+ ions are released into the mixture.

Hydrochloric acid is a typical strong acid, so the dissociation reaction is complete:

HCl H+ + Cl–

HA H+ + A–


The dissociation of a weak acid in water is a reversible reaction:

Weak acids

Weak acids do not dissociate fully, some of their H+ ions stay attached to the acid molecule.

Ethanoic acid is a typical weak acid, with its ions in dynamic equilibrium with the un-dissociated acid. The reaction is moving both directions at the same rate.

HA ⇌ H+ + A–

CH3COOH ⇌ H+ + CH3COO–


Strong or weak?


Sulfuric acid

Sulfuric acid (H2SO4) contains two hydrogen ions per sulphate ion. Both hydrogen ions can dissociate in solution.

It is therefore called a diprotic acid. In general the first H+ ion of a diprotic acid will dissociate more readily than the second.

H2SO4 is a strong acid, so it fully dissociates to release its first H+ ion:

However, HSO4– is a weak acid, so it only dissociates partially

to release the second H+ ion:

H2SO4 H+ + HSO4–

HSO4– H+ + SO4

2–⇌


Properties of strong and weak acids


pH of strong and weak acids

have lower pH values

are better conductors of electricity

react more quickly.

pH is a measure of the number of H+ ions in solution, with a lower pH meaning more H+ ions.

Because strong acids dissociate fully in solution, they contain more H+ ions per molecule of acid, producing a lower pH.

Compared to weak acids of the same concentration, strong acids:

high H+ low H+


pH or concentration?


Properties of strong and weak alkalis

Alkalis can be classified as strong and weak in the same way as acids. A strong alkali, such as sodium hydroxide, fully dissociates in solution.

A weak alkali, such as ammonia, does not fully dissociate, and some of the OH– ions are not released into solution.

NH3 + H2O ⇌ NH4+ + OH–

NaOH Na+ + OH–

Comparable levels of ion dissociation mean that strong and weak alkalis have similar properties to strong and weak acids.


Properties of strong and weak alkalis


Strong and weak acids and alkalis


Uses of sulfuric acid

Sulfuric acid has a role in the manufacture of many products.

fertilizers

paper oil nylon

steelexplosives


Dangers of sulfuric acid

Sulfuric acid will remove water from your skin, leading to bad burns.

Concentrated sulfuric acid is a very powerful dehydrating agent. Dehydrating agents remove water from a compound, in a vigorous exothermic reaction. An example is the reaction of sulfuric acid with glucose.

conc. H2SO4

glucose carbon + water

The sugar is rapidly broken down, producing an expanding mass of steaming carbon.


The contact process


Making sulfuric acid


Conditions for the contact process

The contact process involves a reversible reaction which reaches an equilibrium:

What conditions would you suggest to produce the highest and most economical yield of sulfur trioxide?

Remember: Whenever a change is made to a reversible reaction in dynamic equilibrium, the equilibrium position will shift to try and oppose that change.

2SO2 + O2 ⇌ 2SO3

This reaction is exothermic in the forward direction.


Equilibrium position


Compromise conditions

The conditions used during this stage of the contact process are:

raw materials

equipment

energy

wages

What costs are involved in the manufacture of sulfuric acid?

These provide a compromise in which the cost of making the sulfuric acid is balanced by the yield and speed of production.

400 to 450 °C 10 atmospheres vanadium (V) oxide (V2O5) catalyst

2SO2 + O2 ⇌ 2SO3


Making sulfuric acid


Glossary


Anagrams


Multiple-choice quiz

1 of 36© boardworks ltd 2009. 2 of 36© boardworks ltd 2009

Documents

acids h ions

acids release h ions

hydrogen ions h

strong acids acids

alkalis oh ions

h acceptors

h donors

soluble bases bases