Carbonates

Question 1

What is a precipitation reaction?

Answer 1

When dissolved ions react with each other to form a solid compound.

Question 2

What is a ionisation reaction? What is opposite reaction?

Answer 2

When solid compounds break apart in a solution.
Precipitation is reverse reaction.

Question 3

What is a acid/base reaction?
What is assumed in these reactions?

Answer 3

Special type of ionisation reaction where a hydrogen ion H+ is added to or removed from a solution.
Assumed that water is present.

Question 4

How are hydrogen ions added to water?

Answer 4

Addition of an acid

Question 5

How can hydrogen ions be removed from water?

Answer 5

Addition of a base

Question 6

What is a ion association reaction?

Answer 6

Occurs when dissolved ions form “ties” with each other within a solution.
New species is still soluble but acts differently to individual dissolved ions.

Question 7

What is an oxidation/reduction reaction?

Answer 7

Involves valence changes and transfer of electrons.
Generally releases electrons which another element must accept - commonly produces hydrogen gas

Question 8

What is a base and an acid?
What is water?

Answer 8

Acid - compounds which release hydrogen ions
Base - compounds which accept hydrogen ions
Water - can do both!

Question 9

When does water behave as a base?
When does water behave as an acid?

Answer 9

If a compound is a stronger acid than water then it acts as a base.
If a compound is a stronger base than water, then water will act as an acid.

Question 10

What does pH measure?
What is the pH of an acid and base?

Answer 10

The concentration of ions present.
Acid: pH < 7 (lots of H+)
Base: pH > 7

Question 11

What is chemical equlibrium?

Answer 11

When a reaction can move in both directions at once (double arrow) and rates of reactions are equal.

Question 12

What expression represents equilibrium and what equation can be used to calculate the equilibrium constant?

Answer 12

aA + bB <=> cC + dD

([C]^c [D]^d) / ([A]^a [B]^b) = K

Capitals = chemical species in moles per litre
lower case = no. of moles required for equilibrium

Question 13

Why must we be careful when using K and what is it convenient to introduce?

Answer 13

Natural environments rarely reach chemical equilibrium.
A log scale since are dealing with very large and small numbers: pK = -logK

Question 14

How does a strong acid react with water?
What is important to note about these reactions?

Answer 14

Donate hydrogen ions to water.
One direction reactions.

Question 15

How are strong acids used in water treatment?
What are the 3 important strong acids and where do they stem from?

Answer 15

Used for pH adjustments and coagulation (creation of floc)
Hydrochloric acid
Nitric acid - burning petrol
Sulphuric acid - coal-fired power stations

Question 16

Which acids are found in acid rain? What are there properties?

Answer 16

Sulphuric and nitric
Strong

Question 17

How do weak acids react with water and what equilibrium exists?

Answer 17

Weak acids do not completely dissociate in water.
Equilibrium exists between dissociated ions and the undissociated compound (2 way reaction).

Question 18

Give some examples of how weak acid reactions can be used in water treatment

Answer 18

Acetic acid - anaerobic digestion
Carbonic acid - corrosion, coagulation, softening, pH control
Hydrogen sulphide - aeration, odour control, corrosion
Hypochlorous acid - disinfection
Phosphoric acid - phosphate removal, plant nutrient

Question 19

What is buffering capacity and what causes it?

Answer 19

Ability to resist changes in pH.
Due to the presence of carbonate systems.

Question 20

Why does carbonic acid buffer fresh water systems but sulphuric acid not?

Answer 20

Freshwaters have a pH of 6-9 which means only weak acids/bases contribute to rivers buffering.
Carbonic acid is weak so only partially dissociates in water, the reversible equilibrium means carbonic acid resists pH by absorbing or releasing H+ when a base or acid is added.
Sulphuric acid fully dissociates adding H+ ions which lowers pH.

A buffer needs a weak acid and a conjugate base.

Question 21

What does the carbonate system control? How is it formed?

Answer 21

pH and provides a buffer to resist changes in pH
Formed from geology (chalk/limestone)

Question 22

What does the carbonate system comprise of?
What is reaction?

Answer 22

Carbon dioxide
Carbonic acid
Bicarbonate ions
Carbonate acids

Reaction is from top to bottom and all reactions are 2 way.

Question 23

How does carbon dioxide make all rain weakly acidic?

Answer 23

Dissolves in water and is closely linked with chemical process that determines acidity and alkalinity of water. CO2 dissolves into cloud vapor which forms carbonic acid and makes rain weakly acidic.

Question 24

What are the weak bases in the carbonate system and how do they resist pH change in natural system?

Answer 24

Bicarbonate and carbonate ions resist pH changes if a strong acid is added such as acids in acid rain.

Question 25

How does CO2 naturally get removed and added to a system? What does this affect and how is it resisted?

Answer 25

Removed by photosynthesis and added by atmosphere which will affect the pH. Buffered by bicarbonate and carbonate ions to maintain pH.

Question 26

What 4 conditions determine how a system will behave?

Answer 26

If system can be exposed to the atmosphere (open) or not (closed). If system has a source of solid carbonate or not.

Question 27

How does the carbonate buffer system work to resist change in an open system with a source of carbonate if a strong acid is added? When does buffering stop?

Answer 27

Increase in hydrogen ions. Carbonate combines with hydrogen ions to produce bicarbonate which produces carbonic acid which produces excess CO2. (system moves left) Released into atmosphere since is open. Removes hydrogen ions from water and resists lowering of pH. Stops when all bicarbonate or carbonic acid is neutralised at pH 4.5 or 8.3

Question 28

How does the carbonate buffer system work to resist change in an open system with a source of carbonate if a strong base is added?

Answer 28

Hydrogen ions are consumed but the system moves to the right where CO2 is replenished from the atmosphere and system moves to hydrogen ions to be replaced.

Question 29

Where does source of carbonate come from? What happens if there is no source of carbonate? What are these systems better for?

Answer 29

Calcium carbonate - chalk No carbonate means buffering cannot occur and they are susceptible to acidification, eutrophication, and acidic pollutants. These systems have a naturally low pH. Systems are better for photosynthesis since more CO2 stays dissolved.

Question 30

How does the closed carbonate buffer system compare to an open? What does source of carbonate supply?

Answer 30

The pH changes more quickly since there is no CO2 source/sink from the atmosphere. Provides an alkaline carbonate buffer to resist lowering of pH.

Question 31

How does the carbonate system vary and what does it depend on? What happens when water is acidic and basic?

Answer 31

Depends on pH. Based on the concentration of hydrogen ions, various of the carbonate system reactions are more or less likely to occur. Acidic - H+ attach to carbonate or bicarbonate forming carbonic acid Basic - Carbonic acid breaks apart adding H+ to solution

Question 32

What causes diurnal swings in low buffering waters? What can lower these swings?

Answer 32

Susceptibility to pH changes which upsets ecosystems. Due to photosynthesis pushing pH up by removing CO2 from the water and then respiration reducing pH throughout day. Higher alkalinity since this provides buffering.

Question 33

How does acidification of sea water occur?

Answer 33

From humans releasing more CO2 into atmosphere, making sea water more corrosive which can impact corals and sea life

Question 34

What are the 3 common sources of water?

Answer 34

Upland catchment Lowland catchment Chalk aquifer

Question 35

Discuss why upland catchment needs less treatment than water from lowland or chalk aquifers, Upland has Less TS More DO Less Coliforms Lower pH

Answer 35

Has less total solids since sediments are carried downstream thus higher further down. More DO since has turbulent flow which adds oxygen into water. Lowland rivers have more plants which reduces oxygen, confined aquifer has no link to surface so no way to replenish oxygen. Coliforms show amount of biological activity, high in lowland rivers so needs more treatment.