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Chemistry > Environmental Chemistry-Water > Flashcards

Environmental Chemistry-Water Flashcards

1
Q

What is Hard Water?

A

it is water that will not easily lather with soap. Hardness in water in caused by Ca2+ or Mg2+ ions dissolved in water.

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2
Q

Name the most common substance in soap

A

Sodium Stearate/ C17H35COONa

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3
Q

Write the reaction that occurs when soap is added to hard water and describe as well as name the substance formed.

A

Ca2+/Mg2+ + 2C17H35COO- -> (C17H35COO)2Ca↓
Calcium stearate or scum is a sticky white substance formed because the calcium and magnesium ions react with the soap to form insoluble substances

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4
Q

Define Temporary hardness

A

It is hardness that can be removed by boiling the water.

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5
Q

State the main points in relation to temporary hardness

A

-Rainwater is slightly acidic, water and carbon dioxide react to give carbonic acid (reversible)
-Limestone (calcium carbonate) is insoluble in water but it reacts with carbonic acid. They react to form a soluble calcium hydrogencarbonate.
CaCO3 + H2CO3 -> Ca(HCO3)2
-Water that has temporary hardness is really a solution of dilute Calcium Hydrogencarbonate.
-By boiling the water this reaction takes place:
Ca(HCO3)2 -> CaCO3↓ + CO2 + H2O
limeSCALE is produced.
-the overall effect of heating is that the calcium ions are now precipitated as insoluble calcium carbonate (limescale)

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6
Q

What tests should be carried out in order to test for carbonate ions.

A

-in a reaction with Calcium carbonate and HCl to give CaCl2 , water and CO2
The CO2 gas that is given off turns limewater milky. The presence of hydrogencarbonate ions is confirmed using a solution of MgSO4

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7
Q

Define permanent hardness.

A

It is hardness which cannot be removed by boiling the water.

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8
Q

What causes permanent hardness.

A

the presence of MgSO4(magnesium sulfate) and CaSO4 (Calcium sulfate)

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9
Q

Name some methods of removing hardness and explain.

A
  • Distillation, all dissolved solids/liquids are removed however it is expensive.
  • `Washing Soda, NaCO3.10H2O, to make it easier to form a lather, the Ca2+ ions react with the CO32- ions from the washing soda to give off a Calcium carbonate precipitate.
  • Ion-exchange resin, modern ion-exchange resins are synthetic materials that exchange /swap ions that cause hardness for ions that do not.
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10
Q

Explain what a cation-exchange resin does.

A

it exchanges the Ca2+ and Mg2+ ions with Na2+ ions (ratio 1:2).
represented as:
Ca2+ +2RNa -> R2Ca + 2Na+
it eventually needs to be replenished by passing it through a concentrated solution of sodium chloride. Then the Ca2+ and Mg2+ ions are replaced by Na2+ ions and are ready for use again.

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11
Q

How is deionised water produced for Laboratory use?

A

by passing ordinary water through a water deioniser that contains a mixture of cation exchange resin and anion exchange resin. AKA mixed-bed resin. All the positive ions are replaced by H+ ions and all the negative ions are replaced by OH- ions. These two then react to form water.

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12
Q

What is distilled water?

A

it is the purest form of water as all dissolved and suspended solids as well as dissolved gases have been removed from the water.

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13
Q

State three advantages and disadvantages of hard water.

A

Adv:
Provides calcium for teeth and bones
Nicer taste
Good for brewing and tanning leather
DisAdv:
Blocks pipes, leaves scale on kettles and boilers
Wastes soap
Produces scum

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14
Q

State and explain any two stages of water treatment. (drinking water)

A
  1. Screening-physical process, the water (from rivers, lakes etc) is passed through a wire mesh to remove any floating debris (twigs, cones etc).
  2. Flocculation-Chemical process, the coming together (coagulating) of small suspended particles in water.
  3. Sedimentation/Settlement- physical process, the flocs previously formed sink to the bottle of a settlement tank.
  4. Filtration-Physical process, the water is passed through large beds of sand. These filter beds remove any remaining suspended solids. The water is now clean but not fit to drink.
  5. Chlorination-Chemical process, Chlorine is added in small quantities to kill any harmful micro-organisms in the water.
  6. Fluoridation-Chemical process, helps to reduce tooth decay. Fluorine compounds ( sodium fluoride, NaF, and hexafluorosilic acid,H2.SiF6) are added.
  7. pH Adjustments-Chemical process, is the water is slightly acidic, it may cause corrosion of pipes. Calcium hydroxide is added. If the water is very hard, sodium carbonate is added to soften it. If this causes the pH to increase, carbon dioxide is added.
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15
Q

What purpose does Aluminium sulfate have, in what step is it seen in and what problems are caused by adding an excess of it?

A

Coagulation of small suspended particles
Flocculation
Taste of water affected//corrosion of pipes

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16
Q

What purpose does chlorine have, in what step is it seen and what problems are caused by adding an excess of it?

A

Sterilise water//kill harmful microorganisms
Chlorination
Taste and smell of water affected

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17
Q

What purpose does Fluorine compounds have, in what step are they seen and what problems are caused by adding an excess of them?

A

Reduce tooth decay
Fluoridation
Staining of teeth

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18
Q

What purpose does Calcium hydroxide have, in what step is it seen and what problems are caused by adding an excess of it?

A

Raise pH
pH adjustment
hardness of water

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19
Q

What purpose does Sodium carbonate have, in what step is it seen and what problems are caused by adding an excess of it?

A

Soften water
pH adjustment
taste of water affected

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20
Q

What purpose does Sulfuric acid have, in what step is it seen and what problems are caused by adding an excess of it?

A

Lower pH
pH adjustment
Corrosion of pipes

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21
Q

What happens if a considerable amount of organic waste is present in water?

A

The dissolved oxygen levels will drop so much that fish life in the water may be reduced or killed off completely

22
Q

What happens when the dissolved oxygen levels drop to zero?

A

Anaerobic bacteria will take over and the river will become foul-smelling due to the presence of Hydrogen sulfide (H2S ) (instead of H20)

23
Q

Define the Biochemical oxygen demand (B.O.D).

A

The amount of dissolved oxygen, consumed by biological action, when a sample of water is kept at 20°C, in the dark for five days.

24
Q

Write a balanced equation for the reaction between limescale and carbonic acid in rainwater that gives rise to temporary hardness in water.

A

H2CO3 + CaCO3 -> Ca(HCO3)2

25
Q

Why is it necessary to use diluted water to get the BOD

A

in order to ensure that dissolved oxygen will be present throughout the five-day period of the test and that a measurable amount of oxygen will be left after the five-day period.

26
Q

Under what conditions should the bottle have been kept in for the five days to get the BOD? Explain why each is necessary.

A

At 20°C in order to avoid photosynthesis which would add oxygen to the water.
A constant temperature, which gives a valid comparison of the results as the rate at which oxygen is consumed depends on the temperature.

27
Q

Defin Eutrophication.

A

is the enrichment of water with nutrients, leading to excessive growth of algae and other plants.

28
Q

Where does natural and artificial eutrophication occur and how is it caused?

A

Natural: in lakes and is caused by a gradual increase in nitrogen and phosphorus levels caused by sediments building up in these lakes.
Artificial: is caused by a sudden increase in nitrate and phosphate nutrients caused by artificial fertilisers being washed into rivers and lakes, or domestic sewage or waste. Algal blooms and covers the area and stops sunlight from getting to the plants under the water.

29
Q

give examples of Heavy Metal ions that cause pollution.

A

lead ions (Pb2+), mercury ions (Hg2+) and cadmium ions (Cd2+)

30
Q

Why are heavy metal ions that cause pollution classified as cumulative poisons? Give examples of how different ions can cause harm or damage.

A

their concentration tends to build up in the body upon continual exposure.
Lead is a poisonous substance and is particularly harmful to young children who can suffer brain damage.
Metallic mercury is dangerous when the vapour is inhaled into the lungs.

31
Q

How are heavy metal ions removed from water? Give examples of equations.

A

By precipitation.
Pb2+ + 2HCl -> PbCl2↓ + 2H+ (dilute HCl added)
Pb2+ + 2OH- -> Pb(OH)2↓ (calcium hydroxide added)
Hg2+ +S2- -> HgS↓ (Sulfide salts added)

32
Q

How do you test for heavy metal ions in water

A

AAS// atomic adsorption spectrometry

33
Q

Why is excess KI added in titrations?

A

Because I- is insoluble in water but soluble in Potassium. It also provides I- ions to make I2 for the reaction.

34
Q

Name the chemical used to determine the concentration of calcium and magnesium.

A

Edta/ Ethyldiaminetetraacetic acid

35
Q

Name the primary standard used in the water chapter

A

disodium salt

36
Q

What is a complex

A

A complex is a compound in which a module such as edta forms coordinate bonds (dative bonds) to a metal atom or ion.

37
Q

Define complexometric titration

A

A complexometric titration is a titration involving the formation of a complex between metal ions and a reagent such as edta. In this type of titration the end point is marked by a sharp decrease in the concentration of free metal ions.

38
Q

Define a buffer solution

A

A buffer solution is a solution that resists changes in pH, i.e it keeps the pH constant

39
Q

Why is it necessary to keep the pH at around 10 when determining the total hardness in a water sample using edta

A
  1. The reaction between the Ca2+ and Mg2+ ions and the edta is pH dependent and the edta may not fully complex with these ions if the pH is not kept constant at pH = 10
  2. The Eriochrome Black T indicator requires a pH of 8 to 10 for the colour change to occur.
40
Q

Why is edta solution stored in a plastic container?

A

The edta must be stored in a plastic container because edta reacts with the ions in glass if it is stored in a glass container for a long period of time.

41
Q

Name the colour change in the determination of hardness in a water sample using edta

A

Wine-red to blue.

42
Q

Define the primary treatment of sewage.

A

Primary treatment of sewage is a mechanical process in which large solids are removed by screening and some suspended solids are removed by settlement (sedimentation)

43
Q

Define the secondary treatment of sewage.

A

Secondary treatment of sewage is a biological oxidation process in which the levels of suspended and dissolved organic materials are reduced, i.e. the sewage is decomposed by means of bacteria which use the nutrients from the sewage together with oxygen from the air to break down the sewage.

44
Q

Define the tertiary treatment of sewage.

A

Tertiary treatment of sewage is a process involving the removal of phosphorus compounds by precipitation and the removal of nitrogen compounds by biological and ion-exchange methods.

45
Q

What is the principle of colorimetry?

A

The principle of colorimetry is that the amount of absorbance of light by a coloured solution is proportional to the concentration of the solution.

46
Q

What is screening and why is it done?

A

What- solids that are floating in the sewage are removed by passing the sewage through steel bars.
Why- It is important to remove items such as twigs, leaves, plastic bags and other floating debris as these could interfere with the pumps and other equipment later in the treatment process.

47
Q

How much of the suspended solids and B.O.D does primary treatment of sewage remove.

A

One half of suspended solids and one third of the B.O.D

48
Q

How much B.O.D does secondary treatment of sewage remove.

A

95% of the B.O.D has been removed.

49
Q

Describe the Activated sludge process.

A

The Activated sludge process is a common method of bringing about bacteria breakdown of the sewage.

  • the sewage is pumped into a large aeration tank where it provides nutrients for a large number of growing micro-organisms
  • the micro-organisms use the nutrients from the sewage together with the oxygen from the air to break down the sewage.
  • from the aeration tank, the sewage flows into a settling tank.
  • the sludge removed is rich in nutrients and may be used as fertiliser or stored in a sealed tank where anaerobic bacteria convert to methane.
50
Q

How are phosphates removed in the tertiary treatment of sewage.

A

They are removed by precipitation.
e.g by adding aluminium sulfate Al2(SO4)3
this precipitates aluminium phosphate :
Al3+ + PO4 3- –> AlPO4 ↓
phosphate may also be removed by adding iron (III) chloride (removes as iron(III) phosphate.
lime may also be used.

51
Q

Why is tertiary treatment not performed in all sewage plants

A

Because it is expensive

52
Q

Name the three methods that are used to analyse water.

A 
pH measurement (universal indicator)
AAS (Atomic Absorption Spectrometry)
Colorimetry

Chemistry (15 decks)

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