CHE2: NATURAL WATER AND HYDROGEN

Natural water is never pure it contains dissolved solids and gases and suspended gases

  • Rain water

It is the purest form of natural water and it is fairly poor, it contains dissolved gases, dust bacteria and other impurities

  • Spring or well water

Some rain water percolates into the ground and is called ground water. It passes through soil and rocks and it dissolves some solids. The water forms a lake underground. It can be accessed at the spring or well

  • River water

Some rain water runs along the surface of the ground and is called surfaced water. It contains dissolved solids, gases on plenty of dirt and mud.

  • sea water

Water in streams and rivers flow into lakes and seas. Sea water contains a lot of impurities.

WATER TREATMENT: (PURIFICATION OF WATER)

Removal of pollutants from water involves the following processes

  1. Screening

Water is passed through big and small screen to remove big and suspended solids.

  1. Filtration

Through course sand and fine sand to remove small suspended particles.

  • Segmentation

Water after filtration is not pure enough. It contains fine suspended particles which must be removed

This is done by adding a powered flocculants for example potassium aluminums sulphate of the water. Addition of the alum encourages the fine suspended particles to settle at the bottom of the tank. This is called Segmentation. Addition of alum to water is called flocculation.

  1. Chloride

Is the addition of chlorine to the water to kill disease causing germs?

Chloride is disinfectant

  • The safest water for drinking is boiled water

 

Water treatment Process in the state of Illinios, USA. Internet Photo

HYDROGEN: H2

Laboratory preparation

The gas is prepared by reacting metals like zinc, iron, and magnesium with dilute acids.

Metal + acid              salt + hydrogen (word equation)

The most commonly used metal in the laboratory is zinc metal and the most commonly used acids are dilute hydrochloric/sulphric acid.

Copper (ii) sulphate is added to a mixture to speed up the chemical reaction. The reaction occurs at room temperature. (It doesn’t require heating)

Zinc + dil sulphiric acid              zinc chloride + hydrogen

Zn + H2              ZnSo4 +H2

Zinc + dil hydrochloric acid              zinc chloride + hydrogen

Chemical test for hydrogen              i+i6 a colorless gas which gives a pop sound when a burning wooden splint i6 applied.

 

Note

Pure hydrogen burns quietly with a faint blue flame but mixture of hydrogen and air gives a pop sound when a burning splint is applied

Properties of hydrogen

-Colorless

-Odorless

-Tasteless

-Slightly soluble in cold water

-It is less dense than aired and therefore is collected by upward delivery or downward displacement of air

 

 

-It is neutral to litmus

-It is the lightest gas known (used for filling pillions at weather station)

-It diffuses faster than any other gas

Test for hydrogen

A mixture of hydrogen and air gives a pop sound when a burning splint is applied

Reducing action of hydrogen

  • Reduction

Is removal of oxygen from a substance?

  • Oxidation

Is gain of oxygen?

Hydrogen removes oxygen from hot oxides of iron, lead and copper.

Copper (ii) oxide + hydrogen              copper metal + water

Cuo + H2              Cu(s) + H2O (L)

Lead (ii) oxide + hydrogen             Lead metal + water

PBO (S) + H2                      Pb(s) + H2O (L)

  • A reducing agent

Is the oxygen accepter, an oxidizi1ng agent is oxygen donor.

Trinitron tetra oxide + hydrogen iron                     water

Fe3O4 + 4H2                   3 Fe + 4H2O

Hydrogen combines with chloride to form hydrogen chloride

Chloride + hydrogen               Hydrogen chloride

Cl (g) + H2(g)                       2HCl(g)

Hydrogen reacts with nitrogen gas to form ammonia gas.

Nitrogen + hydrogen                 Ammonia

N2(g) + 3 H2 (g)                       2NH3(g)

Preparation of dry hydrogen gas

Set up of the apparatus

 

The top of the funnel is opened to bring the acid into contact with metal. Zinc reacts with dilute sulphric acid to give hydrogen gas.

Zinc + dilute sulphric acid              Hydrogen + zinc sulphate

Zn + H2SO4                H2+ZnSO4

The metal is catalyzed by copper (ii) sulphate. The hydrogen produced is bubbled through concentrated sulphric acid to dry it and collected by upward delivery, (downward displacement of air). Hydrogen can also be dried by using anhydrous calcium chloride (Fused Calcium Chloride)

 

 

Intermittent supply of acid hydrogen can be obtained by using Kipp’s apparatus refer to the figure above. Fairly big pieces of zinc metal are added to the middle bulb, dilute hydrochloric acid is poured into the bulb until covers the zinc.

The gas bubbled is collected in a washed bottle containing water to remove any fumes of carried over by the hydrogen

Uses of hydrogen

  1. It is used in the manufacture of ammonia gas by the habour process.
  2. Used for filling balloons at a weather station. This is because the lightest gas known.
  • It is with oil in manufacture of blue band margarine
  1. It is used in the conversion of coal to synthetic petrol
  2. It is used in the oxy-hydrogen flame used for cutting and welding steel

Burning of hydrogen in air (Synthesis of water)

Synthesis is a combination of two or more elements to form a compound

Hydrogen + oxygen                water vapor

2H2 (g) + O2 (g)                        2H2O (g)

 

Hydrogen from a cylinder is used.  If it’s not available town gas or bottled by gas is used. The hydrogen is lit at the jet. The flame burns and hot gases from it are sucked up the funnel and through the cooled test tube by filter pump. Part of gases condenses to form a colorless liquid.

Testing to prove that the colorless liquid is water

  1. Chemical test

A little of the liquid added to anhydrous copper (ii) sulphate and it turns blue

  1. Physical test

The boiling point of liquid is determined it is 100oC

Water is formed by burning hydrogen with oxygen

It is a synthesis reaction

H2(g) + O(g)             H2O(L)

 

Hydrogen + Oxygen                water

Action of metals on water and steel

To study reaction of sodium and potassium on water

Sodium reacts violently with cold water.

Potassium is even more reactive and dangerous.

A small piece of sodium metal is dropped into a trough containing cold water, (are must be taken to avoid wetting the walls of trough with water otherwise sodium would stick on them.

Observation 

-The metal floats (sodium is less dense than water)

-It melts to a silvery ball and moves about on the surface of water reacting with it to produce hydrogen gas.

-The resulting solution turns pink/red litmus blue (it is alkaline)

-If touched between fingers the resulting solution has a soapy fed

Sodium + cold water               sodium hydroxide solution + hydrogen

Na(s) + 2H2O (L)                               NaOH (ag) + H2 (g)

Potassium metal

A small piece of potassium is dropped into a trough of cold water.

Observation

-The metal floats on the water (it is less dense than water)

-It melts to a silvery ball which moves about vigorously on the surface of the water producing hydrogen gas which burns with purple flame (lilac flame)

-The resulting solution is alkaline that turns red litmus blue

-Has a soapy fecal

Potassium + cold water           Potassium hydroxide

K(s) + 2H2O (L)                  KOH (aq) + H2 (g)    

 

Note

Potassium reacts more vigorously and dangerously than sodium.

With potassium metal the hydrogen produced, ignites and burns with a lilac flame while in sodium it doesn’t burn unless sodium movement is restricted.

To collect hydrogen gas flame when sodium reacts with water

A small piece of sodium is cut with knife and is wrapped with wire gauze. Space in left for the sodium to move in the gauze.

If cannot move there is a danger of an explosion in the experiment. A boiling tube is filled with water and is inverted in a beaker of water.

If the cage is dropped into the beaker, the boiling tube is moved to bring it over the cage to collect gas formed as shown below.

 

Reaction of calcium and magnesium with cold water

Calcium

A beaker is filled with water, calcium are dropped into the beaker using tongs. The metal sinks and reacts quickly forming hydrogen gas. Sometimes the gas marks the metal float. The resulting solution is alkaline because it turns pink litmus blue. Sometimes the solution turns milky because calcium hydroxide solution, clime water.

Calcium + cold water             calcium hydroxide + hydrogen

Cu(s) +2H2O (L)                     Ca (OH) (ag) + H2 (g)

If carbon dioxide gas is bubbled through the filtrate it turns milky.

Collecting the gas

Calcium metal is dropped into a beaker full of water and metal is covered with a glass filter funnel.

The gas is collected in a boiling tube.

Magnesium

Clean magnesium ribbon by wrapping it with steel wool until it’s brightly shiny. Roll it in a ball and drop it in hot water. Few bubbles of hydrogen slowly form on the metal which becomes dull. Place a boiling tube filed with water over the magnesium metal and leave it for a week. Test any gas formed with a burning splint to prove that its hydrogen.

Magnesium + water Magnesium                     hydroxide + Hydrogen

Note

Magnesium gives a very slow reaction with cold water.

  • Arrange the metals: Potassium, Sodium, magnesium in their order of decreasing reactivity in water.

Potassium

Sodium

Calcium

Magnesium

 

To study the reaction ribbon is cleaned with sandpaper or steel wool, cotton wool or glass wool is moistened with water. The apparatus is set up as shown the figure below;-

 

The magnesium ribbon is heated strongly. Heat conducted down the tube boils the water thus converting it into steam. The magnesium catches fire, and burns brightly. Much heat is given off. The tube sometimes swells or it breaks and the hot hydrogen may burn when it meets air.

  • The residue is white magnesium oxide because the temperature is too high for the formation of magnesium oxide.

Magnesium + steam Magnesium                     oxide + hydrogen

Mg(s)+ H2O(g)                    MgO(S)+ H2(g)

To study reaction of steam on heat iron:

 

The water is boiled: the end of the combustion tube was the steam enters is warmed so that steam does not condense. When all air is out of apparatus, gas jar is put in position. Iron is heated and steam is passed at a low rate. The iron changes to blue-black; tri iron tetra oxide and hydrogen collect in the gas jar.

Word equation

Iron + steam            Hydrogen + Tri iron Tetra oxide

3Fe(s) + 4H2O (g)                   Fe3O4(s) +4H2 (g)

-The reaction is reversible

-Reversible reaction is one which can proceed in either direction depending on temperature, pressure concentration ETC.

Reaction of metals and water

ASSIGNMENT : NATURAL WATER AND HYDROGEN ASSIGNMENT MARKS : 10  DURATION : 1 week, 3 days

 

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