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iii Passing air through the glass tube slowly |
To allow enough time of contact between the active part of and the heated copper turnings |
2 State and explain the observations made in the glass tube |
Colour change from brown to black |
Brown copper metal reacts with the active part of airoxygen to form black copper II oxide |
Chemical equation |
Copper Oxygen Copper II oxide |
2Cus O2 g 2CuOs |
The reaction reduces the amountvolume of oxygen in syringe B leaving the inactive part of air Copper only react with oxygen when heated |
3 Calculate the of |
i Active part of air |
active part of air V1 V2 x 100 308cm3 x 100 19493 |
V1 1580cm3 |
ii Inactive part of air |
Method 1 |
inactive part of air V2 x 100 1272cm3 x 100 80506 |
V1 1580cm3 |
Method 2 |
inactive part of air 100 active part of air |
100 19493 80507 |
4 The of active part of air is theoretically higher than the above while of inactive part of air is theoretically lower than the above Explain |
Not all the active part of air reacted with copper |
5 State the main gases that constitute |
a active part of air |
Oxygen |
b Inactive part of air |
Nitrogen carbon IV oxide and noble gases |
6 If the copper turnings are replaced with magnesium shavings the of active part of air obtained is extraordinary very high Explain |
Magnesium is more reactive than copper The reaction is highly exothermic It generates enough heat for magnesium to react with both oxygen and nitrogen in the air |
A white solidash mixture of Magnesium oxide and Magnesium nitride is formed This considerably reduces the volume of air left after the experiment |
Chemical equation |
Magnesium Oxygen magnesium II oxide |
2Mgs O2 g 2MgOs |
Magnesium Nitrogen magnesium II nitride |
3Mgs N2 g Mg3N2 s |
cTo find the composition of active part of air using alkaline pyrogallol |
Procedure |
Measure about 2cm3 of dilute sodium hydroxide into a graduated gas jar Record the volume of the graduated cylinder V1 |
Place about two spatula end full of pyrogallol1 2 3trihydroxobenzene into the gas jar Immediately place a cover slip firmly on the mouth of the gas jar Swirl thoroughly for about two minutes |
Invert the gas jar in a troughbasin containing water Measure the volume of air in the gas jar V2 |
Sample observations |
Colour of pyrogallol1 2 3trihydroxobenzene change to brown |
Level of water in gas jar rises when inverted in basintrough |
Volume of gas jar air in gas jar V1 800cm3 |
Volume of gas jar air in gas jar after shaking with alkaline pyrogallol1 2 3trihydroxobenzene V2 640 cm3 |
Sample questions |
1 Which gas is absorbed by alkaline pyrogallol123trihydroxobenzene |
Oxygen |
2 Calculate the |
i of active part of air |
V1V2 x 100 800cm3 640 cm3 x 100 20 |
V1 800cm3 |
ii of inactive part of air |
V2 x 100 640 cm3 x 100 80 |
V1 800cm3 |
dTo establish the presence of carbon IV oxide in air using lime water |
Pass tap water slowly into an empty flask as in the set up below |
Sample observation questions |
What is the purpose of paper cover |
To ensure no air enters into the lime water |
What happens when water enters the flask |
It forces the air from the flask into the lime water |
3 What is observed when the air is bubbled in the lime water |
A white precipitate is formed The white precipitate dissolves on prolonged bubbling of air |
4 a Identify the compound that form |
ilime water |
Calcium hydroxide CaOH2 |
ii White precipitate |
Calcium carbonate CaCO3 |
iii When the white precipitate dissolves |
Calcium hydrogen carbonate CaHCO3 |
bWrite the chemical equation for the reaction that tale place when |
i White precipitate is formed |
Calcium hydroxide carbon IV oxide Calcium carbonate water |
Ca OH 2aq CO2 g CaCO3s H2O l |
ii White precipitate dissolves |
Calcium carbonate water carbon IV oxide Calcium hydrogen carbonate |
CaCO3s H2O l CO2 g CaHCO3 aq |
5 State the chemical test for the presence of carbon IV oxide gas based on 4a and b above |
Carbon IV oxide forms a white precipitate with lime water that dissolves in excess of the gas |
6 State the composition of carbon IV oxide gas by volume in the air |
About 003 by volume |
BOXYGEN |
a Occurrence |
1 Fifty 50 of the earths crust consist of Oxygen combined with other elements eg oxides of metals |
2 About 70 of the earth is water made up of Hydrogen and Oxygen |
3 About 20 by volume of the atmospheric gases is Oxygen that form the active part of air |
b School laboratory preparation |
Oxygen was first prepared in 1772 by Karl Scheele and later in 1774 by Joseph Priestly It was Antony Lavoisier who gave it the name Oxygen |
Procedure |
Method 1 Using Hydrogen peroxide |
Half fill a troughbasin with tap water Place a bee hive shelfstand into the water |
Completely fill the gas jar with water and invert in onto the bee hive shelfstand |
Clamp a round bottomed flask and set up the apparatus as below |
Collect several gas jars of Oxygen covering each sample |
Sample observation questions |
1 What is observed when the hydrogen peroxide is added into the flask |
Rapid effervescencebubblingfizzing |
2 Describe the colour and smell of the gas |
Colourless and odorless |
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