Chemistry Exam Form 4 July

1) The list of elements in Period 3 is given below
Sodium
Magnesium
Aluminium
Silicon
Posphorus
Sulphur
Chlorine
Argon

a) Given two physical changes of the element across the period . Explain our answers
i) Change The atomic radius decreases from left to right across the period
Reason Across the period, the numbe of proton increases b one unit from one element to next element but the filled electron shells remains as three. Hence the electrostatic forces between the nucluesu and valance electrons becomes stronger.
ii) Change Electronegatively increases from left to right
Reason Across the period, the electrostatic forces between the nuclues and valance electron become stronger. Hence the tendency to attract electron is higher

b)Name an element that is composed of diatomic molecules Chlorine
c) Name an element that is used in light bulbs Argon
d) Name an element that form an acidic oxide. Sulphur

e) i) Give a chemical property of the amphoteric oxides Amphoteric oxides can react with both acid and bases to form salt and water
ii) State an element that forms an amphoteric oxide Aluminium


2) The transition element are a block of metals in the middle of the Periodic Table. They have similar properties.
i) State 4 typical properties of the transition elements [4m]
The have high melting pints and high densities [1]
They have variable oxidation states in their compounds [1m]
They form coloured compounds. The solution of their compounds in water are always coloured [1m]
The elements and their compund are good catalyst [1m]

ii) By using suitable element, list threee industrial uses of transition metals [6m]
Iron – The manufacture of ammonia in Haber Proces
Nickel – The manufacture of margarine from vegetable iols
Vanadium (V) oxide - The manufacture of sulphuric acid in Contact Process
b) Explain the following statement
i) The reactivity of Group 1 increase down the group [5m]
-One valance electron
- The number of filled electron shells increases by one unit from one element to the next element
-Atomic radius increase down the group
-The electrostatic forces between the nucleus and valance electron become weaker
- It is easier for metal lower in the group to lose its valance electron in a chemical reaction
- Reactivity increase down the group

ii) The electronegativity of Group 17 decreases down the group [5m]
- Is a measurement of the tendency of an element to attract electron
- All element in Group 17 has have seven valence electrons. The number of filled electron shells increase by one unti from one lement to the next
- The atomic radius increases down the group.

4) a) Define the following terms [6m]
i) Diffusion is a random movement of particles from a region of high concentration to a low concentration [2m]
ii) Sublimation is a process where a substance changes from the solid to gaseous state without melting [2m]
iii) Melting is a process where solid change into liquids at a constant tempeature when it is heated [2m]
b) Carbon -12 and carbon 14 are isotopes
i) What do you understand about the word isotopes. [6m]
- Isotopes are atoms of the same elemetn with different numbers of nuetrons
- They have the same proton number but different nucleon numbers.
- There are two type of isotopes, radioactive and non radioactive [2m]
- Radioactive isotopes produce harmful radiation
ii) Give an exmaple of a pair of isotopes other than carbon [2m] Na 23 and Na 24
iii) Explain why carbon 12 and carbon 14 have the same chemical properties [1m]
They have the same number of electrons
iv) Give one use of carbon -14 [2m]
To determine age of organic material through a process called carbon dating. [2m]
v) After undergoing radioactive decay, a sample of 100g of cabon contain 80g of carbon 12 and 20g of carbon13. Calculate the relateive atomic mass of carbon in this sample.
= ( 80(12) + 20(14) ) /100
= 12.4 g