CHEMISTRY TESTLET

FORM 3CHEMISTRY
28/9/2006
Name_____________________________________________________________
What do you understand by:
a) Exothermic reaction.
- Heat produced along with the products and is lost to the surroundings as the system cools to attain room temperature
b) Heat of formation.
- Amount of heat absorbed or liberated when one mole of the substance is formed from its elements.
One mole of butane (C4H10) burns completely in oxygen and liberates 2877kJ.
a) Write the equation for the combustion of butane.
- C4H10 (g) + 13/2O2 (g) 4CO2(g) + 5H2O(l)
b) Draw an energy cycle diagram for these changes.
C4H10 (g) + 13/2O2 (g) 4CO2(g) + 5H2O(l)



4C(s) + 5H2(g)
c) If the following heats of combustion are given.
DHqc (graphite) = -393kJmol-1
DHqc (H2 (g)) = -286kJmol-1
DHqc (C4H10) = -2877kJmol-1
Calculate the heat of formation of butane, C4H10 from its elements in heir normal states at standard conditions of temperature and pressure.
DHfq(C4H10) = {DHqc (graphite) +D Hqc H2 (g)}- DHqc (C4H10)
= {(-393x4)+(-286x5)}-(-2877)
= -125kJmol-1
What is electrolysis?
- Decomposition of an electrolyte to allow passage of current.
b) The set up below represents electrolysis of dilute sulphuric (vi) acid.
Gas M
Gas N

Cathode



Anode
c) Name M and N.
M; Oxygen.
N; Hydrogen
d) Write ionic equations for
i) The production of gas M.
- 4OH-(aq) + 4e- O2 (g) + H2O (l)
j) The production of gas N.
- 2H+(aq) + 2e- H2 (g)
e) At what electrode does oxidation takes place? Explain your answer.
- Anode.
f) What type of electrodes is suitable for this experiment? Explain your answer.
- Platinum, Carbon.
- Un reactive



a) Give the oxidation number of Cr in the CrO42-
- Cr = +6
b) Give the change of oxidation number of the underlined species in the following
equations.
Cr2O72- + 8H+ + 3SO3 2- 2Cr3+ + 4H2O +3SO42-

- Cr = +6 to Cr = +3
c) Why are reactions like the above described as red ox reactions?
- Involves oxidation and reduction reactions.
Study the flow chart below and answer the questions that follow.
Gas Q carbon CO2
Brown gas R
Air heat
Gas T
a) Identify gas R and T.
- R = Nitrogen (iv) oxide.
- T = Nitrogen
b) State and explain the observations made when gas Q is bubbled through Iron (ii) sulphate solution
- It burns from green to dark brown because of the formation of the complex.
- FeSO4 (aq) +NO (g) FeSO4NO(aq)
Study the information in the table below and answer questions that follow (letters are not actual symbols of the elements)
Element
E.C of stable ion
Atomic radius
Ionic radius.
N
2.8.8
0.197
0.099
P
2.8.8
0.099
0181
R
2.8
0.160
0.065
S
2.8
0.186
0.095
T
2
0.152
0.068
U
2.8
0.072
0.136
i) Write the formula of the compound formed when N reacts with P [N=20,P=17].
- NP2
ii) Identify the elements, which belong to the third period of the periodic table. Explain.
- P, R, S
- They have a large/bigger atomic radius than ionic radius hence they lose electron to be stable thus in period 3.
iii) Which of the elements identified in (ii) above come first in the third period? Explain.
- R.
- Have smaller atomic radii than S.
iv) Select two elements, which are non-metals.
- P, U.
Magnesium hydroxide is used as a medication to relieve stomach acidity. Explain why sodium hydroxide is not used for the same purpose.
- Sodium hydroxide produces a lot of heat that may damage body tissues.
Study the diagram below
NaOH HCL Solution B
Zn2+ (aq) Solid A
NaOH Solution C

a) What property does solid A in the reactions above exhibit?
- Amphoterism.
b) Write the chemical reaction leading to the formation of solution C.
- Zn (OH) 2(g) +2OH-(aq) [Zn (OH) 4] 2-(aq)