KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Give two reasons why helium is used in weather balloons
Expected Response
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Starting with copper metal, describe how a solid samples of copper (II) carbonate can be prepared.
The information below relates to elements S,T,U and X. ( the letters do not represents the actual symbols of the elements.
Both T and X are in group II of the periodic Table) (b) Arrange the elements in order of their increasing reactivity
Study the information in the table below and answer the questions that follow. The letters do not represent the symbols of the elements.
a) Write the electrons arrangement for the atom formed by elements and M and Q
b) Select an element which is
d) Element R lodes its outermost electron more readily than I. Explain e) Using dots(.) and crosses (x) to represent outermost electrons show bonding in the compound formed elements N and Q. f) Explain why the melting point elements M is higher than that of element . g) Describe how a solid mixture of sulphate of R and lead sulphate can be separated into solid samples.
Ammonium ion has the following structure:
Label on the structure:
(a) covalent bond;
(b) coordinate (dative) bond.
The table below shows properties of some elements A, B, C and D which belong to the same period of the periodic table. The letters are not the actual symbols of the elements.
(a) Arrange the elements in the order they would appear in the period. Give a reason.
(b) Select the metallic element which is the better conductor of electricity. Give a reason.
ANSWERS
(a) BDACAcross the period the atomic radius decreases.
(b) D Across the period the conductivity increase due to increase in delocalised electrons.
Charcoal is a fuel that is commonly used for cooking. When it burns it forms two oxides.
(a) Name the two oxides. (b) State one use of any of the two oxides.
ANSWERS
(a) Carbon (IV) Oxide and Carbon (II) Oxide
(b) CO2- Fire extinguishers Fizzy drinks Food preservative . Solvay process . CO -Manufacture of fuel (water gas) Reduction in the extraction of metals Manufacture of methanol
(a) Other than their location in the atom, name two other differences between an electron and a proton.
(b) the table below gives the number of electrons ,protons and neutrons in particles A,B, C , D, E, F and G
(i) Which particle is likely to be a halogen?
(ii) what is the mass number of E (iii) write the formula of the compound formed when E combines with G iv) Name the type of bond formed in (iii) above. (v) How does the radii of C and E compare ? Give reason. (vi) Draw a dot (.) and cross(x) diagram for the compound formed between (vii) Why would particle B not react with particle D?
ANSWERS
(a)- electron has I mass while proton has mass of one mass unit. 1840
- proton is positively charged while electron is negatively charged.
(iv)Ionic bond or electrostatic
(v)E has a smaller atomic radius than C E has more protons than C : nuclear attraction stronger.
(vii)Particle B is inert with a stable electronic configuration will not react.
What name is given to elements which appear in group (II) of the periodic table?
ANSWER
Explain why the following substances conduct an electric current.
(a) Magnesium metal. (b) Molten magnesium chloride.
The diagram below shows the bonding between aluminium chloride and ammonia.
(a) Name the types of bonds that exist in the molecule.
(b) How many electrons are used for bonding in the molecule?
The table below gives the number of electrons, protons and neutrons in substances X, Y and Z. Study it and answer the questions that follow.
(a) Which letter represents an ion?
(b) Which of the substances are isotopes? Give a reason.
ANSWERS
(a) Y
(b) Y and Z They have the same number of protons (8) but different atomic masses.
The table below gives some properties three elements in group (VII) of the periodic table .Study it and answer the questions that follow:
(a) Which element is in liquid form at room temperature? Give a reason.
(b) Explain why the boiling point of iodine is much higher than that of chlorine.
ANSWERS
(a) Bromine M
At room temperature (25°C), Bromine is liquid since its MP and BP is between -7 and 59 (b) Atomic mass of iodine is higher than that of chlorine. Van der waal’s forces are stronger in iodine than chlorine hence iodine’s BP is higher than that of chlorine.
Graphite is one of the allotropes of carbon.
(a) Name one other element which exhibits allotropy. (b) Explain why graphite is used in the making of pencil leads.
ANSWERS
(a) Sulphur or phosphorus.
(b) Carbon atoms in graphite are arranged in layers of hexagons which are held by weak van der waal forces. The layers slide over each other when some force is applied on them; hence suitable in making pencil leads.
Distinguish between the terms deliquescent and efflorescent as used in chemistry
ANSWERS
The diagram below represents part of the periodic table. Use it to answer the questions that follow.
a) Write the electronic arrangement for the stable ion formed by W.
(b) Write an equation for the reaction between V and Q. (c) How do the ionization energies of the elements M and T compare? Explain.
The diagram below shows a set up for the laboratory preparation and collection of dry chlorine gas
Expected Response
Molar heats of vaporization increase down the group with increase in van der waals forces of attraction as the number of electrons increase/increases in size/ volume/mass/energy levels/ density/ No. of protons (atomic number)
Use the information in the table below to answer the questions that follow. The letters do not represent that actual symbols of the elements.
a) Give the reasons why the meling point of:
i) S is higher than that of R ii) V is lower than that of U b) How does the reactivity of W with chlorine compare with that of R with chlorine? Explain, c) Write an equation for the reaction between T and excess oxygen
d) When 1.15g of R were reacted with water, 600cm3 of gass was produced.
Determine the relative atomic mass of R. (Molar gas volume = 24000cm3) e) Give one use of element V
ANSWERS
(a)(i) Metallic bonds in S are stronger than in R.
(ii) V is monoatomic (independent) hence weaker van der waals forces while U is diatomic hence stronger van der waals forces V has less van der waals forces while U has more van der waal forces (b) reacts more vigorously/more reactive/reacts faster - reactivity of group 1 elements increases down the group/Ionisation energy is less than that of R - easier to lose outer electron in W than in R W is more electropositive than R
The atomic numbers of phosphorus, sulphur and potassium are 15, 16 and 19 respectively. The formulae of their ions are P3-,S2- and K+. These ions have the same number of electrons.
a) Write the electron arrangement for the ions. b) Arrange the ions in the order of increasing ionic radius starting with the smallest. Give a reason for the order.
Describe how a solid sample of the double salt, ammonium iron(II) sulphate, can be prepared using the following reagents; Aqueous ammonia, sulphuric(VI) acid and iron metal.
ANSWERS
Carbon (II) oxide is described as a “silent killer”
a) State one physical property of carbon (II) oxide that makes it a “silent killer” b) State and explain one chemical property that makes carbon (II) oxide poisonous to human beings
ANSWERS
(a)gas has no colour and smell.
(b)Carbon (II) oxide has high affinity for iron in the haemoglobin in the blood, or displaces oxygen from haemoglobin, the body tissues are deprived of oxygen.
Using electrons in thee outermost energy level, draw the dot (.) and cross (x) diagrams for the molecules H2O and C2H4. (H = 1, C = 6, O = 8)
i) H2O ii) C2H4 b) The formula of a complex ion is Zn(NH3)42+. Name the type of bond that is likely to exist between zinc and ammonia in the complex ion.
a) What is meant by the terms: i. element ii. atomic number
b) The formula for a chloride of titanium is TiCl3. What is the formua of its sulphate?
ANSWERS
(i) Element — substance that consists of one type of atoms.
(ii) Atomic number — number of protons in an atom. Ti3(S04)2
a) Distinguish between a deliquescent and a hygroscopic substance.
b) Give one use of hygroscopic substance in the laboratory.
ANSWERS
(a)Deliquescent - a substance that absorbs water from the atmosphere and changes into a solution.
Hygroscopic — a substance that absorbs water from the atmosphere but just becomes wet. (b)drying agent.
Write the formula of sulphide of an element C, whose atomic number is 5. (C is not the actual symbol of the element)
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