KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
A sample of water drawn from a river passing through an agricultural district was divided into two portions. The first portion gave a white precipitate when acidified barium chloride was added. The second portion when warmed with aqueous sodium hydroxide gave a colourless gas, which turned a moist red litmus paper blue.
0 Comments
Give one advantage and one disadvantage of using petrol containing tetraethyl lead in motor vehicles.
Expected Response
Advantage
Expected response
Add water to the solid mixture A dissolves while B does Not
An isotope of Uranium 234U, decays by emission of an alpha particle to thorium 92
ANSWERS
The structure of a detergent is
a) Write the molecular formula of the detergent.
b) What type of detergent is represented by the formula? c) When this type of detergent is used to wash linen in hard water, spots (marks) are left on the linen. Write the formula of the substance responsible for the spots The reaction between and methanoic acid at 300C proceeds according to the information given below (a) On the grid below, plot a graph of concentration of Bromine (Vertical axis against time) (b) From the graph determine: (i) The concentration of bromine at the end of 3 minutes (ii) The rate of reaction at time ‘t’ where t = 1 ½ minutes (c) Explain how the concentration of bromine affects the rate of reaction (d) On the same axis sketch the curve that would be obtained if the reaction was carried out at 200C and label the curve as curve II. Give a reason for your answer.
(a) The elements nitrogen, phosphorous and potassium are essential for plant growth.
(i) Potassium in fertilizers may be in the form of potassium nitrate Describe how a sample of a fertilizer may be tested to find out if it contained nitrate ions. (ii) Calculate the mass of nitrogen present if a 25kg bag contained pure ammonium phosphate, (NH4)2 HPO4. (N = 14.0, H=1.0, P = 31.0, O = 16.0 (b) The table below shows the solubility of ammonium phosphate in water at different temperatures.
(i) On the grid provided, draw the solubility curve of ammonium phosphate (Temperature on x – axis)
(ii) Using the graph, determine the solubility of ammonium phosphate at 25°C (iii) 100g of a saturated solution of ammonium phosphate was prepared at 25°C I what is meant by a saturated solution? II Calculate the mass of ammonium phosphate which was used to prepare the saturated solution (c) The graph below shows how the PH value of soil in a farm changed over a period of time
(i) Describe how the pH of the soil can be determined
(ii) State one factor that may have been responsible for the change in the soil pH in the time interval AB
(a) Methanol is manufactured from carbon (IV) oxide and hydrogen gas according to the equation:
The reaction is carried out in the presence of a chromium catalyst at 700K and 30kPa. Under these conditions, equilibrium is reacted when 2% of the carbon (IV) oxide is converted to methanol
(i) How does the rate of the forward reaction compare with that of the reverse reaction when 2% of the carbon (IV) oxide is converted to methanol? (ii) Explain how each of the following would affect the yield of methanol: I Reduction II Using a more efficient catalyst (iii) If the reaction is carried out at 500K and 30kPa, the percentage of carbon (IV) oxide converted to methanol is higher than 2% I what is the sign of ΔH for the reaction? Give a reason II Explain why in practice the reaction is carried out at 700K but NOT at 500K (b) Hydrogen peroxide decomposes according to the following equation : 2H2O2(aq) →2H2O(l) + O2 (g) In an experiment, the rate of decomposition of hydrogen peroxide was found to be 6.0 x 10-8 mol dm-3 S-1. (i) Calculate the number of moles per dm3 of hydrogen peroxide that had decomposed within the first 2 minutes (ii) In another experiment, the rate of decomposition was found to be 1.8 x 10-7 mol dm-3S-1. The difference in two rates could have been caused by addition of a catalyst. State, giving reasons, one other factor that may have caused the difference in two rates of decomposition
The flow chart below shows a sequence of chemical reactions starting with copper study it and answer the questions that follow.
(a) In step 1, excess 3M nitric acid was added to 0.5g of copper powder
(i) State two observations which were made when the reactions was in progress (ii) Explain why dilute hydrochloric acid cannot be used in step 1 (iii) I Write the equation for the reaction that took place in step 1 II Calculate the volume of 3M nitric that was needed to react completely with 0.5g of copper powder. (Cu = 63.5) (b) Give the names of the types of reactions that took place in steps 4 and 5 Step 4 Step 5 (c) Apart from the good conductivity of electricity, state two other properties that make it possible for copper to be extensively used in the electrical industry.
(a) Give the systematic names of the following compounds
(b) State the observations made when Propan – 1- ol reacts with:
(i) Acidified potassium dichromate (VI) Solution (ii) Sodium metal (c) Ethanol obtained from glucose can be converted to ethane as shown below
Name and describe the process that take place in steps I and IIStep I Step II
(d) Compounds A and B have the same molecular formula C3H6O2. Compound A liberates carbon (IV) oxide on addition of aqueous sodium carbonate while compound B does not. Compound B has a sweet smell. Draw the possible structures of: (i) Compound A (ii) Compound B (e) Give two reasons why the disposal of polymers such as polychloroethane by burning pollutes the environment.
(a) State two factors that should be considered when choosing fuel for cooking
(b) The diagram below represents a set – up that was used to determine the molar heat of combustion of ethanol
During the experiment, the data given below was recorded
Calculate the:
(i) Heat evolved during the experiment (density of water = 1g/cm3 Specific heat capacity of water = 4.2 Jg-1K-1 (ii) Molar heat of combustion of ethanol (C = 12.0, O = 16.0, H=1.0) (c) Write the equation for the complete combustion of ethanol (d) The value of the molar heat of combustion of ethanol obtained in (b) (ii) above is lower than the theoretical value. State two sources of error in the experiment.
(a) The diagram below shows the extraction of sodium metal using the downs cell. Study it answer the questions that follow
(a) Give the names of the following compounds
(b) Study the information in the table below and answer the questions that follow
(i) Name the reagents used in:
Step I ……………………. Step II …………………….. Step III …………………….. (ii) Write an equation for the complete combustion of CH = CH (iii) Explain one disadvantage of the continued use of items made from the compound formed in step III
(i) Which element is likely to be hydrogen? Give a reason for your answer (ii) What is the Eθ value of the strongest reducing agent? (iii) In the space provided draw a labeled diagram of the electrochemical cell that would be obtained when half – cells of elements B and D are combined (iv) Calculate the Eθ value of the electrochemical cell constructed in (iii) above (b) During the electrolysis of aqueous copper (II) sulphate using copper electrodes, a current of 0.2 amperes was passed through the cell for 5 hours (i) Write an ionic equation for the reaction that took place at the anode (ii) Determine the change in mass of the anode which occurred as a result of the electrolysis process (Cu= 63.5, 1 Faraday = 96,500 coulombs). Aqueous potassium sulphate was electrolysed using platinum electrodes in a cell. a) Name the products formed at the cathode and anode. Anode b) How does the concentration of the electrolyte change during electrolysis. c) Why would it not be advisable to electolyse aqueous potassium sulphate using potassium metal electrodes.
During the electrolysis of aqueous silver nitrate, a current of 5.0a was passed through the electrolysis for 3 hours.
a) Write the equation for reaction which took place at the anode. b) Calculate the mass of silver deposited (Ag = 108; IF=96500C)
The diagram below shows a student's set-up for the preparation and collection of hydrogen gas.
(a) How would the final volume of hydrogen gas produced be affected if 80cm3 of 0,75M hydrochloric acid was used?
(b) Give a reason why helium is increasingly being preferred to hydrogen in weather balloons.
ANSWERS
(a) No change in volume since the number of moles of acid is equal in both cases.
(b) It is less dense and does not burn like hydrogen
a) When brine is electrolyzed using inert electrodes, chlorine gas is liberated at the anode instead of oxygen. Explain this observation.
b) Name the product formed at the cathode.
ANSWERS
(a)Chlorine ions in Brine are high concentration compared to oxide ions in solutions
(b)Hydrogen gas
An alcohol has the following composition by mass: hydrogen 13.5%, oxygen 21.6% and carbon 64.9%
a) Determine the empirical formula of the alcohol(C=12.0; H=1.0)=16.0). b) Draw the structural formulae of the compound,
a) Explain why permanent hardness in water cannot be removed by boiling.
b) Name two methods that can be used to remove permanent hardness from water.
ANSWERS
(a) The calcium and magnesium compounds in this water can not be decomposed by heating i.e. CaCl2, CaSO4, MgSO4 and MgCl2
(b)Ionic exchange Uses sodium carbonate (washing soda) |
Chemistry Topics
All
Archives
December 2024
|
We Would Love to Have You Visit Soon! |
Hours24 HR Service
|
Telephone0728 450425
|
|
8-4-4 materialsLevels
Subjects
|
cbc materialsE.C.D.E
Lower Primary
Upper Primary
Lower Secondary
Upper Secondary
|
teacher support
Other Blogs
|