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​These are chemistry questions and answers categorized according to topics, papers i.e. Paper 1 and 2, Levels i.e. form 1 to form 4, kcse year the examination was done and section A or B
Select topic/category to open topical questions from that particular option provided. ​Chemistry Topics
The following procedure was used to investigate the temperature changes that occur when sodium hydroxide solution is added to dilute hydrochloric acid.
(i) Place the acid in a glass beaker and record its temperature. (ii) Add a known volume of sodium hydroxide solution. (iii) Stir the mixture and record the highest temperature reached. (iv) Repeat steps (ii) and (iii) with different volumes of sodium hydroxide solution. (a) State two factors that must be kept constant in this experiment (b) Explain how the use of a polystyrene cup will affect the results.
ANSWERS
(a) State one characteristic of a reaction where equilibrium has been attained.
ANSWERS
(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 reached 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 in pressure 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 the 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
ANSWERS
(a) What is meant by rate of reaction. (1 mark)
(b) In the space provided, sketch the diagram of a set-up that can be used to determine the rate of reaction between manganese(IV) oxide and hydrogen peroxide. (3 marks)
(e) A student placed a small amount of liquid bromine at the bottom of a sealed gas jar of air as shown in Figure 4.
(i) Describe what will be observed: (1 mark)
I. after two minutes . II. after 30 minutes
(ii) Use the Kinetic theory to explain the observations: (2 marks)
I. after 2 minutes . II. after 30 minutes
(d) Some plants have seeds that contain vegetable oil.
Expected Response
In an experiment on rates of reaction, potassium carbonate was reacted with dilute sulphuric (VI) acid.
(a) What would be the effect of an increase in the concentration of the acid on the rate of the reaction? (b) Explain why the rate of reaction is found to increase with temperature.
ANSWERS
(a)The rate of reaction increases. This is because when the concentration is high: the number of collisions between particles is also high hence reacts faster,
(b)Increase in temperature results in increase in the kinetic energy of the particles. This makes particles move faster and collide frequently leading to faster rate of reaction.
(a) Other than concentration, state two factors that determine the rate of a reaction.
b) In an experiment to determine the rate of reaction, excess lambs of calcium carbonate were added to 2 M hydrochloric acid. The mass of calcium carbonate left was recorded after every 30 seconds. The results are shown in the table below 
i) Write the equation for the reaction that took place
ii) On the grid provided, plot a graph of mass of calcium carbonate vertical axis Against time (iii) Determine the rate of reaction at the 105th second. (c) Why does the curve level off after some time? (d) On the same grid, sketch a curve for the same reaction using 4 M hydrochloric acid and label the curve R.
ANSWERS
(c)Determine the rate of reaction at 105 sec. (Use the graph) Draw a tangent at 105 (d)Curve levels off, because all the acid has been used up. (e)Sketching a curve.
Use the information in Table 2 to answer the questions that follow.
(a) State what is meant by heat of reaction. (1 mark)
(b) Calculate the heat change when one mole of methane reacts completely with excess chlorine in the presence of UV light. (2 marks)
Expected Response
The curve shown below shows the variation of time against temperature for the reaction between sodium thiosulphate and hydrochloric acid.
a) Write the equation for the reaction between sodium thiosulphate and dilute hydrochloric acid
b) Explain the shape of the curve
ANSWERS
(b)Explain:
As the temperature increases, the time taken for the reaction to take place decreases. Explanation Increase in temperature, leads to increase in kinetic energy, thus increasing the frequency of fruitful /successful collision, hence decrease in time taken for the reaction to take place.
When solid A was heated strongly, it gave off water and a solid residue. When water was added to the solid residue, the original solid A, was formed
(a) What name is given to the process described? (b) Give one example of solid A
ANSWERS
(a)Type of reaction: Reversible reaction/temporary reaction.
(b)Copper (II) Sulphate salt (Crystals) Copper (II) Chloride hydrated. Any other hydrated salts e.g. Cobalt (II) Chloride
(a) Other than temperature, state two factors that determine the rate of a chemical reaction.
(b) A solution of hydrogen peroxide was allowed to decompose and the oxygen gas given off collected. After 5 minute, substance G was added to the solution of hydrogen peroxide. The total volume of oxygen evolved was plotted against time as shown in the graph below 
(i)Describe the procedure of determining the rate of the reaction at minute 12.
(ii)How does the production of oxygen in region AB compare with that in region BC? Explain (iii)Write an equation to show the decomposition of hydrogen peroxide. (c) Sulphur (IV) oxide react with oxygen to form Suplhur (VI) oxide as shown in the equation below 
(ii)Name one catalyst used for the reaction.
ANSWERS
(a) Pressure
Concentration Catalyst Particle size/surface area Light intensity (b) (i) Draw a tangent to the graph at 12 min. Determine change in volume /Calculate gradient. Determine change in time. Divide change in volume by change in time 
In an experiment on rates of reaction, potassium carbonate was reacted with dilute sulphuric (Vi) acid
(a)What would be the effect of an increase in the concentration of the acid on the rate of the reaction? (b) Explain why the rate of reaction is found to increase with temperature.
ANSWERS
(a) Rate increases.
(b) Temperature increases the kinetic energy of the particles increasing the number of collisions.
Describe how an increase in concentration increases the rate of a reaction. (2 marks)
Expected Response
As the concentration increases the number of reactivity of particles per unit volume increases thus affecting collisions. This leads to increase in the rate of reaction
In an experiment to study the rate for reaction between duralumin (an alloy of aluminium, magnesium and copper) and hydrochloric acid, 0.5g of the alloy were reacted with excess 4M hydrochloric acid. The data in the table below was recoded.
Use it to answer the questions that follow. 
a) i) On the grid provided, plot a graph of total volume of gas produced (vertical axis) again time. ii) From the graph, determine the volume of gas produced at the end of 2 ½ minutes. b) Determine the rate of reaction between the 3rd and 4th minute. c) Give a reason why some solid remained at the end of the experiment d) Given that 2.5cm3 of the total volume of the gas was from the reaction between magnesium and aqueous hydrochloric acid, calculate the percentage mass of aluminium present in 0.5g of the alloy. (Al = 27.0 and Molar gas volume = 24,000cm3 at 298k) e) State two properties of duralumin that make it more suitable than aluminium in aeroplane construction.
Expected Response
The curves below represent the change in mass when equal masses of powered zinc and zinc granules were reacted with excess 2M hydrochloric acid. Study them and answer the question below.
Which curve represents the reaction with zinc granules? Explain your answer.
ANSWERS
X - Zinc granules
The gradient of the graph is less steep because there is less surface area. |
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December 2024
AuthorMaurice Atika is a teacher and a netprenuer. |
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