20. Reaction rates and reversible reactions and Answers

Reaction rates and reversible reactions

1.   Study the following equilibrium reaction and answer the questions that follow:-

HL_(aq) + H₂O(_l)   H₃O⁺(aq) L^–(aq)

 Given that in an acid solution, H₃O⁺_(aq) act in place of hydrogen ions, H⁺, according to the

equation.

 H₃O_(aq) + OH^–_(aq) 2H₂O_(l)

Explain what would be observed when potassium hydroxide solution is added to the above equilibrium mixture  

2.  The scheme below shows the energy changes that take place between ice, water and steam.

 Study it and answer the questions that follow:-

 (a) What name is given to the energy change H₄?

 (b) What is the sign H₃, give a reason  

3.  The table below gives bond energies for three covalent bonds

(a) Calculate the energy change for the following reaction:

  H_2(g) + Cl_2(g) 2HCl_(g)  

(b) Sketch an energy level diagram for the reaction in (a) above  

4.  The sketch below was obtained when 2g of magnesium was reacted with excess of 2M

hydrochloric acid. The volume of hydrogen evolved was then plotted against time as

shown below:

   (a) On the same axis plot the graph that would be obtained if 1M hydrochloric acid

was used instead of 2M hydrochloric acid. Explain

(b) Explain the significance of the flat portion BC of the curve

5.  In a closed system an equilibrium exists between Nitrogen (IV) Oxide and dinitrogen

tetraoxide as shown in the equation below:

N₂O_{4 (g)} 2NO_{2 (g);} H = + 27.5KJ

(a) State and explain the observation when a glass syringe containing the equilibrium

mixture is immersed in ice-cold water  

 (b) If the piston of the syringe is pushed, state the effect on the position of the equilibrium

6.  The table below gives the volumes of the gas produced when different volumes of 2M hydrochloric acid were reacted with 1.0g of a lump of an alloy of Magnessium and copper at room temperature

 (a) Write an equation for the reaction that occurred

 (b) On the grid provided below, plot a graph of the volume of the gas produced (vertical axis)

against the volume of acid added (Note that before the reaction comes to completion, the

volume of the gas produced is directly proportional to the volume of the acid added)  

(c) From the graph, determine:

(i) The volume of the gas produced if 13.0cm³ of 2M hydrochloric acid had been used  

(ii) The volume of 2M hydrochloric acid required for the reaction to go to completion  

 (d) State and explain the effect on the rate of production of the gas if:

(i) 1.0g of the lump of the alloy were replaced by 1.0g powder of the alloy

(ii) The reaction was carried out at 35^oC.  

 Hydrochloric acid, the rates of evolution of hydrogen gas were found to be as follows:-

(I) (a) Draw a graph of rate of evolution of hydrogen gas against length of ribbon *

  (b) What conclusion can you make from this graph? *

  (c) Determine the rate of evolution of hydrogen gas from a piece of magnesium ribbon

12cm long under the same conditions  *

(d) With dotted line, sketch on the same axis the graph that would be obtained if all the

ribbons were ground into powder  *

(II) (a) The curves below represent the changes in concentration of substances E and F with

time in reaction

E_(g) F_(g)

(i) Which curve represents the change in the concentration of substance F? Give a reason  *

(ii) Give one reason for the shapes of the curves after two minutes  *

8.  A typical electrolysis cell uses a current of 40,000 amperes. Calculate the mass (in Kg of aluminium produced in one hour). (Al = 27) (Faraday = 96500Coloumbs )

9.  The table below shows the volumes of nitrogen (IV) oxide gas produced when different

volumes of 1M nitric acid were each reacted with 0.635g of copper at room temperature.

 a) Give a reason why hydrochloric acid can not be used instead of nitric acid

b) Explain how the rate of the reaction between copper and nitric acid would be affected
if the temperature of the reaction mixture was raised

 c) On the grid provided below, plot a graph of the volume of the gas produced (vertical axis)

against volume of acid

 d) Using the graph, determine the volume of:

  (i) Nitrogen (IV) oxide produced when 30cm³ of 1M nitric acid were reacted with 0.635g

  of copper

  (ii) 1M nitric acid which would react completely with 0.635g of copper  

the volume of gas collected against time when dilute sulphuric acid

  is reacted with Zinc granules:-

 (a) Determine the rate of reaction between the 1^st and 3^rd minute  

  (b) When did the reaction stop?

 (c) Give a possible reason for the reaction to stop

11.  The equation below represents a reaction that takes place in an industrial process

  4NH_3(g) + 5O_2(g) 6H₂O_(g) + 4NO_(g)

 (a) Name the catalyst used

 (b) What are the other conditions for the reaction?

 (c) Why are the products cooled before being oxidised?

12.  Nitrogen reacts with hydrogen according to the equation below:-

N_2(g) + 3H_2(g) 2NH_3(g) H = -92KJ

  (a) How would the yield of ammonia be affected by increase in :-

(i) Pressure

(ii) temperature  

(b) The ammonia produced is isolated form the other gases from time to time. How does

this affect the equilibrium?

13.  Explain how you would obtain an insoluble salt XSO₄ when you are provided with the following :-

 (i) Distilled water

 (ii) Solid YSO₄ which is soluble in water

  (iii) Solid salt X(NO₃)₂

14.  Metal R and S were used to form a cell. The following half equations show the standard electrode

 potentials of the half cells. (R and S are not actual symbols of the element)

R²⁺ +
2e^– R_(s) E^ = ^–2.04V

S²⁺_(aq) + 2e^– S_(s) E^ = ^–0.47V

 Write the full equation for the cell and calculate the e.m.f

15.  The apparatus below were used to study the properties of air

 (a) State two observations made at the end of the experiment  

 (b) Give one simple method that can be used to prevent rusting  

16.  Equal volumes of 1M monobasic acids K and L were each reacted with excess zinc granules.

 The table below shows the volumes of the gas produced after two minutes

 (a) Explain the difference in the volumes of the gas produced  

(b) How can the production of the gas be increased?

17.  The following is a thermochemical equation for the reaction between hydrogen and oxygen

 H_2(g) + O₂(_g) H₂O_(l) H = -287kJmol^-1

 Calculate the bond energy between the elements in water given that:

 O = O = +496kJmol^-1   H-H = + 435kJmol^-1

18.  AgClO_2(s) Ag_(s) + ½ Cl_{2 (g)} + O_{2 (g)} H = 0.00KJ/mol

 What is the effect on the position of equilibrium of the above system if temperature is

decreased? Give a reason

19.  Sodium carbonate was reacted with dilute sulphuric (VI) acid at 25^oC. The volume of

carbon (IV) Oxide gas liberated was recorded at 10seconds interval. Below is a graph of

the volume of carbon (IV) Oxide gas evolved against time.

(a) On the same axes, sketch the curve labelled V that would be obtained if Barium carbonate

was used instead of sodium carbonate. (All conditions remain constant)

20.  (a) What is meant by activation energy?  

 (b) A certain mass of unground compound X1 reacted with excess dilute hydrochloric acid.

The volume of carbon (IV) oxide gas liberated was measured after every 20 seconds.

The results were presented as shown in the graph below:-

 (i) On the same axis, sketch the curve that would be obtained if the experiment was repeated

using ground compound X1

 (ii) Explain the shape of your curve in (b) (i) above

22.  The sketch below shows the rate at which substance T is converted into U. Study it and

answer the questions that follows:-

 When the equilibrium has been established the two curves become horizontal after sometime.

  Explain the effect of the amount of the two reactants and products  

23.  Elements A, B, C, and D are not actual symbols, have atomic numbers 19, 9, 12 and 10 respectively.

 (a) Which two elements represent non-metals

  (b) Write the formula of the compound formed between elements B and C and identity the

bond present in the compound

24.  An equilibrium is established between nitrogen tetra -oxide and nitrogen (IV) oxide as shown below:  State and explain what happens when temperature is increased  

N₂O_4(l) 2NO_2(g)

Pale yellow Red-brown fumes

with time in the reactions:

CaCO_{3 (s)} + 2HCI _(aq) CaCl_{2 (aq)} + H₂0 _(g) + CO_{2 (g)}

(a) Which curve shows the amount of calcium chloride varying with time? (lmk)

(b) Explain why the two curves become horizontal after a given period of time. (lmk)

(c) Sketch on the graph how curve II would appear if the experiment was repeated using a

more dilute hydrochloric acid solution   (lmk)

26 State the effect on the equilibrium when;

a) Pressure is increased

b) Oxygen gas is added

6.  An equilibrium is established between CrO₄ and H⁺ ions as shown below:

 2CrO_{4 (aq)} + 2H⁺_(aq) Cr₂O_7(aq) + H₂O_(l)

(Yellow) (Orange)

27.  State and explain and explain the observation made when aqueous sodium hydroxide is added

to the equilibrium mixture

28.  Two experiments were carried out as follows and the volume of hydrogen gas evolved

measured at intervals of 10seconds for 100seconds.

(i) 8cm of magnesium ribbon was added to 1M hydrochloric acid

(ii) 8cm of magnesium ribbon was added to 0.5M hydrochloric acid

Graphs of volume of hydrogen evolved against time were plotted

(a) Which of the graph was obtained for reaction (i) above? Explain

(b) Explain the general shape of the graph

29. Bromine dissolves in water forming a brown solution, according to the dynamic

equation below.

Br_2
(aq) + H₂O _(l) 2H⁺ _(aq) + Br^–_(aq) + OBr^–_(aq)

State and explain the observation that could be made if a solution of sodium hydroxide

is added to the system

Reaction rates and reversible reactions Answers

1.  colour changes from red to blue

 H₃O⁺ions and L^–_(aq) ions which form red solution.

2.  (a) H₄ – latent heat of fusion

 (b) H₃ – is negative particles lose hat/process is exothermic/heat is given out (any )

3.  a) H-H(g) + Cl – Cl (g) 2H – Cl

  Bonds broken bonds formed

 ∣H-H = 435 KJ 2 H – Cl = 430 x 2

 ∣ Cl – Cl = 240 KJ. = 860 KJ.

 Total = 675 KJ.

∆H_R = 860 + 675 ^√½

= -185KJ ^√½

 b)

4.

• Graph should be less steeper showing lower reaction rate since HCl is less concentrated. ^√½
  

b) Graph flattens out at BC showing that all the magnesium has been used up, hence, no reaction is taking place ^√½ and there is therefore no evolution of hydrogen gas. ^√½ The volume of the gas, therefore, remains constant. ^√½

5.  a) Pale yellow liquid is observed. ^√1 Backward reaction is favoured since ^√½ it is exothermic.

  Dinitrogen tetra oxide liquefies^√½ at very low temperature to pale yellow liquid.

 b) Pressure increase, and favours backward reaction ^√½ which is at lower pressure hence

  equilibrium shifts to the right. ^√½

6.  a) Mg(s) + 2 HCl(aq) MgCl₂(aq) + H₂(g) √1

 b)

 c) (i) Showing on the graph. √½   Answer √½

  (ii) Showing on the graph. √½   Answer √½

 d) i) The rate of reaction increases. √1

  The surface area of particles has been increased √½ thus increasing the area √½ of

contact of the reacting particles.

  ii) The rate of reaction increases. √1

  Increase in temperature results in crease in the kinetic energy of the particles. This makes

the particles move faster and collide more frequently with sufficient energy to cause more

effective collision per given time. √1

7.  I a) Drawn on graph paper  A = ½ mk

S = ½ mk

P = 1 mk

C = 1 mk

 b) Rate of evolution of hydrogen gas increases with increase in length of magnesium ribbon.

 c) Read from the student’s graph. 1 mk – showing on graph

1 mk – for answer.

 d) Shown on the graph paper.

II  a)(i) Curve I   Reason: F increases as E decreases.

  (ii) Equilibrium is achieved.

8.   Q = 40000 x 60 x 60 = 144000000c

Mass of Al = 144000000 x 27

3 x 96500

= 13.43kg

9.  a) Hydrochloric acid is a weaker oxidizing agent which cannot oxidize copper to form

Nitrogen (VI) Oxide gas

 b) It increases  1 mark

Molecules/ particles acquire the necessary activation energy// Kinetic energy. This

increases the frequency of collisions hence the rate of reaction  ½ mark

 c) Graph  – Scale 1 mark with axis well labeled

– Plotting + all points correct 1 mark

5 correct points ½ mark

  Less than 5 points 0 mark

  Correct smooth curve  1 mark

  TOTAL 3 marks

 d)  i) 360cm³ Read correct value from graph + .05

  –

ii) 40cm³ = Value from graph + .05

–

Read where it levels off

10.  (a) 260 – 85 = 175 = 87.5cm₃/mn;

  2 2

(b) 4 ½ min;

(c) Zinc was used up / H₂SO4 used up;  

11.  (a) Platinum / Platinum Rhodium

 (b) Pressure -9atm  ( ½ mk)

  Temp – 700^oC – 900^oC  ( ½ mk)

 (c) Reaction is exothermic

12. (a) (i) Will increase;  

(ii) Decrease;

13.

• Dissolve solid^{√ ½} YSO₄ to obtain^{√ ½} YSO₄ in solution,
  
• Dissolve ^{√ ½} X(NO₃)₂ in water to obtain ^{√ ½} X (NO₃)₂ solution.   3
  
• Mix the two above solutions
  
• Filter to obtain XSO₄
  solid residue, rinse with water and dry by heating^{√
  ½}
  
  
• under asbestos pad.  
  

14.  R_(s) + S²⁺_(aq) R²⁺_(aq) + S_(s)
√1  (1 mk)

 E.m.f  = 0.47 – (-2.04)

= -0.47 + 2.04   2

= 1.57V √1 (1 mk)

15.  (a) – Water level rises. √1  (1 mk)

  Grey Iron wool changes to brown. √1 (1 mk) 3 (any one of these scores)

 (b) – Oiling and greasing. √1  (1 mk)

  – Painting.

16.  (a) L is more ionized √1 than K hence reacts faster √1 producing higher

volume of a gas. Or L is a stronger acid therefore ionized faster than K a weaker acid 3

(b) Increasing the temeprature√1 using zinc powder/increasing the concentration of acid.

17.  Energy of reaction = Bond breakage + Bond formation. √1

 Bond formation = Energy of reaction – Bond Breakage

= -287 – 931 √1 3

= – 1218 K Joules per mole. √1 (3 mks)

18.  – No effect on the position of the equilibrium

– RXM is neither endothermic nor exothermic hence not affected by changes in temperature

enthalpy is zero.  

19.  (a)

21.  a)the minimum energy required by the reaction particles to cause a successful collision

to form product

  b)i)

NB. I) Sketch curve should be to the left and both flatten not at the same final volume

ii)curve is stop to the left due to the size of particles of ground compound x is reduced,√ ½ increasing surface area√ ½ of the particles thus increasing area of contact of

22 At equilibrium there will be very little of T that has reacted.√1At equilibrium there will be a lot

of T and very little V produced hence equilibrium lies to the left or forms the reactants√1

23.  – CB₂

  – Ionic bond  

24.  – Intensity of red-brown fumes increases.

  – High temperature vapourizes liquid nitrogen tetra-oxide to form nitrogen (IV) oxide that

is red-brown.  

25.  a) Curve 1

 b) After sometime, the rate of formation of CaCL₂ or rate of depletion of CaCO₃ become to low

that cant be evaluated

26.  a) Equilibrium shifts o the left, more CO₂ formed

(Increase in pressure favors reaction producing fewer molecules)

 b) Equilibrium shifts to the left, more CO_2(g) formed  

27.  The solution turns yellow. Equilibrium shifts to the left when NaOH is added, the

OH ^– ions react with H⁺ ions forcing more of cr2 O^72- and H₂O to react forming more

H⁺ and crO4^2- ions  the reaction particles causing higher rate of reaction and twice shorter time√ ½

28. (i) B The acid had higher concentration  (½mk)

(ii) The rate of reaction is initially high   (½mk) because of high concentration

of the reactant but decreases ( ½mk) steadily as the concentration also decreases.  

29. Yellow/brown colour of bromine water  ((½mk) fades or becomes colourless because

sodium hydroxide solution provides OH^– ions which reacts with H⁺ ions to form water ( ½mk)

shifts the equilibrium to the right