Kinetics

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Kinetics

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Question 1

• The reaction between excess magnesium carbonate
  and hydrochloric acid can be followed by
  measuring the volume of carbon dioxide produced
  with time. The results of one such reaction are
  shown below. How does the rate of this reaction
  change with time and what is the main reason for
  this change?

Volume of CO2
Time
0
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Wrong Answer!
The magnesium carbonate is present in excess,
hence there is little change in surface area.
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Question 2

• Zinc reacts with sulfuric acid as shown below.
• Zn(s) H2SO4(aq) ? ZnSO4(aq) H2(g)
• Two identical samples of zinc powder were reacted
  with separate samples of excess acid as follows
• Reaction 1 zinc added to 0.5 moldm-3 sulfuric
  acid
• Reaction 2 zinc added to 2.0 moldm-3 sulfuric
  acid
• What is the same for reaction 1 and 2?
• ?

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Wrong Answer!
Zinc is the limiting reagent and hence determines
the volume of the hydroge gas. The 2 samples of
zinc have the same mass.
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Question 3

• Which of the following is true for a catalyst ?

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Wrong Answer!
Catalysts lower the activation energy for the
forward and backward reactions. They provide a
new mechanism or pathway.
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Question 4

• The reaction between nitrogen and chlorine in the
  atmosphere under normal conditions is extremely
  slow. Which statement best explains this?

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Wrong Answer!
Very few colliding molecules have kinetic
energies equal to or in excess of the activation
energy.
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Question 5

• The rate of a reaction between two gases
  increases when the temperature is increased and a
  catalyst is added. Which statements are correct
  for the effect of these changes on the reaction?

Increasing the temperature Adding a catalyst
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Wrong Answer!
Activation energy does not vary with temperature.
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Question 6

• What are the usual units for rate?

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Wrong Answer!
Rate is defined as change in concentration (mol
dm-3 ) with time (s)
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Question 7

• For the reaction
• 2HCl(aq) CaCO3(s) ? CaCl2(aq) H2O(l) CO2(g)
  
• Which gives the fastest rate?

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Wrong Answer!
This combination has acid of the highest
concentration and the greatest surface area for
the calcium carbonate. Both these factors
maximise the collision rate.
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Question 8

• Excess magnesium was added to a beaker of aqueous
  hydrochloric acid on a balance. A graph of the
  mass of the beaker and contents was plotted
  against time (line 1).

• What change in the experiment could
• give line 2?
• The same mass of magnesium
• but in smaller pieces.
• II. The same volume of a more
• concentrated solution of
• hydrochloric acid
• III. A lower temperature

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Wrong Answer!
The addition of hydrochloric acid of higher
concentration means that the reaction will go
faster and hence the loss in mass will be faster.
In addition, since the acid is the limiting
reagent then a greater total volume of gas is
lost and a greater mass loss results.
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Question 9

• Which statement explains why the speed of some
  chemical reactions increased when the surface
  area of the reactants is increased?

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Wrong Answer!
The reaction takes place on the surface. The
collision rate will be directly proportional to
the rate.
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Question 10

• Which of the following reactions do not occur
  rapidly?

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Wrong Answer!
The reaction between reactive metals (including
sodium) with water and acid is generally very
fast. Rusting is a slow process.
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Question 11

• The rate of many reactions increase rapidly with
  small increase in temperature. Which of the
  following best accounts for this behaviour?

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Wrong Answer!
An increase in temperature does increase the
collision rate, but the rise in proportion of
molecules colliding with kinetic energies equal
to or in excess of the activation increases is
the major underlying reason for the rapid
increase in rate.
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Question 12

• In the reaction shown below, which species may be
  acting as a catalyst?
• O3 Br ? BrO O2
• BrO O2 ? Br O2

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Wrong Answer!
The bromine atom is an intermediate and behaves
as both a reactant and a product.
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Question 13

• For which one of the following reactions would
  pressure measurements be the least sensitive for
  measuring the initial rate of reaction?

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Wrong Answer!
Two volumes of gas form two volumes of gas.
Hence, there is no change in pressure.
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Question 14

• All the following statements are correct except

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Wrong Answer!
Increasing the pressure increases the
concentration of reacting gases and hence
generally increases the rate of reaction.
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Question 15

• When 100cm3 of 1.0 moldm-3 methanoic acid,HCOOH,
  are added to 1 gram of magnesium turnings at
  200C, hydrogen gas is slowly produced. All the
  following will increase the initial rate of
  hydrogen production except

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Wrong Answer!
Tripling the volume of the acid will not alter
the initial rate of reaction since there is no
change in concentration and hence there will be
no effect on the collision rate between magnesium
atoms and hydrogen ions from the methanoic acid.
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Question 16

• When ammonia is manufactured commercially a
  catalyst is used. What is the effect of this
  catalyst?
• N2(g) 3H2(g) 3NH3(g) ?H -92kJ
• To shift the position of equilibrium and increase
  the yield of ammonia.
• To increase the rate of only the forward
  reaction.
• To increase the rates of the forward and reverse
  reactions.

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Wrong Answer!
A catalyst will increase the rate at which
equilibrium is reached, as it will speed up both
the forward and reverse reactions equally, but it
will have no effect on the position of the
equilibrium and hence the value of Kc.
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Congratulations!