module 3.5 Flashcards
What is meant by the term average bond enthalpy?
(Average enthalpy change) when one mole of bonds
of (gaseous covalent) bonds is broken
Describe and explain the effect of increasing the pressure on the reaction rate.
Increased rate
AND
greater concentration of molecules / more molecules per
(unit) volume
More collisions per second / more frequent collisions
State le Chatelier’s principle.
The (position of a dynamic) equilibrium shifts to minimise
(the effect of) any change
Explain why the actual conditions used by the chemical industry might be different.
Low temperature gives a slow rate
OR high temperatures needed to increase rate
High pressure is expensive (to generate)
OR high pressure provides a safety risk
Explain why the use of a catalyst can reduce the demand for energy.
Allows reactions to take place at lower temperatures
Apart from heat loss, suggest two reasons for the difference.
Assume that the calculation has been carried out correctly.
incomplete combustion
non-standard conditions
evaporation of alcohol/water
specific heat capacity of beaker/apparatus
A closed system is required for dynamic equilibrium to be established.
State one other feature of this dynamic equilibrium.
Rate of the forward reaction is equal to the rate of the reverse
reaction
OR
concentrations do not change
The student heats the equilibrium mixture keeping the volume constant.
Predict how the composition of the equilibrium mixture changes on heating.
Explain your answer.
More H2 and I2 OR less HI (equilibrium position shifts) to the left AND (Forward) reaction is exothermic OR reverse reaction is endothermic OR in the endothermic direction
Predict and explain what effect, if any, an increase in the pressure would have on the position of the equilibrium.
No effect
AND
Same number of (gaseous) moles on both sides
What is meant by the term standard enthalpy change of formation?
Give the standard conditions.
(Enthalpy change) when one mole of a compound
is formed from its elements
298 K / 25 °C AND 1 atm / 100 kPa / 101 kPa / 1 bar
Explain how increasing the temperature increases the rate of reaction.Include a labelled sketch of the Boltzmann distribution, on the grid below.Label the axes.
axes labelled (number of) molecules and (kinetic) energy
Correct drawing of a two Boltzmann distributions
i.e. both curves must start within the first small square nearest to the
origin AND must not touch the x axis at high energy
Drawing of Boltzmann distribution at two different
temperatures with higher and lower temperature clearly
identified ( ie T2>T1)
QWC - (At a higher temperature) more molecules have energy
above activation energy
OR greater area under the curve above the activation energy
Describe and explain the effect of decreasing the pressure on the rate of a reaction.
(Decreasing the pressure) decreases the rate of reaction AND Decreased concentration of molecules OR Number of molecules remains the same but the volume increases OR Less molecules per (unit) volume Less frequent collisions
What is meant by the term enthalpy change of combustion, ΔHc?
(Enthalpy change that occurs) when one mole of a
substance
completely combusts OR reacts fully with oxygen
The student noticed that compound I burnt with a yellow flame and produced black smoke.
Suggest an explanation for these observations.
incomplete combustion OR not sufficient oxygen available
AND carbon is formed
reaction in catalytic converters?
2NO + 2CO 2CO2 + N2
Propenenitrile is manufactured from propene as shown in the equation.
C3H6(g) + NH3(g) + 1½O2(g) CH2CHCN(g) + 3H2O(g) ΔH = –540 kJ mol–1
The conditions used are 450 °C and 2.5 atmospheres in the presence of a catalyst.Describe and explain, using le Chatelier’s principle, the effect on the position of equilibrium of the following changes:
• a temperature above 450 °C
• a pressure above 2.5 atmospheres
• the absence of a catalyst.
(as temperature rises) position of equilibrium changes to
minimise effect of temperature rise by absorbing energy
OR
(as pressure rises) position of equilibrium changes to
minimise the pressure increase by reducing the
pressure and making fewer gas molecules
as temperature rises the position of equilibrium shifts to
the left AND increase in pressure shifts the equilibrium to
the left
relates change with temperature to the
(forward) reaction being exothermic OR reaction releases
energy or heat
OR
reverse reaction is endothermic OR reverse reaction
takes in heat or energy
change with pressure because there are fewer moles of
reactants OR more moles of products
removing the catalyst does not change the position of
equilibrium
The chemical industry uses temperature and catalysts to control the rate of reactions.Using Boltzmann distribution diagrams, explain the effect on the rate of a reaction of:
• increasing the temperature
• adding a catalyst.
Correct drawing of one Boltzmann distribution (could be
temperature or catalyst)
axes labelled (number of) molecules and energy
Increasing the temperature
Correct drawing of Boltzmann distribution at two different
temperatures with higher and lower temperature clearly
identified
idea that activation energy is lowered with a catalyst
Collision theory
reaction is faster with catalyst AND when temperature is
increased
Greater proportion of molecules with energy above
activation energy (with increased temperature or when
catalyst is used )
more effective collisions OR more successful collisions
(with increased temperature or when catalyst is used)
The pressure of the equilibrium mixture is increased.Explain what happens to the position of the equilibrium.
(equilibrium position shifts) to the left
(because there are) fewer moles (of gas) on the reactant
side
OR
(there are) more moles (of gas) on product side
This explanation mark is dependent on the correct shift of
the equilibrium
The temperature of the equilibrium mixture is increased.
Explain what happens to the position of the equilibrium.
(equilibrium position shifts) to the right
(because forward) reaction is endothermic
OR
reverse reaction is exothermic
This explanation mark is dependent on the correct shift of
the equilibrium
The reaction is actually carried out in the presence of a nickel catalyst at a pressure of 30 atmospheres. Suggest why the manufacturer uses a pressure of 30 atmospheres.
Gives a high rate of reaction OR reaction is fast OR
reasonable rate of reaction without shifting equilibrium too
much to the left
The nickel catalyst increases the rate.
Use a labelled diagram of the Boltzmann distribution of molecular energies to explain why.
y-axis label is ‘(fraction of or number of) molecules’ AND xaxis
label is ‘energy’ AND correct curve
Lowers activation energy
More molecules with energy above activation energy with
a catalyst OR more effective collisions OR more
successful collisions
Outline the stages that allow nitrogen monoxide and carbon monoxide to react in a catalytic converter.
CO and NO are adsorbed (onto surface) OR reactants are
adsorbed (onto surface)
weakening of bonds OR chemical reaction OR new bonds
are made OR carbon dioxide and nitrogen are made
CO2 and N2 desorbs (from the surface) OR products
desorbs (from the surface)
It is not possible to measure the enthalpy change of formation of N2O(g) directly.
Suggest why it is not possible.
Reaction gives NO OR reaction gives NO2 OR reaction
gives a mixture of oxides OR activation energy too high
OR rate of reaction is too slow
The reaction is repeated at a higher pressure.
Describe and explain what happens to the rate of the reaction between H2(g) and Cl2(g).
more concentrated (particles) OR more particles per (unit)
volume
more collisions per second
OR more frequent collisions
The reaction is repeated again. This time the temperature is decreased.
Describe and explain, by drawing appropriately labelled Boltzmann distributions, what happens to the rate of reaction between H2(g) and Cl2(g).
y-axis label is ‘(number of) molecules’ AND x-axis label is
‘energy’ AND one correct curve
Correct curve for lower temperature (labelled)
Activation energy does not change
OR clearly labelled on diagram, e.g. Ea OR E
Fewer molecules have energy above activation energy
OR fewer molecules have enough energy to react
So fewer successful collisions
Write the equation for the aerobic respiration of C6H12O6.
C6H12O6 + 6O2 6CO2 + 6H2O
Explain, in terms of bond breaking and bond forming, why this reaction is exothermic.
Bond breaking absorbs energy AND bond forming releases
energy
More energy released than absorbed
Suggest why the enthalpy change of formation of C6H12O6cannot be determined directly.
Would make carbon dioxide and water instead
OR activation energy (too) high
OR rate is (too) slow
OR do not react together
Explain the meaning of the term endothermic.
Reaction in which energy enters the system (from the
surroundings)
Explain the meaning of the term activation energy.
Activation energy is the minimum amount of energy needed
for the reactants to react
Explain, in terms of the rate of the forward reaction and the rate of the backward reaction, how the mixture of N2(g) and O2(g) reaches a dynamic equilibrium containing N2(g), O2(g) and NO(g).
Rate of forward reaction slows down and rate of backward
reaction speeds up
(Until) rate of forward reaction is the same as the rate of the
backward reaction
The research chemist repeats the experiment at the same temperature using the same initial amounts of N2(g) and O2(g). This time she carries out the experiment at a much higherpressure.
Suggest why
• much less time is needed to reach dynamic equilibrium
• the composition of the equilibrium mixture is the same as in the first experiment.
Reaction is faster
Increasing pressure mean more particles per unit volume
OR increasing pressure gives more crowded particles
OR increasing pressure gives more concentrated (particles)
So more collisions per second
OR higher collision frequency
OR collisions more often
(Changes of pressure) do not change the (position of)
equilibrium
Both sides of equation have same number of moles (of gas)
The reaction between nitrogen and oxygen in a car engine does not reach a dynamic equilibrium.
Suggest why not.
not a closed system
Some scientists believe that it is more important to have a high percentage yield in fermentation but others think that a high atom economy is more important.Is it more important to have high percentage yield or a high atom economy in fermentation?
Explain your answer.
High percentage yield with a simple reason
e.g. because the aim is to manufacture ethanol;
to reduce waste; increases sustainability
BUT
High percentage yield because there is very efficient
conversion from reactant to product
OR to reduce the waste of starting materials
OR
High atom economy with a simple reason
e.g. because it is cheaper or makes less harmful products;
to reduces waste; increases sustainability
BUT
High atom economy to reduce the amount of waste products
OR less by products
OR more desired product
Suggest why poly(ethenol) dissolves in water.
Poly(ethenol) has (many) O–H group(s)
Poly(ethenol) forms hydrogen bonds with water
What is meant by the term enthalpy change of reaction?
(enthalpy change when) the number of moles of reactants
as specified in the (balanced) equation react together
The actual value for the enthalpy change of this reaction is +81 kJ mol–1.
Suggest a reason why the actual value for the enthalpy change of this reaction is different from the calculated value.
Bond enthalpies may not be the same as the average bond
enthalpy
OR
The idea that bonds have different strengths in different
environments
Explain, using an enthalpy profile diagram and a Boltzmann distribution, how the presence of a catalyst increases the rate of reaction.
one activation energy labelled on enthalpy profile diagram
idea that activation energy is lowered
catalyst has a different reaction pathway
OR different reaction mechanism
OR two curves drawn on profile
QWC – correct diagram of reaction profile for endothermic or
exothermic reaction with products and reactants at different
heights – y axis labelled as energy or enthalpy
Drawing of Boltzmann distribution AND axes labelled (number
of) molecules and energy
More molecules with energy above activation energy with a
catalyst OR
More molecules that overcome the activation energy
More effective collisions OR more successful collisions
Explain why many industrial manufacturing processes use catalysts.Include in your answer ideas about sustainability, economics and pollution control.
Enable reactions to occur with less waste
OR enable reactions to take place with higher atom economy
OR fewer undesired products
Enable reactions to happen with less toxic solvents/reactants
OR enable reactions to produce less toxic waste/side products
Reactions can happen at room temperature
OR reactions can happen at atmospheric pressure
OR reactions can happen at a lower pressure
OR reactions can happen at a lower temperature
Saves energy (costs)
Reduce carbon dioxide emissions
OR reduces amount of fuel burnt
OR reduces greenhouse gas emissions
Enable reactions to occur with more specificity
OR enable reactions to produce correct stereoisomer
The relative atomic mass of the sulfur found by the space probe was different from the relative atomic mass of sulfur on Earth.
Suggest why.
Different isotopic abundance
NO and CO react very slowly without a catalyst.The catalyst in a catalytic converter increases the rate of reaction.Explain, using an enthalpy profile diagram and the Boltzmann distribution model, how the use of a catalyst increases the rate of reaction.
One activation energy correctly labelled on enthalpy profile
diagram
Idea that activation energy is lowered
Catalyst has a different reaction pathway OR different
reaction mechanism OR two curves drawn on profile
Correct diagram of reaction profile for exothermic reaction
with product below reactants with y axis as enthalpy or
energy and ∆H label – arrow should go down. Ignore a
small gap between at either end of ∆H line
Drawing of Boltzmann distribution – axes labelled number of
molecules and energy
More molecules with energy above activation energy with a
catalyst
More effective collisions OR more successful collisions
The actual conditions used in the catalytic oxidation of ammonia include 900 °C and an increase in pressure. Suggest why these conditions are a compromise.
Increased pressure speeds up reaction / ora
900 oC increases the rate OR increased temperature
speeds up reaction / ora
Idea that high enough temperature without compromising
yield OR idea that high enough pressure without
compromising yield
