Kinetics. Flashcards
What is meant by collision theory.
A theory used to describe chemical reactions using rate and energy of collisions.
What two requirements must reacting particles meet in order for them to react.
They must collide with enough energy to break bonds and also between parts of the molecule that are going to react together.
Therefore have the activation energy and also correct orientation orientation.
Where is the Emp and mean energy found on a Maxwell Boltzmann distribution. Define both of these terms.
Emp this is the most probable energy (not the same as mean energy) and is found at the curves peak.
Then slightly to the right in a way that divides the area under the graph into half is the mean energy.
What happens to species that are at the top of an enthalpy diagram.
They are called a transition state or activation complex and there are bonds being made and broken , this creates extra enemy and makes such species unstable.
What should be on the x and y axis of a Maxwell boltzman distribution.
On Y : fraction of particles with energy E
On x : Energy E
What type of energies do most particles have on a Maxwell boltzman distribution. Comment also on the shape of the curve.
Most particles have intermediate energies around the peak of the curve.but the distribution is not symmetrical (normal).
Explain why the energy distribution on a Maxwell-Boltzman curve goes through the origin and has no upper limit.
This is as no particles have no energy and very few particles have high energy on the curves right hand side , there is no maximum energy (E) the particles can have and so the curve has no upper limit.
How can a reaction go to completion if few particles have energy greater than EA?
Particles can gain energy through collisions.
Why do a few particles have low energies on the MB energy distribution curve.
A few have low energies because collisions cause some particles to slow down
How many particles have energy greater than EA.
Only a few particles have energy greater than the EA
What does the area under the curve of a MB energy distribution represent.
The area under the curve represents the total number of particles present
What happens to the key values on a MBD when the temperature is increased. State also how the number of molecules that have such new values changes.
As the temperature increases the distribution shifts towards having more molecules with higher energies as you would expect. SO….
At higher temps both the Emp and mean energy shift to higher energy values, although the number of molecules with those energies decrease. There is a wider spread of energies.
Why should the total area under the curve should remain constant.
The total area under the curve should remain constant
because the total number of particles is constant.
Comment on the range of energies at higher temperatures compared to that of lower temperatures.
At higher temperatures the molecules have a wider
range of energies than at lower temperatures.
Explain the need of energy to start a reaction and also define activation energy in your answer.
Reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually NEEDED TO BREAK THE RELEVANT BONDS in one or either of the reactant molecules.
This minimum energy is called the activation energy. The activation energy, EA, is defined as the minimum energy which particles need to collide and have a successful collision.
Define the rate of reaction.
The rate of reaction is defined as the change in concentration of a substance in unit time , Its usual unit is mol dm-3s-1 .
When measuring reaction rates if a graphical method was to be used how cloud reaction rate be calculated. A reaction where the change in concentration isn known.
When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction.Reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a tangent to the curve (at different times) and calculating the gradient of the tangent.
Briefly outline a reaction and hence method in which you could measure the rate of reaction without knowing the change in concentration, state also why this is an approximation and any assumptions that are made. For your answer refer to the experiment between sodium thiosulfate and hydrochloric acid.
In the experiment between sodium thiosulfate and hydrochloric acid we usually measure reaction rate as 1/time. The time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the sulfur .
Na2S2O3 + 2HCl —> 2NaCl + SO2 + S + H2O
This is an approximation for rate of reaction as it does not include concentration. We can use this because we can assume the amount of sulfur produced is fixed and constant.
What needs to be included in any answer that refers to doubling of concentration/rate.
make sure you mention double the number of particles per unit volume and double the frequency of effective collisions.
What is the word perfect answer as to why reaction rate will increase with a greater concentration or pressure ( same answer for both).
At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of successful collisions.
What happens to the shape of a MD curve if you increase the concentration. Draw this out for doubling the concentration. What adjustment needs to be made to the axis.
If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change.
They curves will be higher, and the area under the curves will be greater because there are more particles.
It is now double the number of particles with energy , not a fraction since the fraction would stay the same.
At higher concentrations with reference to the MBD state and explain how rate increases (make the distinction between proportion and raw number of particles .
Can see from the curve that there is a greater number of particles with energy > EA. Since the curve goes up higher and there has not been a temperature change and hence no actual change to energy distribution the proportion of particles with energy > EA is constant.
(unless you change the actual energy of the particles by temperature the distribution and hence PROPORTION with energy > EA will stay constant.
When comparing rate curves of amount of product ( e.g volume of gas produced ) against time what needs to be calculated to find the finishing values of curves. And what is the relationship between this and the amounts of product produced.
Need to calculate/ compare initial moles of reactants to distinguish between different finishing volumes.
e.g. the amount of product is proportional to the moles of reactant.
Given other conditions are the same what will different volumes of the same concentration product have in common and what will differ in terms of their rate curves.
Will have the same initial rate of reaction but will finish at different amounts that is proportional to the initial moles.
Define initial rate of reaction.
The initial rate is the rate at the start of the reaction where it is fastest.
With reference to the MBD state how increasing temperature increases the rate of reaction and make the distinction between proportion and raw values of particles in your answer. And the effect of temperature on rate in general compared to other factors.
As the temperature increases, the graph shows that a SIGNIFICANTLY bigger PROPORTION (it is proportion since the actual energy distribution is changed unlike with concentration) of particles have energy greater than the activation energy, so the frequency of successful collisions increases.
A small increase results in significant increase in particles with E>Ea
IN GENERAL:
At higher temperatures the energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction.
What properties does the rate equation relate together
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Give a generalised rate equation for aA + bB into products.
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r = k[A]m[B]n what do m and n represent here and how can the total order of reaction be calculated from this example.
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Give the meaning of all the possible reaction orders by using [A] as an example. Draw a graph of rate against concentration to show this relationship.
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For the rate equation r = k[A]m[B]n state what the units of K are dependent on.
State also what the actual value of K is independent of and a situation where the value of K would change.
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Although you don’t need to memorise it , what is the unit of K for all of the overall order reactions.
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How are the values of reaction orders found out and what shouldn’t they be confused for.
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What should the units of r be taken as when calculating a unit for the value of the rate constant K.
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For the overall first order reaction Rate = k[A][B]0 find the units of K. And explain working.
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For the following reaction r = k[A][B]2 work out the units for K.
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What is a continuous rate method and give the two variables that are monitored and graphed.
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What will eventually happen to the graph in a continuous rate method and why.
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Give a typical reaction for continuous monitoring of rate and outline the practical method in doing this.
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For measuring the change in volume of gas by way of continuous monitoring why are large excesses of reactants not used. Give two reasons.
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What are the two ways initial rate can be calculated.
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State very briefly the two ways that rate order can be calculated experimentally.
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What special property do zero order reactants have and state also the significance of the gradient of the graph of that particular reactant against time.
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Need to do both table and also graphical order examples.
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