Kinetics and Equilibria Flashcards
Describe what is meant by collision theory
For a collision to be successful, particles have to collide with sufficient energy and the correct orientation
The rate of a chemical reaction can be explained by using collision theory
Collision theory has three main parts
In order for particles to react, they must collide (if particles do not collide, then they cannot react
Secondly, when particles react, chemical bonds must be broken
(breaking chemical bonds requires energy- called Ea)
collide with enough energy to start breaking the chemical bonds in the reactants
In order for a reaction to take place, particles must collide in the correct orientation
e.g. green atoms must collide for a reaction to happen
if particles are in the incorrect orientation - the collision would be ineffective and a reaction will not take place
What is the rate of reaction proportional to
The rate of a reaction is proportional to the number of effective collisions per second/frequency of effective collisions
Use collision theory to explain the effect of reactant concentration and gas pressure on the rate of a reaction
One way to increase the rate of a reaction is to increase the conc. of the reactants
At a higher concentration, there are more reactant particles in the same volume
Because the reactant particles are closer together, there is an increased chance of collisions taking place
There will be an increased frequency of collision
Some of these collisions will be effective, leading to a reaction
This means that the rate of reaction will increase
We can also increase the rate of reaction by increasing the gas pressure
Increasing the pressure, makes the particles closer together
This increases the frequency of collisions leading to an increased rate of reaction
What is the activation energy
activation energy - the minimum amount of energy that particles must have for a collision to result in a reaction
the minimum energy particles must have in order to start a reaction by breaking chemical bonds
Ea - the enthalpy difference between the reactants and the highest point on the curve
Reactions can only occur when collisions take place
between particles having sufficient energy.
This energy is called the activation energy.
activation energy - the minimum amount of energy that particles must have for a collision to result in a reaction
in order for a reaction to happen, particles must collide with enough energy to start breaking the chemical bonds
if collision does not have enough energy - then particles cannot react = simply bounce of each other
when we measure the rate of a cheimcal reaction - we can either measure how quickly the amount of a reactant decreases or how quickly the amount of product is formed
this depends on which is easier to measure
accurately
e.g. if in a reaction you are making CO2 - measuring the volume of a gas is straight forward
One way is to catch the gas in an upturned measuring cylinder filled with water
A more accurate method is to use a gas syringe
in either case - we can read the volume of gas at regular time intervals from the scale and plot the volume of gas against time
explain the shape of the rate graph - products formed / time
Initially we get a lot of product formed rapidly
This tells us that the rate of reaction is fast
This is because we have a high conc. of reactants and a high frequency of effective collisions
Over time, the reaction slows down
This is because HCL is reacting - so the conc. of HCL is decreasing
Since HCL is the limiting reactant
Limiting reactant is running out
Because of this, the frequency of effective collisions is reduced
Eventually, the reaction stops and no more product is formed
This is because all of the HCL has reacted and there are no more effective collisions
Describe how to measure reaction rate from the gradient of a graph
measure the rate of reaction at any point by drawing a tangent
to measure the initialrate - draw a tangent from the zero time point
measure the gradient of tangent
construct a right angled triangle using the tangent
measure the y component (vol of co2) and x component (time)
to calculate the rate divide the volume of gas by time taken
rate of reaction - use a balance to measure mass of co2 released
start with 0
to negative
Describe why reactions with low activation energies will have a higher rate than reactions with high Ea
Reaction has a low Ea
So this reaction could take place quite rapidly at room temp.
This is because, at room temp. , a relatively large proportion of reactant molecules will collide with enough energy to cross the Ea barrier and react
Reactions with high Ea
At room temperature, this reaction should be relatively slow as only a small proportion of reactant molecules will collide with enough energy to cross the Ea barrier
Define catalyst
A catalyst is a substance that increases the rate of a
chemical reaction without being changed in chemical
composition or amount.
Catalysts work by providing an alternative reaction route
of lower activation energy.
Describe and explain the effect of catalysts on the rate of a reaction
One way to increase the rate of chemical reactions is to use a catalyst
Catalysts allow a reaction to take place via an alternative pathway with a lower Ea than the uncatalyzed reaction
Describe the economic and environmental benefits of using catalysts
Catalysts are not used up in a reaction and are not permanently changed
So that means that catalysts can be reused
By using catalysts, we can make reactions take place rapidly even at relatively low temperatures
This reduces the amount of energy needed by the chemical industry which in turn reduces the need to burn fossil fuels to provide this energy
This saves money, providing an economic benefit
It also reduces CO2 emissions, making the chemical industry more sustainable
However some catalysts are toxic, so this negative aspect needs to be weighed against the benefits
Describe the Maxwell-Boltzmann curve of particle energies
Maxwell–Boltzmann distribution of molecular energies in
gases.
Shows the distribution of energies amongst molecules in a reaction
If we measure the energy of all the molecules in a gas, a liquid or a solution, we get a curve
This curve is called the Maxwell-Boltzmann distribution
The curve starts at the origin (0 - 0)
This means that there are no molecules with zero energy
Secondly, some molecules have a very high energy
The curve does not touch the x axis at high energies
This tells us that there is no maximum energy that molecules could have
Lastly, the area under the curve tells us the total number of molecules in the system (reactants)
The most probable energy is directly under the peak of the curve (mode)
The mean energy is slightly to the right of this
That is because particles at very high energies skew the mean to the right
Use the Maxwell-Boltzmann curve to explain the effect of catalysts on the rate of chemical reactions
In order to collide effectively and react, molecules must have at least this energy
These molecules are represented by the area under the curve to the right of the Ea (after Ea)
The activation energy in the presence of a catalyst - will be more left
All of the molecules which now have at least this energy can collide
effectively and react
these molecules - area under the curve
This is a much greater number of molecules than for the uncatalyzed reaction
This explains why the rate of reaction increases in the presence of a catalyst
Use the Maxwell-Boltzmann curve to explain the effect of temperature on the rate of chemical reactions
Another way to increase the rate of reaction is to increase the temperature.
The effect of increasing the temperature on the Maxwell-Boltzmann distribution curve
At higher temperatures, there are more particles with very high energies
That the most probable energy increases, but the number of particles with the most probable energy falls
The area under the curve represents the total number of molecules
The number of molecules has not changed
Temperature has only increased
So the area under the curve cannot change when we increase the temp.
Therefore at higher temperatures, the curve shifts to the right, but the height of the peak falls
There are two effects to increasing the temperature
Firstly, the molecules are moving faster so the frequency of all collisions increases
Secondly at higher temperatures, we can see that the number of molecules with at least the activation energy has increased substantially
Now, a much greater proportion of the collisions are effective and result in a reaction
This increase in effective collisions explains why the rate of reaction increases at higher temperatures
a small rise in temp gives a large increase in rate
The curve broadens and flattens due to a greater spread of values
the area under the two curves is the same (same no. of molecules)
Define rate of reaction
The rate of reaction is typically measured as the change in concentration of the reactants of products per unit time
Explain the effect of increasing temp on rate
Increasing temp
increases the Ek of particles
leading to a greater frequency of successful collisions
explain the effect of increasing pressure/conc. on rate
Increasing pressure/conc. results in more particles per unit volume
leading to a greater frequency of successful collisions
effect of increasing SA on rate
Increasing surface area exposes more particles
leading to a greater frequency of successful collisions
state whether the combustion of methane is reversible or irreversible and explain why
methane + oxygen -> co2 + h2o - irreversible/one way reaction
co2 and h2o are stable and will not react together to form ch4 and o2
an irreversible reaction is shown with a single headed arrow
Describe what is meant by a reversible reaction
Nitrogen + hydrogen ⇌ ammonia
This is the Haber process - which is used to make ammonia
In a reversible reaction, the products can convert back to the reactants
We illustrate a reversible reaction using a double arrow
Double arrow - means a reversible reaction that can reach equilibrium
Describe what is meant by dynamic equilibrium
When the reactants convert to products - called the forward reaction
When the products convert back to the reactants - called the reverse reaction
At the start of the reaction, we have a high conc. of reactants and no products
Because we have a high conc. of reactants, the rate of the forward reaction is very high
Since there are no products present, the reverse reaction has a rate of 0
As the forward reaction proceeds, the conc. of reactants decreases and conc. of products increases
Because the conc. of reactants decreases, the rate of the forward reaction also decreases
Because the conc. of products increases, the rate of the reverse reaction also increases
Eventually, there comes a point when both the forward and reverse reactions are taking place at the same rate
At this point, the conc. of both the reactants and the products remain constant - they stop changing
Scientists say that the reaction has reached equilibrium
When we reach equilibrium - the forward and reverse reactions have not stopped - both are still taking place but at the same rate - this is why the conc. of both the reactants and products are now constant
Because both the forward and reverse reactions are still taking place - this is called a dynamic equilibrium
At equilbrium - the concentrations of both the reactants and the products are no longer changing
This does not mean that the concentration of the reactants is the same as the conc. of the products - you could have greater reactants than products or vice versa at equilibrium - or concentrations could be the same
point where lines plateau /level off - where equilibrium has been achieved z
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Describe what is meant by a closed system
Placing reactants in a sealed container
sealed container - example of a closed system
In a closed system - no atoms can enter or leave the system
Four conditions which apply to all equilibrium
A closed system - the reactants and products can’t escape
It can be approached from either direction
It is dynamic - the rates of the forward and backward reactions, going on at the same time, are equal + the concentration of the reactants and products are constant but not necessarily the same
The macroscopic properties - e.g. density, concentration, colour, pressure) of the system do not change with time
