6 Chemical Kinetics Flashcards
When was kinetic theory developed and why?
The kinetic theory developed in the 18th century out of a need to explain how it is that gases exert pressure inside a container
What type of states of matter does kinetic theory account for?
The kinetic theory of matter accounts for the properties of solids, liquids, and gases in terms of the interactions of particles and their relative energies
What is the average kinetic energy of the particles directly proportional to?
The average kinetic energy of the particles is directly proportional to the temperature of the system in Kelvin
What is kinetic energy?
Kinetic energy refers to the energy associated with movement or motion. It is determined by the mass and velocity of the substance
What is the equation for kinetic energy?
KE = ½mv2
What is there an inverse relationship between due to the kinetic energy of the particles being equal at the same temperature?
As the kinetic energy of the particles at the same temperature is equal, this means there is an inverse relationship between mass and velocity
What does this inverse relationship explain?
This is why substances with a lower mass diffuse more quickly than those with greater mass at the same temperature
What 4 factors determine the rate of a chemical reaction?
collision frequency
collision energy
activation energy
collision geometry
What is the collision frequency?
If a chemical reaction is to take place between two particles, they must first collide
The number of collisions between particles per unit time in a system is known as the collision frequency
how can collision frequency be changed?
The collision frequency of a given system can be altered by changing the concentration of the reactants, by changing the total pressure, by changing the temperature or by changing the size of the reacting particles
What is it called when the collision of two molecules does not result in a reaction?
Not all collisions result in a chemical reaction
Most collisions just result in the colliding particles bouncing off each other
Collisions which do not result in a reaction are known as unsuccessful collisions
When do unsuccessful collisions occur?
Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds
If they do not have sufficient energy, the collision will not result in a chemical reaction
When will a reaction be successful in terms of collision energy? What is collision energy?
If they have sufficient energy, they will react, and the collision will be successful
The combined energy of the colliding particles is known as the collision energy
What is the activation energy?
The minimum energy the colliding particles need in order to react is known as the activation energy
When will a reaction be unsuccessful, in terms of activation energy?
If the collision energy of the colliding particles is less than the activation energy, the collision will be unsuccessful
When will a reaction be successful, in terms of activation energy?
If the collision energy is equal to or greater than the activation energy, the collision will be successful, and a reaction will take place
How can the activation energy of a reaction be changed?
The activation energy can be changed by the addition of a catalyst
What does collision geometry mean in terms of a successful reaction? When is collision geometry particularly important?
Particles have to have the right orientation when they collide for the reaction to be successful
This is particularly the case with large molecules with complex shapes
Why do most collisions not result in a reaction?
Most collisions do not result in reaction because they do not reach the activation energy rather than not having the correct collision geometry
How do the concentrations of reactants and products change as a reaction progresses?
As a chemical reaction proceeds, the concentration of the reactants decreases and the concentration of the products increases
What is the rate of reaction?
The rate of a reaction is the speed at which a chemical reaction takes place and has units mol dm-3 s-1
How can the rate of reaction be calculated?
rate of reaction = change in amount of reactants or products mol dm-3 s-1 / time (s)
(basically the gradient of the graph)
What is the basis of measuring the rate of reaction?
To measure the rate of a reaction, we need to be able to measure either how quickly the reactants are used up or how quickly the products are formed
What does the method used to measure rate depend on? very very basic
The method used for measuring depends on the substances involved
What changes during a reaction, which allows us to measure rate?
There are a number of ways to measure a reaction rate in the lab; they all depend on some property that changes during the course of the reaction
What do we assume this property is? (property that changes during a reaction allowing us to measure the rate)
That property is taken to be proportional to the concentration of the reactant or product, e.g., colour, mass, volume
What are the “easiest” reactions to measure the rate of?
Some reaction rates can be measured as the reaction proceeds (this generates more data);
faster reactions can be easier to measure when the reaction is over, by averaging a collected measurement over the course of the reaction
What are the 3 main techniques used to measure rate?
mass loss
gas production
colorimetry
What does a colourimeter measure?
A colourimeter measures the amount of light that passes through a solution
When can colour change be used?
If a solution changes colour during a reaction this can be used to measure the rate
How does a colourimeter work?
The intensity of light reaching the detector is measured every few seconds and the data is plotted to show how the concentration of the reactants or products changes with time
What does the graph produced by a colourimeter look like?
The light intensity is related to the concentration, so the graph represents a graph of concentration of products or reactants against time
(downwards slope - kinda curved)
What can colourimetry NOT be used to measure?
Note that colorimetry cannot be used to monitor the formation of coloured precipitates as the light will be scattered or blocked by the precipitate
When can changes in mass be used?
When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass decreases
This can be used to measure the rate of reaction
When can changes in mass be used? v. specific example
For example, the reaction of calcium carbonate with hydrochloric acid produces CO2
How is the data recorded for changes in mass?
The mass is measured every few seconds and change in mass over time is plotted as the CO2 escapes
What does the graph for changes in mass look like?
The mass loss provides a measure of the amount of reactant, so the graph is the same as a graph of amount of reactant against time
What is one limitation of the changes in mass method?
However, one limitation of this method is the gas must be sufficiently dense or the change in mass is too small to measure on a 2 or 3 dp balance
So carbon dioxide would be suitable (Mr = 44) but hydrogen would not (Mr = 2)
When can the volume of gases be used to measure the rate? give an example
When a gas is produced in a reaction, it can be trapped and its volume measured over time
This can be used to measure the rate of reaction.
For example, the reaction of magnesium with hydrochloric acid produces hydrogen
What ways can gas be collected?
- gas syringe
- An alternative gas collection set up involves collecting a gas through water using an inverted measuring cylinder ( as long as the gas is not water-soluble)
What does the graph look like for volume of gas produced?
The volume can be measured every few seconds and plotted to show how the volume of gas varies with time
The volume provides a measure of the amount of product, so the graph is a graph of amount of product against time
(curve increasing not straight line, more curved and eventually reaches a plateau)
Is measuring concentration changes to determine rate easy? How can it be achieved?
Measuring concentration changes during a reaction is NOT easy; the act of taking a sample and analysing it by titration can affect the rate of reaction (unless the reaction is deliberately stopped- this is called quenching).
What alternative method is used instead of e.g. titrating the solution for measuring concentration changes? Give examples
Often it is more convenient to ‘stop the clock’ when a specific (visible) point in the reaction is reached
For example when a piece of magnesium dissolves completely in hydrochloric acid
Another common rate experiment is the reaction between sodium thiosulfate and hydrochloric acid which slowly produces a yellow precipitate of sulfur that obscures a cross when viewed through the solution:
Na2S2O3 (aq) + 2HCl (aq) → 2NaCl aq) + SO2 (g) + H2O (l) + S(s)
What is the limitation of waiting when a specific point in the reaction is reached? e.g. sodium thiosulfate and cross
The main limitation here is that often it only generates one piece of data for analysis
What must the reactant particles have for a reaction to occur in terms of activation energy?
For a reaction to take place, the reactant particles need to overcome a minimum amount of energy
This energy is called the activation energy (Ea)
In an exothermic reaction are the reactants/products higher?
In exothermic reactions the reactants are higher in energy than the products
In an endothermic reaction are the reactants/products higher?
In endothermic reactions the reactants are lower in energy than the products
Therefore is the activation energy greater in an endothermic or exothermic reaction?
Therefore, the Ea in endothermic reactions is relatively larger than in exothermic reaction
Describe what conditions are necessary for a reaction to occur, wit regards to activation energy
Even though particles collide with each other in the same orientation, if they don’t possess a minimum energy that corresponds to the Ea of that reaction, the reaction will not take place
Therefore, for a collision to be successful the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction
Give the equation for calculating the activation energy for the reverse reaction of an exothermic reaction
For an exothermic reaction = ∆H + Ea (forward)
Give the equation for calculating the activation energy for the reverse reaction of an endothermic reaction
For an endothermic reaction = Ea (forward) -∆H
How does a catalyst work?
By decreasing Ea, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.
How does a catalyst increase the rate fo reaction?
More particles are able to collide with sufficient energy to react under the lower activation energy
More frequent, successful collisions lead to a faster rate of reaction
What are the reactants converted into during a reaction? therefore how do concentrations change?
During a reaction, the reactants are used up and changed into the products
This means that as the reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing
Therefore what does the changing concentration of reactants mean for the rate of reaction?
Because of this, the rate of the reaction is not the same throughout the reaction but changes
How can the rate of reaction be calculated, i.e. from what graph?
The rate of reaction during the reaction can be calculated from a concentration-time graph.
What should be kept constant when measuring rate and why?
When taking the measurements, the temperature should be kept the same at all times as a change in temperature will change the rate of reaction
How would you calculate the rate of reaction FROM 15 to 20 minutes?
Eg. you can calculate the rate of reaction from 15 to 20 mins during which the concentration of propene increases from 0.68 to 0.83 mol dm-3:
gradient at 0.83 - gradient at 0.68
= 0.0005 mol dm-3 s-1
When will the calculation of rate be more accurate? How can this be achieved?
The smaller the time intervals, the more accurate the reaction rate value is
Even more accurate is to find the rate of reaction at different concentrations of reactant or product at particular time points
This can be done by drawing tangents at several points on the graph
How is the number of particles in a solution affected, when the concentration is greater?
The more concentrated a solution is, the greater the number of particles in a given volume of solvent
How does an increasing concentration affect rate?
An increase in concentration causes in an increased collision frequency and therefore an increased rate of reaction
How does an increased pressure affect the reaction system?
An increase in pressure in reactions that involve gases has the same effect as an increased concentration of solutions
When the pressure is increased, the molecules have less space in which they can move
Therefore how does increasing the pressure affect the rate of reaction?
This means that the number of effective collisions increases due to an increased collision frequency
An increase in pressure therefore increases the rate of reaction
In what two ways does increasing the temperature increase the rate of reaction?
At higher temperatures, the particles are moving faster, so collide more frequently. A higher number of collisions in total mean a higher number of successful collisions
At higher temperatures, a higher proportion of the molecules have the activation energy or more. This means that a higher proportion of collisions are successful
What particles in a solid will participate in a reaction?
Only the particles on the surface of a solid will collide with particles of the other reactant
Therefore how does an increase in surface area affect the rate of reaction?
If the surface area is increased, more particles will be on the surface and able to collide with particles of the other reactant
This means that there will be more collisions in total and therefore more successful collisions.
How can surface area be increased?
Surface area can be increased by decreasing the size of the reactant particles
For example, powders have a very large surface area
What does the Boltzmann distribution curve show?
The Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
What is the energy distribution in a sample of particles?
In a sample of a substance, a few particles will have very low energy, a few particles will have very high energy, and many particles will have energy in between
What does the graph show in terms of successful collisions?
The graph shows that only a small proportion of molecules in the sample have enough energy for an effective or successful collision and for a chemical reaction to take place
What happens when temperature is increased, i.e. how does their energy change?
When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
What does a greater kinetic energy of the particles cause?
This causes the particles to move around faster resulting in more frequent collisions
Therefore what does a greater average kinetic energy of the particles lead to in terms of successful collisions?
Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
How does the boltzmann curve change with increasing temperature?
With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right
To summarise, how does an increase in temperature cause an increased rate?
There being more effective collisions as the particles have more kinetic energy, making them move around faster
A greater proportion of the molecules having kinetic energy greater than the activation energy
What is the mechanism of a catalyst?
A catalyst provides the reactants another pathway which has a lower activation energy
By using a catalyst, how does the energy of the molecules change?
By lowering Ea, a greater proportion of molecules in the reaction mixture have sufficient energy for a successful collision
As a result of this, the rate of the catalysed reaction is increased compared to the uncatalyzed reaction
How is the use of a catalyst represented on a boltzmann curve?
Ea line is placed further to the left
What is catalysis?
Catalysis is the process in which the rate of a chemical reaction is increased, by adding a substance called a catalyst
What is the good definition of a catalyst?
A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction
What two types categories can catalysts be divided into?
Catalysts can be divided into two types:
Homogeneous catalysts
Heterogeneous catalysts
What does it mean if a catalyst is homogenous and give an example?
Homogeneous means that the catalyst is in the same phase as the reactants
For example, the reactants and the catalysts are all liquids
What does it mean if a catalyst is heterogenous and give an example?
Heterogeneous means that the catalyst is in a different phase to the reactants
For example, the reactants are gases but the catalyst used is a solid
How is the use of a catalyst represented on an energy profile diagram?
peak of Ea lowered
