Chapter 10 Reaction Rates And Equilibrium Flashcards
What does the rate of a chemical reaction measure?
How fast a reactant is being used up or how fast a product is made.
How can the rate of reaction be measured?
Rate= change in concentration / time
Mol dm^-3 s^-1 = moldm^-3 / s
Describe how the rate of reaction changes over time
The rate of reaction is fastest at the start of a reaction, as the reactants are at their highest concentration.
However, the rate then slows as the reaction proceeds because the concentration of the reactants decreases.
The reaction plateaus once one of the reactants has been completely used up so the rate of reaction becomes zero.
What factors can alter the rate of reaction?
Concentration/ pressure (when reactants are gases)
Temperature
Catalyst
Surface area of solid reactants
What is Collision theory?
For a reaction to occur particles must collide with sufficient energy that can overcome the activation energy and collide with the correct orientation.
What is an effective collision?
An effective collision is one that leads to a chemical reaction
When is a collision considered effective?
When particles collide with the correct orientation
When the particles have sufficient energy that can overcome the activation energy barrier.
How does a change in concentration affect the rate of reaction?
Rate of reaction increases as concentration increases meaning there are more particles per unit volume, so the particles are closer together and more frequent effective collisions can occur per second.
How does changing pressure affect the rate of reaction?
Rate of reaction increases as the pressure of gases increases, this is because there are more gas particles per unit volume so the particles are closer together and more frequent effective collisions can occur per second.
How does the surface area of a reactant affect the rate of reaction?
Rate of reaction increases as the surface area of reactants increases, this is because there is a greater surface area exposed for collisions to occur from other particles. Therefore, increasing the number of frequent effective collisions occuring per second.
How can the progress of a chemical reaction be measured?
Monitoring the removal (decrease in concentration) of the reactant
Following the formation (increase in concentration) of the products
What is a catalyst?
A catalyst provides and alternative pathway for the reaction to occur by lowering the activation energy.
Catalysts are not used up in a chemical reaction.
What is a Homogenous catalyst?
The catalyst and reactant are in the same physical state.
What is a Heterogenous catalyst?
The catalyst and reactants are in different physical states
How does a homogenous catalyst work?
The catalyst reacts with the reactants to form an intermediate, the intermediate then breaks down to give the product and then regenerates.
How does a heterogenous catalyst work?
The catalyst provides a surface for the reaction to occur as they are usually solids in contact with gaseous reactants or reactants in solution. The reactant molecules are adsorbed onto the surface of the catalyst, the reaction occurs and the products are then desorbed.
Why are catalysts sustainable?
Allows processes to operate at lower temperatures
Less fossil fuels are required
Lower carbon emissions are produced
Higher atom economies are sustained
There is less waste produced because there is less energy resources needed to process the waste.
What is the Boltzmann distribution curve?
A graph which shows the spread of molecular energies of gases
What are the features of the Boltzmann distribution curve?
No molecules will have zero energy- the curve starts at the origin.
The area under the curve is equal to the total number of molecules
There is no maximum energy, the curve does not meet the x axis.
How does temperature affect the rate of reaction?
The rate of reaction increases as temperature increases as this causes the particles in the reaction to move faster because they have more kinetic energy. Therefore, a greater proportion of these particles will have energy that exceeds the activation energy, increasing the number of frequent effective collisions per second.
How does increasing the temperature affect the Boltzmann distribution curve?
As temperature increases, the average energy of the molecules also increases, this means that the graph is stretched to the right as more molecules have higher energy. There will be a smaller proportion of molecules with lower energy so the peak of the curve is reduced.
How does decreasing the temperature affect the Boltzmann distribution curve?
As temperature decreases, the average energy of the molecules decreases, this means that the graph will shift to the left as more molecules have energy lower than the activation energy. There will be a larger proportion of molecules with less energy so the peak of the curve increases.
How does a catalyst affect the Boltzmann distribution curve?
In the presence of a catalyst, a greater proportion of molecules will have energy equal to or greater than the activation energy needed for the reaction to occur. Therefore, more frequent effective collisions take place increasing the rate of reaction.
What is dynamic equilibrium?
A reaction that takes place in a closed system where the rate of the forward reaction is equal to the rate of the reverse reaction so the concentration of the reactants and products remains constant.
What is Le Chateliers Principle?
When a system in dynamic equilibrium is subjected to a change in conditions its position will shift to minimise the change.
How does changing the concentration of the reactant affect equilibrium?
Increasing the concentration of a reactant will shift the position of equilibrium in the forwards direction so that more products form.
Decreasing the concentration of a reactant will shift the position of equilibrium in the reverse direction so more reactants form.
What is homogeneous equilibrium?
An equilibrium in which all the reactants are in the same state.
How does changing the temperature of a system affect equilibrium?
Firstly, identify the sign for enthalpy change
Negative ΔH= exothermic
Positive ΔH= endothermic
Increasing the temperature of the system favours the endothermic reaction
Decreasing the temperature of the system favours the exothermic reaction
How does changing the pressure of a system affect equilibrium?
Refer to the number of moles of gases on each side Increasing the pressure of a system favours the side with the fewest moles
Decreasing the pressure of the system favours the side with the most moles
Pressure will have no affect on equilibrium if there is the same number of moles on each side
How does the addition of a catalyst affect equilibrium?
A catalyst does not change the position of equilibrium, however it will increase the rate of the forward and reverse reactions equally.
What is the equation for the equilibrium constant?
Kc= [C]^c x [D]^d /
—————————-
[A]^a x [B]^b
[A] and [B] = concentration of reactants
[C] and [D] = concentration of products
a,b,c,d = number of moles of each reactant/ product
What is the shorthand representation of concentration?
[A] = concentration of chemical A
Eg [CO2]= concentration of CO2
What does a Kc value of 1 indicate?
The position of equilibrium is halfway between the reactants and products
What does a Kc value of greater than 1 indicate?
Kc > 1
The position of equilibrium shifts towards the products/right.
What does a Kc value of less than 1 indicate?
Kc < 1
The position of equilibrium shifts to the reactants/ left
How can to the position of equilibrium due to concentration be investigated?
1- Add a solution of yellow Potassium chromate, K2CrO4, to a beaker
2- Add dilute sulfuric acid drop wise until no further change can be observed. The solution turns orange.
3- Add aqueous sodium hydroxide until no further change can be observed. The solution changes back to a yellow colour.
Explain why the addition of dilute sulfuric acid causes the yellow Potassium chromate to turn orange.
When dilute sulfuric acid is added, the concentration of H+ ions increases. Therefore, this causes the position of equilibrium to shift to right making more products so the solution turns orange as CrO7^2- forms.
How can the position of equilibrium due to temperature be investigated?
1- Dissolve Cobalt chloride in water in a boiling tube. Add a small quantity of hydrochloric acid and place the boiling tube in iced water. The solution is a pink colour.
2- Set up a boiling water bath and place the boiling tube into the boiling water. The solution will turn a blue colour.
3- Transfer the boiling tube back to the iced water to change the solution back to pink.
Explain why changing the temperature of Cobalt chloride causes a colour change.
When placed into the boiling water, the heat energy being supplied to the system increases causing equilibrium to shift.
As the forward reaction is endothermic, the position of equilibrium shifts to the right to take heat energy in and minimise the temperature rise.
The solution then turns a blue colour.
How can the progress of a chemical reaction be monitored?
By monitoring the removal (decrease in concentration) of a reactant
By following the formation (increase in concentration) of a product.
How can the progress of a chemical reaction that produces a gas be monitored?
-Monitoring the volume of gas being produced at regular time intervals using gas collection (gas syringe or upturned measuring cylinder filled with water)
-Monitoring the loss of mass of reactants using a balance
When bonds are broken, what does this tell you about the reaction?
Bond breaking is an endothermic process because energy is taken in from the surroundings.
When bonds are formed, what does this tell you about the reaction?
Bond making is an exothermic process because energy is released to the surroundings
How does adding a catalyst affect the rate of reaction?
A catalyst provides an alternative pathway for the reaction to occur lowering the activation energy meaning more particles are able to exceed the activation energy and more frequent effective collisions can occur per second.
