OZ4: Rates of Reaction Flashcards
What is meant by the “rate of reaction”?
How quickly reactants are converted into products
Give the formula for calculating the rate of reaction.
Change in property/time taken
Give 5 ways change in property can be measured to determine rate of reaction.
- Chemical analysis
- Volume of gases
- Mass changes
- PH measurement
- Calorimetery
Why is it important for scientists to measure rates of reactions?
- A reaction occurring too quickly can be dangerous
- A reaction occurring too slowly is impractical because it ties up equipment and people, which costs money
Give an example in which the volume of a gas is measured to determine rate of reaction.
- Reaction between calcium carbonate and HCl produces CO2
- Gas is collected in a measuring cylinder
Give an example in which mass change is measured to determine rate of reaction.
- Reaction between calcium carbonate and HCl produces CO2
- Mass lost of calcium carbonate can be recorded against time
Give an example in PH is measured to determine rate of reaction.
- Reaction between calcium carbonate and HCl produces CO2
- HCl concentration falls so the PH of the reaction mixture also falls
What does a calorimeter do?
Measures change in a reaction’s colour
Give an example where calorimetery is used to determine rate of reaction.
- When zinc reacts with copper (II) sulphate, the blue coloration of copper sulphate decreases
- Measure the rate of this colour decreasing against time
Explain how chemical analysis can determine the rate of reaction.
- Involves taking samples of the reaction mixture at regular intervals
- Stopping reaction in a sample (quenching) before analysis
Give an example in which chemical analysis is used to determine rate of reaction.
- Iodine and propanone react in the presence of an acid catalyst
- The sample is extracted and quenched by adding sodium hydrogen carbonate, neutralising the acid
- Amount of iodine remaining is determined by titration
How do you plot a graph for the rate of reaction?
- Time is plotted on the x axis
- Change in property is plotted on the y axis
Give the conditions for a successful collision.
- Must collide in the right direction
- Must collide with correct orientation
- Must collide with the minimum amount of Kinetic energy required
What are liquid and gas particles always doing?
Always moving and colliding
Why is initial reaction the fastest?
- As the reaction continue, the concentration of reactants decreases
- Less frequent collisions between reactant particles
- Decreased rate of reaction
What does an increased rate of reaction mean in terms of collisions?
Increase in collisions in a given time between reacting particles
Define “activation energy”
The minimum amount of kinetic energy required by a pair of colliding particles before a reaction occurs
What is activation energy needed for?
To make or break bonds
What happens at the highest point of an enthalpy profile diagram?
Transition state:
- Old bonds stretch and break
- New bonds form
In an exothermic enthalpy profile are the reactants higher up or lower down than the products?
Higher
In an endothermic enthalpy profile are the reactants higher up or lower down than the products?
Lower
What does a Maxwell-Boltzmann Distribution graph show?
The distribution of different kinetic energies in a gas
What does the area under a Maxwell-Boltzmann Distribution graph represent?
The number of molecules
Are there more particles on the left or the right side of a Maxwell-Boltzmann Distribution graph?
On the right side
Where on a Maxwell-Boltzmann Distribution graph is the max number of particles with a specific energy?
In the middle on the highest peak on the graph
Where on a Maxwell-Boltzmann Distribution graph is the average energy shown?
In between the middle and Ea on the graph
What factors influence rate of reaction?
- Temperature
- Radiation intensity
- Concentration/pressure
- Catalyst
- Particle size
How does increased temperature increase rate of reaction?
- Increased temperature, means particles on average have more kinetic energy and will move faster
- A greater proportion of molecules will have the activation energy and will react
- So there will be a higher frequency of collisions which involve energy equal to activation energy
- So there will be more successful collisions in a given time (e.g per second)
How does increased temperature change a Maxwell-Boltzmann Distribution graph?
- The peak of the curve is lower
- Graph is shifted to the right
- But same total area under the graph
Why is the peak of the curve shifted lower?
To balance out other areas of the graph increasing so there is a same total area under the graph
What happens to the most probable and average energy when temperature increases?
They also increase
How does the amount of particles with the greater than/same energy as activation energy?
More particles
How does increased concentration/pressure increase rate of reaction?
- Particles are closer together on average
- More particles per cm^3
- So they collide more often, leading to more successful collisions per second
How does increased concentration/pressure change a Maxwell-Boltzmann Distribution graph?
- Area under the graph is bigger because they are more molecules
- So same shape of graph but the curve is higher up
How does particle size increase rate of reaction?
- Powder is better than a lump
- Because there is a greater surface area making collisions more likely
How does a catalyst increase rate of reaction?(2)
- Provides an alternative pathway for a reaction
- with a lower activation energy
How does using a catalyst change a Maxwell-Boltzmann Distribution graph?
- Put the Ea line further up the graph
- Because at any given temperature, a larger proportion of particles collide successfully
