Kinetics Flashcards
What is the rate of a reaction?
The change in the amount of product or reactant over time
What does collision theory state that particles must do in order to react?
Particles must collide in the right direction (at the right angle) with at least the minimum activation energy required
Why must particles react at a certain angle?
So that the parts that need to collide are closer
What is the activation energy?
The minimum energy required for a reaction to occur or for a successful collision. It is usually the energy needed to break the relevant bonds in the reactant.
Why do most collisions between particles not lead to a reaction?
Because they do not have the sufficient energy
What is enthalpy?
The total energy content of the reacting materials
What is enthalpy change?
The energy exchange that takes place with the surroundings at a constant pressure
What is the symbol for enthalpy change?
Delta H
What is the transition state?
The point at which the reactant is no longer a reactant but isn’t yet a product
Is the enthalpy change for an exothermic reaction positive or negative?
Negative
Is the enthalpy change for an endothermic reaction positive or negative?
Positive
How does a catalyst affect the activation energy of a reaction?
It lowers the activation energy by providing an alternate reaction pathway
What is the Maxwell-Boltzmann distribution?
A plot of the number of gaseous molecules against the energy they have at a fixed temperature
What are elastic reactions?
Reactions where energy isnt lost
What does Emp on the MBD mean?
The most probable energy which is the most frequently occurring energy observed.
What is the mean energy?
The sum of all the energies observed divided by the number of energies observed.
What does the area under the curve represent on the MBD?
The total number of particles present
What does the point at the origin of the curve show?
That none of the molecules have no energy
What does the curve being asymptotic show?
That there is no maximum energy
What is the rate of a reaction affected by?
Temperature, concentration/pressure, presence of a catalyst
Explain the features of the MBD curve at a higher temperature.
Curve looks like it’s been squashed and pushed to the right.
Bigger fraction of particles have energy greater than or equal to the Ea.
Area under curve stays same bc no of particles doesn’t change. Lower max. peak.
Why does a higher temperature mean a faster rate of reaction?
Because particles have more energy so they move faster and so they collide more frequently so there are more successful collisions.
Particles have more energy so more of them have energy higher than the required activation energy
How does a small increase in temperature affect the rate of reaction?
Increases it bc greater no. of molecules have energy greater than the Ea
Explain the features of the curve at a higher concentration/pressure.
Same basic shape but higher max peak
Same Emp and mean energy bc the temperature is the same
Greater area bc more particles
Why is there a faster rate of reaction at a higher concentration?
More particles in a given volume so more frequent collisions also more particles means more particles with a higher energy than the Ea
Explain the features of the MBD when there is a catalyst present?
Shape of the curve is unchanged and there are the same number of particles, all with the same amount of energy but there are more particles with energy equal to or greater than the Ea
How does the use of a catalyst increase the rate of the reaction?
It provides an alternate reaction pathway which requires a lower activation energy so more particles have energy greater than the required energy
Describe some of the features of a catalyst?
Remains unchanged at the end of the reaction
Don’t affect the enthalpy change
Don’t affect the position of equilibria for reversible reactions
What are heterogenous catalysts?
Catalysts which are in a different phase to the reactants
What are homogenous catalysts?
Catalysts which are in the same phase to the reactants
How can you calculate the rate of a reaction from a graph?
Find the gradient of the line
What equation is used to calculate the rate of a reaction?
Quantity/time
How can you measure the rate of a reaction?
Reactants used up or products formed
What experiments can you use to measure the rate of a reaction?
Time taken for a precipitate to form (disappearing cross)
Change in mass
Gas volume produced
