Rates of Reaction (F6examonly) Flashcards
(18 cards)
What is the Boltzmann distribution?
a graph that shows the distribution (spread) of kinetic energy in gases, liquids, and solutions
What is on the y axis and x axis of the Boltzmann distribution curve?
y axis - number of molecules
x axis - kinetic energy
What is the principle behind the Boltzmann distribution?
particles in a gas, liquid, or solution have varying kinetic energies (they move at different speeds)
the variation is shown in the Boltzmann distribution
What key points about molecular kinetic energies in gases does the Boltzmann distribution show?
- Some molecules move slowly because they have low kinetic energy.
- Others move very fast due to their high kinetic energy.
- The majority of molecules, however, have moderate kinetic energies and speeds.
- Only molecules with kinetic energies greater than or equal to the activation energy have sufficient energy to react when they collide.
What does every Boltzmann distribution have (rules) and why?
- the curve must start at the origin because zero molecules have zero energy
- the curve never touches the X axis (asymptote) because there is no maximum energy for a molecule
What is the total area under the Boltzmann distribution curve equivalent to?
the total number of molecules
What are the key features of the Boltzmann distribution?
- the curve peaks around the middle because a large proportion of molecules have moderate kinetic energies (most move at moderate speeds)
- the point where the curve peaks shows the most probable kinetic energy a molecule has (Emp)
- a small proportion of molecules have kinetic energies greater than the activation energy
Explain the relationship between the Boltzmann distribution and activation energy?
- the activation energy (Ea) can be marked on the x axis (kinetic energy)
- only the molecules that have an E ≥ Ea can react
- so the area under the graph past the Ea on the x axis indicates the number of molecules that have E ≥ Ea (that can react)
Explain how the Boltzmann distribution curve changes with the presence of a catalyst:
- a catalyst provides an alternative reaction pathway with a lower activation energy
- therefore a larger proportion of molecules have E ≥ Ea (the area under the graph past the Ea on the x axis is greater)
- therefore a greater number of molecules can react and so the rate of reaction increases
Does the Boltzmann distribution curve change shape with the presence of a catalyst?
- no
- the curve stays the same because the distribution of kinetic energy in the molecules stays the same
- only the Ea has changed
What is the effect of temperature on the rate of reaction, in terms of frequency of collisions?
- increasing the temperature increases the kinetic energy of the particles so they move faster and are more likely to collide
- there are also more particles with energy greater than the activation energy so more are able to react
- therefore there are more frequent successful collisions between particles
- and this increases the rate of reaction
What is the effect of concentration on the rate of reaction, in terms of frequency of collisions?
- increasing the concentration increases the number of particles in a given volume (the particles are closer together)
- therefore there are more frequent successful collisions between particles
- and this increases the rate of reaction
What is the effect of pressure of gases on the rate of reaction, in terms of frequency of collisions?
- increasing the pressure of gases decreases the distance between the particles (they are pushed closer together)
- therefore there are more frequent successful collisions between particles
- and this increases the rate of reaction
How do you determine the mean rate of reaction from a graph?
Δ mass/conc. / Δ time
(rise over run)
How do you determine the rate of reaction at a specific point in time from a graph?
draw a tangent and find the gradient of that tangent (rise over run)
What is the role of a catalyst in increasing the rate of reaction in terms of activation energy?
- a catalyst increases the rate of reaction
- by providing an alternative reaction pathway with a lower activation energy
- this means more particles have E ≥ Ea and so more are able to react
- therefore there are more frequent successful collisions between particles
- this increases the rate of reaction
What is the role of a catalyst in increasing the rate of reaction in terms of without being used up?
- a catalyst increases the rate of reaction without undergoing permanent chemical change itself
- it can react to form an intermediate or provide a surface on which the reaction takes place
- but it is not used up in the reaction and is always regenerated at the end (so can be reused)
What does an enthalpy profile diagram show in the presence of a catalyst?
the peak is lower
