5. Kinetics Flashcards
What must happen for a reaction to occur between two substances?
Particles must collide with each other, with enough energy during the collision
Do all collisions result in a reaction?
No - most do not
What does the energy of a collision depend on?
- the orientation at which they collide
* the speed at which they are moving
Which molecules have energy?
All molecules have some energy
What is activation energy?
The minimum energy with which particles need to collide to cause a reaction
What is the symbol for activation energy?
Eₐ
On a graph showing an exothermic reaction, is the energy of the reactants or the products higher?
Reactants
On a graph showing an endothermic reaction, is the energy of the reactants or the products higher?
Products
In a sample of liquid or gas where molecules are colliding with each other and the container, are the collisions elastic or inelastic?
Elastic because there is no loss of energy
In a sample at a given temperature, what will be the energy of each molecule?
- small fraction will have high/low energies
* however most will be between the two extremes about the most probable energy
What is the graph showing the spread of energy of molecules?
A Maxwell-Boltzmann Distribution curve
What are the axes on a Maxwell-Boltzmann distribution curve?
Number of particles against energy
What is the most probable energy?
The energy that most particles have around, at any given time
For the average energy on a Maxwell-Boltzmann graph, how should the area be underneath the graph?
If a vertical line is drawn at the point of average energy, the energy either side of the line should be equal
What does the area underneath a Maxwell-Boltzmann graph distribution curve represent?
The total number of particles in the sample
How is the activation energy of a sample represented on a Maxwell-Boltzmann curve?
When Eₐ is marked on the x-axis of the graph, the area to the right of it represents the number of particles with at least this energy
Why is there an asymptote on a Maxwell-Boltzmann distribution curve?
Few particles have that high energy, however there is no limit on how high the energy of these particles can have - so it doesn’t touch the x-axis
How can the rate of reaction be calculated?
- rate = product formed / time
* rate = reactant used up / time
How does an increase in temperature affect the rate of a reaction?
It increases the rate of a reaction
In many reactions, what does a rise of 10°C mean for the rate of reaction?
It causes the rate of reaction to approximately double
Why does an increase in temperature increase the rate of reaction?
- at higher temperatures, particles have higher energy
- meaning they move faster and are more likely to collide with other particles
- when particles, collide, they do so with more energy, increasing frequency of collisions with Eₐ
What happens to a Maxwell-Boltzmann distribution curve when temperature is increased?
The energy distribution moves to the right as average energy of the molecules increases
Why does the shape of a Maxwell-Boltzmann distribution curve change when temperature is increased?
The spread of energies increases
How does an increase in temperature affect the area underneath a Maxwell-Boltzmann distribution curve?
Doesn’t change as the number of particles stays the same
How does an increase in temperature affect the most probable energy on Maxwell-Boltzmann distribution curve?
Increases due to having more energy at higher temperatures
How does an increase in temperature affect activation energy on a Maxwell-Boltzmann distribution curve?
The activation energy will stay the same for both temperatures, so if a vertical line was drawn on x-axis where it is, it would stay in the same place
How does an increase in temperature affect the number of molecules with energy greater or equal to Eₐ on a Maxwell-Boltzmann distribution curve?
A greater number of molecules with energy greater/equal to Eₐ for higher temps.
How does increasing the concentration of a reagent affect the number of particles in a given volume?
It increases the number of particles
How does increasing the concentration of a regent affect the rate of reaction?
It increases the rate of reaction
Why does an increased concentration increase rate of reaction?
- ↑ chance of successful collision
- so ↑ frequency of collisions
- so ↑ frequency of collisions with required Eₐ
How does increasing the concentration of a reagent affect the shape of a Maxwell-Boltzmann distribution curve?
The curve follows the same shape, but the line is above the original, so the area underneath it is larger
How does an increase in concentration affect the area underneath a Maxwell-Boltzmann distribution curve?
The area increases as the number of particles increases when concentration increases
How does an increase in concentration affect the most probable energy on a Maxwell-Boltzmann distribution curve?
Most probable energy stays the same - as there is no increase in energy
How does an increase in concentration affect activation energy on a Maxwell-Boltzmann distribution curve?
Eₐ stays the same - vertical line drawn on x-axis stays in the same place
How does an increase in concentration affect the number of molecules with energy greater or equal to Eₐ on a Maxwell-Boltzmann distribution curve?
For higher concentrations, there’s a greater number of particles with Eₐ or more - due to ↑ frequency of collisions - so ↑ frequency of collisions with Eₐ or more
How does an increase in pressure affect rate of reaction?
Increases rate of reaction
What effect does an increase in pressure have on the Maxwell-Boltzmann distribution curve?
No change of curve - same energy and number of particles
Why does an increase in pressure increase the rate of reaction?
Increasing pressure means decreasing space that gas particles are moving - so ↑ frequency of collisions
What is a catalyst?
A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount
How does a catalyst work?
- reactants interact to form an intermediate compound
- then they form the product
(in order to reach this transition state they need the Eₐ)
How does a catalyst provide an alternative reaction route?
By decreasing the Eₐ required and hence ↑ reaction rate
How does use of a catalyst affect the Maxwell-Boltzmann distribution curve?
- shape of the curve stays the same - same energy and number of particles
- Eₐ lower so greater area of underneath graph with particles having required Eₐ
Why does use of a catalyst mean that reaction rate will increase?
- lower Eₐ so ↑ proportion of particles with energy equal to or greater than Eₐ
- higher proportion of collisions are successful so rate ↑
Do catalysts produce more product?
No - they produce the same amount of product more quickly
How does adding an inert gas affect the rate of a reaction?
Increases rate - as this increases the pressure
