5. Kinetics
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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