Kinetics I Flashcards
What is the rate of a chemical reaction?
The rate of a reaction is how quickly the products are made or how quickly the reactants are used up. Rate of reaction = change in concentration(or volume)/time
What requirements need to be met in order for two particles to react?
Two particles can only react when they collide with sufficient energy (meet or exceed the activation energy). They need to collide in the correct orientation (steric factor), or else they will simply bounce away from each other.
What is the activation energy of a reaction?
The activation energy (Ea) is the minimum energy that colliding particles need to have for a reaction to occur. If they do not have the activation energy, the particles won’t react.
Why is the orientation of the collision important?
Most reactions have a steric factor (the orientation of the colliding molecules is important). This is because some reacting molecules may only be attracted to certain parties of a molecule (e.g. in an alkene electrophilic addition, the delta positive end of the attacking molecule must be approaching the double bond, or a reaction won’t occur).
How does the concentration of the reactants affect the rate of a reaction?
For reactions in solution, an increase in concentration often causes an increase in rate of reaction. This is because an increase in concentration means there are more particles per unit volume, so particles are, on average, closer together. As a result, collisions between particles happen more frequently. As such, the number of successful collisions per unit time increases, so the rate of reaction is increased.
How does pressure affect the rate of a reaction?
For reactions involving gaseous reactants, an increase in pressure will increase the rate of a reaction. This is because an increase in pressure means three are more particles per unit volume and so gas particles are, on average, closer together. Therefore, collisions between particles are more frequent and so more successful collisions occur per unit time. Pressure may have a small impact on reactions in solution but not for reactions involving solids.
What is the effect of surface area on the rate of a reaction?
For heterogenous reactions (reactions in which the reactants are in different phases, e.g. solid and solution), a large surface area of the solid will result on a faster reaction, because there are more solid particles available to be collided with (as particles below the surface of the material are inaccessible for reactant particles). Therefore, a larger surface area means a greater frequency of successful collisions per unit time, so a faster rate. Increasing the surface area of catalysts also benefits their efficacy, as this gives a larger surface for reactant particles to adsorb onto.
What is the effect of temperature on the rate of reaction?
The higher the temperature, the faster the rate of reaction, because the reacting particles have more kinetic energy, so move more frequently. Therefore, they collide more often. However, the main effect of an increase in temperature is that each collision is also more energetic, so a greater proportion of collisions meet or exceed the activation energy, so not only is the frequency of collisions higher, but the proportion of collisions which are successful is also increased.
What does the Maxwell-Boltzmann distribution show?
The Maxwell-Boltzmann distribution represents the energy distribution of molecules in a gas. The y axis shows the number or fraction of particles with a particular energy, while the x axis shows the kinetic energy. The peak of the graph represents the most common (modal) energy of the gas molecules, while the mean energy of the particles lies to the right of this (as it is skewed by extreme values).
What are some important points to note about a Maxwell-Boltzmann distribution?
- the curves are not symmetrical (they reach a peak early, then decrease)
- the curves start at the origin (as no particles have an energy of zero)
- the curves are asymptotic to the x axis because there is no theoretical limit to the energy a particle could have.
Why is there no theoretical limit to the energy a particle could have?
Successive end-to-end collisions in the same direction would cause a particle to gain more and more energy. This could theoretically happen indefinitely, giving the particle a theoretically infinite energy.
What does the area under a curve on a Maxwell-Boltzmann distribution represent?
The number of molecules in the gas.
What happens to the curve of a Maxwell-Boltzmann distribution if you increase the temperature?
When the temperature of a gas is increased, the average energy of the particles will increase. As a result. The peak of the graph will shift to the right. However, the total number of molecules in the gas remains the same, so the peak of the graph must also be lower than the original, so that the total area remains the same.
How do catalysts work?
Catalysts lower the rate of reaction because they provide an alternative reaction pathway of lower activation energy. This means that a greater proportion of collisions are successful, and so the rate is increased.
How does the addition of a catalyst affect a Maxwell-Boltzmann distribution?
A catalyst will shift the activation energy line to the left, but the peak of the curve will be unaffected, so a greater number of molecules now have enough energy to collide successfully (some still use the old pathway if they have sufficient energy though).
