Physical 1 - kinetics

What do you need in order for 2 particles to react?

They must collide with enough energy to break bonds, and in the right orientation, and for rapid reaction you need many rapidly moving particles in a small volume. Most collisions don’t lead to a reaction.

What is activation energy (Ea)?

Minimum amount of energy needed to start a reaction.

Definition of rate of reaction?

Change in the amount/ concentration of reactants or products over time.

What effect does increasing temperature have on the rate of reaction?

Increases kinetic energy of the particles, so more of them have greater Ek than Ea (Ek≥Ea). There is also a higher frequency of successful collisions due to the increased speed.

What effect does increasing concentration of a solution have on the rate of reaction?

If more particles are present in a given volume then collisions are more likely and frequency of successful collisions increases. However as the reaction proceeds reactants are used up and concentration falls, so rate of reaction then drops.

What effect does increasing pressure of a gas have on the rate of reaction?

More particles in a given volume so the frequency of successful collisions increases.

What effect does increasing surface area of solids have on the rate of reaction?

Greater the total surface area of a solid, the more of its particles are available to collide with molecules in a gas or liquid, which increases the frequency of successful collisions.

What is the effect of adding a catalyst on the reaction rate?

Provides an alternative reaction pathway with a lower energy, so frequency of successful collisions increases because more have a greater kinetic energy than the activation energy.

Energy level diagram for exothermic reaction:

Energy level diagram for endothermic reaction:

What is transition state/activated complex?

Some bonds are in the process of being made and some are in the process of being broken.

What are the assumptions of the Maxwell-Boltzmann distribution model?

• Gas molecules have negligible energy compared to the container

• Collisions between the molecules and the walls are elastic so there is no energy change

• Molecules don’t interact with each other

Draw a general Maxwell Boltzman distribution curve:

How is activation energy represented on Maxwell curve?

A line is drawn, and the area under the graph to the right of the Ea line represents the number of particles with enough energy to react.

What happens to the Maxwell curve when temperature is increased, and why?

It shifts to a higher velocity, and the curve gets broader and flatter due to a greater spread of values. The area under the curve stays constant and corresponds to the total number of particles. Small temp increase leads to a large rate increase as many more particles have a Ek greater than Ea, which leads to a large increase in the frequency of successful collisions.

Where are the modal and mean values on a Maxwell curve?

Modal is the highest point, and mean is shifted slightly to the right

Draw a curve showing the effect of increasing and decreasing temperature on the Maxwell curve?

What happens to the Maxwell curve when temperature is decreased, and why?

Shifts to lower velocities, curve gets lower and more pointed due to smaller spread of values, but area under curve

What happens to the Maxwell curve when a catalyst is added?

Moves the Ea to the left resulting in a greater under the curve where particles have a greater Ek than Ea, leads to faster rate of reaction.