HS Chemistry - Reversible Reactions

Collision Theory

Overview of The Page

This page will cover:

• What is collision theory?
• According to collision theory, what factors affect a reaction's rate?

The collision theory is the idea that chemical reactions occur when molecules collide with one another. Molecules have kinetic energy, and thus they move around and collide with one another. However, reactant molecules must collide with a certain amount of energy, called the activation energy, in order to react. If they collide with less energy than this, then they will not react. This is called an ineffective collision. However, if reactant molecules collide with enough or more energy than is needed for the reaction, then they will react. This is called an effective collision. Effective collisions are influenced by several factors.

Temperature

The temperature of a substance is a measure of the average kinetic energy of that substance's molecules. Some molecules have a kinetic energy higher than the average, and some have a kinetic energy lower than the average. Therefore, some of the molecules have enough kinetic energy that when they collide, they will be able to react. If the temperature is increased, the average kinetic energy of the molecules will increase. There will be more molecules $assuming we didn't add or remove matter from the system$ that have enough energy to react when they collide, and therefore more effective collisions will occur when the temperature is higher. Thus, higher temperatures increase the rate of a reaction.

This can be visualized through a Boltzmann diagram:

The Boltzmann diagram shows the kinetic energies of the molecules at different speeds. The height of the graph at a particular point shows how many molecules in the system have that molecular speed. The dashed line labeled "250K" shows that the average kinetic energy of the molecules, the temperature of the system, is 250K, and the dashed line labeled E_a shows the activation energy. These labeled lines are extra - they are only here to provide extra information, and won't necessarily be on all Boltzmann diagrams.

This diagram shows that only a few molecules have enough energy to react when they collide with one another. But if the temperature was 350K instead of 250K:

More of the molecules in the system have enough energy to react when they collide with each other if the temperature is 350K instead of 250K. It's still less than half, since the temperature, which gives the average kinetic energy of the molecules, is 350K, but it's more than it was at 250K.

The area underneath both graphs has remained the same, because the amount of molecules in the system hasn't changed.

Concentration

If we do change the amount of matter in the system, then the rate of reaction will also change. This isn't as simple as more matter = faster reaction; the rate of reaction depends on the concentration of the molecules. For example, if we take the reaction:

NaCl $aq$ + K $s$ ⇌ KCl $aq$ + Na $s$

On the reactant side there's NaCl dissolved in water. If we add more NaCl, we increase the concentration of NaCl in the water, which will increase the reaction, as reactant molecules will be able to collide with one another more often if there are more of them.

But if we instead add more water to the dissolved NaCl solution, the concentration of NaCl in the solution will decrease. This will cause the reaction rate to decrease, as reactant molecules won't be able to collide with each other as often if there are more water molecules present, even though the amount of NaCl molecules hasn't changed.

A higher concentration of reactants increases the rate of reaction.

Catalysts

Catalysts decrease the activation energy required for a reaction to occur. They therefore increase the rate of reaction, as more molecules are able to collide with enough energy to react. Therefore, catalysts always increase the rate of reaction, but don't affect the position of equilibrium in a reversible reaction.

Some of the best and most effective catalysts are found in living organisms, although not all catalysts are. For example, in the Haber process, iron is often used as the catalyst.

Reactions often occur in the same state of matter - that is, all the reactants and products are in the same state of matter in the reaction $for example, a reaction between two dissolved ionic compounds that produces two different dissolved ionic compounds$. However, they also often occur in different states of matter - that is, not all of the reactants and products are in the same state of matter in the reaction $for example, a reaction between two dissolved ionic compounds that produces a precipitate \[a solid\]$.

Catalysts can be classified as homogenous or heterogenous based on this. If the reactants, products, and the catalyst$s$ are all in the same state of matter, then the catalysts is homogenous. Otherwise, the catalyst is heterogenous.

Rate of Reaction

The rate at which a reaction occurs can be determined by measuring the change in concentration of its reactants OR products over time:

Average reaction rate = ∆[Reactants] / ∆t

OR

Average reaction rate = ∆[Products] / ∆t

As a reaction in one direction $non-reversible reaction$ progresses, there are fewer and fewer reactants available as more products are formed. Thus, as the concentration of reactants decreases, so does the reaction rate, which is why the formula above gives an average reaction rate.

Another way of measuring the rate of a reaction is through the reaction's half-life - the amount of time it takes to react 50% of the reactants present in the system. The half-life of a reaction in a given system that experiences no change other than the reaction being observed $the temperature, pressure, concentration, etc. of the substances aren't changed$ always remains constant.

Using the half-life, an equation for the reaction's rate can be formed. This is the reaction's rate law.