# Rate of Reactions

## Key Questions

• Several factors can increase the rate of a chemical reaction. In general, anything that increases the number of collisions between particles will increase the reaction rate, and anything that decreases the number of collisions between particles will decrease the chemical reaction rate.

INCREASE THE CONCENTRATION OF THE REACTANTS
A higher concentration of reactants leads to more collisions per unit time and leads to an increasing reaction rate.

INCREASE THE PRESSURE OF GASEOUS REACTANTS
Changing the pressure of gaseous reactants is, in effect, changing their concentration. The increased number of collisions caused by a higher pressure generally increases the reaction rate.

DECREASE THE PARTICLE SIZE OF SOLID REACTANTS
Reaction depends on collisions. If a reactant is a solid, grinding it into smaller particles will increase the surface area. The more surface area on which collisions can occur, the faster the reaction.

INCREASE THE TEMPERATURE
Usually, an increase in temperature causes an increase in the reaction rate. A higher temperature means that the molecules have a higher average kinetic energy and more collisions per unit time. It also increases the number of collisions that have enough energy to cause a reaction to take place.

CHANGE THE SOLVENT
The rate of a chemical reaction depends on the medium in which the reaction occurs. The reaction may go faster in an aqueous solution than in an organic solvent or in a more polar solvent.

Catalysts lower the activation energy of a chemical reaction and increase the rate of a chemical reaction without being consumed in the process. They do this by an alternative mechanism that has a lower activation energy.

• It doesn't.

Activation energy is a fixed amount for a reaction - a constant which is derived from the bonds in the reactants which need to be broken so that the products can be formed.

It is of thermodynamic importance as to what the numerical value of the activation energy is.

The rate of reaction, however, is of kinetic importance. That is a question of how quickly the bonds break, not how easily the reaction proceeds.

For the same reaction, it is the same binds that need to be broken, so the same amount of energy that will be needed to do it. The only thing that can affect the activation energy is using a catalyst - that introduces a new reaction pathway, so it is not really the same reaction any more.

Hayek explains some of this in a video.

• CHEMICAL REACTION RATES
The reaction rate of a chemical reaction is the amount of a reactant reacted or the amount of a product formed per unit time. Often, the amount can be expressed in terms of concentrations or some property that is proportional to concentration.

For a reaction such as A → 2B, we could measure either the rate at which [B] increases or the rate at which [A] decreases.
Rate of formation of B = increase in concentration/time interval = Δ[B]/Δt
Rate of reaction of A = increase in concentration/time interval = -Δ[A]/Δt

Note that the rate is always expressed as a positive number (that’s the reason for the negative sign in front of the Δ[A]).

Also, the two rates are different. If [A] is decreasing at the rate of 0.1 mol•L⁻¹min⁻¹, [B] is increasing at the rate of 0.2 mol•L⁻¹min⁻¹. The rate of a reaction should be the same, no matter how we measure it. We must account for the stoichiometry of the reaction. So we divide the rate with respect to a component by its coefficient in the balanced equation. Thus, whether we are measuring [A] or [B], the rate of reaction is 0.1 mol•L⁻¹min⁻¹.

For a reaction such as aA + bB → cC + dD, our new definition becomes
Rate of reaction = -(1/a)Δ[A]/Δt = -(1/b)Δ[B]/Δt = (1/c)Δ[C]/Δt = (1/d)Δ[D]/Δt

Depending on the time interval between measurements, the rates are called
initial rate: instantaneous rate at the beginning of an experiment
average rate: rate measured between long time interval
instantaneous rate: rate measured between very short interval

MEASURING REACTION RATES
To measure a reaction rate, we usually monitor either a product or a reactant for its change. We can monitor any physical characteristic related to the quantity or concentration of a product or reactant.

EXAMPLE
N₂O₃(g) decomposes to NO₂(g) and NO(g) according to the reaction
N₂O₃(g) → NO₂(g) + NO(g)
The reaction is followed by measuring [NO₂] at different times. The following data are obtained.
t/s [NO₂]/mol•/L⁻¹
0,,,,,,,,,,,0
884,,,,,,0.193

What is the average rate of formation of NO₂ over this time interval? What is the average rate of reaction over this time interval?

Δ[NO₂]/Δt = (0.193 – 0) mol•L⁻¹/(884 – 0) s = 0.193/884 mol•L⁻¹s⁻¹ =
2.18 × 10⁻⁴ mol•L⁻¹s⁻¹

Since the coefficient of NO₂ in the balanced equation is 1,
Rate of reaction = (1/1)Δ[NO₂]/Δt = 2.18 × 10⁻⁴ mol•L⁻¹s⁻¹