How is reaction rate usually expressed ?
A change in reactant or product concentration per unit change in time
What are the units of concentration ?
Describe the convention with regards to expressing reaction rates
Rates of reactions are expressed as positive numbers, as a result, a negative sign is required for reactants.
What are the units of reaction rate ?
What is the rate expression ?
Rate = K [A]m [B]n
Where K = rate constant
m = order of reaction with respect to [A]
n = order of reaction with respect to [B]
Describe the rate constant K ?
Reaction specific, and directly proportional to the rate of the reaction.
What is the impact of increasing the temperature on the rate constant, K ?
It increases since the proportion of molecules with energy greater than the activation energy Ea of the reaction increases with temperature.
How are the orders of reactions determined ?
They need to be determined experimentally.
What is the value of any number to the power zero ?
Describe a zero order reaction ?
The rate of the reaction is independent of the concentration of reactant [A] and therefore has a constant reaction rate.
The rate of the reaction is therefore equal to the rate constant, K.
How would you calculate the overall order of a reaction ?
[A]order + [B]order
Describe a first order reaction ?
The rate of the reaction is directly proportional to the concentration of the reactant.
Rate = k[A]1 or R=k[B]1
Describe a second order reaction ?
The rate is proportional to the square root of the reactant concentration.
Rate = K[A]2
Sketch and explain the graph of reactant concentration over time for a Zero order reaction
As the concentration of reactant A decreases over time, the slope (rate!) is constant.
The rate is not decreased by a decrease in reactant concentration.
Sketch and explain the concentration over time graph of a first order reaction
As the concentration of reactant [A] decreases, the rate decreases proportionally.
(CF exponential decay)
Sketch and explain the graph of a reactant concentration over time for a second order reaction
The rate of the reaction decreases proportionally to the square of the reactant [A] concentration.
CF exponential decay.
What is the half-life of a reaction ?
The time taken for the concentration of the reactant to decrease by half of its original value.
NB. This is different for each rate order.
What do you need to do to determine the exponents of the rate law for a reaction ?
The only way to do this certainly is through experimentation via the "initial rates" or "isolation method".
If there are two or more reactants involved in the reaction, the concentrations are usually varied independently of each other.
What are the units of a zero order reaction ?
Rate = K [A]0
What are the units of a first order reaction ?
Rate = K [A]1
What are the units of a second order reaction ?
Rate = K [A]2
Name the three steps involved in free radical formation ?
Describe the formation of HCl from H2 + Cl2
What is the rate determining step of a reaction ?
The slowest step of a reaction
How would you predict the rate law from a chemical equation ?
The chemical equation of an elementary step reflects the exact molecular process that transforms its reactants into its products.
In an elementary process, the orders with respect to the reactants are equal to the corresponding stochiometric coefficients
What is the Arrhenius constant (A) ?
Also known as the frequency factor.
What is the collision frequency (z) ?
The number of collisions that molecules acquire per unit time.
What is the orientation factor (p) ?
The proper orientation reactant molecules require for product formation.
What is Ea ?
Describe how it works
For reactants to transform into products, the reactants must go through a high energy "transition state" which is the minimum energy required for reactants to transform into products.
What factors increase K (and therefore, rate?)