5.1 RATES OF REACTIONS Flashcards
What is the equation to work out rate?
Rate = change in concentration / change in time
What are the orders of reactions?
Orders of reactions:
- zero order
- first order
- second order
What is zero order?
Zero order is when the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant.
- any number raised to the power of zero is 1
- concentration does not influence the rate
What is first order?
A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one. [A]^1
- if the concentration of A is doubled (x2), the reactions rate increases by a factor of 2^1 = 2
- if the concentration of A is tripled (x3), the reactions rate increases by a factor of 3^1 = 3
What is second order?
A reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two. [A]^2
- if the concentration of A is doubled (x2), the reaction rate increases by a factor of 2^2 = 4
- if the concentration of A is tripled (x3), the reactions rate increases by a factor of 3^2 = 9
What is the rate equation?
Rate equation:
rate = k [A]^m[B]^n
where-
rate = rate of reaction
k = rate constant
A = concentration of A
m = order of reaction with respect to A
B = concentration of B
n = order of reaction with respect to B
What is the rate constant?
The rate constant, k, is the proportionality constant. It is the number that mathematically converts between the rate of reaction and concentration and orders.
What is the overall order?
The overall order of a reaction gives the overall effect of the concentrations of all reactants on the rate of reaction.
overall order = sum of orders with respect to each reactant
rate = k [A]^m[B]^n
overall order = m+n
What are the units of the rate constant and how can they be determined?
The units of the rate constant, k, depend upon the number of concentration terms in the rate equation.
Determined by:
1. rearranging the equation to make k the subject
2. substitute units into the expression for k
3. cancel common units and show the final units on a single line
How do you work out orders for experimental results?
Orders of reaction must be determined experimentally by monitoring how a physical quantity changes over time. Orders cannot be found directly from the chemical equation.
When comparing, the effect of different concentrations of reactants on reaction rates, it is important that the rate is always measured after the same time, ideally as close to the start of the experiment as possible.
What is continuous monitoring?
Continuous monitoring is when concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction.
