Module 5.1 Flashcards
Define rate of reaction
The change in concentration of a reactant or product per unit of time
List ways in which we can measure the rate of reaction?
Measure the loss in mass Measure how fast a gas in produced Measure how long it takes for a solution to go cloudy Measure the pH change of a reaction Watch for colour change
When measuring a change in mass, what should you use?
A weighting scale
When measuring how fast a gas is produced, what should you use?
A gas syringe
When measuring how long it takes for colour change, what should you use?
Colourimetry
What is meant by order?
With respect to a reactant, the order is the power to which the concentration of a reactant is raised to in the rate equation
What 3 orders must we be aware of?
Zero order
First order
Second order
What does the concentration-time graph of a zero order reactant look like?
Straight line downards
What does the rate-concentration graph of a zero order reactant look like?
Straight line
With zero order reactants, what will happen to the rate, if the concentration of the reactant is changed?
Nothing
With zero order reactants, what pattern does its half-life follow?
DECREASES with time
What does the concentration-time graph of a first order reactant look like?
Slopes downwards
What does the rate-concentration graph of a first order reactant look like?
Straight line upwards
With first order reactants, what will happen to the rate, if the concentration of the reactant is changed?
The rate changes by a proportionally equal amount
With first order reactants, what pattern does its half-life follow?
Constant
What does the concentration-time graph of a second order reactant look like?
Slopes downwards
What does the rate-concentration graph of a second order reactant look like?
Slopes upwards (exponential growth)
With second order reactants, what will happen to the rate, if the concentration of the reactant is changes by X?
The rate will change by X^2
With second order reactants, what pattern does its half-life follow?
Increases with time
Define overall order
The sum of the orders of all the different reactants
If the overall order of a reaction is 0, what units does the rate constant have?
mol dm^-3 s^-1
If the overall order of a reaction is 1, what units does the rate constant have?
s^-1
If the overall order of a reaction is 3, what units does the rate constant have?
dm^3 mol ^-1 s^-1
If the overall order of a reaction is 4, what units does the rate constant have?
dm^6 mol ^ -2 s^-1
What is meant by the rate constant?
The constant that links the rate of reaction with the concentration of the reactants raised to the powers of their orders in a rate equation
What form do rate equations take?
r = k[A]^m x [B]^n
Where A is the concentration of reactant A
Where B is the concentration of reactant B
Where m is the order of reactant A
Where n is the order of reactant B
Define the term half-life
The time taken for HALF of the reactant to be used up
What is the half-life pattern for zero order reactants?
Decreases with time
What is the half-life pattern for first order reactants?
Remains constant throughout the reaction
What is the half-life pattern for second order reactants?
Increases with time
Define the term, the rate determining step
The slowest step in the reaction mechanism of a multi-step reaction
Are chemicals in the rate equation in the rate determining step?
Yes
In the rate equation, if a chemical is second order, how many moles of it will be in the rate determining step?
2 moles
If identical chemicals appear on the same side of the reaction mechanism steps, what do we do?
ADD them together
If identical chemicals appear on opposite side of the reaction mechanism steps, what do we do?
Subtract them
Give the overall equation for this reaction
Step 1) 2N2O5 → N2O4 + 2NO3
Step 2) N2O4 + NO3 → NO + O2 + 2NO2
Step 3) NO + NO3 → 2NO2
2N2O5 → 4NO2 + O2
What equation to find k, can only be used when the overall order is 1?
k = ln(2) / Half-life (seconds)
What are the units for half-life?
Seconds (unless stated otherwise)
Why does the equation k = ln(2) / Half-life only work when the overall order is 1?
First order reactants have a constant half-;life
How can we investigate reaction rates using the initial rates method?
Step 1) Carry out the reaction, continuously measuring one reactant
Step 2) Use this to draw a concentration-time graph
Step 3) REPEAT the experiment using a different initial concentration
Step 4) REPEAT the process for each reactant at regular time intervals and find the rates and orders
