Chapter 18 Rate Orders Flashcards
What is the equation for rate?
(Quantity reacted or produced) / time
Change in Concentration / Change in time
What do square brackets mean?
Concentration of
What is the relationship between rate and concentration?
Rate is proportional to concentration to the power of the order
What is order of a reactant?
The measure of impact of changing concentration on rate
What does zero, first, and second order mean in terms of concentration and rate?
Zero- changing concentration has no effect upon rate
First- the change in concentration is equally reflected in the rate
Second- the change in concentration squared is reflected in the rate
What is the general form for the rate equation?
rate= k[m]^a [n]^b
How do you find the overall order a reaction?
Add the powers of the reactants in the rate equation
How do you find the units of k?
Substitute into the rearranged equation for k, and cancel
How can rate orders be established? Where do catalysts fit in?
Only experimentally, not through the stoichiometric equation
Catalysts can appear in the rate equation
From experimental data, how can you determine the order of a reactant?
Pick two rows where only one concentration changes, and see how the change changes rate, determine order
ENSURE ONLY ONE CHANGES, OR USE PREVIOUS INFO
What are 3 methods for continuous monitoring?
With a gas- Measure the volume of gas collected or mass loss as gas escapes in a certain time
With acid/base/redox- Take samples at regular intervals and calculate conc.
With a colour change- colorimetry, take samples regularly and compare to calibration curve
What is the procedure for colourimetry? [6]
Prepare standard solutions of the coloured reactant. (This can be done by mixing 1 molar solution with water. e.g 0.8 sol + 0.2 water, 0.6 sol + 0.4 water. Conc= original conc x vol coloured / vol total )
Choose a colour complimentary to the coloured substance, opposite on the colour wheel, for the filter
Zero the colorimeter with water
Measure the absorption readings for the different concentration of the coloured substance and plot a calibration curve. Increased conc increased absorption
Carry out the reaction and take samples at regular intervals. Measure the absorption of these samples in the colorimeter.
Convert these readings to concentrations using the calibration curve and plot a concentration time graph. Which can be used to find order.
What is the shape for a zero order concentration-time graph? How can k be calculated?
Constant gradient downwards
Gradient= k
What is the shape for a first order concentration-time graph? How can k be calculated?
Reciprocal graph- half lives constant, calculate at least 3 half lives to verify
k= ln2/half life
Or draw a tangent at a point, calculate the gradient for rate, rearrange the rate equation and use the concentrations to calculate k
What is the different between first and second order concentration-time graphs?
Second order steeper at first and tails of more slowly
Half lives not constant for second order
What is half life and how can it be calculated?
The amount of time taken for the concentration of a reactant to half
Look the time elapsed between the start concentration and half of this concentration
Repeat three times to verify half life
What type of mathematical graph is first order concentration-time graphs?
Exponential decay
What is the shape for a zero order rate-concentration graph? How can k be calculated?
Horizontal line, does not go through the origin
Y intercept= k
What is the shape for a first order rate-concentration graph? How can k be calculated?
y=mx + c format straight line, through the origin upwards
Gradient= k
What is the shape for a second order rate-concentration graph? How can k be calculated?
Quadratic curve through the origin
Plot a graph of rate against concentration squared
k= gradient of this second graph
Or, use values from the graph to find k from rearranging the rate equation, assuming orders of others are known
Rate against Conc
Rate on y
Conc on X
What are the equations used in iodine clock reactions?
S2O82- + 2I- –> 2 SO4 2- +I2
I2 + 2 S2O3 –> 2I- + S4O6 2-
Describe how the reactions in the iodine clock practical lead to initial rate determined?
Iodide ions are colourless. Peroxydisulphate ions reacts with these ions to form iodine, orange brown, and 2 sulphate ions
The iodine produced is immediately used up by the small amount of Thiosulphate ions to produce Iodide ions which are colourless and tetrathionate ions.
When the thiosulphate ions run out, iodine is no longer used up, giving a sharp end point
Rate= 1/t
What is the procedure for initial rates reaction with iodine clocks?
Add 5cm3 iodide ions, 1cm3 of thiosulphate ions, and 1cm3 of starch to a conical flask. In a separate flask, add 5 cm3 of peroxydisulphate
Add the peroxydisulphate to the conical flask and immediately start timing
Stop the timer when the blue/black colour appears.
Repeat the experiment but use 4cm3 of peroxydisulphate and 1cm3 of water. Then 3 and 2. And so on, varying the concentrations of the ions.
Repeat the experiments with 5cm3 of Peroxydisulphate but varying amounts of Iodide ions, working with varying amounts of water as with the peroxydisulphate.
Work out the concentrations of the peroxydisulphate and Iodide by Bottle conc x vol of iodide / total vol of reactants
Calculate rate by 1/t
Plot a graph of rate against concentration for each of the reactants