Chaotwr 18 Flashcards
What is rate of reaction formula
Units
CHANGE IN Concentration / time
CAN be increase conc if a product or decrease conc if a reactant
In which case the graph Will start high finish low
2) units = moles-3 s-1
Orders
This is if doubke conc and order is one, the rate double 2 too, if doubke and order 2, then sqaure
If doubling has no effect, rate is 0
Overal, rate is the sum of the indicies in rate equation
How to compare orders question
How to find k?
Say comparing reaction 1 and 2, and say how one thing is constant (or work it out to be constsnt)
When conc increase y, rate increase by, therefore the order of reactions I x with respect to y
2; find k by once write rate equation sub a row in
Why do we need continous methods of monitoring to find conc time graphs WHATS the use?
As conc/ time = rate, the gradient tells us the rate, and the change of gradient with respect to the conc will tell us the orders
Also with a conc time graph we can take instaneoues rates and use that to find RATES, that allow us to plot against conc for rate conc graph which gives us perfect picture
Can also maybe tell directly from a conc time graph the order, assuming everything else constsnt
SHAPES FOR CONC TIME
If it’s flat low= 0 order
- bevause the gradient is constsnt = rate is constant, doesn’t change with a change of conc
If it’s curved = 1
- gradient constantly changing, meaning with a change of conc the rate changes proprtinaly
If it’s even more curved = 2
- this because now change of conc will cause gradient to change by that factor sqaured
But we don’t need to know this
Why does 1st order conc time follow exponential decay?
How can we tell if it’s expoential usign half lives
Because for the same given time interval, the factor of decrease is CONSTANT
2) if the time taken for conc to drop to half is the SAME in successive jumps = expoential decay and FIRST ORDER
Why can there be two ways of rate EQUATUIN fir same reaction?
Once for the products and one for the reactants
How ti find the k constsnt assuming that rate equation only has ONE reactant and it has constant half life
K = ln2 / half life
OR 1) rate = k x
Find rate of x at any conc using gradient
And re arranbe
Shapes of rate conc and how to find k assuming only reactant
HOW TO PLOT RATE CONC GRAPH?
0 order straight , as conc increases nothing happens to rate
K = y intercept
1st = linear
As conc increase so does rwte, gradient = k
2nd = quadratic
And if you plot rwte/ conc 2, gradient will be the k
2) to plot a rate conc graph, need to find gradients of conc time graph at different concentrations and record them!
How ti do continous monitoring experiments which type
Why do we use mass decrease when clearly a gas is produced?
1). Monitoring as mass decreases with time (
2) monitoring the increase in volume / time (upturned flask ) for something that produces gas/ gas tube
NEED MASS DECREASES TECHNIQUE when the gas being produced DISSOLVES IN THE LIQUID, meaning you can’t collect it!
3) using colorimeter
Need Coleoptera because not all reactions produce a gas!
How to do these with Colorimtery
1st need to plot a calibration curve.
Of known concs of the product, test with colorimeter
- opposite wavkentgh of light
-zero with distilled water between esch redwing
- take readings of either absorbsnce transmittance whatever , cinc agsisnt this
Now at integrals take out samples of your thing, and measure the absorbsnce
Can now find concs at the different times, now have conc time graph
Can then go on to make a rate conc graph etc
What is the intial rates method and why is it better
WHAT IS ASSUMPTION
To find orders etc, you have to compare the INITAL RATES with different concentrations
This is tedious cuz it means you have to keep drawing new graphs
Instead if you observe the time taken for a colour change to happen, and assume that less than 15% of the reaction has happened, then you can make ASSUMPTION THAT THE INITAL RATE = TO THE RATE YOU CALCULATED
As the rate will be PROPORTIONAL to 1/t taken for a colour change to appear ( more conc = quicker rate = less time), you can use these values and plot against concentration to give you the SHAPE of the graph that will allow you to find the order of reaction very easily
However if you acc want to find k will need to do calculations to find concs
Again what is assumption with intial rates method
That the colour change took place e before 15% if reavtion done, because then the intial rwte is roughly the same as this rate
What happens in iodine clock?
First reavtion iodine is made
Second reaction reacts with thisulfste to go back to iodine ions
You have starch , and starch shoud remain colourless as iodine is converted till ions
Only until ALL THE THISUKFSTE IS USED, will the colour change happen and it remains BLUE as iodine is no longer converted
Reason for using thisukfste is because colour change would else happen STRAIGHT AWAY, it’s needed to delay
What side is the rate equation normally , reactant or product?
Normally reactant because you measure decrease in conc
