Topic 16: Further Kinetics COPY Flashcards
How do you find reaction rate from a graph?
RR is measured by recording how quickly chemical curves change. As this varies throughout the reaction we always consider the initial rate, which is the gradient at 0.
What is the rate eqn?
The rate eqn tells us the relationship between the concs of the reagents and the reaction rate. eg
Rate= k[A]^x[B]^y
k= rate eqn.
x and y= order of the reaction with respect to A and B
What does the rate eqn tell us?
If you increase the concentrations of A or B, the rate would increase by some factor. X and Y tell us by how much changing the concentration will affect the rate.
We cannot tell X and Y from the overall equation, only from experimental data.
What are the types of orders?
First order: if you double A the rate is doubled. The value of X=1. Rate is directly proportional to [A]^1
Second order: if you double A the rate is squared, increasing by a factor of 4. X=2. Rate is directly proportional to [A]^2
Zero order= if I change the concentration of a it has no effect on the overall rate. Rate is directly proportional to [A]^0
State the definition for the order of a reaction
Is the power to which the concentration of a given reagent is raised in the rate equation.
What is the overall order of a reaction?
The overall order of reaction equals the sum of the orders, x+y. eg overall order for [A][B]^2 = 3
Describe k, the rate constant.
k is large for fast reactions and small for slow reactions. Increase in temperature is accompanied by an increase in the value of k.
k is a temperature dependent constant (like Kc!)
what are the units for k?
The unit of k will change depending on the expression for the rate eqn.
Deduce the units for k in the following rate eqn:
rate= k[B][C]
Units for rate= moldm^3s-1 units for [B]= moldm^-3 units for [C]= moldm^-3 Substitute units into equation and solve: moldm^3s-1= k x moldm^-3 x moldm^-3 s-1 = k x moldm^-3
therefore k = mol^-1dm^3s-1
Do catalysts appear in the rate equation?
Don’t appear in chemical equations but may appear in the rate of expression or rate eqn.
Describe reagent CONCENTRATION time graphs for 0, 1st and 2nd orders
see sheet- 0 order is a straight line gradient downwards
1st order is a more curved downwards line
2nd order is most curved
Describe RATE-CONCENTRATION graphs for 0, 1st and 2nd orders
On the x axis 1/time is plotted. on y axis, plot the conc of a REAGENT.
0 order= a flat line
1st order= a straight, positive gradient
order>1= a curved, positive gradient
2nd order= a straight, positive gradient
To find an order>1, plot 1/time against the concentration SQUARED. If the line is straight, then it is second order
How can we find the orders from the INITIAL rate method?
A series of experiments are carried out at constant temp, changing only the concs of the reagents. By comparing initial concs and rates of pairs of experiments the order can be found
What is the half life of a reaction?
The time taken for the concentration of the reagent to become half of its initial value.
How can we find the orders using half lives in RATE CONC graphs?
To find the half life, find half of the highest concentration. Dot along the graph until you hit the curve. Mark down the time taken, eg 10mins. Find half of the concentration again, and do the same again. Is the half lives are constantly the same (always 10mins), the order of the reagent is one