Rate Of Reaction Flashcards
(17 cards)
What is rate of reaction and how is it calculated? What is the definition of initial rate of reaction?
The change in concentration of a reactant or product per unit of time.
Rate= change in concentration of reactants or products / time taken for the change.
Units - rate (Moldm-3 s-1 ), conc (moldm-3), time (s)
Initial rate of reaction is the change in concentration at the beginning of the reaction per unit time.
What is the general rate equation? And what does the rate constant depend on?
Rate= k[A]^m [B]^n
K - rate constant.
A- conc of reactant A
B- conc of reactant B
m - order of A
n - order of B
Rate constant depends on temp of reaction so if temp increases, value of k increases.
What is the definition for the overall order of a reaction? What is the significance of order of reaction?
The sum of the individual orders.
E.g. rate = k [A]1 [B]2 [C]0
Order of A is 1, order of B is 2 and order of C is 0. If B doubled, rate would x4. If C doubled, rate would be unaffected.
If AB+C doubled, rate would x8 (as 2^1 x 2^2 x 2^0 = 8)
What are the 3 orders?
Zero order: rate unaffected by changing
1st order: is conc doubles, rate doubles as well.
2nd order; if conc double, rate x4 (2^2), if conc triples rate x9 (2^3)
How can conc-time graphs be plotted an what can they be used to deduce?
Plotted from continuous measurements. A conc-time graph shows change in concentration with time.
Used to deduce: rate of reaction at a given time, half life of a reactant, whether order of reaction is 1st or not.
How do you find rate of reaction at time ‘t’ from conc-time graph?
Draw tangent to curve at time ‘t’.
Find gradient of tangent to curve.
Rate at time ‘t’ = gradient of tangent to curve.
How do you find the initial rate of reaction from conc-time graph?
Initial is time 0 and is the quickest point of reaction as conc of reactants is the highest. Draw tangent at 0.
How do you measure reaction rates?
- measure gas vol at regular intervals for reaction that produce gases: use gas syringe or measuring cylinder in water bath.
- measure loss in mass at regular intervals for reaction that produce gases: use balance and add cotton wool to top of conical flask.
- measurement of colour intensity using colorimeter at regular intervals for reaction involving colour change: in colourimeter the wavelength of light passing through coloured solution is controlled using a filter. Amount of light absorbed by solution measured. Reaction monitored and analysed: select filter complementary colour to coloured chemical, calibrate colorimeter with water, measure absorbance, plot calibration curve of absorbance against conc, carry out reaction and take absorbance readings at time intervals. Use curve to measure conc at each absorbance, then plot second graph of conc against time.
How would you recognise orders from a conc-time graph?
Zero order: straight line with negative gradient. Rate doesnt change at all during course reaction, value of gradient=to rate constant.
First order: downward curve with decreasing gradient over time. As gradient decreases, reaction gradually slows down. Rate constant of reaction determined using half life.
Whats half life and how do you find it from a conc-time graph?
The half life is the time taken for conc of reactants to fall by half. (Half life written as t1/2)
To find read from time axis the time taken for conc to fall to half.
E.g. reactant as conc of 10 so draw a line at 5moldm-3 (10/2=5) to the curve and draw down to the time axis, this is the 1st half life. For 2nd half life, do same bt at 2.5moldm-3 (5/2=2.5)
What is the Arrhenius equation and the units for the equation?
The equation is in the equation sheet. (K=Ae^-Ea/RT)
A-pre exponential factor (frequency of collisions) (same units as constant.)
Ea- activation energy (Jmol-1)
R-8.314
T- temp in K
K- rate constant
e^-Ea/RT represent proportion of molecules that have sufficient energy for a reaction take place.
How do you find the half life of a first order reaction from a conc-time graph?
The half life of a first order reaction is independent of conc.
Half life remains constant so first order is one.
e.g. draw conc-time graph for reactant a (1st order). The intial conc is 100moldm-3 and has half life of 10mins. To draw it draw line from conc axis at 50 (100/2=50) and draw a cross at 10mins this is the first half life. Continue this for 25moldm-3 at 20mins, 12.5moldm-3 at 30mins and 7.25moldm-3 at 40 mins until curve is drawn.
How do you calculate rate constant from half-life?
For 1st order reaction.
More accurate than drawing tangent.
K=(In 2 ) / t1/2
k- rate constant
t1/2 - half life.
How do you find the order from a rate-conc graph?
Order 1: rate directly proportional to concentration. So if conc doubles, rate doubles. Graph is straight line through origin.
Order 0: rate independent of concentration. If conc changes, rate unaffected. Graph straight line which cuts x axis and has 0 gradient.
Order 2: rate is exponential function of concentration. If conc doubles, rate x4. Curve must pass through origin.
What is a convenient way of obtaining the initial rate of reaction and what can be assumed from the results?
A clock reaction is convenient way as involves taking single measurement only. Measurement of time taken for visible colour change. If no sig change in rate during time, assumed that average rate of reaction over time will be same as intial rate so initial rate proportional to 1/t. Repeated several time with different concs.
Iodine clock used for formation of iodine.
From results, order with respect to each reaction determined and rate equation can be written.
What is the rate determining step and which step in a mechanism is the slowest?
RDS- the slowest step in the reaction mechanism (usually step 1)
The rate equation gives a clue about RDS. E.g. rate=k[C6H6][Br+] - this tells that 1mole of C6H6 (as 1st order), 1mole of Br+ (as 1st order) so C6H6 and BR+ are involved in slowest step of mechanism.
What does the logarithmic form of Arrhenius equation allow you to determine and how do you find Ea using graph?
Logarithmic form in equation sheet.
Determine both Ea and A to be determined graphically. Plot of In K against 1/T gives a straight line graph. When given a table of k and t values use In K to convert K and 1/T to convert T. (In k= -Ea/R 1/T +In A.)
The gradient of the line is -Ea/R
Y intercept is In A
