Rates

Question 1

what are units for rates?

Answer 1

moldm-3s-1

change in conc/time

Question 2

at what time is the initial rate

Answer 2

0s
this is directly related to the initial concentration of the reactant

Question 3

the rate equation for a general reaction
A + B -> C + D

Answer 3

rate = k [A]m[B]n

m = order of reaction with respect to reactant A
n = order of reaction with respect to reactant B

Question 4

what is the order of reaction

Answer 4

m + n
the power to which the concentrations of this reactant is raised in a rate equation

Question 5

zero order

Answer 5

the conc of a zero order reactant has no effect on the rate of reaction
[A]0 = 1

Question 6

how to work out the rate of a reaction

Answer 6

it can only be found experimentally

Question 7

first order

Answer 7

rate is proportional to A
so doubling A, doubles the rate of reaction

Question 8

second order

Answer 8

rate is proportional to [A]2
if we double [A], the rate is quadrupled

Question 9

what is the order of reaction

Answer 9

the power which [A] is raised with respect to A

Question 10

what is the rate determining step

Answer 10

the slowest step that holds everything else up

Question 11

measuring rates of reaction experimentally
Collecting gas

Answer 11

1) gas syringe is attached to a sealed reaction vessel and measure the volume of gas produced
2) measured on a graph against time
3) gradient of tangent shows the rate of reaction

Question 12

measuring rate of reaction experimentally:
measure colour change using a colorimeter

Answer 12

• if a blue solution you should use a red filter, as the solution is absorbing red light
• amount of red absorbed relates directly to the concentration
• draw a calibration curve, so absorbance can be converted to conc
• several experiment take place with differ conc

Question 13

measuring rate of reaction experimentally:
quenching and titrating

Answer 13

1. The reaction in each of the samples is stopped (quenching)
2. Quenching ‘freezes’ the reaction to allow the concentration to be determined by titration
3. with collected data, the ror can be calculated by determining the change in concentration with time

Question 14

measuring rate of reaction experimentally
loss of mass

Answer 14

• measuring mass over a period of time. curve decrease until initial tangent T=0s gives the initial rate
• conc will be changed and initial rate is measured

Question 15

measuring rate of reaction experimentally
ph monitoring

Answer 15

pH = -log10[H]+, so [H]+ = 10(-pH)
- pH values can be taken over time, they can be directly converted to [H]+ by using the equation
-graph of [H]+ can be drawn
- if [H]+ is a reactant, the curve can give the rate or gradients of tangents at various [H]+ can be taken which equal the rate

Question 16

the Arrhenius equation

Answer 16

links the rate constant with activation energy and temperature
K= Ae -Ea/RT

K = rate constant
A = frequency factor
e = mathematical quantity
Ea = activation energy (Jmol-1)
R = gas constant
T = temperature (Kelvins)

Question 17

relationship between K and the rate

Answer 17

whatever k increase by the rate increase by the same factor
e.g. double

Question 18

Arrhenius equation
finding the activation energy (Ea)

Answer 18

Ea = (InA x RT) - InK

Question 19

Arrhenius equation
finding the frequency factor (A)

Answer 19

A = K/e -Ea/RT

Question 20

Arrhenius equation
finding the temperature (T)

Answer 20

InA - Ea / Ink x R

Question 21

Arrhenius equation
finding InK

Answer 21

InK = InA - Ea/RT