module 5 - physical chemistry and transition elements

Question 1

what is short hand for concentration of reactant ‘A’

Answer 1

[A]

Question 2

what is the correlation between rate and concentration

Answer 2

directly proportional

Question 3

define zero order

Answer 3

when the concentration has no effect on rate of reaction

Question 4

in a zero order reaction:

Answer 4

• concentration of a reactant is to the power of 0
• any number raised to the power of 0 =1
• doesn’t appear in the rate equation
• concentration does not influence the rate of reaction

Question 5

in a first order reaction:

Answer 5

if the concentration of A is doubled (x2) the reaction rate is increased by a factor of 2^1
(directly proportional)

Question 6

in a second order reaction:

Answer 6

if the concentration is of A is doubled, the reaction rate increases by a factor of 2^2 = x4

Question 7

what is the rate equation

Answer 7

rate = K [A]^m [B]^n

k = rate constant
[ ] = concentration
m&n are orders

Question 8

what is the trend in concentration-time graphs for:
zero order
first order
second order

Answer 8

zero: straight line with negative gradient
first order: downward curve with a decreasing gradient over time - half life is constant
second order: also a downward curve but steeper at the start and tails off more slowly

Question 9

define exponential decay

Answer 9

first order reactions have a constant half life (time taken for half a reactant to be used up)

Question 10

what are the trends in rate-concentration graphs for:
zero order
first order
second order

Answer 10

zero: straight horizontal line with zero gradient
first: straight line directly proportional
second: upward curve with increasing gradient

Question 11

how can you find the rate constant (k) from rate-concentration graphs for:
zero order
first order
second order

Answer 11

zero - y intercept
first: gradient
second: cannot be determined

Question 12

how do you work out the initial rate of reaction from a concentration-time graph

Answer 12

draw a tangent at T=0 and calculate the gradient

Question 13

what does the rate equation only include?

Answer 13

reacting species involved in the rate-determining step.

Question 14

what is the Arrhenius equation? And what do the letters stand for?

Answer 14

k = A e^(-Ea/RT)
k = rate constant
A = frequency factor
Ea = activation energy
R = gas constant (8.314)
T = temperature (Kelvin)

Question 15

what is the logarithmic form of the Arrhenius equation

Answer 15

Ln k = - Ea/R (1/T) + Ln A

Question 16

why is the logarithmic form of the Arrhenius equation useful?

Answer 16

so we can determine both Ea and A to be determined graphically

a plot of Ln k against 1/T gives a straight line graph in the form of y=mx+c
gradient m = -Ea/R

intercept c of Ln A on y axis

Question 17

Bromine reacting with an excess of methanoic acid
Suggest how the concentration of the bromine could be monitored

Answer 17

Measure the reduction of colour of bromine (turns from brown-orange to colourless)