How fast?

Question 1

What is the rate of chemical reaction defined as?

Answer 1

the change in concentration of reactant or product per unit time

Question 2

What does changing the concentration of substances taking part in a reaction do?

Answer 2

usually (but not always) change the rate of reaction (rate equation shows this mathematically)

Question 3

What is the general rate equation?

Answer 3

Rate = k[A]m [B]n
k= rate constant
A] = concentration of reactant A (moldm-3)
[B] = concentration of reactant B (moldm-3)
m = order of reaction with respect to A
n = order of reaction with respect to B

Question 4

What is the order of reaction?

Answer 4

a measure of how a reactant affects the rate of reaction

Question 5

what can the orders only be?

Answer 5

zero[A]0, first[A]1 or second[A]2

Question 6

what are rate constants dependent on?

Answer 6

temp

Question 7

How is order with respect calculated?

Answer 7

1st order- doubles, rate doubles
2nd order- doubles, rate quadruples
0 order- no effect on rate

Question 8

Why does increasing temp without changing conc result in a fast rate of reaction?

Answer 8

At a higher temperature more particles have Ea or above. Hence there are more successfül collisions per unit time.

Question 9

What happens if the 2 elements that are 1st order are doubled?

Answer 9

Rate quadruples

Question 10

What does decreasing by a factor of 4 mean?

Answer 10

1/4

Question 11

How does 2nd order affect rate?

Answer 11

If [C]× 2
rate × 4
If [C]× 3
rate × 9
If [C]× ½
rate × ¼

Question 12

What are the units for rate?

Answer 12

moldm-3s-1

Question 13

How can the rate of a chemical reaction be followed?

Answer 13

by measuring either the rate of increase
in the concentration of a product or the rate of decrease in the concentration of a reactant

Question 14

What is the rate of reaction defined as?

Answer 14

the change in concentration of a reactant or a product
per unit time.
Rate = Change in concentration/time
Units = (moldm-3)/s= moldm-3s-1

Question 15

What is half life?

Answer 15

the time taken for the concentration of a reactant to fall to half the original value.

Question 16

What is the half life of a first order reaction?

Answer 16

A first order reaction has a constant half-life. The half-life of a first order reaction is independent of the concentration of the reactants.

Question 17

Uranium-238 decays by a first order process with a constant half-life of 4.5x10 9 years.
The rate equation can be written as rate=k [238U].
(i) How long will it take a 200g sample of 238U to decay to 25g?

Answer 17

half life x 3

Question 18

(c) Archaeologists can determine the age of organic matter by measuring the amount of 14C in a sample. The decay of 14C is first order (t1/2 of 14C = 5600 years).
Estimate the age of a piece of wood found to contain 1/16th as much 14C as living material.

Answer 18

1/16=4 half lives
4x5600

Question 19

what does t1/2 stand for?

Answer 19

half life

Question 20

How can you calculate the rate constant from its half life?

Answer 20

K=ln2/t1/2

Question 21

What does a constant half life mean?

Answer 21

1st order

Question 22

What is half life independent of?

Answer 22

initial conc

Question 23

How is half life in a graph worked out?

Answer 23

the time at which the initial conc half’s

Question 24

How do we produce a rate-concentration graph and what does it
show us?

Answer 24

We can draw a concentration-time graph and calculate the gradient (rate) at
different concentrations; hence we can plot the rate against concentration. Using two tangents on a concentration-time graph can therefore also be used to prove that a reaction is first order.
(b) We can record the overall time (t) taken for completion of a reaction at different concentrations of reactant and then use 1/t to represent rate. e.g. disappearing cross

Question 25

What happens in the reaction between sodium thiosulfate and HCL?

Answer 25

This is a useful reaction to study because two colourless solutions react together and a yellow precipitate is observed. Each time the experiment is carried out, the concentration of one of the reactants is varied whilst keeping the concentration of the other reactant constant. The time taken for the precipitate to form is measured.

Question 26

How do you work out rate for the disappearing cross experiment?

Answer 26

Since the appearance of a product is being monitored in this case, the rate is inversely proportional to time. By calculating the value of 1/t at the time the precipitate appears, an approximation of the rate of each individual reaction can be found

Question 27

To determine the time taken for reaction to reach completion (colour change) what is used?

Answer 27

coloromiter

Question 28

What is a fist order reactant directly proportional to when calculating rate using time taken?

Answer 28

change in conc

Question 29

what axis is rate on on a rate conc graph?

Answer 29

y axis

Question 30

what are the two types of graphs?

Answer 30

conc (y) time (x) rate (y) conc (x)

Question 31

If the rate determining step (slow step) is the first step then what happens?

Answer 31

then the species on the left hand side of that step will appear in the rate equation, and their balancing numbers (stoichiometry) become the powers of the concentrations.

Question 32

What is the definition of the rds?

Answer 32

The slowest step in the reaction mechanism of a multi-step reaction which determines the overall rate

Question 33

How do you identify the rds?

Answer 33

the stoichiometry of the second step and preceding steps match the rate equation.

Question 34

What is an intermediate?

Answer 34

doesn’t appear in main equation

Question 35

What are rate constants dependent on?

Answer 35

changes in temperature. An increase in temperature by 10OC approximately doubles the rate of the reaction.

Question 36

What do the letter stand for in the arhenius equation?

Answer 36

A = constant of proportionality known as the pre-exponential factor. e = mathematical constant with the value of 2.71828. This is the inverse natural log. Ea = activation energy of the reaction – units J (not kJ) R = gas constant (J K−1 mol−1) T = temperature – units K (not oC).

Question 37

Which parts of LnK = LnA - Ea/RT equate to y=mx + c?

Answer 37

Y= LnK m(gradient)= -Ea/R x= 1/T c= LnA

Question 38

What can the arrhenius plot be used to identify?

Answer 38

An Arrhenius plot can be used to identify: • A: the intercept on the graph is equal to In A • The activation energy, Ea: the gradient is equal to –Ea/R, where R is the known gas constant, so this can be rearranged easily to find Ea

Question 39

what is the inverse of Ln?

Answer 39

e ln

Question 40

How do you calculate activation energy from the arrhenius equation?

Answer 40

From Arrhenius equation: Rearrange: Therefore Gradient (m) = −Ea / R −Ea = Gradient × R Ea = −Gradient x R