Rate and Equilibria A2

Question 1

What are the units of rate?

Answer 1

Mol dm^-3 s^-1

Question 2

On a conc-time graph, what does the gradient represent?

Answer 2

Rate of reaction

Question 3

On a rate-conc graph, what is the shape and direction of the line for a zero, first and second order reaction?

Answer 3

Zero = straight horizontal line
First = straight positive line (directly proportional)
Second = curved positive line

Question 4

How does a zero, first and second order reactant affect rate if its concentration is doubled?

Answer 4

Zero = no effect
First = doubles if doubled
Second = quadruples if doubled

Question 5

What reactants are shown in the rate equation? and how?

Answer 5

Zero order - not shown.
First order and second order are shown.
The order of a specific reactant is the power to which the concentration of the reactant is given.

Question 6

What is K? What is the only thing that affects it?

Answer 6

Rate constant.

Temperature

Question 7

How do you calculate the overall order of reaction?

Answer 7

Sum all the orders of the reactants.

Question 8

How are orders of reactions found?

Answer 8

Experimentally

Question 9

Give three methods of monitoring the rate of a reaction.

Answer 9

Mass loss, gas collection and colorimetry

Question 10

Describe the conc-time graphs for zero, first and second order reactions

Answer 10

Zero = straight negative line
First = curved negative line - constant half life
Second = (more) curved negative line

Question 11

What is the equation linking first order k and half-life?

Answer 11

k = ln2 / half-life

Question 12

what is a clock reaction?

Answer 12

The time taken for a visual change in a reaction - generally measured by a colorimeter.

Question 13

What is the rate determining step?

Answer 13

The slowest step in a reaction / mechanism

Question 14

What is the link between the rate equation and RDS?

Answer 14

Rate equation only includes species up to and in the RDS.

Question 15

What equation links the rate constant and activation energy?

Answer 15

Arrhenius equation - (ln k = ln A - Ea/RT)

Question 16

On a graph of ln k against 1/T what does the gradient give you?

Answer 16

-Ea/R

Question 17

On a graph of ln k against 1/T what does the y-intercept give you?

Answer 17

ln A

Question 18

In the Arrhenius equation, what is R and A?

Answer 18

R = gas constant (8.31 J mol^-1 K^-1)
A = Arrhenius constant

Question 19

How can you work out activation energy from a ln k against 1/T graph?

Answer 19

look at Arrhenius equation as if y=mx+c
y = ln K
m = -Ea/R
x = 1/T
C = ln A

Rearrange m so Ea = m x -R

Question 20

How do you figure out A from y-intercept on 1/T against ln k graph?

Answer 20

A = e^y-intercept

Question 21

What is the name for equilibrium when all the species have the same state? And different state?

What state species does the Kc expression include?

Answer 21

Same states = homogeneous equilibrium
Different states = heterogeneous equilibrium

Only includes aqueous or gases.

Question 22

What method do you use to find the moles of species at equilibrium?

Answer 22

RICE tables

Reaction, Initial moles, Change in moles, Equilibrium moles

Moles/volume to calculate concentration at equilibrium.

Question 23

How do you calculate partial pressures?

Answer 23

From moles at equilibrium (RICE table) calculate mole fraction (mol of substance / total moles).

Multiply total pressure by mole fraction to calculate partial pressure.

Question 24

How do you write expression for Kp? What are the units?

Answer 24

p(C) / p(A) p(B)^2 - example
Always p and curved brackets

Generally, units are Pa or atm

Question 25

How does temperature affect K?

Answer 25

Exothermic reactions: Increasing temperature decreases K

Endothermic reactions: Increasing temperature increases K

Question 26

What happens to equilibrium position if temperature increases for an exothermic reaction? Explain why.

Answer 26

Equilibrium shift to the left.

K decreases so ratio of product:reactant is greater than Kc. So reactants must increase and products decrease so shift to the left.

Question 27

What happens to equilibrium position if temperature increases for an endothermic reaction? Explain why.

Answer 27

Equilibrium shift to the right.

K increases so ratio of product:reactant is smaller than Kc. So reactants much decrease and products increase so shift to the right.

Question 28

How does altering concentration and pressure effect K?

Answer 28

Does not affect K.
But does affect equilibrium position as concentrations/pressures must change accordingly.

Question 29

How does increasing concentration of reactant affect equilibrium position?

Answer 29

Kc remains constant but ratio is smaller than Kc (as reactants on bottom of fraction). So to maintain Kc value, reactant must decrease and product must increase so equilibrium shifts to the right until ratio = Kc.

Question 30

How does increasing pressure affect equilibrium position?

Answer 30

Kp remains constant but each partial pressure increases, so ratio is greater than Kp. So p(reactant) must increase and p(product) must decrease so equilibrium shifts to the left until ratio = Kp.

Acception = when you have p()^2 involved