Chapter 10 Reaction rates and equilibrium

Question 1

rate of chemical reaction?

Answer 1

how fast a react is being used up or how fast a product is being formed

rate= change in conc/ time (units= moldm-3 s-1)

Question 2

How does rate of reaction change as the reaction progresses?

Answer 2

rate is fastest at start of reaction (each reactant is at highest conc)

rate slows down as reaction proceeds (reactants are used up)

once all reactions are completely used up, rate=0

Question 3

Why are some collisions effective and others ineffective?

Answer 3

an effective collision only occurs if:
- particles collide with correct orientation
- particles have sufficient energy to overcome activation energy barrier of reaction

Question 4

rate of reaction: changing concentration?

Answer 4

increased conc= increased rate
–> more particles in the same volume (more crowded)
–> more frequent collisions
–> more effective collisions in the same given period of time
–> rate increases

Question 5

rate of reaction: changing pressure?

Answer 5

Increased pressure = gas particles become more crowded
–> closer together and collide more frequently, leading to more effective collisions

Question 6

Two ways of determining rate of reaction for gas producing reactions?

Answer 6

1. monitor volume of gas produced at regular time intervals
   –> underwater measuring cylinder / gas syringe
2. monitor loss of mass of reactants using balance

Question 7

rate of reaction: changing temperature?

Answer 7

higher temp= increased rate
–> more kinetic energy= particles move around faster
–> collision frequency increases= more effective collisions
–> rate increases

Question 8

What is a catalyst

Answer 8

substance that speeds up the rate of reaction without being used up
–> provides an alternative reaction pathway with lower activation energy

Question 9

What is a homogeneous catalyst?

Answer 9

one that has the same physical state as reactants
–> catalyst reacts with reactants to form an intermediate and then breaks down to give product

Question 10

Examples of homogeneous catalysts

Answer 10

making esters with sulfuric acid
C2H5OH + CH3COOH = CH3COOC2H5 + H2O
–> all substances here are in liquid state

OZONE depletion with CL radical as a catalyst
–> 2O3 = 3O2
All substances are in gaseous state

Question 11

Heterogeneous catalysts

Answer 11

one that has a different physical state from reactants
–>reactant molecules are adsorbed (weakly bonded) onto surface of catalyst
–> product molecule leaves surface by desoprtion

Question 12

Economic important and sustainability of catalysts

Answer 12

• catalysts increase rate by lowering activation energy
  –> reduces temperature needed and energy requirements
  –> meaning less electricity or fossil fuel is used
  –> cutting down on costs and producing fewer pollutants

Question 13

Boltzmann distribution

Answer 13

a graphical representation of the spread of molecular energies in gases
–> Ea= activation energy and normally only a small proportion of molecules have enough/ more energy than Ea

Question 14

Features of a Boltzmann distribution

Answer 14

• no molecules have 0 energy (curve starts at origin)
• area under curve= total number of molecules
• no maximum energy (curve does not meet x-axis at high energy)

x axis= energy/ y axis= number of molecules with a given energy

Question 15

How does temperature affect boltzmann’s distribution?

Answer 15

At higher temp:
1. more molecules will have greater energy than Ea
–> greater proportion of collisions will lead to reaction
–> collisions will be more frequent as molecules are moving faster

Question 16

Boltzmann temp graph

Answer 16

At T2 (higher temp), the peak is much lower and shifted to right

on the end of x axis, it is higher than T1 but Ea is still same

Question 17

How does catalysis affect Boltzmann’s distribution?

Answer 17

Alternative reaction route with a lower Ea
–> greater proportion of molecules have equal/ more than Ea
–> more molecules will cause effective collisions
–> increased rate

Question 18

Boltzmann catalysis graph

Answer 18

Same trend but lower activation energy so new line is shifted to the left

Question 19

What is dynamic equilibrium?

Answer 19

when the forward reaction rate is equal to backwards reaction rate

-concentration of reactants and products do not change (ARE NOT EQUAL)

Question 20

Closed system

Answer 20

isolated from surroundings so temp, pressure and other conditions are unaffected

Question 21

Le Chatelier’s principle

Answer 21

when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the change

Question 22

Concentration and equilibrium

Answer 22

Increasing reactants conc/ decreasing products conc = rate of reaction shifting to the right
–> more product needs to be made to minimise change

Decrease reactant conc/ increase product conc = rate of reaction shifts left
–> more reactants need to be formed

Question 23

Investigating changes to equilibrium with concentration (theory)

Answer 23

Equilibrium between aqueous chromate ions (CrO4 2-) and dichromate ions (Cr2O7 2-) is sensitive to changes in acid conc

2CrO4 2- (aq) + 2H+(aq) = Cr2O7 2- (aq) + H2O (aq)

chromate is yellow and changes to orange once H+ ions are added as dichromate is orange

Question 24

What happens when we add H2SO4 to potassium dichromate or we add NaOH

Answer 24

Adding H2S04 turns the solution orange
Adding NaOH turns solution yellow again

Question 25

How does adding sulfuric acid to the potassium dichromate affect the equilibrium

Answer 25

More H2SO4= increasing conc of H+ ions
–> system wants to counter change so it shifts to the right
–> shift decreases conc of added reactant H+ by making more products
–> solutio turns orange as Cr2O7 2- forms

Question 26

How does adding sodium hydroxide to the potassium dichromate affect the equilibrium

Answer 26

more NaOH= increased conc of OH+ ions which react with H+ to form water
–> reduces conc of H+ so system shifts to the left to minimise the change, making more reactants
–> solution turns yellow as CrO4 2- is made

Question 27

Tmperature change and equilibrium

Answer 27

Shift direction depends on delta H

Increase temp shifts equilibrium to endothermic direction (delta H positive)

Decrease in temp shifts equilibrium to exothermic direction (delta H negative)

Question 28

If forward rate is exothermic, what do temperature changes do

Answer 28

Temp increase= position moves left= more reactants made

Temp decrease= position moves right= more products made

Question 29

If forward rate is endothermic, what do temperature changes do

Answer 29

Temp decrease= position moves left= more reactants made

Temp increase= position moves right= more products made

Question 30

Pressure change and equilibrium?

Answer 30

Increase pressure= position shift to side with fewer gaseous moles
–> to reduce pressure

Question 31

Catalyst and equilibrium?

Answer 31

catalyst does not change position at all

Question 32

What does Kc tell us?

Answer 32

Kc value of 1 indicates position is halfway between reactants and products

Kc value > 1 means position is towards products

Kc value < 1 means position is towards reactants