2.2 rates of reaction

Question 1

What is the collision theory?

Answer 1

The theory that chemical reactions only occur when particles collide with sufficient energy

Question 2

Define activation energy

Answer 2

the minimum amount of energy needed to start a chemical reaction between two particles

Question 3

describe and explain the gradient of the curve on a rate of reaction graph

Answer 3

initially the gradient is very steep because the rate of reaction Is very fast at the start (there are more particles so more successful collisions)

the gradient decreases overtime as reactants are used up
the curve eventually levels off when the reaction is complete (one or all of the reactants have been completely used up)

Question 4

how can you calculate rate of reaction?

Answer 4

Rate of reaction = Amount of reactant used or amount of product formed / Time

Question 5

What units could be used for rate of reaction?

Answer 5

g/s, cm3/s, mol/s

Question 6

what conditions can be changed to increase the rate of reaction?

Answer 6

increase In temperature
increase in pressure
increase in surface area of reactants
increase in concentration of reactants

Question 7

How does temperature affect the rate of reaction?

Answer 7

reactants have more energy so more particles have energy greater than or equal to activation energy so more successful collisions
collisions also occur more frequently because the particles have more kinetic energy

Question 8

How does surface area affect the rate of reaction?

Answer 8

it increases the rate of reaction because increasing the surface area of solid reactants increases the frequency of collisions

Question 9

How does concentration affect the rate of reaction?

Answer 9

more reacting particles in the same volume so more frequent successful collisions occur

Question 10

How does pressure affect the rate of reaction?

Answer 10

The higher the pressure, the more the kinetic energy.

• Creates more collisions between particles
• These collisions are more energetic, so have more kinetic energy and greater ability to react
  same number of particles in a smaller volume

Question 11

draw a reaction profile diagram for exothermic reaction (label activation energy)

Answer 11

check quilzlet

Question 12

draw a reaction profile diagram for an endothermic reaction

Answer 12

check quizlet

Question 13

Maxwell-Boltzmann distribution for two temperatures

Answer 13

check quizlet

Question 14

what can be added to speed up rate of reaction?

Answer 14

a catalyst

Question 15

what is a catalyst?

Answer 15

substance that speeds up the rate of a chemical reaction without being chemically changed by the end

Question 16

How does a catalyst affect the rate of reaction?

Answer 16

A catalyst offers an alternative pathway that requires less activation energy in the reaction, as it helps breaks the bonds, but at the end of the reaction it is still there and not used up.

Question 17

how can catalysts be identified in a reaction?

Answer 17

they are chemically unchanged so can be seen in the product
also not used up

Question 18

Maxwell-Boltzmann distribution catalyst

Answer 18

same shape, greater number of particles in a higher Ea region

Question 19

Maxwell - Boltzmann Distribution concentration

Answer 19

check quizlet

Question 20

how can you measure the rate of reaction when a gas is given off?

Answer 20

collect the gas produced in an upside down measuring cylinder in a trough of water or in a gas syringe to measure the vol of gas produced
- measure the amount of gas collected over regular intervals

Question 21

how can you measure the rate of reaction when a precipitate is formed?

Answer 21

put a black cross below a beaker containing one reactant. Tim how long it takes for the cross to disappear after the second reactant is added

Question 22

why is the precipitate method not very accurate?

Answer 22

qualitative
people will disagree when the cross disappeared

Question 23

Marble chips react with hydrochloric acid to produce calcium chloride, water and carbon dioxide. How could you increase the rate of reaction?

Answer 23

increase the surface area by turning marble chips into powder
- increased concentration of acid
- increased temperature

Question 24

What is a heterogeneous catalyst?

Answer 24

A catalyst in a different physical state to the reactants

Question 25

Why are transition metals good catalysts?

Answer 25

They can exist in variable oxidation states, so can provide alternative pathways easily (availability of 3d and 4s shells)

Question 26

what do heterogenous catalyst do to reactants?

Answer 26

weakens bonds
- brings molecules closer
- more favourable orientation

Question 27

what are the 4 stages of heterogenous catalyst? (H.A.R.D)

Answer 27

• 1 = the heterogenous catalyst is solid whilst the reactants are gaseous
  2 = The reactants adsorb into the active sites on the catalyst and bonds are weakened. Reactant molecules are held in position.
  3 = The reactants move closer together and react forming new products which are held on the surface.
  4 = The new products desorb from the surface and leave the active site open to react with another molecule.

Question 28

why are partially filled d orbitals better for catalysts and reactions?

Answer 28

Partially filled d orbitals are able to accept bonding pairs of electrons from reactants. The more d orbital space available to accept electrons, the stronger the bond will be. Less availability will equate in weaker bonds which may not have time for the reactants to react before they are desorbed.

Question 29

equation 1 of the contact process ( SO2 oxidised by vanadium oxide)

Answer 29

Sulphur dioxide oxidised to sulphur by vanadium (v) oxide
SO2 + V2O5(⁺⁵) → SO3 + V2O4 (⁺⁴)

Question 30

equation 2 (The vanadium has been reduced, oxidised back to the starting vanadium (V) oxide by oxygen-O)

Answer 30

The vanadium has been reduced to vanadium(IV). It is oxidised back to the starting vanadium (V) oxide by oxygen
2V2O4 + O2 → 2V2O5
The vanadium(v)oxide is unchanged at the end of the reaction.

Question 31

what is the overall equation of the contact process?

Answer 31

Overall reaction : 2SO2 + O2 → 2SO3

Question 32

How are catalysts poisoned?

Answer 32

Impurities are strongly adsorbed on the active sites on the surface of the catalyst.

In heterogeneous catalysis the ‘poison’ molecules are adsorbed more strongly to the catalyst surface than the reactant molecules, the catalyst becomes inactive.

Question 33

what will happen to bonding molecules if active site is poisoned?

Answer 33

Incoming reactant molecules will have nowhere to bond to if the active site is already occupied/bonded to a reactant.

Question 34

What catalysts are used in catalytic converters?

Answer 34

• Platinum, rhodium, palladium (highly expensive)

Question 35

what is the support medium of catalytic converters?

Answer 35

honeycomb ceramics

Question 36

What does a catalytic converter do?

Answer 36

processes the toxic gases in engine exhaust, turning them into carbon dioxide, water, and other less harmful substances

Question 37

what is the catalyst in the haber process?

Answer 37

iron (Fe)

Question 38

what is the equation for haber process?

Answer 38

N2(g) + 3H2(g ↔ 2NH3(g) ΔH = -92.4kjmol-1

Question 39

describe sulphur poisoning in the haber process

Answer 39

the hydrogen is obtained from natural gas which is contaminated by sulphur, which if not removed will poison the Fe catalyst by adsorbing onto the surface of the iron.

Question 40

how can sulphur poisoning be detected?

Answer 40

sulphur is added to natural gas to give it odour so leaks can be smelt

Question 41

what is a homogenous catalyst?

Answer 41

The catalyst is in the same physical state as the reactants

Question 42

what phase do reactions occur with homogenous catalysts?

Answer 42

reaction takes place in aqueous phase, e.g. aqueous acid with aqueous reactants
In this case the catalyst may lead to another mechanism or it may stabilise an intermediate compound. The new mechanism will have a lower activation energy.

Question 43

what is the overall reaction of persulphate ions?

Answer 43

S2O8 2- (aq) + 2I - (aq) → 2 SO4 2- (aq) + I2 (aq)

Question 44

why does this reaction have a high activation energy?

Answer 44

they are both negatively charged ions so therefore they repel each other which requires more energy

Question 45

mechanism stage one equation of S2O8

Answer 45

2 Fe 2+ (aq) + S2O8 2- (aq) → 2 Fe 3+ (aq) + 2 SO4 2- (aq)

Question 46

mechanism stage two of S2O8

Answer 46

2 Fe 3+ (aq) + 2 I - (aq) → 2 Fe 2+ (aq) + I2 (aq)

Question 47

Explain why Fe3+ ions are as effective as Fe2+ ions in catalysing this reaction.

Answer 47

because both of them could occur in any order during these reactions

Question 48

what are the disadvantages of a homogenous catalyst?

Answer 48

catalyst actively involved, get intermediate, transition state, but then keep reacting to reform catalyst
- hard to separate

Question 49

what is an autocatalyst?

Answer 49

A product of the reaction has to catalyse the reaction

Question 50

equation for permanganate ( potassium manganate) reacting with ethanedioate ions

Answer 50

2MnO4⁻ + 5C2O4²⁻ + 16H⁺ ͢ 2Mn²⁺ + 10CO2 + 8H2O

Question 51

what is the catalyst of this reaction?

Answer 51

2Mn²⁺

Question 52

graph to show auto catalysed

Answer 52

around half way through the catalyst is generated = steeper graph
by end, used up most of reactants so rate is slow again

Question 53

oxidation of manganate ions p1 equation

Answer 53

4Mn²⁺ + MnO4⁻ + 8H⁺ ͢. 5Mn3⁺ + 4H2O

Question 54

reduction of manganate ions p2 equation

Answer 54

2Mn³⁺ + C2O4⁻ ͢. 2CO2 + 2Mn²⁺