3.2.2 Reaction rates

Question 1

what does the rate of a chemical reaction measure

Answer 1

• measures how fast a reactants is used up or how fast a product is formed

Question 2

what can the rate of a reaction be defined as + give formula + units

Answer 2

• the change in concentration of a reaction or product in a given time
• rate = change in conc / time
• moldm-3 / s = moldm-3s-1

Question 3

when is the rate of a reaction the fastest

Answer 3

• at the start of a reactions as each reactant is in its highest conc

Question 4

why does the rate of reaction slow down as reaction progresses

Answer 4

• as reactants are being used up their conc decreases + rate of reaction slows

Question 5

when is the rate of reaction zero

Answer 5

• when the reactant has been completely used up, the reaction stops + rate is zero

Question 6

what are the factors that can change the rate of a chemical reaction

Answer 6

• concentration (or pressure is gases)
• temperature
• use of catalyst
• surface area of solid reactants

Question 7

what does the collision theory state

Answer 7

• two reacting particles must collide for a reaction to occur

Question 8

what does an effective collision lead to

Answer 8

• leads to a chemical reaction

Question 9

a collision will be effective if it meets 2 conditions:

Answer 9

• particles collide with correct orientation
• particles have sufficient energy to overcome the activation energy barrier of the reaction

Question 10

how does increasing concentration affect rate of reaction

Answer 10

• increase in conc causes increases in number of particles per unit volume
  > particles are closer together
  > leads to higher frequency of collisions
  > more successful collisions per unit of time
• causes an increased rate of reactions

Question 11

how does increases surface area of solids affect rate of reaction

Answer 11

• more reactant particles are exposed/available for collision
  > higher frequency of collisions
  > more successful collision per unit time
• causes increased rate of reaction

Question 12

how does increases temp affect rate of reaction

Answer 12

• higher temp means particles have higher kinetic energy
  > more particles have energy above the activation energy
  or
  > particles move faster
  > higher frequency of collisions
  > more successful collisions per unit time
• increased rate of reaction

Question 13

how does a catalyst affect rate of reaction

Answer 13

• provides an alternative route with lower activation energy
• more particles have energy above the activation energy
• more successful collisions per unit time
  > increased rate of reaction

Question 14

how can a rate of reaction be measured

Answer 14

• using gas syringe
• inverted measuring cylinder
• change in mass

Question 15

what is a catalyst

Answer 15

• a substance that speeds up the rate of a reaction by providing an alternative reaction pathways of lower activation energy
  > its not used up in the chemical reaction

Question 16

catalysts aren’t used up in reactions but they may do what

Answer 16

• react with a reactant to form an intermediate in a multistep reaction but will be regenerated by the end of the reaction

Question 17

what are homogenous catalysts + how do they work

Answer 17

• catalysts that has the same physical state as the reactants
• catalyst reacts with reactants to form an intermediate which breaks down to give the product
  > catalyst is regenerated

Question 18

what are heterogenous catalysts + how do they work

Answer 18

• catalysts that has different physical state from the reactants
• they tend to be solids in contact with gaseous or solutions of reactants
• reactant molecules are adsorbed onto surface of catalyst (weakly bonded), where reaction takes place
  > after reaction, product molecules leave surface of the catalyst by desorption

Question 19

why does catalysis serve great economic importance

Answer 19

• products made faster (higher rate) + at lower cost (less energy)
  > this increases profitability

Question 20

why is catalysis good for sustainability

Answer 20

• catalysts lower temp + energy needed for reactions by lowering activation energy
• lower energy requirements mean less fossil fuels are used
  > this reduces CO2 emissions

Question 21

what is the Boltzmann distribution

Answer 21

• the spread of molecular energies in gases

Question 22

what does the area under the Boltzmann distribution represent

Answer 22

• the total number of molecules

Question 23

why does the line of a Boltzmann distribution never meet the x-axis

Answer 23

• there is no maximum energy for a molecule

Question 24

what does the shaded region of a Boltzmann distribution represent

Answer 24

• where the successful collisions + reactions take place
  > shows the molecules that have more energy than Ea, that is enough energy to react

Question 25

what happens to the Boltzmann distribution as you increase temp

Answer 25

- as temp inc, so does average energy of molecules > more molecules have higher energy - number of molecules same, so area under curve is same - more molecules have energy greater than or equal to Ea > peak is lower + shifted to the right

Question 26

what happens to the Boltzmann distribution when you introduce a catalyst

Answer 26

- catalyst provides alternative reactions route with lower Ea > doesn't change energy of molecules themselves - because Ea is lower, more molecules will have energy equal to / greater than Ea > on graph curve stays same but activation energy moves left