R2.2 How fast? The rate of chemical change

Question 1

rate of reaction

Answer 1

the change in concentration of a particular reactant/product per unit time

Question 2

rate of reaction units

Answer 2

mol dm^-3 s^-1

Question 3

due to what do species react

Answer 3

• Collisions of sufficient energy (kinetic energy due to temperature)
• Proper orientation (collision geometry)

Question 4

collision theory

Answer 4

• Particles must collide with sufficient kinetic energy in the correct orientation
• Considers particles to be hard spheres that react with each other with sufficient kinetic energy

Question 5

successful collision

Answer 5

• Reactant particles have the correct orientation, enough kinetic energy (equal to or exceeding Ea) and collide
• Change into product particles when they collide
• Results in a chemical reaction and chemical change

Question 6

unsuccessful collision

Answer 6

• Reactant particles collide
• Bounce off each other
• Wrong orientation, not enough kinetic energy

Question 7

steric effect

Answer 7

The rate/path of a chemical reaction depends on size/arrangements of groups in a molecule

Question 8

how does pressure influence the rate of reaction

Answer 8

• Increase pressure = increase rate
• Forces the particles closer together
• More particles per unit volume
• More frequent successful collisions

Question 9

how does concentration influence the rate of reaction

Answer 9

• Increase concentration = increase rate
• Particles are closer together as there are more pre unit volume
• Frequency of successful collisions between reactant particles in solution increases

Question 10

how does surface area influence the rate of reaction

Answer 10

• Increase surface area = increase rate
• More particles on the surface that can react
• More frequent successful collisions

Question 11

how does temperature influence the rate of reaction

Answer 11

• Increase temperature = increase rate
• Molecules have an average greater velocity → greater distance per unit time travelled → more successful collisions per unit time
• Larger proportion of the colliding molecules have kinetic energy equal to or exceeding activation energy → larger proportion of successful collisions

Question 12

what is a catalyst

Answer 12

A substance that increases the rate of reaction but remains chemically unchanged at the end of the reaction

Question 13

how does the presence of a catalyst influence rate of reaction

Answer 13

• Provides an alternate reaction pathway with a lower activation energy
• Therefore, a greater proportion of sample particles have sufficient kinetic energy to react when they collide
• Rate is directly proportional to the concentration of the catalyst

Question 14

homogeneous catalyst

Answer 14

Catalyst and reactants in the catalysed reaction are in the same phase

Question 15

heterogeneous catalyst

Answer 15

• Catalyst and reactants in the catalysed reaction are in a different phase
• Provides a surface on which the reaction can occur

Question 16

biological catalyst

Answer 16

enzymes

Question 17

activation menergy

Answer 17

• the minimum energy that colliding particles need for a successful collision leading to a reaction
• the minimum amount of combined total kinetic energy required by a colliding pair of ions, atoms or molecules for a chemical reaction to occur; the energy barrier that has to be overcome to form the transition state

Question 18

energy barrier of a fast reaction

Answer 18

low, low activation energy

Question 19

energy barrier of a slow reaction

Answer 19

high under standard conditions

Question 20

transition state

Answer 20

• Forms before the products, cannot be easily isolated and studied as it is unstable
• The partially bonded, short lived chemical species of highest potential energy, located at the top of the activation energy barrier as a reaction proceeds from reactants to products

Question 21

what is distributed over a Maxwell-Boltzmann distribution

Answer 21

the distribution of kinetic energies of molecules at a constant temperature

Question 22

what is the total area under the curve of a MBD directly proportional to

Answer 22

directly proportional to the total number of molecules

Question 23

what is the area under any portion of the curve is directly proportional to

Answer 23

the number of molecules with kinetic energy in that range

Question 24

do particles in a substance all have the same energy, explain

Answer 24

• no
• energy constantly changes
• when particles collide with each other, they gain/lose energy
• particles with insufficient energy may gain enough to react in a later collision

Question 25

graph of concentration vs rate

Answer 25

directly proportional

Question 26

graph of pressure vs rate

Answer 26

directly proportional

Question 27

graph of surface area vs rate

Answer 27

directly proportional

Question 28

graph of temperature vs rate, explain

Answer 28

• exponential
• a small temperature increase has a large effect on rate (higher kinetic energy value)

Question 29

how can light (involving reactions of gas mixtures including halogens) influence rate of reaction

Answer 29

Sunlight/UV can potentially cause a very large increase to rate of reaction

Question 30

rate determining step

Answer 30

• the slowest elementary step in a chemical reaction that determines the rate of reaction

Question 31

many reactions occur in…

Answer 31

…a series of elementary steps

Question 32

what can energy profiles be used to show

Answer 32

the activation energy and transition state of the rate-determining step in a multistep reaction

Question 33

molecularity of an elementary step

Answer 33

the number of reaction particles taking part in that step

Question 34

unimolecular

Answer 34

• 1 chemical specie
  A –> B

Question 35

bimolecular

Answer 35

• 2 chemical species
  A + A –> B
  A + B –> C

Question 36

termolecular

Answer 36

• 3 chemical species
  2A + B –> C

Question 37

intermediate

Answer 37

made in one step and used in another

Question 38

what do rate equations depend on and how can they be determined

Answer 38

• depend on the mechanism of the reaction
• can only be determined experimentally

Question 39

order of a reaction with respect to a reactant

Answer 39

• the exponent to which the concentration of the reactant is raised in the rate equation
• can describe the number of particles taking part in the rate determining step

Question 40

overall reaction order

Answer 40

the sum of the orders with respect to each reactant

Question 41

rate constant (k)

Answer 41

• temperature dependent
• units are determined from the overall order of the reaction

Question 42

graph of rate vs concentration, 0 order

Answer 42

never (0,0)

Question 43

graph of rate vs concentration, 1st order

Answer 43

• directly proportional
• positive linear gradient
• (0,0)

Question 44

graph of rate vs concentration, 2nd order

Answer 44

• directly proportional to [A]^2
• (0,0)

Question 45

graph of concentration vs time, 0 order

Answer 45

• negative linear gradient
• half life decreases
• constant half life decrease at a constant rate

Question 46

graph of concentration vs time, 1st order

Answer 46

• constant half life

Question 47

graph of concentration vs time, 2nd order

Answer 47

• half life increases with time
• concentration
• half life is inversely proportional to concentration

Question 48

what does the Arrhenius equation use, and what does it do with this

Answer 48

• uses the temperature dependence of the rate constant
• to determine the activation energy

Question 49

what does the Arrhenius factor (A) take into account

Answer 49

the frequency of collisions with proper orientations

Question 50

what happens to rate of reaction and rate constant when temperature increases

Answer 50

increase exponentially

Question 51

frequency factor (A)

Answer 51

• combination of the steric factor (p) and the collision number (z)
• an account of the frequency of collision with the correct orientation
• how often the activation energy barrier is met