Rates of Reactions (Chapter 10.1, 10.2, 10.3 + Chapter 18)

Question 1

What is the rate of a reaction?

Answer 1

The change in concentration of a reactant or product in a given time

Question 2

What does the rate of reaction show?

Answer 2

How fast a reactant is used up or how fast a product is formed

Question 3

What is the equation for rate?

Answer 3

Change in concentration/time

Question 4

What are the units of rate?

Answer 4

Moldm-3/s-1

Question 5

What are the four factors that affect rate?

Answer 5

1) concentration/pressure
2) temperature
3) use of a catalyst
4) surface area of solid reactants

Question 6

What is collision theory?

Answer 6

That two reacting particles must collide for a reaction to occur

Question 7

When is collision effective?

Answer 7

Only when particles 1) collide with the correct orientation and 2) have enough energy to overcome the activation energy barrier of a reaction

Question 8

In terms of collision theory, why does rate increase as concentration increases?

Answer 8

1) the increase in concentration increase the number of particles in the same volume
2) ∴ the particles are closer together and collide more frequently
3) ∴ in a given period of time there are more effective collisions and an increased rate of reaction

Question 9

In terms of collision theory, why does rate increase as pressure increases?

Answer 9

1) the concentration of gas molecules increases as the same number of gas molecules occupy a smaller volume
2) ∴ gas molecules are closer together and collide more frequently
3) ∴ in a given period of time there are more effective collisions and an increased rate of reaction

Question 10

What is a catalyst?

Answer 10

• A substance that changes the rate of a chemical reaction without undergoing any permanent change itself
• It is not used up in a reaction
• At the end of a reaction, the catalyst is regenerated

Question 11

How may a catalyst work?

Answer 11

It may react with a reactant to form an intermediate or provide a surface on which the reaction can take place

Question 12

How does a catalyst increase the rate of a chemical reaction?

Answer 12

By providing an alternative reaction pathway of a lower activation energy

Question 13

What is a homogenous catalyst?

Answer 13

A catalyst with the same physical state as the reactants

Question 14

How does a homogenous catalyst work?

Answer 14

1) the catalyst reacts with the reactants to form an intermediate
2) the intermediate breaks down to give the product and regenerates the catalyst

Question 15

What is a heterogenous catalyst?

Answer 15

A catalyst with a different physical state from the reactants (usually solids with gaseous/aqueous reactants) often used in industrial processes e.g. Haber process

Question 16

How does a heterogenous catalyst work?

Answer 16

1) the reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place
2) after the reaction, the product molecules leave the surface of the catalyst by desorption

Question 17

What does using a catalyst do to an industrial process?

Answer 17

It reduces the temperature and energy requirements for the process

Question 18

What are the benefits of using a catalyst?

Answer 18

1) if the process requires less energy, less electricity and fossil fuels are used
2) fewer fossil fuels being used reduces CO2 emissions ∴ less global warming
3) cuts costs and increases profitability (as product is made faster and using less energy)

Question 19

Why do industries use catalysts?

Answer 19

1) they have to use processes with high atom economies and less pollutants
2) the economic advantage of using a catalyst outweighs the costs of developing a catalytic process

Question 20

Describe the movement and energy of molecules in a gas, liquid or solution

Answer 20

• Some molecules move fast with high energy, some move slow with low energy
• Most molecules move close to average speed and have close to the average energy
• Only a small proportion of molecules have more energy than the activation energy (enough energy to react)

Question 21

What is the Boltzmann distribution?

Answer 21

The spread of molecular energies in a gas

Question 22

Describe the Boltzmann distribution

Answer 22

1) the curve starts at the origin bc no molecules have zero energy
2) the curve does not meet the x-axis as there is no maximum energy for a molecule (infinite energy)
3) area under curve = total number of molecules

Question 23

What happens to the Boltzmann distribution at higher temperatures?

Answer 23

• The peak is lower and shifted to the right

- The number of molecules stays the same ∴ the area under the curve remains the same

Question 24

Why does rate increase as temperature increases (Boltzmann distribution)?

Answer 24

1) more molecules have an energy more than or equal to the activation energy (a greater proportion of molecules can overcome the activation energy)
2) ∴ a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
3) collisions will also be more frequent as molecules are moving faster with more kinetic energy (increased energy is more important than increased frequency)

Question 25

What happens to the Boltzmann distribution in the presence of a catalyst?

Answer 25

• The activation energy is decreased

- ∴ a greater proportion of molecules exceed the new lower activation energy

Question 26

Why does rate increase in the presence of a catalyst (Boltzmann distribution)?

Answer 26

1) a catalyst provides an alternative reaction route with a lower activation energy (Ec)
2) compared to Ea, a greater proportion of molecules now have an energy more than or equal to Ec
3) ∴ on collision, more molecules will have the energy to react to form products, increasing the rate of reaction

Question 27

What is there rate of reaction proportional to and what is the power?

Answer 27

• The rate of reaction is proportional to the concentration of a particular reactant raised to a power
• For each reactant, the power is the order of reaction for that reactant

Question 28

What is zero order?

Answer 28

• When the concentration of a reactant has no effect on the rate
• Any number raised to the power of 0 is 1 ∴ the concentration does not influence the rate

Question 29

What is first order with respect to the reactant?

Answer 29

• When the rate depends on its concentration raised to the power of one
• Concentration x 2, rate x 2

Question 30

What is second order with respect to the reactant?

Answer 30

• When the rate depends on its concentration raised to the power of two
• Concentration x 2, rate x 4

Question 31

What is the overall order of a reaction?

Answer 31

• The sum of orders with respect to each reactant (m+n)

- It gives the overall effect of the concentrations of all reactants on the rate of reaction

Question 32

What is the rate equation and what does it give?

Answer 32

• rate = k[A]^m[B]^n

- It gives the mathematical relationship between the concentrations of the reactants and the reaction rate

Question 33

What is the rate constant?

Answer 33

• The proportionality constant

- The number that mathematically converts between the rate of reaction, concentration and orders

Question 34

What are the units of k for each order?

Answer 34

0 = moldm-3s-1
1 = s-1
2 = dm3mol-1s-1

Question 35

How can concentration-time graphs be plotted?

Answer 35

From continuous monitoring - continuous measurements taken during the course of a reaction

Question 36

What are the three methods of continuous monitoring?

Answer 36

1) gas collection - use gas syringe to measure volume of gas produced
2) mass loss
3) colorimetry - the wavelength of light passing through a coloured solution is controlled using a filter and the amount of light absorbed by a solution is measured

Question 37

What is the gradient of a concentration-time graph equal to?

Answer 37

The rate of reaction

Question 38

What must be true to deduce the order of a reaction (if the order is 1 or 0)?

Answer 38

All other reactant concentrations must remain effectively constant

Question 39

Describe a zero order concentration-time graph

Answer 39

• Straight line with a negative gradient
• The reaction rate does not change during the course of the reaction
• Value of the gradient = k (rate of reaction)

Question 40

Describe a first order concentration-time graph

Answer 40

• Downward curve with decreasing gradient over time
• As the gradient decreases with time, the reaction gradient slows down
• The time taken for the concentration of the reactant to halve (successive half lives) is constant and can be used to determine k
• Gradient of the curve at a concentration = the rate at that concentration

Question 41

What is the half life?

Answer 41

The time taken for half of a reactant to be used up

Question 42

How do you calculate k from the rate?

Answer 42

k = rate (gradient of the tangent at [A]) / [A]

Question 43

How do you calculate k from the half life?

Answer 43

k = ln2 / half life

Question 44

Describe a zero order rate-concentration graph

Answer 44

• Horizontal straight line (gradient = 0)
• rate = k (y-intercept)
• Reaction rate does not change with increasing concentration

Question 45

Describe a first order rate-concentration graph

Answer 45

• Straight line graph through origin
• rate = k[A]
• k = gradient

Question 46

Describe a second order rate-concentration graph

Answer 46

• Upward curve with increasing gradient
• rate = k[A]^2
• bc of the curve, k cannot be directly obtained (only by plotting rate against concentration squared, k = gradient)

Question 47

What is the initial rate and how is it found?

Answer 47

• Instantaneous rate at the start of a reaction when t=0

- It is found by measuring the gradient of tangent at t=0 on a concentration-time graph

Question 48

Describe a clock reaction

Answer 48

• The time is measured from the start of an experiment for a visual change (colour/precipitate) to be observed
• It is a more convenient way of obtaining the initial rate of reaction by taking a single measurement

Question 49

How does a clock reaction work?

Answer 49

1) provided that there is no significant change in rate during the time taken for a visual change to occur, it can be assumed that the average rate of reaction over this time will be the same as the initial rate
2) The initial rate is then proportional to 1/t (units=K-1)
3) the reaction is repeated several times with different concentrations and values of 1/t are calculated for each experimental run (and can be plotted against concentration)

Question 50

When is the clock reaction reasonably accurate?

Answer 50

As long as less than 15% of the reaction has taken place because at the start of the reaction the curved is less curved, so more accurate for the initial rate

Question 51

Describe the iodine clock (formation of iodine)

Answer 51

• As iodine solution is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured
• Starch is usually added bc it forms a complex with iodine which is dark blue-black

Question 52

Why are reactions likely to take place in a series of steps?

Answer 52

• A reaction can only take place when the right amount of all particles collide (very unlikely)
• It is unlikely that more than 2 particles will collide together

Question 53

What is a reaction mechanism?

Answer 53

The series of steps that make up an overall reaction

Question 54

What is the rate determining step?

Answer 54

The slowest step in the sequence of steps in a reaction mechanism (the steps in a multi-step reaction take place at different rates, each with different rate constants)

Question 55

Describe the rate equation in relation to the rate-determining step

Answer 55

• The rate equation only includes reacting species involved in the rate-determining step
• The orders in the rate equation match the number of species involved in the rate-determining step

Question 56

Explain why the rate and k increase with temperature

Answer 56

Rate increases because the value of k increases

1) the Boltzmann distribution is shifted to the right, increasing the proportion of particles that exceed the activation energy
2) particles move faster and collide more frequency ∴ there are more successful collisions (as more energy)
3) the change in rate is mainly determined by activation energy bc with increasing temperature, the effect of increasing frequency is comparatively small to the the effect of increasing the proportion of molecules exceeding the activation energy

Question 57

What is the Arrhenius equation?

Answer 57

k = Ae^-EA/RT

Question 58

What is A?

Answer 58

The pre-exponential factor - takes into account the frequency of collisions with the correct orientation (essentially constant over small temp range)

Question 59

What is e^-Ea/RT?

Answer 59

The exponential factor - represents the pupation of molecules that exceed Ea and that have sufficient energy for a reaction to take place

Question 60

Describe the components of the Arrhenius equation

Answer 60

R = 8.314 J/mol/K
T = temp in K
Ea = activation energy in J/mol

Question 61

How can Ea and A be determined graphically?

Answer 61

1) lnk = -Ea/RT = lnA
2) lnk = -Ea/R x 1/T + lnA
y = mx + c
3) ∴ plot of ink against 1/T gives onwards straight line with gradient = -Ea/R and y-intercept = lnA

Question 62

What must appear in the rate equation?

Answer 62

A catalyst

Question 63

What should rate never be?

Answer 63

Negative

Question 64

Why might the rate of reaction decrease at the end of a zero order reaction?

Answer 64

1) the temperature decreased ∴ the rate decreased
2) if measuring the loss of a gas before the apparatus is sealed, the rate will be greater at the start
3) the catalyst becomes saturated ∴ the rate decreases