5.1.1 - Rates Of Reactions

Question 1

What is a rate equation?

Answer 1

A mathematical expression showing how the rate of a chemical reaction depends on the concentrations of the reactants

A rate equation typically takes the form Rate = k[A]m[B]n.

Question 2

What is the general form of the rate equation for the reaction A + B ➔ C + D?

Answer 2

Rate = k[A]m[B]n

In this equation, k is the rate constant, [A] and [B] are the concentrations of reactants, and m and n are the orders of reaction.

Question 3

What does the variable ‘Rate’ represent in the rate equation?

Answer 3

The reaction rate measured in mol dm-3 s-1

It indicates how fast the reactants are converted into products.

Question 4

What is the significance of the rate constant ‘k’?

Answer 4

It relates the rate of a reaction to the concentrations of the reactants at a specific temperature

A larger k value indicates a faster rate of reaction.

Question 5

What does a zero order reaction imply about the rate?

Answer 5

The rate is independent of the reactant concentration

Doubling or tripling the concentration has no effect on the rate.

Question 6

What is the impact of doubling the concentration in a first order reaction?

Answer 6

It doubles the rate of the reaction

The rate is directly proportional to the reactant concentration.

Question 7

How does a second order reaction respond to changes in concentration?

Answer 7

The rate is proportional to the square of the reactant concentration

Doubling the concentration quadruples the rate.

Question 8

How is the overall order of a reaction determined?

Answer 8

It is the sum of the individual orders of the reactants, m and n

This determines how the rate changes with changes in concentration.

Question 9

Can reaction orders be determined from balanced chemical equations?

Answer 9

No, they can only be determined experimentally

This is a crucial aspect of kinetic studies.

Question 10

What happens to the rate constant ‘k’ when the temperature increases?

Answer 10

It increases exponentially

Higher temperatures lead to more frequent and more energetic collisions between reactant molecules.

Question 11

What do the units of ‘k’ depend on?

Answer 11

The overall order of the reaction

Different orders lead to different units for the rate constant.

Question 12

What are the two main methods to determine the reaction order with respect to a specific reactant?

Answer 12

1. Initial rates method
2. Continuous monitoring method

The initial rates method measures how the initial rate changes with the concentration of the reactant, while continuous monitoring tracks concentration changes over time.

Question 13

What does the initial rate of a reaction refer to?

Answer 13

The rate at the very beginning of the reaction (time = 0)

It is calculated by finding the gradient of the tangent to the concentration-time graph at t = 0.

Question 14

What is a key assumption when using the initial rates method?

Answer 14

The concentrations of any other reactants remain substantially constant throughout the experiment

This ensures that changes in rate are solely due to the reactant being studied.

Question 15

What is a clock reaction?

Answer 15

A type of reaction where the time taken to produce a fixed amount of product is measured as initial concentrations of reactants are varied

A clear observable change signals the endpoint of the reaction.

Question 16

In a clock reaction, how is the initial rate determined?

Answer 16

The initial rate is inversely proportional to the time taken for the observable change to occur

A faster clock reaction indicates a higher initial rate.

Question 17

What is the classic example of a clock reaction?

Answer 17

The iodine clock reaction

Involves the reaction of hydrogen peroxide, iodide, and thiosulfate, resulting in a color change.

Question 18

What is the purpose of colorimetry in monitoring reactions?

Answer 18

To measure the absorbance of a specific wavelength of light by a solution to quantify changes in concentration

Different reactants and products may have different absorbances.

Question 19

What is the first step in preparing a calibration curve for colorimetry?

Answer 19

Prepare a series of standard solutions with known concentrations of the analyte

This is followed by measuring their absorbance.

Question 20

What does a zero-order reaction rate-concentration graph look like?

Answer 20

A horizontal line indicating the reaction rate is independent of the reactant concentration

The rate equation is: rate = k.

Question 21

What indicates a first-order reaction on a rate-concentration graph?

Answer 21

A straight line passing through the origin

The rate equation is: rate = k[X].

Question 22

What shape does a second-order reaction rate-concentration graph have?

Answer 22

A curved plot indicating the reaction rate is proportional to the square of the reactant concentration

The rate equation is: rate = k[X]².

Question 23

What is the relationship between half-life and first-order reactions?

Answer 23

The half-life is independent of the initial concentration and each successive half-life is the same duration

The half-life can be read directly from the concentration-time graph.

Question 24

How is the rate constant (k) related to the half-life (t1/2) for first-order reactions?

Answer 24

k = ln(2)/t1/2

This equation shows that the rate constant can be calculated using the half-life.

Question 25

What is the rate-determining step in a reaction mechanism?

Answer 25

It is the slowest step in a multi-step reaction mechanism which determines the overall rate of the reaction.

Question 26

How does the rate-determining step affect the overall reaction rate?

Answer 26

The slowest step in a reaction restricts the rate of the entire reaction.

Question 27

Can each step in a reaction mechanism have the same rate?

Answer 27

No each step can have different rates

Question 28

What information can the rate equation provide about the rate-determining step?

Answer 28

The rate equation can help identify which reactants are involved in the rate-determining step.

Question 29

What does it mean if a reactant appears in the rate equation?

Answer 29

If a reactant appears in the rate equation it must be involved in the rate-determining step

Question 30

What can be inferred if a reactant is absent from the rate equation?

Answer 30

If a reactant is not in the rate equation it does not participate in the rate-determining step.

Question 31

Is the rate-determining step always the first step in a reaction mechanism?

Answer 31

No the rate-determining step is not necessarily the first step in the mechanism.

Question 32

Can the balanced equation of a reaction alone reveal the reaction mechanism?

Answer 32

No the reaction mechanism often cannot be deduced from the balanced equation alone.

Question 33

How does the order of reaction relate to the rate-determining step?

Answer 33

The order of reaction with respect to a reactant indicates the number of molecules of that reactant involved in the rate-determining step.

Question 34

If a reaction is second-order with respect to reactant X what does this indicate about the rate-determining step?

Answer 34

It indicates that two molecules of X are involved in the rate-determining step.

Question 35

In the reaction Cl• + O3 ➔ ClO• + O2 (slow) followed by ClO• + O3 ➔ Cl• + 2O2 (fast) which species must appear in the rate equation?

Answer 35

Both Cl• and O3 must appear in the rate equation since they are involved in the slow step

Question 36

How can knowledge of the rate-determining step help in deducing a reaction mechanism?

Answer 36

By identifying which reactants are involved in the rate-determining step scientists can better understand and deduce the overall reaction mechanism.

Question 37

What does the rate equation rate = k[CH3Cl] suggest about the reaction mechanism for chloromethane and OH- substitution?

Answer 37

It suggests that OH- is not involved in the rate-determining step supporting a two-step mechanism where OH- appears in the faster second step.

Question 38

Why might an intermediate be involved in the rate-determining step even if it does not appear in the overall balanced equation?

Answer 38

Intermediates can form and be consumed in reactions affecting the rate-determining step without being present in the final balanced equation.

Question 39

What does the Arrhenius equation describe in a chemical reaction?

Answer 39

It links the rate constant (k) with temperature (T) and activation energy (Ea) showing how these factors affect the reaction rate.

Question 40

Write the Arrhenius equation.

Answer 40

k=Ae (−Ea/RT)

Question 41

In the Arrhenius equation what does k represent?

Answer 41

The rate constant of the reaction.

Question 42

What is the pre-exponential factor (A) in the Arrhenius equation?

Answer 42

Also known as the Arrhenius constant it represents the frequency of successful collisions with the correct orientation.

Question 43

What does Ea stand for in the Arrhenius equation?

Answer 43

Activation energy measured in J mol−1

Question 44

Define the gas constant R in the Arrhenius equation.

Answer 44

R is the gas constant with a value of 8.31 J K−1mol−1.

Question 45

How does activation energy (Ea) affect the rate constant (k)?

Answer 45

Higher Ea lowers k because fewer molecules have enough energy to overcome the barrier resulting in a slower reaction rate.

Question 46

What effect does temperature (T) have on the rate constant (k) in the Arrhenius equation?

Answer 46

Increasing temperature exponentially increases k as more molecules gain sufficient energy to overcome Ea.

Question 47

Why does a small increase in temperature cause a large increase in reaction rate?

Answer 47

It significantly raises the proportion of molecules with energy greater than or equal to Ea leading to more successful collisions.

Question 48

What is the logarithmic form of the Arrhenius equation?

Answer 48

lnk=lnA(−Ea/RT)

Question 49

​What kind of graph is used to determine Ea and A from the Arrhenius equation?

Answer 49

An Arrhenius plot which plots lnk (y-axis) against 1/T (x-axis)

Question 50

In an Arrhenius plot what does the gradient of the line represent?

Answer 50

The gradient is -(Ea/R)

Question 51

How can the y-intercept of an Arrhenius plot be used?

Answer 51

The y-intercept gives lnA and by taking e^intercept

Question 52

In calculating Ea how do you convert from J mol−1 to kJ mol−1?

Answer 52

Divide the activation energy in J mol−1 by 1000

Question 53

What factors do NOT affect the values of Kc and Kp?

Answer 53

The values of Kc and Kp are not affected by changes in concentration, pressure, or by the use of a catalyst.
##Footnote
They are only affected by changes in temperature.

Question 54

How do concentration and pressure changes affect a reaction?

Answer 54

Concentration and pressure changes affect the rate of the reaction (the kinetics), but not the position of equilibrium. Therefore, they have no impact on the equilibrium constant.

Question 55

What effect does a catalyst have on Kc and Kp?

Answer 55

A catalyst speeds up the rate of reaction but does not change the position of equilibrium or affect the values of Kc or Kp.

Question 56

Which factor affects the position of equilibrium and equilibrium constants (Kc and Kp)?

Answer 56

Temperature changes affect the position of equilibrium, leading to changes in the concentrations of reactants and products and affecting Kc and Kp.

Question 57

What happens to Kc or Kp if the forward reaction is exothermic and temperature increases?

Answer 57

For an exothermic reaction, an increase in temperature shifts the equilibrium to the left (favoring the reverse reaction), which decreases the concentrations of products and increases the concentrations of reactants. Thus, Kc or Kp decreases.

Question 58

What happens to Kc or Kp if the forward reaction is endothermic and temperature increases?

Answer 58

For an endothermic reaction, an increase in temperature shifts the equilibrium to the right (favoring the forward reaction), which increases the concentrations of products and decreases the concentrations of reactants. Therefore, Kc or Kp increases.

Question 59

What is the effect of decreasing temperature on Kc or Kp?

Answer 59

*For exothermic reactions, a decrease in temperature will increase Kc or Kp.
*For endothermic reactions, a decrease in temperature will decrease Kc or Kp.

Question 60

What is a homogeneous reaction?

Answer 60

A reaction where all the reactants and products are in the same state

Question 61

What is a heterogeneous reaction?

Answer 61

When the reactants and products are in different states eg. gas and solid.
##Footnote
Only gases are used for Kc and Kp