3.1.9 - Rate Equations

Question 1

How would you measure changes in a volume of gas produced over time?

Answer 1

Using a gas syringe.

Question 2

What are the limitations of a typical gas syringe?

Answer 2

100ml of gas can be collected; where collecting gas cannot be exceeded.

Question 3

What 2 factors does the Arrhenius constant take into consideration?

Answer 3

Frequency of collisions (rate of molecular encounters) and orientation of molecules (spatial arrangement of molecules).

Question 4

Explain the rate-determining step.

Answer 4

Each step has a different rate of reaction; the slowest step controls the rate of reaction. This is called the RDS (rate determining step).

Question 5

What is meant by order of reaction?

Answer 5

Power of concentration term in the rate equation.

Question 6

What do you add to conduct the Iodine Clock Experiment?

Answer 6

Sodium thiosulfate and starch.

Question 7

What is the role of a colorimeter?

Answer 7

Measures the absorbance of light in a coloured sample; the more concentrated a sample, the darker its colour, hence the more light absorbed.

Question 8

What must be done to each sample before titrating with alkali?

Answer 8

Quenching / Stop the reaction by dilution.

Question 9

What is a rate equation and how is it generally expressed?

Answer 9

A rate equation relates the rate of a chemical reaction to the concentrations of reactants. For a reaction aA + bB → products, it is expressed as r = k[A]^m[B]^n.

Question 10

Explain the meaning of reaction orders in the context of a rate equation.

Answer 10

Reaction orders (m, n) in a rate equation indicate how the rate of reaction depends on the concentrations of reactants.

Question 11

How do you calculate the units of the rate constant (k) in a rate equation?

Answer 11

The units of k depend on the overall order of the reaction. For a first order reaction, the units are s^-1; for second order, mol^-1dm^3s^-1; for third order, mol^-2dm^6s^-1.

Question 12

What is the continuous monitoring method in the context of rate of reaction?

Answer 12

Continuous monitoring involves observing and recording the change in concentration of a reactant or product over time, typically by measuring the volume of a gas produced or consumed using a gas syringe.

Question 13

How is the initial rate of a chemical reaction determined?

Answer 13

The initial rate is determined by measuring the rate of reaction at the very beginning (time zero) and can be calculated from the gradient of the concentration vs. time graph.

Question 14

How does changing the concentration of a reactant affect the rate of reaction according to its order?

Answer 14

For a zero-order reactant, changes in concentration have no effect on the rate. For a first-order reactant, the rate is directly proportional to its concentration. For a second-order reactant, the rate is proportional to the square of its concentration.

Question 15

What is the RDS and how does the rate-determining step affect the rate equation of a reaction?

Answer 15

The rate-determining step is the slowest step in a reaction mechanism and controls the overall rate of reaction; the molecularity of the reactants in this step determines their orders in the rate equation.

Question 16

What is the Arrhenius equation and what does it describe?

Answer 16

The Arrhenius equation, k = Ae^(-Ea/RT), describes how the rate constant (k) changes with temperature. A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin.

Question 17

How can reaction order be determined graphically?

Answer 17

The order can be determined by plotting log(rate) versus log(concentration); the slope of this line gives the reaction order with respect to the reactant.

Question 18

How does temperature affect the feasibility of reactions with different entropy changes?

Answer 18

For reactions with positive entropy changes (∆S > 0), increasing temperature generally makes the reaction more feasible; for reactions with negative entropy changes, increasing temperature makes the reaction less feasible.

Question 19

Describe what happens in a zero-order reaction regarding the rate and concentration.

Answer 19

In a zero-order reaction, the rate of reaction is constant and does not depend on the concentration of the reactant; the rate equation is r = k.

Question 20

What characterises a first-order reaction in terms of rate and reactant concentration?

Answer 20

In a first-order reaction, the rate of reaction is directly proportional to the concentration of one reactant; the rate equation is r = k[A].

Question 21

Explain a second-order reaction.

Answer 21

A second-order reaction is one where the rate of reaction is proportional to the square of the concentration of one reactant, or the product of the concentrations of two reactants; the rate equation can be r = k[A]^2 or r = k[A][B].

Question 22

How is the order of a reaction determined experimentally?

Answer 22

Reaction order is determined experimentally by observing how changes in reactant concentrations affect the rate of reaction.

Question 23

How are the units of the rate constant (k) determined for different orders of reactions?

Answer 23

The units of the rate constant depend on the overall order of the reaction; for example, for a first-order reaction, the unit is s^-1.

Question 24

How does temperature affect the rate constant (k) in chemical reactions?

Answer 24

The rate constant (k) increases with temperature; this relationship is quantified by the Arrhenius equation.

Question 25

What is the initial rate method and its significance in kinetics?

Answer 25

The initial rate method involves measuring the rate of reaction at the very start where the concentrations of reactants are known; it’s crucial for determining the reaction order and rate constant accurately.

Question 26

Define the rate-determining step in a reaction mechanism.

Answer 26

The rate-determining step is the slowest step in a reaction mechanism that controls the overall rate of the reaction.

Question 27

Explain the principle of a clock reaction in studying reaction kinetics.

Answer 27

Clock reactions are used to determine reaction kinetics by measuring the time it takes for a visible change to occur, indicating a specific point in the reaction.

Question 28

What are 4 properties you can record changes in via continuous monitoring?

Answer 28

1. Gas volume 2. pH change 3. Colour change 4. Loss of mass

Question 29

What piece of apparatus would you use to track colour change during a chemical reaction?

Answer 29

Colorimeter.

Question 30

For the equation A + B —> C + D, what would the rate equation be?

Answer 30

Rate = k[A]ⁿ[B]ⁿ.

Question 31

How does increasing the temperature affect the rate constant?

Answer 31

It would increase the rate constant.

Question 32

What are 2 features of all clock reactions?

Answer 32

1. Sudden increase in the concentration of one of the products as a limiting reactant is used up. 2. Clearly defined end point.

Question 33

What does a straight, non-horizontal line on a rate-concentration graph show?

Answer 33

The reaction is first order.

Question 34

Why does doubling the temperature generally have a greater effect on reaction rate than doubling the concentration of a reactant?

Answer 34

Doubling the temperature significantly increases the number of molecules with enough energy to overcome the activation energy, exponentially increasing the rate.

Question 35

How to calculate the activation energy Ea using the Arrhenius equation from experimental data?

Answer 35

Use the slope from a plot of ln(rate constant) vs. 1/T (inverse temperature); multiply the slope by -R to get Ea in joules, then convert to kJ/mol if needed.

Question 36

How can a graph indicate a zero-order reaction with respect to a reactant?

Answer 36

A plot of reactant concentration vs. time for a zero-order reaction shows a linear decrease.

Question 37

How to determine the reaction order from a plot of reaction rate versus reactant concentration?

Answer 37

If the plot is a straight line passing through the origin, the reaction is first-order with respect to that reactant.

Question 38

Why is sodium hydrogencarbonate used in reactions involving iodine and propanone?

Answer 38

Sodium hydrogencarbonate neutralizes the acidic catalyst, stopping the reaction by removing the acidic environment needed for the reaction’s progress.

Question 39

Why might the order of a reaction with respect to a particular reactant be considered negligible?

Answer 39

If a reactant is in large excess or its concentration changes very little during the reaction, its effect on the rate can be considered negligible.

Question 40

What does a square relationship between reactant concentration and reaction time indicate about the reaction order?

Answer 40

If the time to complete a reaction becomes four times shorter when the concentration of a reactant is doubled, it suggests a second-order reaction with respect to that reactant.

Question 41

Describe a method to collect and measure hydrogen gas produced in a reaction.

Answer 41

Use a gas syringe or an inverted measuring cylinder in a water bath to collect the hydrogen gas.

Question 42

Define the term ‘overall order of a reaction.’

Answer 42

The overall order of a reaction is the sum of the powers of the concentration terms in the rate equation.

Question 43

How does temperature affect the rate constant and consequently the rate of a reaction?

Answer 43

As temperature increases, the rate constant generally increases, making the reaction proceed faster.

Question 44

How can kinetic data from variable concentration experiments determine the reaction order?

Answer 44

By observing how changes in the concentration of reactants affect the rate of reaction.

Question 45

How can you determine the effect of pH change on the rate of a reaction?

Answer 45

By comparing the rates of the reaction at different pH levels.

Question 46

Discuss the importance of experimental design in the determination of reaction kinetics.

Answer 46

Proper experimental design ensures accurate, reliable data collection, which is crucial for determining the rate law, reaction order, and rate constant of a reaction.

Question 47

Explain in general terms how a catalyst increases the rate of a reaction.

Answer 47

A catalyst increases the rate of a reaction by lowering the activation energy required for the reaction to proceed.

Question 48

What does the concentration of hydrogen ions [H+] reflect?

Answer 48

The concentration of hydrogen ions [H+] reflects the pH.

Question 49

What does the rate change ratio indicate?

Answer 49

The rate change ratio gives insight into the reaction’s sensitivity to pH changes.

Question 50

Why is experimental design important in reaction kinetics?

Answer 50

Proper experimental design ensures accurate, reliable data collection, which is crucial for determining the rate law, reaction order, and rate constant of a reaction.

Question 51

What variables should be controlled in experimental design?

Answer 51

Variables such as temperature, reactant concentrations, and measurement techniques should be controlled.

Question 52

How does a catalyst increase the rate of a reaction?

Answer 52

A catalyst creates an alternative reaction pathway with a lower activation energy.

Question 53

What are the axes labeled on the Maxwell Boltzmann distribution?

Answer 53

x axis = collision energy

y axis = fraction of molecules with energy

Question 54

Why can a small increase in temperature lead to a large increase in reaction rate?

Answer 54

Many more particles have enough energy to reach activation energy (Ea).

Question 55

What is meant by activation energy?

Answer 55

Activation energy is the minimum amount of energy required for a reaction to occur.

Question 56

Which change involves a different activation energy: increasing reactant concentration, increasing temperature, or adding a catalyst?

Answer 56

The addition of a catalyst involves a different activation energy.

A catalyst creates an alternative pathway with a lower Ea.