Unit 7: Chemical Kinetics

Question 1

rate of reaction

Answer 1

The decrease in one reactant’s concentration, or, the increase in one product’s concentration per unit time.

Question 2

how is the rate of a reaction measured?

Answer 2

While technically it is measured in:
change in concentration per unit time

it can be approximated by many other methods and units, such as:
* change in mass per time
* change in volume per time.

Question 3

Why do the starting points for reactants and products make sense?

Answer 3

At time = 0, the reactants (reagents) start at their maximum concentration because the reaction hasn’t started yet — none of the reactants have been used up.

The products start at zero concentration because no product has been formed yet at the beginning of the reaction.

✅ This makes sense because the reaction hasn’t had any time to convert reactants into products.

Question 4

When is the reaction happening fastest? What makes you say that?

Answer 4

The reaction is fastest at the very beginning, just after time = 0.

You can tell because the slope of the reactant line is steepest (decreasing quickly) and the product line is increasing most rapidly there.

✅ The steeper the slope, the greater the rate of change in concentration, which means the reaction is proceeding quickly.

Question 5

When is the reaction finished? What makes you say that?

Answer 5

The reaction is finished when the concentration curves level off (become flat).

This means the reactant concentration stops decreasing and the product concentration stops increasing — so no more net change is occurring.

✅ A flat line (zero slope) indicates that the concentrations are no longer changing, meaning the reaction has reached completion or equilibrium.

Question 6

What value on a graph represents its rate of reaction?

Answer 6

The rate of reaction is represented by the slope (gradient) of the concentration vs. time curve.

For reactants: rate = –Δ[reactant]/Δt

For products: rate = +Δ[product]/Δt

✅ A steeper slope means a faster reaction rate, and the sign (positive or negative) indicates whether it’s a reactant being consumed or a product being formed.

Question 7

In order for a successful reaction to occur, reactant particles must…

collision theory

Answer 7

1) Particles must collide
→ No collision, no reaction.

2) Particles must collide with the correct orientation
→ The kinetic energy must be equal to or greater than the activation energy (Eₐ) to break bonds.

3) Particles must collide with sufficient kinetic energy (speed) to overcome activation energy

ineffective collision = no reaction

Question 8

activiation energy

Answer 8

In order for reactant particles to turn into products, the bonds in the reactant particles must be broken first. This takes energy (is an endothermic process), and therefore, there needs to be enough energy present to break these bonds apart if the particles are going to react with one another.

This energy, the minimum energy required for a reaction to take place, is known as the activation energy. On a potential energy diagram (enthalpy diagram), the activation energy is shown as an energy hurdle.

Question 9

how to determine whether a reaction is endothermic or exothermic from an enthalpy diagram?

Answer 9

If the products are lower in energy than the reactants, then:
It’s an exothermic reaction:
* Energy is released to the surroundings.
* The ΔH (enthalpy change) is negative.
* The activation energy (Eₐ) appears as a “hump” between reactants and the peak of the transition state.

For the opposite reaction (endothermic):
* The products are higher in energy than the reactants.
* Energy is absorbed from the surroundings.
* The ΔH is positive.
* The activation energy is still shown as a hump, but now the final energy level (products) is above the initial level (reactants).

Exothermic: Products lower than reactants → -ΔH
Endothermic: Products higher than reactants → +ΔH

Question 10

catalysts

Answer 10

Catalysts are things that can be added to reactions that speed up the reaction rate. They do this by interacting with the reactants in such a way that they provide an alternative pathway for the reaction to take place with a lower activation energy. Catalysts do not get consumed by reactions, so the same amount is present before and after the reaction.

Examples of common catalysts include platinum metal, concentrated sulfuric acid and enzymes (protein molecules).

Question 11

catalyst on enthalpy diagram

Answer 11

The catalyst lowers the activation energy: this makes the reaction proceed faster.

The ΔH remains the same (the energy difference between reactants and products does not change).

The catalyzed curve starts and ends at the same points, but its peak is lower than the uncatalyzed one.

Question 12

specifically distribution

maxwell-boltzmann distribution

Answer 12

Maxwell-boltzmann distribution give a visual for showing the proportion of reactant particles in a liquid or gas moving with a given speed / energy. they are used to help illustrate and explain the effects of temperature and catalysrs on the rate of a reaction.

the total area under the curve represents the total number of reactant particles. the area under a given spot on the graph/curve represents the number/percentage of reactant particles that have that speed/energy.

Question 13

key points on a maxwell-boltzmann distribution

Answer 13

General Shape
* Temperature is a measure of the average kinetic energy of a substance’s particles.
* At any given temperature, all of the particles of a substance are not moving with the same speed.
* The particles of a substance move with a great variety of speeds, with a small percentage at very low speeds, a small percentage at very high speeds, and the largest proportion close to the average speed.
* This results is a distribution that is somewhat close to a bell-curve or a normal distribution.

Beginning of the Curve
As no particles have zero speed/ kinetic energy (no particles are at Absolute Zero, Temp = 0 Kelvin), the line of the graph always begins at the origin.

End of the Curve
While absolute zero represents an exact limit at the low end of energies, there is no theoretical maximum speed of reactant particles, so the right side of the graph should not be shown ever touching the x-axis.

Question 14

maxwell-boltzmann and reactions (specifically with activation energy and catalysts)

Answer 14

In kinetics, a Maxwell Boltzmann can be used to illustrate the concepts of activation energy and catalysts.

A vertical line can be added to the Maxwell-Boltzmann Distribution to represent the activation energy of the reaction. The exact point where it is added does not matter, though it is usually shown about 2/3 to 3/4 of the way down the x-axis. The particles to the right of this line represent the particles with sufficient energy to overcome the activation energy. This helps explain how even at somewhat cool temperatures reactions occur, as it is still possible for a percentage of reactant particles to have sufficient energy to overcome the activation energy of a reaction.

The impact of a catalyst can be illustrated by moving the activation energy line towards lower energy, which on this graph, is furhter left on the x-axis. Moving the line further left illustrates how more particles have energy greater than or equal to the activation energy when a catalyst is used.

Question 15

Maxwell-Boltzman: The Effect of Temperature

Answer 15

As temperature changes, the average speed/kinetic energy of a substance’s particles changes, however, this does not mean that each particle’s speed changes by the same amount.

Lower Temperatures
* With a decreasing temperature, the average KE decreases, so the peak of the MB Curve should be further to the left.
* The range of likely speeds also decreases, so the curve becomes narrower.
* Since the area under the curve must remain constant (to represent a constant number of particles), the curve gets narrower and taller, as
more particles have each of the more limited speeds.

Higher Temperatures
* With increasing temperature, the average KE increases, so the peak of the MB Curve should be further to the right.
* While the average increases, there are still some particles with very low speeds, so the range of likely speeds also increases and the curve becomes wider.
* Since the area under the curve must remain constant (to represent a constant number of particles), the curve gets wider and lower, as fewer particles have each of the more varied speeds.

Question 16

The Impact of Temperature on Reaction Rate

Answer 16

At higher temperatures, it is visually shown that more particles have greater energy than the activation energy. This is a big reason why reactions occur faster at higher temperature.

Question 17

what factors affect the rate of reaction?

Answer 17

we cannot influence what proportion of collisions are effective / have the righ orientation, we can however make reactions more frequent.

1) temperature (all states of matter):
a) more particles have enough energy to overcome activation energy has temp increases
b) increases the frequency of collisions

2) surface area (solids): by breaking solids into smaller pieces or powders increases the particles exposed, increasing the frequency of collisions. hence size decreases, sa increases, and frequency of collisions increase.

3) concentration (aqueous solutions and gases): results in more frequent collisions.
3.5) for gases, an increase in pressure or a decrease in volume are alternative ways to change its concentration.

4) catalyst: provides an alternative pathway with a lower activation energy, so a greater proportion of collisions become sucessful.
*Catalyst particles are not consumed by reactions, however, they do take part in reactions. Therefore, they also collide with reactant particles during a reaction. This means that the concentration, temperature and/or surface area of a catalyst are also influential on the reaction rate.

Question 18

how to experimentally measure reaction rates?

Answer 18

Usually, to experimentally determine a reaction rate, you need to determine a property that will measurably change over time as the reaction happens. This is easiest if the reaction involves the production of a gas, or involves a color change.

Question 19

Common methods used to measured rate of reaction:

chemical reactions producing gases

Answer 19

• Collect volume of gas produced over time (connect sealed reaction vessel to a gas syringe or with a tube to an inverted-water-filled measuring cylinder)
• Measure pressure of gas produced over time (connect sealed reaction vessel to a pressure probe)
• Measure mass change of reaction vessel over time (place reaction vessel onto a digital balance)

Question 20

Common methods used to measured rate of reaction:

chemical reactions with changing colour

Answer 20

Measure color change over time with a colorimeter

Question 21

Common methods used to measured rate of reaction:

chemical reactions with changing concentration of H+ or OH-

Answer 21

measure pH change over time with a pH meter

Question 22

Common methods used to measured rate of reaction:

chemical reactions with chaning concentration of ions

Answer 22

measure conductivity change over time with a conductivity probe.

Question 23

how to determine the rate of reaction graphically?

no matter what method is used

Answer 23

the rate of reaction is determined using a trendline. The equation of that line can be figured out either by data processing software (Capstone, Excel, etc.) or by finding two points on the line and calculating it (slope = rise/run).

The trendline at a specific point, known as the tangent line, can be used to determine the instantaneous rate at that time, while the trendline across a larger portion of data gives the average rate of reaction for over that time.

The most important feature of the equation of the trendline is the slope, or gradient, which represents the rate of reaction for that time of the reaction.
Faster Reaction Rates = Steeper Slopes
Slower Reaction Rates = Less Steep Slopes

In an investigation the trendline is usually made to fit the beginning of the data, as the slope there represents the initial rate of reaction. This is when the reaction rate is fastest and when the amounts/ concentrations of reactants are known and best compared.

Question 24

3 distinct sections of a curve on a rate of reaction graph

Answer 24

Section 1: Reaction Fastest (Steepest Slope)
Why it’s fast:
* At the start, the concentration of reactants is highest, so particles collide more often.
* There are more frequent and effective collisions.
* If a catalyst is present or the temperature is high, the rate is even faster.
Key idea: High number of reactant particles = high collision frequency = fast reaction rate.

Section 2: Reaction Slows Down (Gentle Slope)
Why it slows:
* As the reaction proceeds, reactants get used up, so there are fewer particles available.
* Fewer collisions per second, so the reaction rate decreases.
* Product may also accumulate and interfere, especially in closed systems.
Key idea: Lower reactant concentration = fewer collisions = slower rate.

Section 3: Reaction Over (Horizontal Line)
Why it stops:
* One or more reactants are used up completely.
* No more collisions that can lead to product formation.
* The reaction has reached completion.
Key idea: No more reactants = no more collisions = reaction stops.

Question 25

Experimentally Determining Reaction Rate: Non-Continuous Methods

Answer 25

The general idea for a non-continuous method is to measure the total time it takes for a given change to happen. examples: Measuring the time it takes for a piece of metal or a carbonate to completely disappear in an excess amount of acid. Measuring the time it takes for the sudden color change of the iodine clock. Measuring the time it takes for the sulfur produced in a reaction to block out a dark spot/X written underneath the reaction vessel.

Question 26

Calculating Rate from a Non-Continuous Method

Answer 26

With time data collected, you can get a quick sense of how a factor affects reaction rate (longer times = slower rates). To calculate a rate, you need to take the inverse of time. Rate = 1 / time (s-1) Since you are using the total time value, this will give you a value for the average rate of the whole reaction to go from start until the endpoint you decided upon. While not ideal, these non-continuous methods of calculating rate of reaction are still very useful and relevant for comparing the rate of a given reaction under varying conditions (such as different temperatures, concentrations, etc.).

Question 27

Key Graphing Considerations

Answer 27

**Initial Slope** - The initial slope of a curve represents the initial rate of reaction. Any change in conditions that affects reaction rate should affect the initial slope of a graph. **End Y-value of Curve** - The y-value at which the curve ends is dependent upon the amount of limiting reactant. Only if the amount of limiting reactant changes does the graph end at a different y-value. Factors that could affect the amount of limiting reactant are mass, volume and concentration.

Question 28

# reaction orders and reaction mechanisms Why **doesn’t** increasing the concentration of all reactants always increase the rate of reaction equally?

Answer 28

* According to Collision Theory, increasing concentration generally increases reaction rate by increasing collision frequency. However, experimental rate data show that not all reactants affect the rate equally. * This is because many reactions do not occur in a single step—they involve **reaction mechanisms**. * In multi-step reactions, not all reactants are involved in the **rate-determining step**. * Therefore, increasing the concentration of a reactant not involved in the slowest step has little or no effect on the rate. * Reactions involving multiple reactants are unlikely to occur through simultaneous collisions, especially with correct orientation and energy.

Question 29

What are reaction mechanisms?

Answer 29

Reactions are thought to take place in a series of steps (sort of like mini-reactions). These steps are known as **elementary steps** or component reactions, and they add up to the entire stoichiometric chemical equation (sort of like how half equations add up to overall redox equations). These component steps are collectively known as the Reaction Mechanism and are meant to illustrate HOW the reaction proceeds.

Question 30

How is a reaction mechanism determined?

Answer 30

A reaction mechanism can only be suggested after **experimentally** studying the rate of a chemical reaction, and even then, the mechanism is really just a **theory**, or **one possible explanation** of how the reaction proceeds. there are often many mechanisms that can be proposed that match the collected rate data, and they can never be proven, only supported by data as a possibility.

Question 31

the rate determining step

Answer 31

In reactions with many elementary steps, the step with the **highest activation energy is the slowest step**. As a result, it determines the overall rate of the reaction (it "bottle-necks" the overall reaction rate). This highest Ea/ slowest step is known as the **Rate Determining Step** (RDS).

Question 32

Why must catalysts be involved in the rate-determining step (RDS)?

Answer 32

As the rate determining step determines the overall reaction rate, catalysts must be involved in the rate determining step. They provide an alternative pathway to the RDS, with a lower activation energy. By lowering the activation of the rate determining step, the overall rate can be increased.

Question 33

Why doesn’t lowering the activation energy of a non-rate-determining step increase the overall reaction rate?

Answer 33

Lowering the Ea of a non-RDS step would have no effect, since the RDS Ea would remain unchanged and continue to slow down the overall reaction rate.

Question 34

How do different reactants in a reaction mechanism affect the rate of reaction?

Answer 34

In reaction mechanisms, different reactants are involved in different steps of the mechanism. Depending on whether the component step occurs before, after, or is the RDS, the concentrations of the reactants in the step can have different effects on the rate of the chemical reaction. The way in which a reactant's concentration affects the reaction rate is called its **reaction order**.

Question 35

How is reaction order determined?

Answer 35

The order of each reactant in a chemical reaction must be determined experimentally. Orders can be integers or fractions, but for IB Chemistry, only integer orders of reaction will be considered. Orders do not have to match the coefficients of the stoichiometric chemical equation. However, orders for a specific reactant can match the coefficients of reactants within the rate determining step.

Question 36

types of reaction orders (in IB)

Answer 36

Zero Order Reactants are reactants whose concentration has no effect on the rate of reaction. First Order Reactants are reactants whose concentration has a directly proportional effect on the rate of reaction. Second Order Reactants are reactants whose concentration has a squared effect on the rate of reaction.

Question 37

# reaction orders What happens to the concentration of a reactant over time during a reaction, and how can this be monitored?

Answer 37

The concentration of any reactant will decrease over the course of a reaction, and this can often be monitored over time. With other reactants in great excess, the concentration of a given reactant over time changes in specific ways based on its reaction order.

Question 38

How does the concentration of a zero order reactant change over time?

Answer 38

The concentration of a zero order reactant has no effect on the rate of the reaction, therefore, the reactant is used up at a constant rate throughout the course of the reaction. the graph would be a slightly downward sloping linear curve. ## Footnote *x axis time y axis concentration

Question 39

How does the concentration of a first order reactant affect rate and change over time?

Answer 39

The concentration of a first order reactant has a directly proportional effect on the rate of the reaction. Therefore, as the reactant is used up, the rate slows down proportionally. 1/2 the concentration = 1/2 the rate, etc. This leads to a **constant half-life** for first order reactants. It takes the same amount of time to reduce the concentration from: initial to 1/2, 1/2 to 1/4, 1/4 to 1/8, etc. exponentital graph downward sloping ## Footnote *x axis time y axis concentration

Question 40

How does the concentration of a second order reactant change over time?

Answer 40

The concentration of a second order reactant has a squared effect on the rate of the reaction. Therefore, its concentration gets used up quickly at first, and then much more slowly once its concentration has been lowered, since its concentration drastically affects the overall reaction rate. a steeper exponential curve than first order reaction graph. ## Footnote *x axis time y axis concentration

Question 41

How can you distinguish between 1st and 2nd order concentration–time graphs?

Answer 41

1st and 2nd order graphs look very similar for concentration vs. time. The key to distinguishing them (important for Paper 1) is to look for the constant half-life, which is only present for 1st order reactants.

Question 42

determining orders of reaction and the rate constant

Answer 42

The classic reaction order question gives a series of trials in which the concentration of each reactant is changed (one at a time) while the concentration of the others is held constant. By comparing the rate of reaction values as each reactant concentration is changed, the orders of each reactant can easily be determined. Once the rate orders are known, the values for concentration and rate from any specific trial can be plugged into the equation and the rate constant can be solved for.

Question 43

rate equation

Answer 43

The rate equation (or rate law) is a mathematical equation that equates the rate of a reaction to the concentrations of the reactants (and possibly a catalyst). rate = k [A]^x[B]^y [C]^z rate = rate of reaction, as change in concentration per change in time (mol dm^-3 time^-1) [ ] = Concentration (mol dm^-3) x, y & z = order of reaction of each individual reactant/catalyst. x + y + z = the overall order of reaction = sum of reactant orders k = the rate constant.

Question 44

rate constant

Answer 44

The rate constant is different for every reaction and must be determined experimentally once the rate and orders of reaction are known. Its units change as needed to make the two sides of the equation have the same units, as such, it always has time^-1 as part of its unit. Bigger rate constants imply a faster rate of reaction and smaller values imply slower rates of reaction. The rate constant is only affected by temperature. It increases with increasing temperature, but non-linearly (more on this relationship later). In a mechanism, each component reaction can be written as having its own rate equation and its own rate constant. The RDS should have the smallest rate constant since it is the slowest step.

Question 45

intermediate

Answer 45

An intermediate is a reacting species that is formed by an earlier reaction step and also goes on reacting to form another species (be that a product or another intermediate). Intermediates can have a range of stabilities and lifetimes, and are always shown on potential energy diagrams as troughs.

Question 46

transition state

Answer 46

A transition state exists between each distinct reactant/ intermediate/product change. It contains partially formed and broken bonds and exists for barely an instant. Transition states are always shown as energy maxima on potential energy diagrams.

Question 47

# consider elementary steps, intermediates, and the addition of a catalyst conditions of chemical equations

Answer 47

While stoichiometric equations may contain many reactant particles, each elementary step in a reactionn mechanism consists of only **1 or 2 particles at a time**, as the probability of more than 2 particles colliding successfully is very small. In the first step of a mechanism, only reactants or catalysts can be used. Steps can produce **intermediates** that do not show up in the overall equation. Intermediates get used in the next step after they are formed and cancel out of the overall stoichiometric equation. If a **catalyst** is involved in the mechanism as a **reactant**, it must be involved in the RDS and it must be re-produced at some point later in the mechanism so that it cancels out from the overall stoichiometric equation. Each elementary step must be balanced and overall the elementary steps must add up to the stoichiometric equation.

Question 48

rules for evaluting possible mechanisms

Answer 48

1) **The Rate Equation only depends on the rate determining step.** This means a rate equation can be initially written where rate equals the rate constant, times the concentration of each species from the RDS. The exponent (order of reaction) of each species is the same as its coefficient in the RDS. 2) **Rate equations cannot show the concentration of intermediates** (they can only show reactants or catalysts). Intermediates involved in the RDS must have their concentrations in the rate equation replaced by the concentrations of the reactants/catalyst that created them in previous steps.

Question 49

# shows on maxwell boltmann distributions temperature's effect with different activiation energies

Answer 49

The higher the activation energy of a reaction, the greater the impact of temperature on its reaction rate. The relationship between changing temperature and reaction rate should allow us to determine that reaction's activation energy. In the graph with the higher activation energy, the percentage of particles with energy greater than or equal to the activation energy increases more than in the case of a relatively lower activation energy.

Question 50

arrhenius equation (N/F)

Answer 50

Temperature has a positive, quantitative relationship with the rate constant, k, but this relationship is non-linear (in fact, it is exponential). The exact quantitative impact is related to the magnitude of the Ea. Reactions with higher activation energies are more affected by changes in temperature than those with lower activation energies. The Arrhenius Equation relates these 3 values together (in section 1 of the data booklet): k: rate constant T: temperature in K Ea: activation energy R: gas constant e: 2.718... A: Arrhenius Constant The Arrhenius equation is an **empirical relationship**, meaning it has been determined based on experimental data. It cannot be proven mathematically/ theoretically.

Question 51

# what is it related to? A (the arrhenius constant)

Answer 51

It is related to the frequency of collisions/ the fraction of collisions that have the correct orientations. Simpler reactant species are more likely to have the correct orientation when colliding, so the value of A generally increases for simpler reactants/ reactions.

Question 52

arrhenius equation graphical method

Answer 52

The Arrhenius Equation can be reworked to 2 more easily used equations (both of which are available in section 1 of the data booklet) through the use of logarithms. Taking the natural log of both sides of the Arrhenius Equation converts it into the second form, shown below. lnk = -Ea/RT + lnA This second form can be used to achieve a linear relationship, where (1/T) is used on the x axis and lnk is used on the y-axis.

Question 53

solving for activation energy from a graph of arrhenius equation (using logs)

Answer 53

By plotting 1/T vs. ln k data and fitting a trendline, the slope of the trendline can be used to solve for activation energy. m = -Ea / R -m R = Ea * Since activation energy is always positive, the slope of these graphs is always negative. * Higher activation energy values result in steeper lines. * The lines never go all the way to the y-axis, since 1/T cannot equal zero.

Question 54

Subtle Details to Watch For (on arrhenius equation graph)

Answer 54

If you are evaluating a graph, note the x 10-3 is usually added to the x-axis. This has been done to simplify the presentation of 1/T values, and means the actual values of 1/T have to be divided by 1000 (or multiplied by x10-3) if you need to use them in an equation. Since the Ideal Gas Law Constant, R, has the units J mol-1 K-1 and the slope has the unit of K, the units of Ea end up as J mol-1. Often the Ea is asked for in kiloJoules, so be ready to convert to kJ by dividing by 1000.