How Fast? The Rate of Chemical Change (DP IB Chemistry)

Colorimetry is a technique that measures the amount of light that passes through a solution to determine the concentration of a colored species.

Colorimetry cannot be used to monitor the formation of colored precipitates as the light will be scattered or blocked by the precipitate.

Hydrogen gas is not suitable for measuring reaction rates using mass loss due to its low molecular mass, which means that the mass loss is negligible.

A gas syringe is used to trap and measure the volume of gas produced over time in a reaction.

Quenching is the process of deliberately stopping a reaction and taking samples at regular intervals to allow the concentration to be determined by titration.

Conductivity can be used to measure the rate of a reaction by monitoring changes in the electrical conductivity of the reaction mixture over time, as the concentration of ions changes.

A clock reaction is a non-continuous method for measuring reaction rates in which the time taken to reach a fixed point is measured.

A graph to show how light intensity changes over time, as a coloured solution reacts to form a colourless solution is:

True.

Gas loss is an appropriate method to measure the rate of reaction for the reaction of a metal carbonate with acid because the molecular mass of the carbon dioxide produced will give a significant / appropriate change of mass.

A graph to show how the mass of the reaction mixture changes over time, as a metal carbonate reacts with acid is:

True.

The volume of gas produced is an appropriate method to measure the rate of reaction for the reaction of a metal with acid.

A graph to show how the volume of gas produced changes over time, as a metal reacts with an acid is:

Collision theory explains how chemical reactions occur based on collisions between reactant particles.

According to collision theory, the four factors that influence the rate of a chemical reaction according to collision theory are:

• Collision frequency,

• Collision energy,

• Activation energy,

• Collision geometry.

Collision frequency is the number of collisions between particles per unit time in a system.

Collision energy is the combined energy of two colliding particles.

False.

Not all collisions between reactant particles result in a chemical reaction. Most collisions are unsuccessful.

An unsuccessful collision is when particles collide in the wrong orientation or when they don't have enough energy and bounce off each other without causing a chemical reaction.

A successful collision is where the particles collide in the correct orientation and with sufficient energy for a chemical reaction to occur.

False.

Collision geometry becomes increasingly important in large complex biomolecules such as proteins and carbohydrates.

The rate of reaction depends on the number of successful collisions that happen per unit time.

Three ways that the collision frequency of a given system can be changed include:

• Changing the concentration of the reactants

• Changing the total pressure

• Changing the temperature

• Changing the surface area of the reacting particles

The activation energy of a chemical reaction can be changed by the addition of a catalyst.

False.

Most collisions do not result in a reaction because they do not reach the activation energy / have energy greater than or equal to the activation energy.

True.

Increasing the concentration of a solution will increase the number of reactant particles in a given volume, allowing more frequent and successful collisions per second.

Increasing temperature increases the kinetic energy of the particles, leading to more frequent and successful collisions with energy greater than the activation energy.

Increasing the surface area of a solid reactant increases the rate of reaction because more surface area is exposed to the other reactant, producing a higher number of collisions per second.

The number of collisions is proportional to the number of particles present.

The five factors affecting the rate of reaction are:

• Concentration,

• Pressure,

• Temperature,

• Surface area,

• The use of catalysts.

False.

An increase in pressure affects reactions involving gases, having the same effect as an increased concentration of solutions.

A catalyst is a substance that provides the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalyzed reaction, increasing the rate of reaction.

False.

A catalyst does not undergo permanent chemical change and is chemically unchanged at the end of the reaction.

Surface area can be increased by decreasing the size of the reactant, e.g. from large pieces to a fine powder.

Increasing temperature affects reaction rate by:

• Making particles move faster, leading to more frequent collisions,

• Increasing the proportion of particles with activation energy or more.

Catalysts reduce the environmental impact of industrial processes by reducing energy requirements, reducing waste products, and increasing the selectivity of processes.

The symbol for activation energy is E_a.

Activation energy is the initial increase in energy, from the reactants to the peak of the curve.

Activation energy (E_a) is the minimum amount of energy that reactant particles need to overcome for a reaction to take place.

In exothermic reactions, the reactants are higher in energy than the products.

If a reversible reaction is exothermic in the forward direction, the activation energy of the reverse reaction is:

E_a (reverse) = ∆H + E_a (forward)

False.

The activation energy can be higher or lower than the enthalpy change of the reaction, depending on whether the reaction is exothermic or endothermic.

If a reversible reaction is endothermic in the forward direction, the activation of the reverse reaction is:

E_a (reverse) = E_a (forward) - ∆H

True.

In endothermic reactions, the reactants are lower in energy than the products.

Energy profiles are graphical representations of the relative energies of the reactants and products in chemical reactions.

An energy profile shows that a reaction is exothermic because the energy of the products is lower than the energy of the reactants.

True.

The difference in height between the energy of reactants and products on an energy profile represents the overall enthalpy change of a reaction.

An energy profile shows that a reaction is endothermic because the the energy of the products is higher than the energy of the reactants.

The energy of the reactants and products is displayed on the y-axis of an energy profile.

The progress of the reaction is shown on the x-axis of an energy profile.

A downwards arrow on an energy profile indicates an exothermic reaction.

An upwards arrow on an energy level diagram indicates an endothermic reaction.

False.

Catalysts remain chemically unaltered by the end of the reaction.

The two types of catalysts are homogeneous catalysts and heterogeneous catalysts.

A homogeneous catalyst is a catalyst that is in the same phase as the reactants.

A heterogeneous catalyst is a catalyst that is in a different phase to the reactants.

Enzymes are biological catalysts that control many biochemical reactions within cells.

False.

Transition metals are often used as catalysts due to their ability to form more than one stable oxidation state.

Catalysts lower the activation energy of a reaction by providing an alternative reaction pathway.

The energy profile diagram to show the effect of a catalyst on an exothermic chemical reaction is:

False.

Catalysts do not change the overall enthalpy change of a reaction; they only lower the activation energy.

The energy profile diagram to show the effect of a catalyst on an endothermic chemical reaction is:

A Maxwell-Boltzmann distribution curve is a graph that shows the distribution of energies of particles in a sample at a certain temperature.

The peak of a Maxwell-Boltzmann distribution curve represents the most probable energy of a particle, sometimes written as EMP or E_MP.

Increasing temperature causes the Maxwell-Boltzmann distribution curve to flatten and the peak to shift to the right.

True.

An increase in temperature results in a higher proportion of particles with energy greater than the activation energy.

A catalyst does not change the Maxwell-Boltzmann distribution curve.

It lowers the activation energy, resulting in a greater proportion of molecules having sufficient energy for a successful collision.

The shaded area represents the number of particles with energy greater than the activation energy.

False.

The Maxwell-Boltzmann distribution curve must start at the origin and it approaches, but never touches the x-axis.

An increase in temperature increases the rate of reaction by:

1. Causing more effective collisions as particles have more kinetic energy

2. Increasing the proportion of molecules with (kinetic) energy greater than the activation energy.

A Maxwell-Boltzmann distribution for a chemical reaction, including labelled axes, most probable energy, E_mp, and activation energy, E_a, is:

In the Maxwell-Boltzmann distribution curve with a catalyst, the shaded area represents the extra particles which have enough energy to react when a catalyst is present compared to without a catalyst.

The rate equation is an expression that shows how the rate of a reaction depends on the concentration of reactants and a rate constant.

The rate constant (k) is a proportionality constant that links the rate of reaction to reactant concentrations.

False.

The rate equation can only be determined experimentally, not from the balanced chemical equation.

The overall order of a reaction is the sum of the powers of the reactant concentrations in the rate equation.

Zero order means that the rate of reaction is independent of the concentration of a particular reactant. So, changing the concentration will have no effect on the rate of reaction.

First order is where the rate of reaction is directly proportional to the concentration of a single reactant. So, doubling the concentration will have double the rate of reaction.

Second order is where the rate of reaction is directly proportional to the square of the concentration of a single reactant. So, doubling the concentration will have increase the rate of reaction by a factor of 4.

The rate of reaction is typically measured by:

• Decrease in the concentration of a reactant over time

• Increase in the concentration of a product over time.

The units for the rate of reaction are mol dm⁻³ s⁻¹.

The rate equation for a reaction that is first order with respect to [A] and zero order with respect to [B] is:

Rate = k [A]

False.

The rate equation only includes reactants that affect the rate of reaction, which may not be all reactants from the balanced chemical equation.

Concentration change from 0.15 mol dm^-3 to 0.30 mol dm^-3

• Concentration doubles

Rate of reaction change from 1.2 x 10^-2 mol dm^-3 s^-1 to 4.8 x 10^-2 mol dm^-3 s^-1

• Rate of reaction increases by a factor of 4

Therefore, A is second order.

The overall order of reaction is 2 + 1 = 3.

Rate = k [A]² [B]

If the concentration of both chemicals is doubled, the rate of reaction increases by a factor of 8.

False.

3A + 2B → C + 2D

The rate equation cannot be taken directly from the balanced equation.

The rate equation for a reaction that is zero order with respect to [A] and second order with respect to [B] is:

Rate = k [B]²

False.

Products and catalysts may feature in rate equations.

Intermediates do not feature in rate equations.

Rate = k [A] [B]

If the concentration of both chemicals is halved, the rate of reaction decreases by a factor of 4.

A change in [B] that does not result in an increase/decrease in the rate of reaction means that the B is zero order.

The order of reaction with respect to a reactant is the power to which the concentration of that reactant is raised in the rate equation.

A concentration-time graph is a graph showing how the concentration of a reactant or product changes over time during a reaction.

A horizontal line on a rate-concentration graph indicates a zero-order reaction with respect to that reactant.

True.

A first-order reaction shows a straight diagonal line on a rate-concentration graph.

A second-order reaction shows a curved line on a rate-concentration graph.

A rate-concentration graph is a graph showing how the rate of a reaction changes with the concentration of a reactant.

The shape of a concentration-time graph indicates the order of the reaction:

• A straight line for zero-order

• A shallow curve for first-order

• A steep curve for second-order.

The gradient of a rate-concentration graph can provide information about the order of the reaction with respect to the reactant being studied.

Reaction orders can be determined experimentally by measuring initial rates at different reactant concentrations and analysing the relationship between rate and concentration.

A first order rate-concentration graph is:

A second order concentration-time graph is:

A zero order concentration-time graph is:

The rate constant, k, is a proportionality constant in the rate equation that relates the rate of reaction to reactant concentrations.

True.

The rate constant remains constant if only the concentration of reactants is changed.

The value of the rate constant can be changed by altering the temperature or using a catalyst.

Increasing temperature increases the value of the rate constant.

The rate constant is directly proportional to the rate of reaction.

Activation energy is the minimum energy required for a reaction to occur, and it affects the value of the rate constant.

True.

The units of the rate constant depend on the overall order of the reaction.

The units of the rate constant for a zero-order reaction are mol dm⁻³ s⁻¹.

The units of the rate constant for a first-order reaction are s⁻¹.

The units of the rate constant for a second-order reaction are mol⁻¹ dm³ s⁻¹.

A reaction mechanism is the sequence of elementary steps that describe how a chemical reaction occurs at the molecular level.

An elementary step is a single, simple step in a reaction mechanism that involves a small number of particles.

An intermediate is a species produced in one elementary step and consumed in a subsequent step of a reaction mechanism.

True.

The sum of elementary steps must equal the overall reaction equation.

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

The rate-determining step determines which reactants appear in the rate equation and their respective orders.

False.

Chemical kinetics can suggest or disprove a reaction mechanism, but it cannot prove one.

To predict a possible reaction mechanism, you need:

• The overall reaction equation

• The experimentally determined rate equation.

Step 1:   NO₂ + NO₂ → NO₃ + NO

Step 2:   NO₃ + CO → NO₂ + CO₂ 

The overall reaction equation, using the proposed elementary steps, is:

NO₂ + CO → NO + CO₂

True.

The proposed elementary step for a one-step reaction mechanism is the same as the overall reaction equation.

True.

2NO₂ + F₂ → 2NO₂F Rate = k [NO₂] [F₂]

A possible mechanism for the above reaction could include the following elementary steps:

NO₂ + F₂ → NO₂F + F

NO₂ + F → NO₂F

The elementary steps combine, losing F on both sides, to give the overall equation.

2N₂O₅ + O₂ → 4NO₂ + O₂ Rate = k [N₂O₅]

Step 1: N₂O₅ → 2NO₂ + O

Step 2: N₂O₅ + O → 2NO₂ + O₂

Step 1 is the rate-determining step because it is the elementary step containing the species and stoichiometry that match the rate equation.

A transition state is an unstable, high-energy state formed temporarily when reacting molecules collide, corresponding to the activation energy.

False.

The transition state always has higher energy than both reactants and products.

The rate-determining step is represented by the highest activation energy barrier on an energy profile diagram for a multi-step reaction.

False.

In a multi-step reaction, the first elementary step does not always have the highest activation energy.

The rate-determining step has the highest activation energy.

The overall shape of an energy profile diagram can indicate whether a reaction is exothermic or endothermic.

True.

The activation energy is always positive on an energy profile diagram, representing the energy barrier that must be overcome for the reaction to proceed.

Molecularity is the number of reacting particles taking part in an elementary step of a reaction.

In terms of molecularity, the three classes of chemical reaction are:

• Unimolecular

• Bimolecular

• Termolecular.

A unimolecular reaction involves one reactant particle in an elementary step, while a bimolecular reaction involves two reactant particles.

Three reactants are involved in a termolecular elementary step.

2N₂O₅ + O₂ → 4NO₂ + O₂ Rate = k [N₂O₅]

Step 1: N₂O₅ → 2NO₂ + O

Step 2: N₂O₅ + O → 2NO₂ + O₂

Step 1 is unimolecular.

2N₂O₅ + O₂ → 4NO₂ + O₂ Rate = k [N₂O₅]

Step 1: N₂O₅ → 2NO₂ + O

Step 2: N₂O₅ + O → 2NO₂ + O₂

Step 2 is bimolecular, because there are two reactants in elementary step 2.

Termolecular elementary steps are not common in reaction mechanisms because it is unlikely that three different reactants will collide in the correct orientations and with sufficient energies to react.

In the Arrhenius equation, the terms k, A, E_a, R and T represent:

• k is the rate constant

• A is the Arrhenius factor

• E_a is the activation energy

• R is the gas constant

• T is the temperature in Kelvin.

The Arrhenius factor, A, represents the frequency of collisions with proper orientations for a reaction to occur.

True.

The activation energy, E_a, in the Arrhenius equation is always constant for a given reaction.

According to the Arrhenius equation, increasing temperature increases the rate constant.

Increasing the activation energy decreases the rate constant, according to the Arrhenius equation.

True.

The natural logarithm form of the Arrhenius equation, ln(k) = ln(A) - (E_a/RT), is used to simplify calculations and for analysing graphs.

When comparing ln(k) = ln(A) - (E_a/RT) to y = mx + c:

• ln(k) = y

• -E_a/R = m

• 1/T = x

• ln(A) = c

To calculate the activation energy using the Arrhenius equation, you need the rate constants at different temperatures.

The Arrhenius factor, A, can be determined experimentally by plotting ln(k) against 1/T and finding the y-intercept, which equals ln(A).

A graph of ln(k) against 1/T is used to determine activation energy using the Arrhenius equation.

False.

The slope of the Arrhenius plot, ln(k) vs 1/T, is equal to -E_a/R, where E_a is the activation energy and R is the gas constant.

The activation energy is calculated by multiplying the negative of the gradient of the Arrhenius plot by the gas constant R.

The y-intercept of the Arrhenius plot represents ln(A), where A is the Arrhenius factor.

An Arrhenius plot is a graph of ln(k) against 1/T that is used to determine the activation energy and Arrhenius factor of a reaction.

True.

A linear Arrhenius plot indicates that the reaction follows Arrhenius behavior.

Temperature should be in Kelvin (K) when constructing an Arrhenius plot.

The Arrhenius factor, A, can be calculated by taking the exponential of the y-intercept of the Arrhenius plot.

True.

The activation energy calculated from an Arrhenius plot is typically expressed in kJ/mol.

To construct an Arrhenius plot, you need rate constant values measured at different temperatures.