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.