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Chapter 17

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How Fast? The Rate of Chemical Change (SL)

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Study guide

Understanding Reaction Rate

Definition: Change in concentration, mass, or volume of reactant/product per unit time.
Units: mol dm−3s−1 (aqueous conc.), g s−1 (solid mass), cm3s−1 (gas volume).
Rate is fastest at the start (max reactant conc.) and decreases as reactants are consumed.
Instantaneous rate = gradient of tangent to concentration-time curve.
Rate becomes zero when a limiting reactant is used up.

Measuring Reaction Rates

Gas evolution: Gas syringe or inverted graduated cylinder.
Dense gas escape: Monitor mass loss on an electronic balance (use cotton wool plug).
Color change/light absorbance: Colorimetry or spectrophotometry.

Collision Theory Fundamentals

Reactions occur via molecular collisions.
Successful collision: Sufficient energy AND correct orientation.
Activation Energy (Ea): Minimum kinetic energy for a collision to be successful.
Conditions for success: Kinetic Energy (≥ Ea) and correct geometry/orientation.

Factors Affecting Reaction Rate

Concentration (aqueous): Higher conc. -> more particles/volume -> higher collision frequency -> higher rate.
Pressure (gaseous): Higher pressure -> smaller volume -> higher particle conc. -> higher collision frequency -> higher rate.
Surface Area (solid): Larger surface area -> more particles exposed -> higher collision frequency -> higher rate.
Temperature: Higher temp. -> higher average kinetic energy -> particles move faster -> *significantly larger fraction* of particles meet Ea -> higher rate.
Catalyst: Provides alternative pathway with lower Ea -> larger fraction of particles meet Ea -> higher rate.

Maxwell-Boltzmann Distribution

Graph showing kinetic energy distribution of particles at a given temperature.
Area under curve = total number of particles.
Peak = most probable kinetic energy.
Area to the right of Ea = fraction of particles with sufficient energy.
Increasing temperature shifts curve right, flattens peak, increases area beyond Ea.

Role of Catalysts

Increase reaction rate without being consumed.
Lower activation energy (Ea).
Shift Ea line left on Maxwell-Boltzmann graph.
Increase fraction of successful collisions.
Affect forward and reverse reactions equally.
Do not alter overall enthalpy change (ΔH).

Flashcards (21)

Rate of a chemical reaction: The change in the concentration, mass, or volume of a reactant or product per unit of time.
Successful collision: A collision between reactant particles that possesses sufficient energy and proper orientation to result in a chemical reaction.
Activation energy (Ea): The minimum kinetic energy that colliding particles must possess in order for a collision to result in a chemical reaction.
Maxwell–Boltzmann energy distribution curve: A graph that shows the distribution of kinetic energies among particles in a sample of gas or liquid at a specific temperature.
Catalyst: A substance that increases the rate of a chemical reaction without undergoing any permanent chemical change itself.
Rate: how rapidly reactants are consumed or how rapidly products are generated throughout the course of a chemical transformation.
Collision theory: a theoretical model that explains how chemical reactions occur at the molecular level and why different reactions proceed at different rates.
energy threshold: required to overcome electrostatic repulsions between electron clouds, deform existing chemical bonds, and reach the peak of the energy barrier.
Temperature: a measure of the average kinetic energy of the particles in a substance.
kinetic energy: This indicates that a much larger proportion of the particles possesses kinetic energy exceeding the activation energy threshold, which leads to a marked increase in the frequency of successful collisions and a significantly higher reaction rate.
chemical reaction: Collision theory is defined as a theoretical model that explains how chemical reactions occur at the molecular level and why different reactions proceed at different rates.
per unit: Because the particles are packed more closely together, the collision frequency between reactant particles increases, which raises the frequency of successful collisions per unit time and increases the overall reaction rate.
Maxwell Boltzmann: A Maxwell - Boltzmann energy distribution curve is defined as a graph that shows the distribution of kinetic energies among particles in a sample of gas or liquid at a specific temperature.
particles must: Second, the particles must collide with the correct collision geometry or spatial orientation, ensuring that reactive sites line up correctly so that old bonds can break and new bonds can form.
energy barrier: A catalyst lowers the activation energy barrier for both the forward and reverse reactions equally, meaning it accelerates both directions equally and does not alter the overall enthalpy change (ΔH) of the reaction.
energy equal: The area under the curve located to the right of the activation energy threshold (Ea) represents the exact fraction of particles that possess kinetic energy equal to or greater than the activation energy.
colliding particles: Activation energy (Ea) is defined as the minimum kinetic energy that colliding particles must possess in order for a collision to result in a chemical reaction.
total number: Increasing the concentration of reactants in an aqueous solution increases the total number of particles present per unit volume.
light absorbance: For reactions involving a distinct color change or a change in light absorbance, colorimetry or spectrophotometry measures how light absorbance varies over time as the concentration of a colored reactant or product changes.
one another: However, not all collisions result in chemical change; many collisions cause particles to bounce off one another unchanged.
average kinetic: Increasing the temperature of a system increases the average kinetic energy (Ek=21mv2) of its constituent particles, causing them to move at higher average speeds.

Practice questions (30)

What is the rate of a chemical reaction defined as?
What is the standard SI-derived unit for changes in aqueous concentration per second?
Which method can be used to monitor the rate of a reaction that generates a dense gas escaping from an open flask?
The rate of a chemical reaction is typically fastest at the very end of the reaction.
What is the instantaneous rate of reaction at any specific time defined as?
Collision theory explains how chemical reactions occur at the molecular level and why different reactions proceed at different rates.
For a collision to be successful, what two essential conditions must be satisfied simultaneously?
What is activation energy (Ea)?
Increasing the concentration of reactants in an aqueous solution leads to:
How does increasing the pressure of a gaseous system affect the reaction rate?
Subdividing a solid reactant into smaller pieces increases its reaction rate primarily by:
What is the primary reason why an increase in temperature causes a large increase in reaction rate?
What does the total area under a Maxwell–Boltzmann energy distribution curve represent?
When the temperature of a system increases, how does the Maxwell–Boltzmann distribution curve change?
What is a catalyst?
How does a catalyst increase the rate of a chemical reaction?
What are the SI-derived units for changes in solid mass or evolved gas volume per second?
Order the following factors that influence reaction rate from least to most effective in increasing rate, based on the material's emphasis:
The rate of a chemical reaction is not defined as the change in the concentration, mass, or volume of a reactant or product per unit of time.
The rate of a chemical ______ is defined as the change in the concentration, mass, or volume of a reactant or product per unit of time.
Select every statement your material supports about “Particles Must”.
In your own words, define “chemical reaction”.
Rate describes how rapidly reactants are not consumed or how rapidly products are generated throughout the course of a chemical transformation.
Which of these best describes “total number”?
Put these steps in the right order.
In your own words, define “energy equal”.
When a reaction evolves a gaseous product, the rate can be determined continuously by collecting the gas in a gas syringe or an inverted graduated cylinder over water.
Which term matches this description? “Collision theory is defined as a theoretical model that explains how chemical reactions occur at the molecular level and why different reactions proceed at different rates.”
Match each term to its definition.
In your own words, define “total number”.

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