Chemistry Chapter: Reaction Rates and Concentration

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Questions and Answers

Which of the following chemical reactions can be sped up by increasing the concentration of reactants?

The electrolysis of water

The decomposition of hydrogen peroxide solution (correct)

The neutralization reaction between hydrochloric acid and sodium hydroxide (correct)

The reaction between solid sodium and chlorine gas

How would you change the volume of the container to produce HCl(g) quicker from the following reaction: H2(g) + Cl2 (g) -> 2HCl (g)?

Change the volume based on the temperature

Keep the volume of the container constant

Decrease the volume of the container (correct)

Increase the volume of the container

Which of the following factors is NOT directly affected by an increase in the concentration of reactants?

Activation energy required for the reaction to occur (correct)

Number of successful collisions

Rate of the reaction

Frequency of collisions between reactant particles

What is the main reason why increasing the concentration of reactants leads to an increase in the rate of a reaction?

The frequency of collisions between reactant particles increases Signup and view all the answers

Which of the following is NOT a correct explanation for the role of concentration in reaction rate?

Higher concentration increases the activation energy, hence speeding up the reaction Signup and view all the answers

How does increasing the pressure of a gaseous system affect the rate of a reaction?

It increases the frequency of collisions between reactant molecules Signup and view all the answers

Which of the following is an example of a reaction that can be speeded up by increasing the concentration of a liquid reactant?

The reaction between hydrochloric acid and sodium bicarbonate Signup and view all the answers

Which of the following statements is an accurate description of the relationship between concentration and reaction rate?

Higher concentration generally results in a faster reaction rate Signup and view all the answers

What is true about an endothermic reaction?

Energy acts as a product. Signup and view all the answers

Which of the following correctly represents the molar enthalpy of decomposition of water?

H2O + energy → H2 + O2 Signup and view all the answers

What characterizes an exothermic reaction?

The change in enthalpy, ∆H, is negative. Signup and view all the answers

How is molar enthalpy used in an exothermic reaction expression?

Reactants → Products + ∆H Signup and view all the answers

For a reaction with a molar enthalpy of -261 kJ/mol, which of the following is true?

The reaction releases energy. Signup and view all the answers

What effect does a catalyst have on the activation energy of a reaction?

It decreases the activation energy for both forward and reverse reactions. Signup and view all the answers

In a reaction mechanism, what is an elementary step?

A single balanced chemical reaction within the overall reaction. Signup and view all the answers

How does the presence of a reaction intermediate affect a multi-step reaction mechanism?

It acts as a product and reactant in different steps. Signup and view all the answers

If a reaction has an uncatalyzed activation energy of 50 kJ/mol for the forward reaction and a reverse reaction enthalpy change of +40 kJ/mol, what is the maximum possible catalyzed activation energy for the reverse reaction?

90 kJ/mol Signup and view all the answers

Which statement about the change in enthalpy (∆H) when a catalyst is added to a reaction is true?

The ∆H remains unchanged regardless of the presence of a catalyst. Signup and view all the answers

What is the correct unit for measuring the rate of change in gas volume?

ml/s Signup and view all the answers

Which of the following macroscopic properties cannot be measured quantitatively?

Change in color Signup and view all the answers

How does increasing the temperature affect the rate of reaction?

Increases kinetic energy and collision frequency Signup and view all the answers

What is the effect of increasing concentration on reaction rates?

Increases the frequency of collisions Signup and view all the answers

Which reaction could be accelerated by increasing the concentration of the solution?

Combustion of CH4 (g) Signup and view all the answers

What is the role of average kinetic energy in determining reaction rates?

It relates to collision intensity and frequency Signup and view all the answers

Which of the following is a metric for measuring the change in color over time?

Colorimeter reading Signup and view all the answers

What primarily influences the rate of a chemical reaction according to the factors mentioned?

Temperature and concentration Signup and view all the answers

How many elementary steps are involved in a reaction if the number of activated complexes is 4?

4 Signup and view all the answers

In an Ep diagram, which step is most likely to be catalyzed?

The slowest step Signup and view all the answers

If a step in a reaction is described as endothermic, what can be inferred?

It absorbs energy Signup and view all the answers

Which piece of information is NOT typically determined from an Ep diagram?

The concentration of reactants Signup and view all the answers

What effect does increasing the concentration of a reactant have on the overall rate of reaction?

It increases the rate Signup and view all the answers

In the context of an Ep diagram, what does RDS stand for?

Rate-Determining Step Signup and view all the answers

In exothermic reactions, how is the change in enthalpy (∆H) characterized?

∆H is negative Signup and view all the answers

What is the primary characteristic of the activated complex in a reaction?

It is an unstable group that quickly breaks apart Signup and view all the answers

When calculating the overall ∆H for a reaction, which calculation method is NOT appropriate?

Considering only the first step's ∆H Signup and view all the answers

Which of the following correctly describes the heat of reaction (∆H) for an endothermic process?

∆H is positive as heat is consumed Signup and view all the answers

What is the primary purpose of sketching an Ep diagram?

To visualize energy changes during a reaction Signup and view all the answers

What does the activation energy (Ea) represent in a chemical reaction?

The minimum energy needed to initiate a reaction Signup and view all the answers

How can endothermic reactions be represented in a chemical equation?

Reactants + energy → products Signup and view all the answers

In the context of potential energy diagrams, which term refers to the heat released or consumed during a reaction?

Heat of reaction Signup and view all the answers

What does a negative ∆H value indicate about a reaction?

Reaction is exothermic Signup and view all the answers

What is indicated by the maximum point on a potential energy diagram?

The activated complex Signup and view all the answers

Study Notes

Chemistry 3202 Introduction

Instructor email: [email protected]
Course materials located on Google Classroom
Seating plan based on last name
Cell phones, earbuds, and bathroom use guidelines
Testing and in-class assignments (ICA's)
Attendance and homework are important
Course description and formula sheet handouts provided

Unit 1: From Kinetics to Equilibrium - Section 1.1 - Skill #1

Four factors affecting reaction rate:
- Surface area
- Temperature
- Concentration
- Catalyst addition

1. Surface Area

Surface area is the exposed matter of a solid substance.
Splits of wood have more surface area compared to a whole piece of wood
A 5.0 g of coal dust has a greater surface area than a 5.0 g lump of coal, reacting faster.
Rate of reaction is directly proportional to the surface area

2. Temperature

Temperature, in Kelvin, is proportional to the kinetic energy of particles.
Doubling the Kelvin temperature doubles the average kinetic energy of particles.
Higher temperatures lead to more frequent and intense particle collisions, increasing the reaction rate.

3. Concentration of Reactants

Important for gases and aqueous solutions, not pure solids (e.g., Zn).
Higher concentration results in more frequent collisions, leading to a faster reaction rate.
Example: Vinegar on fish and chips reacts more effectively than glacial ethanoic acid.
Concentration of HCl solution affects the reaction rate with Zn.

4. Catalyst

A substance that speeds up a chemical reaction without undergoing permanent changes.
The mass of the catalyst remains constant before and after the reaction.
Common catalysts include MnO2 in H2O2 decomposition and Fe in NH3 production
Catalysts provide an alternate pathway with lower activation energy, increasing reaction rates.
Example: Decomposition of hydrogen peroxide to water and oxygen with KI as catalyst.

Core Lab #1 Rates of Reaction - Section 1.1 - Skill 2

Measuring reaction speed involves observing/controlling a measurable factor (e.g., gas volume).
Rate of reaction is calculated as the change in quantity during a specific time period (e.g., ml/s).

Core Lab #1 Cont' Investigating the Rates of Reaction

Maintaining constant conditions (except for the manipulated variable) is crucial for accurate experiments.
Manipulable factors in reaction rate experiments include: H2SO4 concentration, surface area of marble chips, and temperature of flask.

1.2 Collision Theory - Kinetic Molecular Theory

All matter forms atoms, ions, or molecules, constantly moving randomly and colliding with each other/container walls.
Kinetic energy (Ek) associated with each collision remains constant.
Temperature of the substance remains consistent
Pressure exists in gases because of this characteristic.

1.2 Collision Theory – Skill #2

Pressure: Gas molecule collisions with the container walls cause pressure. Inflating a balloon, fluid pressure increasing with depth, are examples.
Diffusion: Gas molecules spread out as far as possible. Examples include scents, dyes in water, osmosis in body cells.

1.2 Collision Theory – Skill #3

Essential Conditions for Reaction: Three factors determine collisions leading to reactions
- Reactant particles must collide
- Colliding particles must have proper orientation for reactions
- Sufficient collision intensity/energy needed for bond breaking / formation. (To overcome activation energy)

1.2 Collision Theory - Example

Illustrates successful vs. unsuccessful reactions based on collisions:
- Atom contact
- Correct orientation of atoms for reactions
- Minimum energy requirement for effective reactions.

1. Using Observables to Increase Reaction Rate

Increasing the surface area of a reactant (e.g., breaking up a solid).
Increasing the concentration of a reactant.
Adding a catalyst.
Increasing reaction temperature.

Jeopardy! Question 2

The correct method for safely removing a stuck lid from a bottle of pickles is to run ice-cold water over the lid.

Jeopardy! Question 3

Water molecules move faster at higher temperatures (80°C) compared to lower temperatures (15°C).

1.2 Collision Theory – Skill #4: Macroscopic Properties

Measurable physical properties of chemicals during reactions.
Variables include mass, pH, electrical conductivity, gas pressure, color change, gas volume, and concentration.
Units for reaction rates involve quantities per time.

Example #4

Units for Macrosopic Properties: Mass (grams/sec), Volume (mL/sec), pH (pH/sec), Gas pressure (kPa/sec), Gas volume (mL/sec)
Cannot be measured quantitatively: Concentration and colour change

Section 1.3 Factors Affecting Reaction Rate (Part II) – Skill #1

Temperature:
- Average kinetic energy per particle increases with temperature.
- Particles collide with more energy and greater frequency
- Increased chance of successful collisions with correct orientation, leading to faster reaction
Explanation of milk spoiling faster in a warm place
Temperature increase means particles collide more often and with more energy this more effective collisions increasing reaction rate.

Section 1.3 Factors Affecting Reaction Rate (Part II) – Skill#2: Concentration

Concentration is the number of particles per unit volume.
Higher concentration; greater frequency of collisions and particles are closer this increase chances (and frequency) of successful collisions with enough energy (Ea) and correct orientation - leading to faster reaction rate
Example*
Which reaction could be “sped up” by increasing the concentration? Combustion of CH4 (g), A-B neutralization. NOT a solid.

Section 1.3 Factors Affecting Reaction Rate (Part II) – Skill #3: Pressure

Pressure in a gas relates directly to the concentration of a gas.
Decreasing volume increases pressure, increasing the concentration of reacting particles.
Increasing pressure leads to more frequent collisions and higher chances of successful collisions, leading to a faster reaction rate.
Example: How to speed up the reaction H₂(g) + Cl₂(g) → 2HCl(g) by changing the container volume?
Decreasing the volume to increase pressure in the container.

Section 1.3 Factors Affecting Reaction Rate (Part II) – Skill #4: Surface Area

Surface Area of solids affects reactions.
Smaller particles (larger surface area) have more exposed surface, increasing collision frequency with other particles and, thus, the reaction rate.
Example: Checking your car's rusting areas.

Skill 2: Addition of a Reaction-Specific Catalyst

Catalysts provide an alternative reaction mechanism with lower activation energy.
This reduces the energy threshold for collisions, thereby increasing the chances (and frequency) of successful collisions, leading to faster reaction rates.

Section 1.4 Potential Energy – Skill #1: Endothermic Reactions

Potential energy is defined as the energy due to position.
Reactions that absorb energy are called endothermic.
The change in energy (enthalpy) is calculated by the difference in energy between the products and reactants. Endothermic reactions have a positive ΔH value.
Molar Enthalpy (ΔH) – change in energy for a reaction

2 ways to represent Endothermic reactions

A) Including energy as a reactant.
B) Writing the enthalpy change separately

Skill #2: Exothermic Reactions

Potential energy reactions that release energy called exothermic.
Exothermic reactions have a negative ΔH value.

1.4 Skill #3 Potential Energy Diagrams

Terminology:
- Activation energy: The energy required to start the reaction. Ea(forward) or Ea(reverse)
- Heat of Reaction: (∆H) is the difference of energy between products and reactants
- Endothermic vs Exothermic (energy absorbed vs released)

Hunt the Letters (Potential Energy Diagrams)

Identify specific parts (activation energy, enthalpy, activated complex) of a potential energy diagram for a chemical reaction.

Example #4 (Exothermic Graph): Calculate values

Identify activation energy (Ea forward and Ea reverse), and enthalpy changes (ΔH forward and ΔH reverse) from an exothermic graph.

Extra Practice (Potential Energy Diagram)

Identify features/symbols (e.g., activation energy) on a given graph.
Calculate activation energy for a reverse reaction using heat of reaction and activation energy for forward reaction.

Ex 3: More word Problems (Potential Energy Diagrams)

Sketch an energy profile graph, classify (endo or exo) a reaction based on the energy profile, and determine enthalpy change for a reaction.

Skill #4: Catalysts with Reactions

Catalysts speed up reactions by providing alternate reaction pathways with lower activation energy.
Enthalpy (ΔH) for a reaction remains unchanged during the presence of a catalyst.

Example #2: Catalysts

Identify minimum catalyzed activation energy from a given uncatalyzed reaction and heat of reaction.
Find maximum possible catalyzed activation energy for the reverse reaction.

Section 1.5 Reaction Mechanisms – Skill #1: Overall Reaction

Most reaction occur in multiple small reaction steps, called elementary steps.
Overall reaction is the sum of elementary steps.
Reaction intermediates are unstable substances formed and consumed during intermediate steps of multiple reactions. They are not part of the overall reaction equation.

Examples for Hess's Law

Calculate overall enthalpy change for reaction using values from elementary steps.
Find missing enthalpy change for elementary steps using Hess's law.

Skill #2: Identifying Reaction Intermediates as catalysts

Identify reaction intermediate from multiple elementary reaction steps, and overall Reaction.

Catalysts

Catalysts are substances that speed up a chemical reaction without being consumed in the process.
Catalysts provide alternate reaction pathways that have lower activation energies compared to uncatalyzed reactions.
Reaction mechanism for catalyzed reaction is different from uncatalyzed.

Example: Catalyst

Identifies the overall reaction, reaction intermediates, and the catalyst given multiple elementary steps.

Rate Determining Step

The slowest step in a multi-step reaction mechanism determines the overall reaction rate.
Each elementary reaction step has its own specific activation energy. (The Ea of the RDS is the highest)

R.D.S Example

Determine overall reaction, reaction intermediate, catalyst, and rate-determining step for a given set of chemical equations.

1.5 Skill #2 Continued (Level 3 question)

Determining missing reactants and products/intermediate from the overall reaction and provided elementary steps.
Identifying the substances and what shouldn´t be in the overall reaction.

Example (Rates & Reaction Mechanisms)

Determine missing elementary step and its enthalpy from given information of Overall Reaction, R.I, and RDS, if any.
Determine which substances (reactants or products) should increase concentration, increasing the overall reaction rate.

1.5 Skill #3 Ep diagrams for multi-step reactions

Relationship between # of elementary steps vs # of activated complexes (peaks) on an energy profile graph.
Identify a) Rate-determining step, b) Enthalpy change (∆H) for each reaction step based on a given energy profile graph. c) Classify reaction (endothermic or exothermic) based on energy changes.
Find which step is most likely to be catalyzed in the given energy profile graph.

Style #2 (Energy Profile Graph construction):

Construct a potential energy diagram from a provided reaction mechanism.
Determine the reaction intermediate(s), catalyst(s), rate-determining step(s)
Calculate the missing enthalpy changes (∆H) for each step in the mechanism.

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Test your understanding of how concentration affects reaction rates with this quiz. Explore concepts such as the relationship between reactant concentration and the speed of chemical reactions, as well as factors influencing rate changes. Perfect for students studying the principles of chemical kinetics.