Driving Force of Reactions Teacher Guide

Unit

Driving Forces of Reactions

Subject

Chemistry

Grade Level

HS

Activity Name(s)

Chemical Reaction Rates

Making and Breaking Bonds

Reaction Rates

Being Prepared

Only a computer is required for "Chemical Reaction Rates" and "Making and Breaking Bonds".

Materials required for Reaction Rates:

• Computer
• Temperature sensor
• 3 - Alka-Seltzer™ tablets
• 3 - clear plastic cups (about 300 ml)
• Warm tap water
• Cool tap water
• Ice water

Since materials needed for all these activities need computers, so having students 1-2 per computer is ideal. Plan significantly in advance of these activities to ensure you have access to computers (state testing requisitions, other teacher use, etc.)

Liquids can be placed lower than computers (first activity) so that if there is any accidental spill, then it would not spoil the computers.

Getting Started

The controls for the simulations are not always intuitive to new users. However, they always have very descriptive assistance provided in the text preceding the simulations. Instruct students to answer the questions before moving on with the next section. Encourage frustrated students to review the written instructions before attempting to interact with the model further.

There is no equipment required beyond a computer with sufficient power to run the simulations for the first and second activities.Try and place water lower than the computer so that if there is accidental spill, it will not affect the computers. The third activity would need a temperature sensor which can be hooked to the computer directly.

Suggested Timeline

The three activities that are well connected and together can fit into either two 45-50 minute period or a 90 minute block. The three activities could all be delivered separately also as about half hour period for each activity . Rest of the period could be used to involve students in a group discussion to gaze the understanding of each activity and the underlying concept.

Thinking about the Discovery Questions

The activities in this unit are motivated by the discovery questions:

• What are the driving forces of a reaction?
• What causes a bond to make or break?
• Will a chemical reaction ever reach equilibrium? How can reaction rate be changed?

These questions can be answered using the concepts of entropy and free energy. These activities, on the other hand, use the approach of potential energy diagrams, making and breaking of bonds and rates of reactions.

Misconceptions

• They do not think of energy changes or energy requirements when they think about a driving force of a reaction. For a student, if one mixes two chemicals there would be a reaction.
• The thermal energy of an object depends on the number and motion energy of the molecules that make up the object.
• The thermal energy of an object depends on the speed and number of molecules that make up the object.
• Stoichiometry is what a student usually has trouble with. For them, greater the amount of reactants, greater would be the amount of products. They would think that all the reactants would combine to give products.
• Students believe a chemical change is always irreversible.

Learning Objectives

• NGSS
  • Performance Expectations
    • HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
  • Disciplinary Core Ideas
    • PS1.B: Chemical Reactions
      • Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy. (HS-PS1-4),(HS-PS1-5)
  • Practices
    • Developing and using models
      • Develop or modify a model - based on evidence - to match what happens if a variable or component of a system is changed.
  • Crosscutting Concepts
    • Stability and Change
      • Students explain stability and change in natural or designed systems by examining changes over time, and considering forces at different scales, including the atomic scale. Students learn changes in one part of a system might cause large changes in another part, systems in dynamic equilibrium are stable due to a balance of feedback mechanisms, and stability might be disturbed by either sudden events or gradual changes that accumulate over time.

Discussion: Setting the Stage

• What affects the speed of a chemical reaction?

  Using prior knowledge, students might think about only temperature and/or catalyst

• Can you think of chemical reactions that takes longer to occur?

  Rusting of iron, Digestion of food etc.

Discussion: Formative Questions

• How does concentration(molecules per volume of container)affect reaction rate?

  Increase in concentration would increase the reaction rate.

• How does the temperature affect the rate of the chemical reaction?

  Increase in temperature speeds up a reaction. It can be observed by the amount of time it takes for reaction to occur

• How does container size (volume) affect reaction rate?

  Increase in container size would decrease the reaction rate as the number of molecules colliding would decrease.

• How did changing the temperature for each container affect the reaction?

  Increase in temperature would increase the reaction rate.

• What is the influence of temperature on the breaking of bonds?

  Increase in temperature would favor breaking of bonds.

• Describe the dissociation energy for molecules that can't be broken at high temperatures. Include in your description information about the graph and what it means.

  The dissociation energy is high for molecules that can't be broken at high temperatures. The length between the low point and the high point in the graph which is labelled as dissociation energy ( can be called activation energy also) is way longer for molecules that can't be broken even at high temperature.

Discussion: Wrapping Up

• What are the various factors that affect speed of a reaction?

  There are several factors that affect the speed of a reaction: nature of the reactants, particle size of the reactants, concentration of the reactants, volume of the container, pressure of gaseous reactants, temperature, catalyst

Additional Background

Not every reaction is spontaneous. Most of them have to be activated to make them cross the activation barrier. Not every reaction results in release of energy. If the energy released during bond formation is more than the energy required for bond formation then the overall reaction would end up with release of energy.

Analysis

Chemical Reaction Rates

1. Take a look at the "chemical potential energy" graph of the normal reaction. You should see that if you start with 2A, it takes very little energy to bond together. However, if you start on the right side of the graph with A₂, then you have to climb up a steep energy hill in order to go back to being 2A. What did you see happen when you ran the normal model, and why does this graph help to explain your observations?

   The graph shows the amount of energy required to cross the barrier(peak) to form the products. The graphs shows that 2A to A₂ needs less energy than A₂ to 2A.

2. Explain the connection between concentration, collisions, and reaction rate.

   Increase in concentration would increase the number of collisions and hence the increase in reaction rate.

3. What affects the equilibrium of a system?

   Change in concentration, volume, pressure and temperature (LeChatlier's Principle)

Making and Breaking Bonds

1. Why does an increase in temperature make it easier for certain molecules to break bonds?

   Increase in temperature decreases the dissociation energy shown in graph which makes it easier for the bonds to break and to form.

2. After a bond is made in a system where energy is not conserved, would one expect the molecule to move slower or faster than the individual atoms? Explain your reasoning.

   The molecules would move slower when they are bonded. This happens because the number of collisions decrease when the atoms are bonded.

3. If a bond is stronger, would one expect to have the molecule move slower or faster after bonding than if the molecule’s bond is weak? Explain your reasoning.

   If the bond is stronger the molecule would move slower because it would decrease the speed of the individual atoms when bonded.

Reaction Rates

1. Make a table of reaction time versus temperature. What is the relationship?

   Reaction time decreases as temperature decreases.

2. Compare the reaction times to your prediction. Was it roughly what you expected?

   Answers will vary depending on the students prediction.

Further Investigation

This basic concept of driving forces of a reaction could be easily extended into advanced learning about the concept where we can talk about enthalpy, entropy and Free energy. When students have the understanding of potential energy diagram, we can easily incorporate the concept of enthalpy. After students work on making and breaking bonds, entropy can be easily understood. Once students understand factors affecting rate of a reaction, spontaneity of a reaction and free energy can be taught.

• What is a limiting reactant?

  The reactant that is used up in a reaction and it controls the amount of products formed.