The Arizona STEM Acceleration Project

Photochemical Reactions

Photochemical Reactions

An 11th grade STEM lesson

Donna Brunjes

12/30/23

Notes for teachers

• Context: This lesson takes place in a classroom over a two-day period for one hour each day
• This lesson explores chemical reactions which require light as activation energy
• Students should have prior basic knowledge about photon emission, catalysts, and activation energy in chemical reactions
• Students work individually on their assignments (Lesson #1)
• Students volunteer to read aloud summary of photochemical reaction laws and definitions (Lesson #1); students answer questions
• Students work in small groups of 2-4 students modeling and summarizing photochemical reactions (Lesson #2)
• Student groups present their model and explanations to their class (Lesson #2)

List of Materials:

• Assignment on how molecules absorb light, the types of light absorbed and how the temporary excited states of molecules can change the physical and chemical properties of molecules
• Written summary (for student distribution) of the two laws governing photochemical reactions - Grotthuss-Draper Law and Stark-Einstein Law - and information on new terms like “photoexcitation,” “transitory,” “quantum yield, ” and “chain reactions”
• Examples of how this technology is used in everyday applications in diverse fields
• Color-printed electromagnetic radiation charts (for student distribution)
• Demonstration of photochemical reaction using thionin, iron (II) sulfate, sulfuric acid solution and aluminum foil
• Demonstration of photochemical reaction using fluorescent dyes mixed in tonic water and exposed to black light
• Black light and white light fixtures for teacher demonstration
• Ball and stick models for student groups to model and present assigned photochemical reactions
• Access to laptop or desktop computers for online research
• Students need a copy of this recording sheet: Click here

​

Arizona Science Standards

Science Standards:

​

HS+C.P1U1.2: Obtain, evaluate, and communicate the qualitative evidence supporting claims about how atoms absorb and emit energy in the form of electromagnetic radiation.

​

HS.P1U1.2: Develop and use models for the transfer or sharing of electrons to predict the formation of ions, molecules, and compounds in both natural and synthetic processes.

​

HS+C.P1U1.4: Develop and use models to predict and explain forces within and between molecules.

​

​

​

• ​
• ​

Standards

Arizona Science Standards - Science and Engineering Practices for High School:

• Developing and Using Models
• Analyzing and Interpreting Data
• Using Mathematics and Computational Thinking
• Planning and Carrying Out Investigations
• Obtaining, Evaluating, and Communicating Information
• Constructing Explanations and Designing Solutions
• Engaging in Argument from Evidence
• Asking Questions and Defining Problems

Objectives:

Today we will learn about chemical reactions called “photochemical reactions” that require light as activation energy for the reaction to occur. Chemical compounds absorb light emissions which enable them to form new structures, combine with other molecules, trigger chain reactions, and decompose existing molecules. Photochemical reactions like photosynthesis and the human body’s synthesis of Vitamin D are both examples of life-sustaining photochemical reactions.

​

Photochemical reactions, which involve light to initiate chemical transformations, have a myriad of modern-day applications across various fields. These include photovoltaic cells used in solar light panels to transform sunlight into electricity, photocatalysts such as titanium dioxide (which we have explored earlier) which catalyze chemical reactions in baked goods and candies like Skittles, and optical sensors used in manufacturing to monitor temperature, pressure and chemical compositions of items being produced.

​

​

Objectives (continued):

Tomorrow we will create ball and stick models of various photochemical reactions. These photochemical reactions include: (1) photosynthesis, (2) photography, (3) photochemical smog, and (4) the synthesis of hydrogen chloride. Hydrogen chloride is an important chemical compound used in industrial and commercial applications due to its strong acidic properties and its chemical reactivity.

​

Student groups will present and explain their models, and they will demonstrate how light energy activates the chemical reactions taking place and the outcomes of those reactions.

Agenda

During Lesson #1 (60 minutes):

What are photochemical reactions?

What laws and principles govern how we identify photochemical reactions?

What types of radiation serve as activation energy for photochemical reactions?

What are some examples of photochemical reactions?

​

​

​

During Lesson #2 (60 minutes):

Student groups are assigned different photochemical reactions.

Students create ball and stick models of these reactions.

Students present and explain how light activates their designated photochemical reaction, and, the outcome of their reaction.

Intro/Driving Question/Opening

​

​

What role does light play in photochemical reactions?

What steps take place in a photochemical reaction?

What type of light is typically absorbed in a photochemical reaction?

What are some examples of photochemical reactions?

What new applications for photochemical reactions might be developed in the future?

How is a photochemical reaction different from a chemiluminescence reaction?

​

Hands-on Activity Instructions

During Lesson #1:

​

• Teacher introduces the subject of photochemical reactions and explains the student assignment, reading and group modelling activities to be undertaken
• Students work independently to complete the photochemical assignment which the teacher helps them complete through guided instruction. Teacher reviews and checks student assignments for completion and accuracy
• Teacher asks for student volunteers to read aloud parts of the distributed summary about the laws that govern photochemical reactions and key definitions. Students answer questions about the summary
• Teacher demonstrates a photochemical reaction through the use of thionin, iron (II) sulfate, sulfuric acid solution, aluminum foil and exposure to white light
• Teacher then demonstrates other photochemical reactions using fluorescent dyes, tonic water and exposure to black light
• Teacher explains the types of electromagnetic radiation that are used to activate photochemical reactions

​

During Lesson #2:

​

• Students working in groups of 2-4 students are assigned photochemical reactions which they need to recreate using ball and stick models
• Student groups research details about their photochemical reactions using their laptops
• Student groups complete their ball and stick models and prepare summaries of how light is activating the photochemical reaction modelled and the outcome (products) of that reaction
• Student groups present and share their models and information with other students in the class

​

Hands-on Activity

​

Sample Photochemical Reaction Assignments for Student Groups

​

1. Photosynthesis

​

2) Photography

​

​

2) Photography (continued)

​

3) Photochemical Smog

“Pans,” “Aldehydes” and “Ozone” are classified as secondary pollutants. Students can make ball and stick models of these compounds as well as the primary pollutants of Nitrogen Dioxide (NO₂), Nitric Oxide (NO), Nitric Acid (HNO₃) and hydrocarbons (CH_x or RH).

Hands-on Activity

​

Sample Photochemical Reaction Assignments for Student Groups (continued)

​

3) Photochemical Smog (continued)

4) Hydrogen-Chlorine Chain Reaction

Peroxyacetyl Nitrate (PAN or Pans)

Aldehyde Functional Group

​

In this photochemical reaction, Hydrogen gas (H₂) combines with Chlorine gas (Cl₂) in the presence of sunlight (hv) to make Hydrogen Chloride (HCl).

​

Assessment

Students complete the following:

​

Formative Assessment:

• Students complete an assignment on how molecules absorb light, the types of light absorbed and the transitory excited states of molecules that can change their physical and chemical properties. Teacher checks student assignments for completion and accuracy.

Summative Assessment:

• Students working in groups of 2-4 students complete a modelling activity using ball and stick models of assigned photochemical reactions. Students research additional information on their photochemical reactions online. Students then present their models and findings to their classmates. Teacher grades student presentations.

​

​

Differentiation

Students should understand that photochemical reactions all require light. Light provides the activation energy for photochemical reactions to occur. Light sources for these reactions typically include visible, infrared and ultraviolet sources of electromagnetic radiation.

​

Students should review how electrons in a molecule when exposed to light will absorb that light as photons and transition to high-energy, excited states. Light is emitted when these same electrons return to their ground states and release photons.

Remediation

Extension/Enrichment

For further enrichment, students might explore how light drives different life-sustaining processes carried out in the human body. Examples include the formation of Vitamin D by exposure of one’s skin to sunlight, the photochemical reaction of rhodopsin which enables our eyes to see, and the production of life-saving drugs to prevent diseases like malaria through photochemical processes.

Most students are familiar with photosynthesis and its role in sustaining plants and creating oxygen, but there are other processes students might study to fully understand the extent and beneficial impact of additional photochemical reactions. These include photovoltaics used in solar light panels to generate electricity, and photolithography used in semiconductor manufacturing.