Heat and Rate of Reaction: Conduction

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Objective

SWBAT will be able to describe how heat transfer by way of conduction effects the rate of a chemical reaction.

Big Idea

Help your students visualize how heat effects the rate of a chemical reaction using seltzer tablets and water.

Getting Started

Students observe the reaction rate of Alka-Seltzer tablets dissolving in water of different temperatures to illustrate that the molecules of water and particles of the tablets move faster when water is heated as well as provide a visual example of how heat is the motion of molecules.  

STEM

Built into these lessons are activities where students use technology to collect data, learn and apply the science of energy transfer, use math skills in design and construction of a model incorporating STEM principles. 

Science and Engineering Practices

Planning and Carrying Out Investigations

Planning and carrying out investigations in 6-8 builds on K-5 experiences and progresses to include investigations that use multiple variables and provide evidence to support explanations or solutions.

  • Plan an investigation individually and collaboratively, and in the design: identify independent and dependent variables and controls, what tools are needed to do the gathering, how measurements will be recorded, and how many data are needed to support a claim.  

Constructing Explanations and Designing Solutions

Constructing explanations and designing solutions in 6–8 builds on K–5 experiences and progresses to include constructing explanations and designing solutions supported by multiple sources of evidence consistent with scientific ideas, principles, and theories.

  • Apply scientific ideas or principles to design, construct, and/or test a design of an object, tool, process or system.

Engage in Argument From Evidence

Engaging in argument from evidence in 6–8 builds on K–5 experiences and progresses to constructing a convincing argument that supports or refutes claims for either explanations or solutions about the natural and designed world(s).

  • Construct, use, and/or present an oral and written argument supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon or a solution to a problem. 
Disciplinary Core Ideas

PS3.A Definitions of Energy
PS3.B Conservation of Energy and Energy Transfer
ETS1.A Defining and Delimiting an Engineering Problem
ETS1.B Developing Possible Solutions
Potential Misconceptions (from Children's Misconceptions about Science)
  1. Heat is a substance.
  2. Heat is not energy.
  3. Temperature is a property of a particular material or object. (Metal is naturally cooler than plastic).
  4. The temperature of an object depends on its size.
  5. Heat and cold are different, rather than being opposite ends of a continuum.
  6. Objects of different temperature that are in contact with each other, or in contact with air at different temperature, do not necessarily move toward the same temperature.
  7. The kinetic theory does not really explain heat transfer. (It is recited but not believed).
  8. Objects that readily become warm (conductors of heat) do not readily become cold.

Elicit

10 minutes

Before moving on in the lesson progression, stop and discuss with the class their thoughts on conduction after having completed the metal washer and water investigation.  

Share the following scenario with the class:

Which cup’s temperature will change the most?

Introduction: We have all had the experience of touching a hot frying pan handle or felt the cold chill by touching an icicle.  In both of these examples heat is being transferred from one object to the other. Heat transferred by contact between objects of different temperatures is called conduction.  Energy will be transferred from the hotter object or heat source to the cooler object or heat sink.  Since everything has stored energy there is always energy being transferred through objects when they touch.  Even in equilibrium, when the temperature of those objects is the same energy is exchanged.  Write a hypothesis to predict which cup’s temperature will change the most.  Be sure to explain your reasoning.

 

Show the image and give them time to think, brainstorm their ideas then ask them to turn and share their thoughts with a partner. This model may be difficult for students. I address this, and provide examples of student work, in my reflection.

 

Bring the class back together and call on a few students to share their thoughts.

Use this time to add the word conduction to your class word wall and student science journals.  If you are looking for some best practices with word walls, here is an excellent article from NSTA's Science Scope (11/10) on creating interactive word walls. 

Engage

10 minutes

In this lab students will explore how heat and surface area effects the rate of reaction building on prior knowledge of heat transfer by means of conduction. 

Students measure rate of reaction and the temperature change resulting from the endothermic reaction of Alka-Seltzer tablets with water. They will also observe physical evidence of the reaction and determine if the temperature of the water causes a difference in the rate of reaction.

Alka-Seltzer neutralizes the acids in the gastrointestinal system by combining with hydrochloric acid in the stomach. It reduces gastric acidity and thereby reduces the activity of pepsin. Alka-Seltzer contains both citric acid (C6H8O7-- anhydrous) and sodium bicarbonate (baking soda -- NaHCO3). When you drop a tablet in water, the acid and the baking soda react, producing the famous fizz. You can think of an Alka-Seltzer tablet as compressed baking soda mixed with a little aspirin.

Endothermic reactions absorb heat energy. Because of this, the surroundings lose heat to the reaction and a temperature decrease can be recorded.

Explain to the students what they will be doing in this lab by going over the lab first.

I use this time to help them construct their hypothesis. Since we have just completed activities exploring conduction and defining both heat and temperature , I found it good practice to bring in these understandings and apply them to the lab. Ask students to imagine the movement of the hot and cold water molecules. Have them tell you again what heat and temperature are defined as then ask them to think about how the speed of these molecules might effect the time it takes for a chemical reaction to run. 

Give them time to share out their ideas and push their thinking by asking them to explain their ideas out loud. Once you have a sense that they "get it" ask them to construct a hypotheses grounded in this same science reasoning.

"I think the that at the hot water will cause the reaction to happen faster because the molecules are moving at a faster average rate and will ......" etc. 

I ask my students to read out their predictions and we edit them together. This kind of verbal/visible editing helps every student with their ideas. 

Explore

30 minutes

Purpose:

Students will measure the rate of reaction and the temperature change resulting from the endothermic reaction of Alka-Seltzer tablets with water. They will also observe physical evidence of the reaction and determine if the temperature of the water causes a difference in the rate of reaction.

In part one the observe the effect of temperature on the rate of reaction and in part two they measure the rate of reaction when the surface area is changed. 

Equipment: 

  • Thermometers
  • (2) Alka-Seltzer tablets
  • 250-mL beakers
  • Graduated cylinder
  • balance
  • spaulta
  • Hot water
  • Cold water
  • mortar/pestle (optional)

The student lab sheet (Alka-Seltzer Lab) is included here in the resources.

Teacher Hints:

  1. Try various water temperatures and see if the observed temperature changes are linear or exponential.
  2. Use two Alka-Seltzer tablets and determine what the temperature extremes are and the rate of reaction. How do your results compare to using a single tablet at the equivalent water temperature?

    Explain to your students that in order for a chemical reaction to occur, the reactants (particles, atoms or ions) must physically come into contact with one another. Any variable that increases the frequency of these encounters will increase the rate at which products are formed. The rate of a chemical reaction can be increased by increasing the temperature of these reactants.

Explain

10 minutes

This lab can be completed over two days if needed. On the second day I like to have students report out their data and then we discuss the results. Focus your questions here on tying together heat transfer via conduction and the rate of reaction. 

Ask students to address any experimental errors they may have encountered along with any questions they generate. I like to make sure that they end this lesson with a good sense that students can explain heat transfer by way of conduction.

Here is a video of two of my students sharing their understanding: