We are working in Chapter 5 for the next few weeks. Chapter 5 contains some difficult content and three labs. I am not sure if we will complete all three labs, but we will try. You will have to write up one of those labs, We will go through the schedule for this month and May to figure out when we can get the material covered.
Because this is a lot of material, it is expected that you will work outside of class to keep up with the material. The first part of 5.1 discusses how energy changes are measured and specific heat. The section is 13 pages. We will spend today working on this section. The next day I will see you is Thursday and I plan to cover the second half of 5.1, so it is up to you to make sure you have mastered the content from the first half of 5.1. I will provide practice questions and work packets for you to work on outside of class.
1. 5.1 Discussion and explanation of energy changes.
Lesson Objectives. By the end of this lesson, you should be able to:
1. Explain how heat is a form of energy
2. Describe temperature as a measurement of kinetic energy
3. Explain the conservation of energy in chemical reactions
4. Compare endothermic and exothermic reactions
5. Describe enthalpy changes and how they are expressed under standard conditions.
6. Calculate specific heat problems.
Text: Chapter 5.1 (p. 165-171)
Links: The Physics Classroom The Nature of Energy Measuring Energy Changes
Mission 1: Energy. Some Days Are Better Than Others.
Some things you should know. Temperature and heat are not the same thing.
Heat is energy that results from a temperature change and produces an increase in disorder in how particles behave. Heat increases the average kinetic energy of molecules, which is measured by temperature. The above video discusses the differences between temperature and heat.
Heat contents of a system is called enthalpy A system is a heat reserve. Changes in energy are denoted by "delta H." Delta H is positive when heat is added and negative when heat is released.
Enthalpy is stored in chemical bonds and intermolecular forces as potential energy. When substances react, the difference in the enthalpy between reactions and products produces an enthalpy change that can be observed.
Most chemical reactions are exothermic; they g ive off heat and result in a transfer of energy from system to surroundings. Delta H is negative in exothermic reactions. Endothermic reactions absorb heat and result in a transfer of energy from surroundings to system. Delta H is positive in endothermic reactions.
Standard enthalpy changes are a set of conditions (which should be in your data booklet) are the following: 100 kPa, a concentration of 1 mol/dm3 (1M) for all solutions, and all substances in their standard states. Temperature is usually not a part of these conditions, but is normally included as 298K.
Mission 2: Thermochemical Equations. It's better to read this brief section on page 168 and then we will discuss it as a class. These are shorthand ways of conveying information.
Mission 3: Let's Be Specific About This! Specific heat is the property of a substance which gives heat needed to increase the temperature by 1K. It depends on the number of particles present.
The formula for specific heat is
Homework: Review the material and finish the specific heat problems. Also complete p. 171 #1-6.
Mission 4: Enthalpy Changes. The direction of change is in the direction of lower stored energy. Chemicals change in a way that reduces their enthalpy. It is expected that a reaction will occur if it leads to a reduction in enthalpy. Products in an exothermic reaction are more stable than the reactants, but stability is a relative term. The sign of delta H is a guide for the likely direction of change, but it is not 100% reliable. Endothermic reactions do occur when there is an increase in the disorder of a system; for example, when gases are produced.