Thermo - The Laws of Thermodynamics (Lesson)
The Laws of Thermodynamics
Introduction
Thermodynamics explains the science behind the relationship between heat, temperature and energy. Comprehensively, thermodynamics deals with the transfer of energy from one place to another and from one form to another. Let’s take a closer look at more real life applications of thermodynamics, so that we can apply them to the specific branch of thermochemistry.
Laws of Thermodynamics
The first segment in this unit of Chemistry Matters looks at the laws of thermodynamics. The students examine heat-related phenomena of chemical thermodynamics using hot and cold packets, illustrating how thermodynamics work in the real world.
Download the note taking guide for Chemistry Matters Unit 8 Segment A. Links to an external site.
Law of Thermodynamics Practice
Things to Know About Thermodynamics Activity
Reactions
In this segment of Chemistry Matters, the students formulate hypotheses for the chemical process that makes the hot and cold packets from segment A. Our host explains the difference between exothermic and endothermic reactions, and our students explore the concept of specific heat capacity by predicting whether ice cubes will melt faster when placed on metal or plastic.
Download the note taking guide for Chemistry Matters Unit 8 Segment B. Links to an external site.
Exothermic Reaction Presentation
Endothermic Reaction Presentation
What Did You Learn About Exothermic and Endothermic Reactions? Activity
Enthalpy
In order to calculate the changes in energy associated with chemical reactions, we need to introduce a new term, enthalpy or H. When a system absorbs energy from the surroundings, an endothermic reaction, Hproducts is larger than Hreactants, so ΔH is positive. When a system loses energy to the surroundings, an exothermic reaction, Hproductsis smaller than Hreactants, so ΔH is negative.
Standard Heat of Reaction
The ΔH for any reaction depends upon the number of moles of reactants you start with. It should make sense that burning 2 mol of C would release twice as much energy as burning 1 mol of C. Because other factors can affect ΔH as well, chemists have agreed to a set of standards to use to measure ΔH. The standard conditions include a pressure of 1 atm, a concentration of 1 M (for aqueous substances), and a temperature of 25oC (298 K). Note that this temperature is different than the standard temperature for gas law problems.
(Recall that STP means a standard temperature of 0 oC. So why do we have a different standard temperature here? It is because 25oC represents average room temperature where these ΔH values are measured.)
To indicate that a heat of reaction is the standard heat of reaction, the symbol has a degree sign added, ΔH°.
A chemical equation that is written to show the heat of reaction is called a thermochemical equation. These can be written for any heats of reaction, standard or not. The thermochemical equation below represents the combustion of carbon in oxygen.
This thermochemical equation can be reversed. In the reverse reaction, the value of ΔH° is the opposite sign.
If we double the number of moles of each chemical, we double the value of ΔH° as well.
Hess' Law
Hess' Law states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.
To better understand this concept, think about climbing a mountain. To get to the top, you can take the road that goes around the mountain. Or, you can hike straight up the side.
Regardless of the path you choose, your change in elevation will be the same. ∆H is the same way in that its value is independent of the path taken to establish its value.
The advantage of Hess' Law is that it allows us to calculate the enthalpy change for a chemical reaction in which the enthalpy data is not directly known. Take a look at the following tutorial to learn more about solving Hess’ Law problems.
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