Reaction Energy a/k/a Thermochemistry

Energy(heat) flows in a chemical reaction.

Content Objectives

Chemistry students wbat describe the three laws associated with thermochemistry.

Chemistry students wbat analyze a chemical reaction to determine if it is exothermic or endothermic.

Language Objectives

  LO: SW apply Hess’s law as it pertains to chemical reactions by manipulating chemical equations individually and in pairs.

  You will determine the heats of reaction of three related exothermic reactions in groups of 3.

Heat

  The energy transferred between samples of matter because of a difference in their temperature.

Heat of Combustion ΔHc

  The energy released as heat by the complete combustion of one mole of a substance.

Heat of Formation ΔHf

  The energy change when 1 mole of a compound is formed from elements

Heat of Reaction

  The quantity of energy released or absorbed as heat during a chemical reaction.

  Enthalpy change (ΔH)=

ΔH= Hproducts - Hreactants Can be:   Exothermic--energy released -ΔH   Endothermic--energy absorbed + ΔH

Conservation of Energy

  Energy in an isolated system remains constant over time.

  Energy can neither be created or destroyed it can only be transformed from one state to another.

Three Laws of Thermochemistry

1.  The heat flow associated with a reaction is directly proportional to the amount of material undergoing the same reaction.

Three Laws of Thermochemistry

2. The heat flow for a reaction is equal in magnitude but opposite in sign for the reverse reaction.

Three Laws of Thermochemistry

3. Hess’s Law: If a reaction can be expressed as the sum of two or more independent reactions, then the heat flow for the reaction is equal to the sum of the heat flows for the independent reactions.

(Think relay race)

Enthalpy

  ΔH = enthalpy change = heat energy gained or lost during a reaction

ΔH = Hf (product) – Hf (reactants)

  (Product = energy tied up in the bonds of the products)

  (Reactants = energy tied up in the bonds of the reactants)

It can be…..

Or it can be….

Endothermic   Absorbs energy in

the form of heat.   ΔH is positive

We measure the change…

Exothermic Reaction Endothermic Reaction

Hess’s Law

If a reaction can be expressed as the sum of two or more independent reactions, then the heat flow for the reaction is equal to the sum of the heat flows for the independent reactions.

(Think relay race-it all adds together!!!)

Hess’s Law

Basic formula for Hess’s Law

ΔHf(product) = Hf(reactants) + Hf(reaction)

Rules for using Hess’s Law 1. Make sure to rearrange the given

equations so that reactants and products are on the appropriate side of the arrows.

2.  If you reverse equations, you must also reverse the ΔH.

3.  If you multiply the equation to obtain a correct coefficient, you must also multiply the ΔH by this coefficient.

bibliography   http://www.sparknotes/testprep/books/sat2/chemistry/

chapter9section5.html   http://www.science.uwaterloo.ca/~cchieh/cact/c120/hess.html   http://www.wbateman.demon.co.uk/asa2sums/sum2.1/

sum2.12.gif   http://www.btinternet.com/~chemistry.diagrams/

endothermic.html   http://dbhs.wvusd.k12.ca.us/webdocs/Thermochem/

HessLawIntro1.html   http://www.chem.queensu.ca/people/faculty/mombourquette/

Chem221/3_FirstLaw/Thermochemistry.asp   Httpp://www.utc.edu/Faculty/Gretchen-Potts/chemistryhelp/

hess.htm