1. Energy and Chemical Reactions Ms. Grobsky

2.  So far, we have discussed the various types of chemical reactions, the driving forces behind them, and how to quantitatively predict amounts of reactants and products using stoichiometry  In order to properly understand chemistry, we must understand the energy changes that accompany these chemical reactions  The study of thermodynamics is concerned with the question: Can a Reaction Occur? Introduction to Thermodynamics

3.  Unlike matter, energy does not have mass and cannot be held in our hands  However, its effects can be observed and measured  When discussing energy, one must assign the following labels:  System  Area of the universe we are focusing on (i.e., the experiment)  Surroundings  Everything else outside the system System and Surroundings

4. Open Mass & EnergyExchange: Closed Energy Isolated Nothing SYSTEMSURROUNDINGS

5.  Defined as the capacity to do work or to produce heat  Units are calorie (cal) or Joule (J)  Before we can make use of this definition, we must understand the concepts of work and heat  Work is the energy used to cause an object to move against a force  Heat is the energy used to cause the temperature of an object to increase  Before we examine these definitions more closely, let’s first consider the ways in which matter can possess energy and how that energy can be transferred from one piece of matter to another So, What is Energy?

6. There are many different forms of energy: Thermal energy is the energy associated with the random motion of atoms and molecules Kinetic energy is the energy due to motion Potential energy is the energy available by virtue of an object’s position Chemical energy is the energy stored within the bonds of chemical substances Type of potential energy All forms of energy are able to interconvert! Types of Energy

7. Total Internal Energy = Kinetic Energy + Potential Energy E = E K + E P Ball thrown upwards slows & loses kinetic energy but gains potential energy The reverse happens as it falls back to the ground Example of Energy Conversion – Kinetic Energy and Potential Energy Energy can also be transferred back and forth between a system and its surroundings in the forms of work and heat

8. Why Can Energy Interconvert?  All of these conversions and transfers proceed in accord with one of the most important observations in science: Energy can neither be created not destroyed – it is conserved!  This important observation is known as the First Law of Thermodynamics

9. What Does Energy Have to Do with Chemical Reactions? Introducing Thermochemistry!

10.  During both parts of the lab, you felt striking temperature differences between yourself (the surroundings) and the baggie (the system)  Well, what do you think caused this temperature difference?  It was due to the exchange of heat!  What exactly is heat?  It is the exchange of thermal energy between two systems at different temperatures that are in contact with one another  Heat flows from a warmer object to a cooler object  Heat is represented by the letter “q”  But, temperature is NOT a measure of thermal energy  It just reflects the kinetic energy (motion) of the particles Tying in the Citric Acid/Sodium Bicarbonate Lab

11.  Come to find out, many physical changes involve release and absorption of energy in the form of heat  Dissolving of salts  Changes in state  Compression/expansion of gases Energy and Physical/Chemical Changes

12. Energy Changes in Chemical Reactions Almost every chemical change either releases or absorbs energy in the form as heat as chemical bonds are broken and formed during the course of a reaction Energy is consumed when bonds are broken Energy is released as new bonds are formed Net total energy change for a reaction will either be a negative value with the release of heat (exothermic) or a positive value with the absorption of heat (endothermic) Thermochemistry is the study of heat exchange in chemical reactions

13. An exothermic process is any process in which energy (heat) flows out of the system into the surroundings “q” is negative Energy of reactants is greater than that of the products Energy can be thought of as a product in a chemical reaction because it is “given off” An endothermic process is any process in which energy (heat) flows into the system from the surroundings “q” is positive Energy of products is greater than that of the reactants Energy can be thought of as a reactant in a chemical reaction because it is being absorbed H 2 O (g) H 2 O (l) + Energy Energy + 2HgO (s) 2Hg (l) + O 2 (g) Heat Exchange between System and the Surroundings

14. Endothermic vs. Exothermic Reactions