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Energy and Chemical Reactions Ms. Grobsky Introduction to Thermodynamics So far, we have discussed the various types of chemical reactions, the driving forces behind them, and how to quantitatively predict amounts of product produced 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? System and Surroundings 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 SURROUNDINGS SYSTEM Exchange: Open Closed Isolated Mass & Energy Energy Nothing So, What is Energy? 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 exchange of thermal energy between two systems at different temperatures that are in thermal contact with one another 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 Types of Energy • 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! Example of Energy Conversion – Kinetic Energy and Potential Energy Total Internal Energy = E Ball thrown upwards slows & loses kinetic energy but gains potential energy Kinetic Energy = EK + + Potential Energy EP The reverse happens as it falls back to the ground • Energy can also be transferred back and forth between a system and its surroundings in the forms of work and heat 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 Relating the First Law of Thermodynamics to Chemical Reactions Time for a mini-lab! Only answer Analysis Questions #1-6 What Does Energy Have to Do with Chemical Reactions? Introducing Thermochemistry! Tying in The Big Chill Lab During the lab, you felt striking temperature differences between yourself (the surroundings) and the beaker (the system) Well, what do you think caused this temperature difference? It was due to the exchange of heat! What exactly is heat again? 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 Energy and Physical Changes 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 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 Heat Exchange between System and the Surroundings • 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” H2O (g) H2O (l) + Energy • 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 Energy + 2HgO (s) 2Hg (l) + O2 (g) Endothermic vs. Exothermic Reactions Time to Finish Post-Lab Questions