11Chapter 15aThermodynamics: The First Law2The Nature of Energy Thermodynamics is the study of the transformation of energy. There are two laws that govern the study of thermodynamics.1. The first law keeps track of energy changes by allowing us to calculate how much heat is generated during a reaction.2. Provides explanations for why some reactions occur and others do not.3The First Law of Thermodynamics This law can be stated as, “The combined amount of energy in the universe is constant.” The first law is also known as the Law of Conservation of Energy. Energy is neither created nor destroyed in chemical reactions and physical changes. 4The Nature of Energy English physicist, James Joule, showed that both heat and work are forms of energy in the middle 1800’s. Energy is the capacity to do work Energy is the capacity to produce heat Energy may be  Transformed from one form to another Transferred from one place to another5The Nature of Energy The law of conservation of energy states that energy can be converted from one form into another but can neither be created or destroyed. The energy of the universe is constant. Energy may be classified as: Potential energy Kinetic energy6The Nature of EnergyPotential Energy – is the energy due to position or composition.Objects tend toward a state of minimum potential energy. Examples of this include: H2O flows downhill. Objects fall when dropped. The energy change for these two examples is: Epotential= mgh ∆Epotential= mg(∆h)27The Nature of EnergyChemical systems tend toward a state of minimum potential energy. Examples of this include: Combustion reactions release heat. Acid/base reactions release heat.8The Nature of Energy Exothermic reactions generate specific amounts of heat. This is because the potential energies of the products are lower than the potential energies of the reactants.9The Nature of EnergyKinetic Energy – is the energy due to motion. The kinetic energy of an object is due to its motion and is dependent on the its mass and its velocity.K.E. = ½mυ2m = massυ = velocity10Some Thermodynamic TermsTo keep track of energy, the world is divided into: A system – the substances involved in the chemical and physical changes that we are interested in studying. The reactants and products of a reaction. The condition of a substance before and after a change of state. The surroundings – everything in the system’s environment. A water bath in which a reaction mixture may be immersed. The surroundings are where observations are made on the energy transferred into or out of the system. 11Some Thermodynamic TermsThe universe – is the system plus the surroundings. However, the part of the actual universe that is affected usually consists of a sample, a flask, and a water bath.12The system is the sample or the reaction mixture of interest.• Reactants and products of the reactionOutside the system are the surroundings.• The reaction container, the room, and everything else other than the reactants and products.The system plus its surroundings is called the universe.313Systems A system can be open, closed or isolated. An open system can exchange both matter and energy with its surroundings. A closed system has a fixed amount of matter but can exchange energy with its surroundings. An isolated system has no contact with its surroundings. Usually a sealed inside rigid, thermally insulated walls. E.g. hot coffee inside a sealed vacuum flask.14Energy and matter can be exchanged with its surroundingsEnergy but not matter can be exchanged with its surroundingsNeither energy nor matter can be exchanged with its surroundings15State Functions The set of conditions that specify all of the properties of the system is called the thermodynamic state of a system.  For example the thermodynamic state could include: The number of moles and identity of each substance. The physical states of each substance. The temperature of the system. The pressure of the system.16State Functions The properties of a system that depend only on the state of the system are called state functions. A state function is the property of the system that depends only on its current state. State functions are always written using capital letters. The value of a state function is independent of pathway. The value of a state function depends only on the state of the system and not on the way in which the system came to be in that state.17State FunctionsLets say I find and move two bricks from the sidewalk outside tothe roof of the building so that the change in elevation of bothbricks is exactly the same. However, lets say I don’t take the same route to get to the roof in each case.  Brick A Æ I carry this brick directly up the stairs to the roof. Brick B Æ I first carry this brick over to the student lounge because I have to stop and get money from the ATM. Then, having money, I buy a cup of coffee. Then I wander over to my office to check my e-mail. Finally I go back outside and take the brick to the roof of the building.18State Functions The change in height, and thus the change in gravitational potential energy is exactly the same for both bricks, whereas the amount of work expended to complete the task is much greater for brick B. Now if you come along the next day and see that the bricks have been moved to the roof, by measuring the change in elevation you can immediately determine what the change in gravitational potential energy was.  However, unless you know exactly what I did you cannot know the work involved in the process of moving the bricks.  Thus, the change in potential energy is a state function, while work is not.419State Functions Consider two beakers of water. I raise the temperature of beaker A to 80°C using a Bunsen burner.  I attempt to raise the temperature of beaker B in the same manner, but accidentally heat up to 90°C instead.  At this point I turn the burner off to let the water cool.  I unexpectedly get called away and when I return the temperature of beaker B is now 50°C, so I have to heat it again to get to 80°C. You can see that while the temperature is a state function (a sample at 80°C is at 80°C regardless of what temperature it was 2 hours ago), while the amount of heat transferred into or out of a system is not