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Chapter 7 Thermodynamics.docx

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McMaster University
Jeff Landry

Chapter 7: Thermochemistry  Heat is given off by a reaction: combustion (burning reaction) --> exothermic (energy is given off -- going out of the system, anytime energy leaves a system it's considered to be negative and visa versa  Other ways that can give off energy from a reaction: light  When we're emitting photon, energy is transferred around, electrons are dropped to lower levels, and gives off energy  Slow combustion takes place in respiration Energy given off More on a larger scale: Global Energy Sources  #1 is oil, humans suck at storing energy, we can't even store enough energy in our cells to go to Montreal :)  Coal: natural gas  Total volume of oil, coal is larger so it's cleaner,  Hydro is not very well used, Fossil fuels non renewable:oil, coal, gas The Energy We get from fuels  Comparing heats of combustion: Units: kJ/gram per gram basis Hydrogen looks really really high in term of energy Sources of energy that we can actually store: Natural Gas, Gasoline, Coal Hydrogen releases oxygen, we can capture it and use it as stored energy but we need to put in electricity, we need fossil fuels to construct that electricity--> not a fuel efficient process  Methanol is easy to produce from CO2 -->available sources. We take CO2 and in the presence of Hydrogen we can get Methanol  Why we don't like to use Hydrogen as fuel?  Huge balloon full of Hydrogen demo Capturing and Using Energy from the Sun Where do these fossil fuels come from--> from the sun   All our energy comes from the sun at some point  (Plants) In the presence of sunlight , they can produce sugar and oxygen in an endothermic energy-->trapping energy  Problem with photosynthesis process: putting a lot of water glucose is very wet so it's really hard to burn glucose If you want all the energy that you stored in your sugar, we can't really use it very quickly   We need to chop down the tree, let it dry for years…… long process…  We want to take energy from the sun and we want to use it now, relatively quickly Delta H > 0 --> Endothermic: Formation of Glucose Delta H < 0 Exothermic : Combustion reaction Issues with Burning Fossil Fuels  It takes 300 years to take trees, chop them slowly releasing the water to use the stored energy Burning Fossil Fuels-CO2 Emissions System versus Surroundings  Definitions: -System is anything we define: the chem classroom, our body is a system, you need to define a system and you can define it what ever you want Usually the beaker in which the chemical reaction is happening is the system  systems that we can have: Open System: Our system can release energy and material Closed System: capped Erlenmeyer flask, can't pour anything out but the energy can change Isolated system: similar to thermos, neither material nor energy exchange Energy  Energy is the capacity to do work  Potential Energy: Energy of state, lower PE at lower state  Kinetic energy is energy of motion  Thermal Energy: with respect to heat: energy transfer that is associated with a temperature gradient Heat Capacity  Is defined as the amount of energy/heat that is required to change the temperature of a system  Heat capacity: how much energy is going to be transferred as the temperature gradient will be equal  Water around the human body is taking the energy away so you die faster in a 0^0 C  If you go out in a windy that, it feels a lot colder than a really cold day  Wind factor  Paraffin wax takes a huge amount of energy to change the temperature of paraffin-> we use it in candles, we want a loot of energy before it starts melting  Where does heat capacity come from: It comes from molecular motion You put a molecule, it moves around spatially, x/y/z it's also moving internally, vibrating   Example: Water Molecule  H2O the bond can stretch (internal movement) Bond angle will vibrate back and forth  Internal motions require energy Higher the internal motion higher the heat capacity  Origin of Heat Capacity  Everytime you add another level of complexity you need more energy  More complex a system, a molecule the higher the heat capacity it's going to have but water is an exception , Comes from intermolecular forces: Hydrogen bonds. Molecules that are polar stronger intermolecular forces will have higher heat capacity. Defining a System's Capacity to Store Heat  q= m x specific heat x delta T = C(heat capacity) x delta T  Heat capacity (C)=the quantity of heat required to change the temperature of a 'system' by one degree  Specific heat capacity: 'system' is 1 g of material  Molar heat capacity: 'system' is 1 mole of material : amount o energy required to change 1 mole of substance by one degree celcius Sample Problem  A 100.0 g copper sample  We got a temperature gradient We're put
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