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Chapter 7

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Gillian Goward

Chapter 7: Thermochemistry Combustion: - Combustion is an exothermic reaction between a fuel and an oxidant o Rapid combustion: produces heat or both heat and light in the form of either a glow or flames o Slow combustion: takes place at low temperatures. E.g. respiration Heat of Combustion: - It is heat evolved during combustion - This heat is stored between the bonds - Ultimately all the energy to get these high energy molecules comes from SUN o Sun gives off energy in the form of photons  plants take this energy in an ENDOTHERMIC reaction to do photosynthesis to produce glucose and oxygen o Plants are the most efficient solar cells  leaves are the cheapest solar cells o Energy is extracted by combustion of the plant material.  Therefore, the forward process is ENDOTHERMIC ∆H> 0 (because heat is consumed) and the reverse reaction is EXOTHERMIC, ∆H<0 (because heat is given off). - Burning leaves does not generate energy because it is consumed by water in the leaves, while things like oil, water is removed and they are much more efficient. Issues with Burning fossil fuels - Non – renewable and contributes Global warming - North Americans burn more fossil fuels more CO2 emissions  b/c we need it to heat our houses and since our days are also shorter, we need frequent lighting compare to other countries near equator Systems versus Surroundings - Open system: material and energy exchange. E.g. beaker - Closed system: only energy exchange. E.g. capped flask - Isolated system: neither material nor energy exchange. E.g. thermos Energy - It is ability to do work. Types of energies: potential (energy within a molecule  increases with the distance), kinetic (energy of the motion) and thermal (heat energy  heat flow from a system to its surroundings  creates a temperature gradient) Heat capacity v/s Specific Heat capacity - Heat capacity  an extensive property  depends on how big your system is o the amount of heat required to change the temperature of a system by one degree or one kelvin o the more complex a molecule is  the higher the heat capacity o thermal energy is expressed as a molecule’s internal motions  expressed in thermal motion  if you put more energy  it will vibrate more (bonds will stretch, compress)  not translate (like resonance  rearrange. ) o q = m x specific heat x ∆T (or C x ∆T)  changes the temperature of a “system by 1 degree”  q is quantity of heat  C is the heat capacity of the system = C = m x specific heat  c= specific heat capacity  to understand which one you need to use  look at the units - specific heat capacity  energy available from these internal degrees of freedom contributes to a substance’s specific heat capacity o system or (C) is 1g of material - molar heat capacity  system (or C) is 1 mol of material - if we have two substance and the specific heat capacity is given, the one with the lower specific heat capacity will lose heat and the one with higher will gain heat Heat of the Reaction - q rxn - qrxn energy flows from the hotter area to the cooler area. the heat of the reaction is the quantity of the heat exchanged between a system and its surrounding when a chemical reaction occurs within the system at constant temperature o q 0 endothermic reaction (Heat Required) rxn o q rxn= 0 Isolated system: thermal energy is transferred between components of the system o q rxn+ qcalorimeter Bomb Calorimetry - q of the system = 0 - involves a combustion reaction  that involves gases - can’t be done this in coffee cup because gasses are produced and thus in coffee you will lose matter to the surroundings - equation q = C x ∆T  where C is the sum of both the substance is the calorimeter - 1 g = 1.010 kJ Calorimeters at a Constant Pressure - Ice – calorimeter (like coffee cup)  no change in temp, no change in pressure o q system0 Enthalpy Change, ∆H and phase change - after a certain amount of heat applied to a substance, it will change phase rather than temperature. o For example: till 100 degree Ce
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