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Lecture 7

CHEM 1A03 Lecture 7: Thermochemistry (Chapter 7).docx

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Jeff Landry

Chapter 7: Thermochemistry - Combustion or burning is a complex sequence or exothermic chemical reactions between a and an oxidant - Rapid combustion: accompanied by the production of hat or both heat and light in the form of either a glow or flames - Slow combustion: takes place at low temperatures - Respiration is an example of slow combustion - The higher the heat of combustion, the better the fuel - Hydrogen economy, methanol economy – using hydrogen / methanol to store energy - Photosynthesis is an endothermic process - Sunlight (energy) is used to build glucose and other simple sugars - 6CO2 + 6H2O  C6H2O6 + 6O2, ΔH◦ = +2.8 x 10^3 (pos because energy going in ) - Energy is extracted by combustion of the plant material - As the forward process is ENDOTHERMIC, ΔH > 0 - The reverse reaction is EXOTHERMIC, ΔH < 0 System versus Surroundings - Open system: material and energy exchange - Closed system: only energy exchange - Isolated system: neither matieral nor energy exchange Energy - Energy = the capacity to do work - Potential energy: energy of a state - Kinetic energy: energy of motion - Thermal energy: kinetic energy on a microscopic scale, movement on a molecular basis / within a molecule - Heat (q): energy transferred between a system and its surroundings as a result of a temperature gradient Heat Capacity - Heat Capacity (C): the amount of heat required to change the temperature of a system by one degree (J◦C ) or (J K ) - Water has a higher heat capacity than air – you wouldn’t last as long in the cold water vs cold air, ability to remove heat from you - Thermal energy is expressed as a molecule`s internal motions - Molecular-level complexity correlated to heat capacity; more complex, higher heat capacity - Energy available from these internal degrees of freedom contributes to a substance`s specific heat capacity - Things that are complex have higher internal degrees of freedom - Water`s high heat capacity is attributed to its bonding Defining a System`s Capacity to Store Heat - Q = m x specific heat x ΔT = C x ΔT - Heat capacity (C): the quantity of heat (q) 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 mol of material - 100.0g copper (specific heat = 0.385Jg-1C-1) at 100C is added to 50.0g water at 26.5C… final temp of copper-water mixture? o q(water) = -q(copper) o m(water) x c(water) x (ΔT) = - m(copper) x C(copper) x (ΔT) = 37.9 C o - Heat of Reaction – qrxn - Qrxn = the quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system at constant temperature - Qrxn < 0 exothermic reaction (heat produced) - Qrxn > 0 endothermic reaction (heat required) - Qsystem = 0 Bomb Calorimetry (Constant Volume): - Isolated system where heat is not transferred outside boundaries of that system and you do a reaction inside of it – reaction nside either gives off or takes in heat and we measure the physical change – temp , volume change, etc.. that we can equate back to how much energy was released or taken in by the reaction - Calorimetry – qsystem = 0 assumption. - Q calorimC calorimeterT - All energy that goes into the calorimeter sums up to zero - Qsys = qrxn + qcalorim = 0 - Qrnx = -qcalorim = -16.71 kJ (from example on slide 18) - Measured on a per mol basis Enthalpy Change, ΔH, Phase Change - When water boils or ice melts, what is the temp of the water during phase transition - Heat is required for phase transition (which occur at constant T!)  latent (hidden) heat of fusion - Molar enthalpy of fusion H2O (s) H2O(l) ΔH = 6.01kJ (at 273.15K) - Phase change = energy associated with it Differentiating Work and Heat - Familiar with ΔH  endothermic or exothermic, indicating energy required, or given off during a chemical reaction - Energy can also be input or output from a system via “work” Pressure-Volume Work, w - Explosives - Gases formed on combustion of gasoline o Chem reaction burning gasoline, releasing energy as heat and work – chem rxn explosion , rapid change in volume.. internal combustion engine? - How much work is done by an expanding gas? - Work = force x distance - W = - (m)(g)/A x Δh x A = -P ΔText - External pressure - what kind of force are we putting on the outside.. eg how much force exerted when you try to expand something - Larger your etxernal force, less work you can have done - The First law of Thermodynamics - Internal energy, U - ΔU sys
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