CHEM H90 Lecture Notes - Lecture 16: Kilowatt Hour, Joule
Midterm Results and Energy
• energy and power
o energy is conserved in all physical processes
o first law of thermodynamics
o since true on atomic scale, also true on everyday scale
o since true on everyday scale, true for planet
o power is the rate at which energy is generated or consumed, i.e., energy per time
• SI units
o SI unit of energy is Joule = 1 kg m2/s2
o SI unit of power is Watt = 1 J/s
o solar flux: overhead sun delivers 1.368 kW/m2 to Earth
o small apple (0.1 kg) dropped from 1m on Newton’s head delivers 1 J
o a 100W bulb consumes a kJ every 10s
• chemical reactions
o combustion of methane
• estimate heat of combustion of methane
o reactants have 4 x (C-H) + 1 x (O=O)
▪ 4x410 + 498 = 2138 kJ/mol
o products have 2 x (C=O) + 4 x (O-H)
▪ 2x732 + 4x460 = 3304 kJ/mol
o so more chemical energy in products than reactants
o so reaction is exothermic (gives off energy)
o estimate 1166 kJ/mol, actually 808 kJ/mol
• your personal energy consumption
o SI unit of energy is Joule = 1kg m2/s2
o much more useful here is the kilowatt-hour
o 1kWh = 60x60x1000J = 3.6MJ
o a 100W bulb uses 0.1 kWh in an hour
o when running, your desktop uses similar amount
o your daily personal consumption is about 50 kWh, as an average American
• amount of energy you use (50 kWh/day)
o appliances
▪ portable heater needs 1.5 kW
▪ water heater about .5 kW
▪ oven needs 2.3 kW
▪ central AC 3.5 kW
▪ slice of toast needs 0.02 kWh
▪ fridge uses about 1 kWh/day
o travel
▪ car uses 1 kWh per mile
▪ plane uses 1.5 kWh per passenger per mile
• everyday scale
o 1 mole of methane is 16 gms
o if I burn 2 kg of methane, this releases 882/8 = 110 MJ of energy = 30.5 kWh
o enough energy for 18 hours of your day
o probably would rather not run cell phone by burning methane
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