Download Thermochemistry

Energy change that occurs during a chemical
reaction and/or changes in state
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A form of energy
Flows from warmer objects to cooler objects
Represented by the variable “q”
Measured by units: calorie or joule
1 J = 0.2390 cal
4.184 J = 1 cal
1000 cal = 1 kilocalorie = 1 Cal
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Exothermic Process
◦ Energy flows from the system into the surroundings
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Endothermic Process
◦ Energy flows from the surroundings into the system
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Conservation of Energy
◦ Energy is not created or destroyed in a physical or
chemical process
◦ If energy in a system decreases, then the energy of
the surroundings increases by the same amount
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Amount of heat needed to increase the
temperature of an object exactly 1 ˚C
◦ Depends on mass and chemical composition
◦ Greater mass = greater heat capacity
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Specific Heat Capacity, Cp (Specific Heat)
◦ Amount of heat needed to raise 1 g of a substance
by 1 ˚C
◦ Metals have low specific heats
◦ Water has a relatively high specific heat
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q= m x Cp x ΔT
Cp is the variable for specific heat
◦ Could be in units of J/(g•˚C) or cal/(g•˚C)
q is the variable for heat (joules or calories)
M is the variable for mass (grams)
ΔT (Tf – Ti) is the variable for change in
temperature (˚C)
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Enthalpy (H) is the heat content of a system at
constant pressure
◦ Heat absorbed or released by a system (constant
pressure) is the change in enthalpy, ΔH
◦ We will only work with a constant pressure
scenario…therefore, q = ΔH
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Calorimetry
◦ Measurement of heat flow into/out of a system
◦ Calorimeter is the device used to measure heat flow
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If a system gains heat, that amount of heat
was lost by the surroundings…
qsys = - qsurr (the negative sign represents
loss)
ΔH = qsys = - qsurr = m x Cp x ΔT
Exothermic reactions have a negative value
for ΔH
Endothermic reactions have a positive value
for ΔH
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A chemical equation that includes enthalpy change
Can be either a “reactant”
◦ Endothermic
2NaHCO3 + 129 kJ  Na2CO3 + H2O + CO2 or
2NaHCO3  Na2CO3 + H2O + CO2 ΔH = 129 kJ
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Or a “product”
◦ Exothermic
CaO + H2O  Ca(OH)2 + 65.2 kJ or
CaO + H2O  Ca(OH)2 ΔH = -65.2 kJ
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How much heat in joules is required to raise
1000g of water from 20 °C to 55 °C ?
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When 435 J of heat is added to 3.4 g of
olive oil at 21 °C, the temperature increases
to 85 °C. What is the specific heat of olive
oil?
How much heat in kilojoules is required to
raise the temperature of 250.0 g of mercury
52 °C? (Cp of Hg = 0.14 J/g* °C)