A form of energy Flows from warmer objects to cooler objects Represented by the variable q Measured by units calorie or joule 1 J 02390 cal 4184 J 1 cal 1000 cal 1 kilocalorie ID: 934991
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Slide1
Thermochemistry
Energy change that occurs during a chemical reaction and/or changes in state
Slide2A form of energyFlows from warmer objects to cooler objects
Represented by the variable “q”
Measured by units: calorie or joule1 J = 0.2390 cal 4.184 J = 1 cal1000 cal = 1 kilocalorie = 1 Cal
Heat
Slide3Exothermic Process
Energy flows from the system into the surroundings
Endothermic ProcessEnergy flows from the surroundings into the systemConservation of EnergyEnergy is not created or destroyed in a physical or chemical processIf energy in a system decreases, then the energy of the surroundings increases by the same amountExothermic and Endothermic
Slide4Amount of heat needed to increase the temperature of an object exactly 1 ˚C
Depends on mass and chemical composition
Greater mass = greater heat capacitySpecific Heat Capacity, Cp (Specific Heat)Amount of heat needed to raise 1 g of a substance by 1 ˚CMetals have low specific heatsWater has a relatively high specific heat
Heat Capacity
Slide5q
= m x
Cp x ΔTCp is the variable for specific heatCould 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)
Δ
T (
T
f
– Ti) is the variable for change in temperature (˚C)
Calculating Specific Heat
Slide6Enthalpy (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,
ΔHWe will only work with a constant pressure scenario…therefore, q = ΔHCalorimetryMeasurement of heat flow into/out of a systemCalorimeter is the device used to measure heat flow
Measuring Enthalpy Changes
Slide7If a system gains heat, that amount of heat was lost by the surroundings…
q
sys = - qsurr (the negative sign represents loss)ΔH = qsys = -
q
surr
=
m
x Cp x
Δ
T
Exothermic reactions have a negative value for
ΔHEndothermic reactions have a positive value for ΔH
Put it all together…
Slide8A chemical equation that includes enthalpy change
Can be either a “reactant”
Endothermic 2NaHCO3 + 129 kJ Na2CO3
+ H
2
O + CO
2
or
2NaHCO
3
Na
2CO3
+ H
2O + CO2 ΔH = 129 kJOr a “product”Exothermic CaO + H2O Ca(OH)2 + 65.2 kJ or CaO + H2O Ca(OH)2 ΔH = -65.2 kJ
Thermochemical Equation
Slide9How much heat in joules is required to raise 1000g of water from 20
°C to 55 °C ?
Example Problems
Slide10When 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)
Example Problems