Thermochemistry
Thermo = heat Chemistry = study of matter
Thermochemistry
•Study of heat changesand energy thataccompany chemicalreactions and phasechanges
Review of Energy
•Capacity to do work or tocreate heat and or generateelectricity
•Types:
•Chemical
•Nuclear
•Thermal
•Radiant (light)
•Electrical
•Mechanical
Law of conservation ofEnergy
•Energy can be convertedfrom one form toanother but it cannot becreated or destroyed
Forms of energy:
•Potential
–Stored energy
•Kinetic
–Energy of motion
Chemical potentialenergy
•Energy stored within thestructural units ofchemical compounds
Heat is energy (Q)
•flows from an area ofhigh heat to an area oflow heat
•Can be measured incalories, Calories, orJoules
•1000 cal = 1 Cal =4.184 J
Heat energy (Q)
•System: part of theuniverse being studied
•Surroundings:everything outside ofthe system
•Universe: system +surroundings
Heat energy (Q)
Endothermic:
•feels cold to touch
•temperature decreases
•heat absorbed by system
•Q = +
Heat energy (Q)
Exothermic:
•feels warm to touch
•temperature increases
•heat released by system
•Q = -
Specific heat capacity
•C or Cp
•The amount of energyrequired to raise thetemperature of 1 gramof a substance 1ºC
•Water= 4.18 J/g 0C
•Glass=.50 J/g 0C
Specific heatcapacity
•Measure of asubstance’s resistanceto temperature change
•High=resistant (water)
•Low=easy (metals,glass, etc.)
Equation
Q = m C ∆T
- Q is the heat absorbed orreleased in Joules
-m is mass in grams
-C is the specific heat inJ/g/ºC
-∆T is the temperature changein ºC
mass
Temperaturechange
(final tempminus initialtemp)
Specificheatcapacityin J/g-oC
(Q) = m C (T)
When 1982 g of water underwent atemperature change from 23.677oCto 27.482oC, how much energy inkJ did the water absorb? Thespecific heat of water is 4.184 J/(g-oC).
Show all work!
How much energy in J isrequired to raise thetemperature of 500.0 g ofcopper from 22.8oC to 100.0oC?The specific heat of copper is0.387 J/g/oC.
Show all work!
Measuring heatexchange
Calorimeter-
insulated device used formeasuring the amount ofheat absorbed or releasedduring a reaction
Calorimeter
Calorimeter
In a closed system:Heat loss = Heat gain
Q lost = Q gained
m C ∆ T = m C ∆ T
Enthalpy
•(∆H) heat content of asystem at constantpressure
•Can measure changes inenthalpy during areaction
Enthalpy heat of reaction∆ Hrxn
∆ Hrxn = Hfinal - Hinitial
Or
∆ Hrxn = Hproducts - Hreactants
Thermochemical Equations
•Used to calculate and express heatchanges.
•The physical states of the reactantsand products in this equation isimportant!
•Example:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
∆H = -890.2 kJ
Exothermic rxn:
∆ Hrxn is < 0
∆ Hrxn is negative
Endothermic rxn:
∆ Hrxn is > 0
∆ Hrxn is positive