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Thermochemistry
• Thermochemistry is the study of heat changes that occur during chemical reactions.
• Heat (q) - energy that is transferred from one object to another because of a temperature difference between them. • Heat always flows from a warmer object to a cooler
object. • Heat cannot be measured directly.
• Temperature – the average kinetic energy of the particles in a sample of matter
• Energy - the capacity for doing work or supplying heat.
Energy in Reactions
• Endothermic process - system absorbs heat from the surroundings. (Energy must be added to the reaction for the reaction to take place)
• Exothermic process - system releases heat to the surroundings.
Ex. Endothermic or exothermic process?-evaporating alcohol-leaves burning-boiling water-water cooling-melting ice-freezing water
To calculate the heat energy required for a temperature change, use the following formula:
Q = (m)(C)(T)
Q = heat energy absorbed or released (J)• + q = endothermic (heat absorbed)• - q = exothermic (heat released)
m = mass (g)C = specific heat of the object (J/goC)
• The amount of heat it takes to raise the temperature of 1 g of a substance 1oC.
• Water has a uniquely high specific heat compared to other substances.
T= change in temperature of object (oC)
Example• How much energy is required to heat
an iron nail with a mass of 7.0 g from 25oC until it becomes red hot at 752oC?
C Fe= 0.46J/goCT = 752-25 = 727oC
q = mC Tq = 7.0g(0.46J/goC) 727oCq = 2300 J
Phase Change• Heat energy can be used to not only change
the temperature of matter, but also its phase.• The energy goes into separating or organizing
the molecules into a new state• The amount of heat energy necessary to
cause a phase change can be calculated using the formula:
Q = mLf (solid/liquid)or
Q = mLv (liquid/gas)
• Q = heat required for phase change (J)• m = mass (kg)• Lf = latent heat of fusion (J/kg)
• Energy required to change one kilogram of a material from a solid to a liquid
• Water Lf = 334,000 (J/kg)
• Lv = latent heat of vaporization (J/kg)• Energy required to change one kilogram of a
material from a liquid to a gas• Water Lv = 2,260,000 (J/kg)
Example• Find the amount of heat needed for
melting 1.3 kg ice at 0 ºC?
Notice you calculate each change separately and then add them all up.
Enthalpy ( H)
• The amount of heat energy absorbed or lost by a system• Exothermic reactions have - H• Endothermic reactions have + H H is also called the heat of reaction.
Calculating Heat Changes• Bond energy is the amount of energy
required to break or make a bond• Breaking a bond is endothermic (+ H)• Forming a bond is exothermic (- H)
H = Hproducts – Hreactants
Example 1• Calculate the energy of the reaction using
heats of formation. H2(g) + Br2(g) 2 HBr(g)
H HBr = -36.29 kJ/mol• All elements and diatomic molecules have
an enthalpy of zero. H = Hproducts – Hreactants H = [2 x -36.29 kJ/mol] – [0] H = -72.58 kJ/mol• Exothermic
Example 2
H CH4 = -74.9 kJ/mol
H CO2 = -393.5 kJ/mol
H H2O = -285.8 kJ/mol
H = Hproducts – Hreactants H = [-393.5 kJ/mol + (2 x -285.8
kJ/mol)] – [-74.9 kJ/mol + (2 x 0)] H = -890.2 kJ/mol• Exothermic
• Calculate the energy of the reaction using heats of formation.
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)