Thermochemistry. Thermochemistry is the study of heat changes that occur during chemical reactions....

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)