“Why do we want to burn propanol?”

CC33HH88 + + H H22O + COO + CO22

There is another non chemical product….There is another non chemical product….

HEAT!

»The science of chemical reactions that involve HEAT is called

THERMODYNAMICS

Thermochemistry

The study of energy changes that accompany physical or The study of energy changes that accompany physical or chemical changes in matterchemical changes in matter

All thermodynamics depends on the LAW OF All thermodynamics depends on the LAW OF CONSERVATION OF ENERGY: the total energy of a CONSERVATION OF ENERGY: the total energy of a systemsystem and its and its surroundingssurroundings remains the same remains the same

Changes that occur in matter may be classified as Changes that occur in matter may be classified as physical, chemical or nuclearphysical, chemical or nuclear

Heat and Energy ChangesPhysical, chemical and nuclear changes are all accompanied by a Physical, chemical and nuclear changes are all accompanied by a

change in energychange in energy–PhysicalPhysical: a change in the form of a substance. No chemical : a change in the form of a substance. No chemical bonds are broken.bonds are broken.Ex. hydrogen boils at -252Ex. hydrogen boils at -252°C °C HH2 (l)2 (l) + heat + heat H H2 (g)2 (g)

–ChemicalChemical: a change in the chemical bonds between atoms : a change in the chemical bonds between atoms resulting in new substances resulting in new substances Ex. hydrogen is burned as fuel in the space shuttle’s engines.

2H2 (g) + O2 (g) 2H2O (l) + heat

–NuclearNuclear: a change in the protons or neutrons in an atom : a change in the protons or neutrons in an atom resulting in a new atom.resulting in a new atom.Ex. hydrogen undergoes nuclear fusion in the Sun, producing Ex. hydrogen undergoes nuclear fusion in the Sun, producing helium.helium.

He e2 H 4 42

01-

11

Heat and Energy Changes cont’d Chemical system – the substances undergoing a

change

Surroundings – the system’s environment

Thermal Energy – energy available from a substance as a result of the motion of its molecules

Temperature (T) – the average kinetic energy of the molecules in a sample, measured in °C or K

Heat (q) – amount of energy transferred between substances, measured in Joules (J)

Energy flows between substances because of their difference in temperature.

Heat and Energy Changes cont’dA. A. ExothermicExothermic: releasing thermal energy, heat (q) flows from the : releasing thermal energy, heat (q) flows from the

system to the surroundings, usually causing an increase in the system to the surroundings, usually causing an increase in the temperature of the surroundings. temperature of the surroundings.

q has a negative valueq has a negative value

B. B. EndothermicEndothermic: absorbing thermal energy, heat (q) flows into the : absorbing thermal energy, heat (q) flows into the system from the surroundings, usually causing a decrease in the system from the surroundings, usually causing a decrease in the temperature of the surroundings.temperature of the surroundings.

q has a positive valueq has a positive value

AA BB

Measuring Energy Changes

CalorimetryCalorimetry: : experimental technique used to measure energy changes/transfers (the quantity of heat (q) in chemical systems

different substances vary in their ability to absorb amounts of heatdifferent substances vary in their ability to absorb amounts of heat

Substances used in a system have a specific heat capacity:

amount of energy required to raise the temperature of amount of energy required to raise the temperature of 1 g 1 g of a of a substance by substance by 1 1 °C or 1K°C or 1K

(see Table 1, pg.301)(see Table 1, pg.301)

Equation used to determine the value of q:

The amount of heat transferred (The amount of heat transferred (qq) depends on) depends on

» measured in grams

» measured in J/g•°C or J/g•K

–measured in °C or K

Measuring Energy Changes

Sample Problem 1Many water heaters use the combustion of natural gas (assume Many water heaters use the combustion of natural gas (assume

methane) to heat the water in the tank. When 150.0 L of water methane) to heat the water in the tank. When 150.0 L of water at 10.0°C is heated to 65.0°C, how much heat flows into the at 10.0°C is heated to 65.0°C, how much heat flows into the water?water?

m = 150L x 1kg/L = 150 kg = 150 000 gc = 4.18 J/g°C T = (65-10) = 55°C

q = mcΔT = 150 000g x 4.18 x 55.0 = 34485000q = 3.45 x 107 J

Sample Problem 2

If 25.0 g of aluminum cools from 310If 25.0 g of aluminum cools from 310°C to 37°C, how many joules °C to 37°C, how many joules of heat energy are lost by the sample?of heat energy are lost by the sample?

m = 25.0 gc = 0.902 T = 37 – 310 = -273

q = 25.0 x 0.902 x –273 = -6156.15q = -6.2 x 103 J* Negative sign indicates loss of energy

Sample Problem 3

Calculate the Calculate the molarmolar heat capacity (C heat capacity (C)) of water (J/mol of water (J/mol•°C).•°C).

c = 4.18 J/g •°CM = 18.02 g/molCp = c x M = 4.18 J/g •°C x 18.02 g/mol Cp = 75.3 J/mol •°C