Ch 6: Thermochemistry

thermodynamics

study of energy and its transformations

thermochemistry

branch of thermodynamics that deals with the heat involved in chemical and physical changes

when energy is transferred from one object to another, it appears as ______ and/or ______

work, heat

a meaningful study of any transfer of energy requires that we first clearly define both the __________ and its _________________.

system, surroundings (system + surroundings = universe)

internal energy E

system is the sun of the potential and kinetic energies of all the particles present

the total energy of the universe remains __________

constant

a change in the energy of the system must be accompanied by an ______ and ___________ change in the energy of the surroundings

equal, opposite

transfer of internal energy (E) between a system and its surroundings

ΔE = E_final - E_initial = E_{products -} E_reactants

two forms of energy transfer

heat and work

heat (q)

is the energy transferred as a result of the difference in temperature between the system and surroundings

heat (w)

the energy transferred when an object is moved by a force

equation for the total change in a system’s internal energy is the sum of the energy transferred as heat and/or work

ΔE = q + w

q + (heat absorbed) + (+ (work done on))

ΔE + (energy absorbed)

q + (heat absorbed) + (- (work done on))

ΔE depends on the sizes of q and w

q - (heat absorbed) + (+(work done on))

ΔE depends on the sizes of q and w

q - (heat absorbed) + (-(work done on))

ΔE - (energy absorbed)

first law of thermodynamics

total energy of the universe is constant

unit of energy is the joule (J)

1 J = 1 Kg/m²/s²

1 cal

4.184 J

1 Cal = 1000 cal

1 kcal = 4184 J

British thermal unit (Btu)

1 Btu = 1055 J

If the expanding gases do 451 J of work on the pistons and the systems releases 325 J to the surroundings as heat, calculate the change in energy (ΔE) in J, kJ, and kcal

-451 - 325 = -776 J

Two Different Paths for the Energy Change of a System

even though q and w for the two paths are different, the total ΔE is the same for both

pressure-volume work

done when the volume of the system changes in the presence of an external pressure P

w = -PΔV

work of expansion

the volume of the systems increases if the temperature is raised or if a chemical reaction results in a net increase in the number of moles of gas. the system expands and does work on the system, losing energy

work of contraction

the volume of the system decreases if the temperature is lowered if a chemical reaction results in a net decrease in the number of moles of gas. the system contracts and has work done on it by the surroundings, gaining energy as work

a reaction taking place in a container with a piston-cylinder assembly at constant temperature produces a gas, and the volume increased from 125 mL to 652 mL against an external pressure of 988 torr. calculate the work value in this process ( 1 atm L = 101.3 J)

-69.4 J

enthapy (H)

ΔH = ΔE + PΔV, if a system remains at constant pressure and its volume does not change much then ΔH = ΔE

ΔH is the change in heat for a system at constant pressure

q_p = ΔE + PΔv = ΔH, ΔH = ΔE

for reactions that do not in involve gases, for reactions in which the total amount (mol) of gas does not change, for reactions in which q_p is much larger than PΔV, even if the total mol of gas does change

exothermic

releases heat, ΔH < 0

endothermic

absorbs heat, ΔH > 0

calorimetry

q = c x m x ΔT

specific heat capacity ©

the quantity of heat required to change the temperature of 1 gram of the substance by 1 K

You heat 22.05 g of a solid in a test tube to 100.00C and then add the solid to 50.00g of water in a coffee-cup calorimeter. The water temperature changes from 25.10C to 28.49C. Find the specific heat capacity of the solid.

q_{H =} -q_C → q_c = m_cc_cΔT_c = 0.449 J/g.K

constant-volume calorimetry

• constant-volume calorimetry is carried out in a bomb calorimeter

• used to measure heat of combustion

• the heat capacity, C, of the entire calorimeter is known

• q_{calorimeter =} C_calorimeter x ΔT_calorimeter