chem 1025 (exam 3) chapter 6

thermodynamics (heat/work)

is the study of energy and its transformations

• when energy is transferred from one object to another it appears as HEAT and/or WORK

thermochemistry

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

system + surroundings =

universe

system

• The part of the universe we are studying

• typically, the reactants and products

surroundings =

• everything else

• typically the immediate surroundings

  • ex. container & air around the container

energy E

the capacity to do work (w) or to produce heat (q)

potential energy

• stored energy

• energy associated with the position

• in chemical systems PE is in the chemical bonds

kinetic energy

energy of motion

• KE = ½ (mass)(velocity)²

First law (law of conservation of energy)

Energy is neither created nor destroyed

• but can be transferred form one kind of energy to another

total energy of universe is

constant

• delta E _universe = delta E _system + delta E _surroundings = 0

internal energy of a system is the sum of

all the potential and kinetic energies of all the particles present

change in internal energy =

E final - E initial

units of energy

1 J = 1 Kg x m²/s²

1 cal =

4.184 J

the British Thermal Unit (BTU)

is used to rate appliances

• 1 BTU = loss J

delta E =

q (heat) + w (work)

heat (q)

The energy transferred as a result of the difference in temperature between the system & surrounding

ex. hot to cold

work (w) =

the energy transferred when an object is moved by a force

q_sys = + means

systems gains (absorb) heat from the surroundings

• ENDOTHERMIC

absorbing heat =

ENDOTHERMIC

q_sys = - means

System loses (releases) heat to the surroundings

• EXOTHERMIC

releasing heat =

EXOTHERMIC

w_sys = + means

Work done BY the surrounding ON the system

• system is GAINING energy as work

w_sys = - means

work done BY system ON the surroundings

• system is losing energy as work

w =

-P delta V

units for w (work)

atm x L or L x atm

1 L x atm =

101.3 J

work of expansion =

The volume of the system increases if the temperature is raised or if a chemical reaction results in a net increase in the # of moles of gas

• the system expands DOES work on the surroundings

• losing energy

• PV = nRT (inverse)

work of contraction =

The volume of the system decreases if the temperature is lowered or if a chemical reaction results in a net decrease in the number of moles of gas

• The surrounding does work on the system

• system contracts

• gaining energy as work

• PV = nRT (direct)

delta H =

q

delta H = - then…

… its is exothermic

delta H = + then…

… its is endothermic

exothermic reaction

• gives off (produces) heat

• heat is transferred from the system to the surroundings

• the system loses energy as heat (-q)

• the temp of the surroundings increases since the surroundings gain heat (feels hot)

  • ex. instant hot packs

endothermic reaction

• absorbs heat

• heat + reactants → products

• heat is transferred from the surroundings to the system

• the system gains energy as heat (+q)

• the temp of surroundings decreases since the surrounding looses heat

calorimetry

• the science of measuring heat (q)

• based on temperature change when a substance absorbs or releases heat

amount of heat depends on (q = mc (delta) T)

• m = mass

• c = specific heat capacity

• delta T = temperature change

capacity (C) =

The amount of heat energy required to raise the temperature of 1.0 g by 1 degree C

high value of C

lots of heat

low value of C

little heat

q _system =

-surroundings

enthalpy delta H =

the heat absorbed or released by a reaction @ constant pressure

delta H formula

delta H = delta E + P(delta V)

@ constant pressure, delta H =

q_p

when heat is absorbed delta H is

delta H is positive

when heat is released delta H is

Delta H is negative

the sign of delta H is changed when

equation is revered

thermochemical equation =

balanced equaiton that includes delta H reaction

the sign of delta H indicates whether

its is exothermic or endothermic

• neg = exo

• pos = endo

Hess’s Law (overall enthalpy of reaction) =

The sum of the enthalpy values of the individual steps of the reaction

• used to determine enthalpy of reaction that cent be measured by calorimetry