Thermochemistry

Chemical reactions almost always involve a transfer of energy between the reaction __________ and its _________________

Transfer of energy between the reaction system and its surroundings

system

The reaction system includes all the chemicals involved in the reaction

Surroundings

Consist of everything else such as containers, water baths and the atmosphere

Two ways in which energy is transfered between a reaction system and its surroundings

Either as work or heat

Thermodynamics

the study of energy transformations or transfers

Work (w)

The transfer of energy as work involves the action of a force through a distance

-Work done on a system is expressed as w=-P(Vf-Vi)

The minus sign occurs becasue were using stating that the work done on a system is a positive quantity

-Si unit is the Joule, pressure in pascals, and volume in cubic meters (m^3)

1 J=

1 kg m^2/s^2

1 Pa=

1 J / m^3

1 L atm = ___ J

101.325 J

work formula

w=-P(Vf-Vi)

Also:

w= -FΔh

w=-PAΔh

w=-PΔV = -P(Vf-Vi)

w=ΔnRT

When the work is positive, the system...

gains energy

When the work is negative, the system...

loses energy

At constant pressure, which of these systems do work on the surroundings

-A(s) +2 B(g)----> C(g)

-2A(g) + B(s) ---->3C(s)

-A(g) +2 B(g)---->2C(s)

-A(g) +B (g)----> 3C(g)

A system that does work on the surroundings has a negative work, w= -PΔV or w=-ΔnRT

--ΔV & Δn are positive if there is an increase in the # of moles of gas as the reaction proceeds

---2A(g) + B(s) ---->3C(s) & A(g) +B (g)----> 3C(g) produce more moles of gas than they consume so ΔV and Δn are both positive and w is negative

w=-P(Vf-Vi) equation gives the work done on a system that is...

compressed or expanded at a constant pressure P

--especially useful in thermochemistry because many reactions take place in vessels open to the atmosphere and the aatmosphere exerts a constant pressure suring the course of the reaction

The work done on a system as a result of compression is a _________ quantity

Positive

The work done on a system as a result of expansion is a _________ quantity

Negative

heat (q)

The energy transferred between objects that are at different temperatures

--When energy as heat is lost by the system to the surroundings, sign of q is negative

Unlike the transfer of energy as work, the transfer of energy as heat does not require...

The application of a force

energy flows spontaneously as heat from a region of ______ temperature to a region of ______ temperature

High temp ------> low Temp

Energy transfer as heat occurs whenever there is a...

temperature difference between a system and its surroundings

exothermic reaction

a reaction that releases energy as heat into the surroundings, will feel hot

- exo=out

-negative ΔH value

ex: a burning match, burning rocket fuel, the reaction inside a chemical heat packet

endothermic reaction

a reaction that absorbs energy as heat from the surroundings, will feel cold

--when energy as heat is transferred from the surroundings into the system, the sign of q is positive

(endo=in)

-positive ΔH value

ex: popcorn popping in a microwave oven (heat absorbed from microwave), boiling water (heat taken from surroundings to turn water from liquid to gas)

Phase changes for endothermic reactions

-solid to liquid (melting)

-liquid to gas (boiling)

-solid to gas (sublimation)

-think of it as going "UP" in phases

-require the addition of heat

Phase changes for exothermic reactions

-liquid to solid (freezing)

-gas to liquid (condensation)

-gas to solid (deposition)

--think of it as going "DOWN" in phases

-require the removal of heat

A system does not contain heat or work, rather heat and work are ways in which....

energy is transferred

In thermodynamics, the energy of a system is denoted by the symbol...

U

energy change of a system

Is denoted by ΔU where ΔU = Uf- Ui

Application of the law of conservation of energy to a reaction system yields...

ΔU = q + w

q= energy trasnfer as heat to or from the system

w= energy trasnferred as work to or from the system

--Also mathematical expression for first law of thermodynamics

state functions

In thermodynamics, functions that depend only on the state of a system and not on how that state is achieved

--The value of ΔU does not depend on how the system goes from the initial to the final state

-state functions denoted with capital letters

ex: energy and enthalpy are state functions

----Work and heat are not state functions!

Energy transfer functions aka path functions

Their values depend onhow the process is carried out or the conditions under which the energy is trasnferred

-denoted with lowercase letters

-ex: heat and work and distance travelled

energy change of the reaction formula

ΔUrxn = q + w

rxn = reaction

enthalpy (H)

The heat content of a system at constant pressure

state function (does not depend on how that reaction is carried out)

-

ΔHrxn =

(the enthalpy change of the reaction)

ΔUrxn + PΔVrxn @constant P

difference between ΔUrxn and ΔHrxn

Just the Energy required for the system to expand or contract against a constant external pressure

-Small difference between values about 1-2 %

enthalpy change for a reaction has the useful property that

ΔHrxn = qp (energy evolved or absorbed as heat at constant pressure

-the transfer of energy as heat at constant pressure is a state function

For an exothermic reaction, the enthalpy of the reactants is _______ than the enthalpy of the products

Greater

-energy is evolved as heat as the reaction proceeds

For an endothermic reaction, the enthalpy of the reactants is _______ than the enthalpy of the products

Less

-Energy in the form of heat must be supplied in order to drive the reaction up the enthalpy "hill"

The enthalpy change for a chemical reaction is equal to the total enthalpy of the products minus the total enthalpy of the reactants. formula that describes this is:

ΔHrxn = Hprod - Hreact

ΔH°rxn

Used when specifying a value for the enthalpy change of a PARTICULAR CHEMICAL REACTION that takes place at a pressure of 1 bar

--The superscript degree sign tells us that the enthalpy change is the standard enthalpy change for the reaction

-The subscript rxn means that ΔH°rxn is expressed in units of kilojoules per mole for the chemical equation as written .

For ex with 2H2 (g) + O2 (g) ---> 2H2O (l), the value of ΔH°rxn stated is the enthalpy change for the reaction of 2 moles of H2(g) with 1 mole O2 (g) at 25C and a pressure of one bar

standard enthalpy change

refers to pure reactants at a pressure of one bar and to pure products at a pressure of one bar

standard states

Pure form of a substance at specified temperature at pressure of 1 bar

-standard state for a solute is one molar (1M) solution of the solute

enthalpies of reactions may be used to predict...

The amount of energy transferred as heat during a given process