Thermochem Quick review

General Thermochemistry Terms

•Thermodynamics -Study of NRG changes & flow of NRG.Tells whether a reaction is possible.

•system - That portion of the universe on whichwe focus.

•surroundings - Everything outside the system.

•Isothermal- A change that occurs at constanttemperature.

•state functions Properties that depend on the initial andfinal state, not on how the change was made. ex: DH, temp, but not work

Energy changes can be measured using calorimetry. Oftenthis involves heating water under controlled conditions (abomb calorimeter).

Three closely related terms are:1. heat capacity is the amount of heat needed to change asystem by 1°C.2. molar heat capacity is the amount of heat needed to changea mole of a substance by 1°C.3. specific heat is the amount of heat needed to change 1 gramof a substance by 1°C. (Water: 1 cal/g°C = 4.184 J/g°C)

Note that heat capacity is an extensive property whereas theother two are intensive.

Coffee cup calorimeter q=mcpDT

Enthalpy

•Heat content of a substance•Symbol H•Unit kilojoules/mole•D H : Hproducts - Hreactants

•Hess’ Law If several reactions add up to give an overall reaction, the D H’s of the reactions will add up to the overall DH.

•Standard Heats of Formation, DHf ° , are useful for thispurpose. This is the energy involved in making one mole of asubstance from its elements at 25°C and 1 atm pressure.

•This law is often written as:DH° reaction = DHf ° products - DHf ° reactantsNote: DHf° for elements is 0.

•If you are NOT using heats of formation, you need to writeout the equations to see how they combine.

BOND ENERGY•Bond energy is the amount of energy needed to BREAK acertain bond. •Breaking a bond is always endothermic

•You can determine the approximate energy change in achemical reaction by adding the bond energies of thereactants and subtracting from this the bond energies of the products. Reactants-products

Note: this is opposite to the Hess’s Law (products -reactants) because bond energy involves breaking bondswhereas DH°f involve forming bonds.

Entropy•Symbol is S (DS)•Unit J/K mol or J.mol-1K-1

•The degree of disorder (randomness) in a system•Spontaneity tends toward + DS•The entropy of a pure crystalline substance at absolute zero is 0.•Larger and more complex molecules have greater entropies

FREE ENERGY

•There is an upper limit to how much work can be derivedfrom any system. That maximum work is called the Gibb’sFree Energy, DG.

•The theoretical maximum work can be achieved if the stepsare so small that they can go either way, if they arereversible.

DG = DH – TDS• This represents the theoretical maximum work that can be done by a system.•A reaction is spontaneous when DG is negative.•When DG = 0, the reaction is at equilibrium.•MAKE SURE THAT YOUR DH AND DS HAVE THE SAME UNITS (CONVERT J IN DS INTO kJ

DH D S Spontaneous…- + … at all temperatures- - … at LOW temperatures+ + … at HIGH temperatures+ - ..reverse rxn spontaneous

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

 For the reaction of methane represented above, H is -

1,523 kJ mol-1. What is the value of H if the combustion produced liquid water H2O(l), rather than water vapor, H2O(g)? (H for the phase change H2O(g) H2O(l) is -44 kJ mol-1).

a. -1,479 kJ/molb. -1,567 kJ/molc. -1,323 kJ/mold. -1,611 kJ/mole.-1,411 kJ/mol

MC 1

Which gives the thermodynamic parameters of the phase change in a system that begins as an open container of liquid water placed in a constant temperature environment of 230 K?

I. G < 0II. H < 0III. S < 0

a) I onlyb) III onlyc) I and III onlyd) II and III onlye) I, II and III

MC 2

Which gives the thermodynamic parameters of the phase change in a system that begins as an open container of liquid water placed in a constant temperature environment of 230 K?

I. G < 0II. H < 0III. S < 0

a) I onlyb) III onlyc) I and III onlyd) II and III onlye) I, II and III

MC 3

I2(g) + 3 Cl2(g) 2 ICl3(g)

 According to the data in the table below, what is the value of Ho

f for the reaction represented above?

 Average Bond Energy

Bond (kilojoules/mol)I-I 150Cl-Cl 240I-Cl 210

 a) +450 kJ/molb) +1,260 kJ/moc) -870 kJ/mold) -390 kJ/mole) +180 kJ/mol

MC 4

H2(g) + O2(g) H2O(l) Ho = x

2 Na(s) + O2(g) Na2O(s) Ho= y

Na(s) + O2(g) + H2(g) NaOH(s) Ho= z

 

Based on the information above, what is the standard enthalpy change for the following reaction?

Na2O(s) + H2O(l) 2 NaOH(s)

a. x + y + zb. x + y - zc. x + y - 2zd. 2z - x - y e. z - x - y

MC 5