Chapter 9 - Energy, Enthalpy, and Thermochemistry

Question 1

What is energy

Answer 1

• capacity to do work or to produce heat

Question 2

What is work

Answer 2

• due to a force acting over a distance

Question 3

What is the law of conservation of energy

Answer 3

• first law of thermodynamics
• energy can neither be created nor destroyed
• energy can be converted from one form to another
• the energy of the universe is constant

Question 4

What are 2 methods of transferring energy

Answer 4

Heat: energy transferred due to a temp difference
Work: transfer of energy by the action of a force through a distance

Question 5

Where is the energy coming from in an exothermic reaction?

Answer 5

• potential energy is stored in chemical bonds
• products have a lower PE than reactants
• energy released when forming bonds is greater than the energy absorbed when breaking bonds
• chemical energy is converted into thermal energy

Question 6

When is a system endo or exothermic? explain using q (heat)

Answer 6

when q>0, endothermic, system energy increases
when q<0, exothermic, system energy decreases

Question 7

Explain expansion and compression using work

Answer 7

expansion: work done by the gas (w<0), exothermic, system energy decreases

compression: work done on the gas (w>0), endothermic, system energy increases

Question 8

Calculate the work associated with the expansion of a gas form 46L to 64L at constant external pressure of 15atm

Answer 8

Gas expansion
- system doing work on surrounding
- therefore, work is negative

W = -P(Vf-Vi)
= (15atm)(64L-46L)
= - 270L*atm

convert to joules

-270Latm * (101.325J)/1Latm
= -2.7*10^-4 J

energy flows out of the system! exothermic

Question 9

What is a change in enthalpy equal to?

Answer 9

• equal to the amount of heat given off or absorbed at a constant pressure

Question 10

Calculate the enthalpy required to heat 1kg of ethane gas (C2H6) from 25 to 75C under condition of constant volume.

Cp=52.92 J/Kmol
Cv= 44.60 J/Kmol

Answer 10

we know constant volume, therefore, change in P increases
change in T = 75-20 = 50C = 50K

moles: 1kg = 1000g/30.07g/mol = 33.3 mol C2H6

Change in H = nCpT
= (33.3mol)(52.92J/Kmol)(50K)
=88112J
= 88.1kJ

Change of H is directly proportional to the change of T

OR could use this eq

change in E = nCvT

Question 11

What is heat capacity?

Answer 11

• represents the amount of energy necessary to increase the temperature by 1C (1K), in J/K or J/C

Heat capacity = (heat absorbed) / (increase in temperature)

Question 12

The combustion of 1.010g sucrose C12H22O11 in a bomb calorimeter causes the temperature to rise from 24.92 to 28.33C. The heat capacity of the calorimeter assembly is 4.90 kJ/C

What is the heat of combustion of sucrose in kJ per mole of sucrose?

Answer 12

bomb calorimeter - constant volume - combustion - exothermic

• heat absorbed by calorimeter

qcal = CT
=(4.90kJ/C)(28.33-24.92C)
=16.7kJ

change in H = qrxn = -qcal = -16.7kJ

qrxn = -16.7kJ / 1.010g sucrose = -16.5kJ/g sucrose

qrxn = -16.5kJ/g sucrose * 342.3g/mol
=-5.65*10^3 kJ/mol