Thermochemistry

Spring Semester Final Exam Review

THERMOCHEMISTRY

The study of heat released or required by chemical reactions

Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt)

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

What is Energy?

Energy

Kinetic energy (KE)

Potential energy (PE)

Energy due to motion

Stored energy

Total Energy = Kinetic Energy + Potential Energy

E = KE + PE

Temperature measures the average

Kinetic energy & potential energy are interchangeable

Heat is the total energy of a system:

Kinetic energy + potential energy

Systems & Surroundings

In thermodynamics, the world is divided into a system and its surroundings

A system is the part of the world we want to study (e.g. a reaction mixture in a flask)

The surroundings consist of everything else outside the system

EXOTHERMIC & ENDOTHERMIC REACTIONS

Exothermic process: a change (e.g. a chemical reaction) that releases heat to the surroundings.

A release of heat corresponds to a decrease in enthalpy

Exothermic process: H < 0 (at constant pressure)

Burning fossil fuels is an exothermic reaction

Exothermic Reactions

Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat from the surroundings.

An input of heat corresponds to an increase in enthalpy

Endothermic process: H > 0 (at constant pressure)

Photosynthesis is an endothermic reaction (requires energy input from sun)

Endothermic Reactions

Endothermic or Exothermic?

exothermic

endothermic

exothermic

endothermic

endothermic

Heating Curves

A plot of temperature vs. time that represents the process in which energy is added at a constant rate

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Heating Curves Animation

Tem

pera

ture

(o C

)

40

20

0

-20

-40

-60

-80

-100

120

100

80

60

140

Time

Melting - PE

Solid - KE

Liquid - KE

Boiling - PE

Gas - KE

Heating Curves

A plot of temperature vs. time that represents the process in which energy is added at a

constant rate

The standard enthalpy of reaction (DH0 ) is the enthalpy of a reaction carried out at 1 atm.

rxn

aA + bB cC + dD

DH0rxn dDH0 (D)fcDH0 (C)f= [ + ] - bDH0 (B)faDH0 (A)f[ + ]

DH0rxn nDH0 (products)f= S mDH0 (reactants)fS-

Calculate the heat of combustion of methane, CH4

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

H◦f CH4 (g) = -74.86 kJ/mol

H◦f O2(g) = 0 kJ/mol

H◦f CO2(g) = -393.5 kJ/mol

H◦f H2O(g) = -241.8 kJ/mol

15

pg. 316

Example Problem

2 mol(-241.8 kJ/mol) = -483.6 kJ

Step #1: multiply the H◦f H2O(g) by 2 since there are two moles of water in the products .

Calculate the heat of combustion of methane, CH4

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

H◦f CH4 (g) = -74.86 kJ

H◦f O2(g) = 0 kJ/

H◦f CO2(g) = -393.5 kJ

H◦ fH2O(g) = -483.6 kJ

16

pg. 316

Example Problem

Step #2: sum up all the H◦ f. :

H◦f = [-393.5 kJ + (-483.6 kJ)]- [-74.86 kJ + (0 kJ )]

H◦f = -802.2 kJ

Hrxn = Hf(products) - Hf(reactants)

Calculations Involving Specific Heat

TmCq

C = Specific Heat Capacityq = Heat lost or gainedT = Temperature

changeTf - Ti

ORTm

qC

Choose all that apply...

C(s) + 2 S(g) CS2(l) H = 89.3 kJ

Which of the following are true?

A) This reaction is exothermic

B) It could also be writtenC(s) + 2 S(g) + 89.3 kJ CS2(l)

C) The products have higher energy than the reactants

D) It would make the water in the calorimeter colder