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CH302 Vanden Bout/LaBrake Fall 2012

Vanden Bout/LaBrake

CH301

THERMODYNAMICS Quantifying Heat Flow – Chemical

Change

UNIT 4 Day 3

CH302 Vanden Bout/LaBrake Spring 2012

Important Information

LM30 DUE T 9AMHW10 DUE T 9AM

EXAM 3 WRAPPER POSTED DUE T 9AM

UNIT4DAY3-VDBThursday, November 15, 20128:19 AM

UNIT4DAY3-VDB


What are we going to learn today?

Use calorimetry to calculate ΔHrxn

Use different methods to calculate ΔHrxn

Heats of FormationHess’s LawBond Energies


Quiz: CLICKER QUESTION 1

A bomb calorimeter measures heat at constantVolume, which is equivalent to.a) ΔUb) ΔHc) Work

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First Law of Thermodynamics –

LAW OF CONSERVATION OF ENERGY

UNIVERSE = SYSTEM + SURROUNDINGS

YOU DEFINE YOUR SYSTEM, EVERYTHING ELSE IS

SURROUNDINGS

ΔU = q + w


Enthalpy Change w/ Chemical Change

ENER

GY

REACTION PATH

2 CH3OH + 3 O2 2 CO2 + 4 H2O + heat



• Thermochemical equation- a chemical change together with the corresponding enthalpy change

• CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ mol rxn-1

• 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(l) ΔH = -1780 kJ mol rxn-1

• CO2(g) + 2H2O(l) CH4(g) + 2O2(g) ΔH = +890 kJ mol rxn-1

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• Thermochemical equation- a chemical change together with the corresponding enthalpy change

• CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ mol rxn-1

• 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(l) ΔH = -1780 kJ mol rxn-1

• CO2(g) + 2H2O(l) CH4(g) + 2O2(g) ΔH = +890 kJ mol rxn-1


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Standard Enthalpies, ΔHrxnº

• Reaction enthalpy based on all reactants and products being in their standard state (1 bar pressure). Tabulated data can be assumed to be at 25°C.


IT IS POSSIBLE TO CALCULATE THE ENTHALPY OF CHEMICAL CHANGE USING TABULATED DATA

MANY REASONS FOR THIS:

ESTIMATE RXN ENTHALPYTOO COSTLY TO RUN EXPERIMENTWANTING TO PREDICT SPONTENEITY


Hess’s Law – Combine a series of chemical reactions to estimate the change in enthalpy for the net equation.

CO2(g) CO(g) + ½O2(g) ΔHº = + 283 kJ mol rxn-1

C (s) + O2(g) CO2 (g) ΔHº = - 393 kJ mol rxn-1

USE THIS EXAMPLE TO DEMONSTRATE HESS’S LAW

C (s) + ½ O2 (g) CO (g) ΔHº = ?

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CO2(g) CO(g) + ½O2(g) ΔHº = + 283 kJ mol rxn-1

C (s) + O2(g) CO2 (g) ΔHº = - 393 kJ mol rxn-1


C (s) + ½ O2 (g) CO (g) ΔHº = ?



• Methanol is a clean burning liquid fuel proposed as a replacement for gasoline. Suppose it could be produced by the controlled reaction of the oxygen in air with methane. Find the standard reaction enthalpy for the formation of 1 mole CH3OH(l) from methane and oxygen, given the following information:

CH4(g) + H2O(g) CO(g) + 3 H2(g) ΔHº = +206.10 kJ

2 H2(g) + CO(g) CH3OH(l) ΔHº = -128.33 kJ

2H2(g) + O2 (g) 2H2O (g) ΔHº = -483.64 kJ



Hess’s Law -example



2H2(g) + O2 (g) 2H2O (g) ΔHº = -483.64 kJ

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Hess’s Law -example



2H2(g) + O2 (g) 2H2O (g) ΔHº = -483.64 kJ


Standard Enthalpy of Formation, ΔHfº

• ΔH for the formation of 1 mole of a compound from its elements in their most stable form at standard conditions

• 2 C (gr) + 3 H2(g) + ½ O2(g) C2H5OH(l) ΔHfº = -277.67 kJ


Use ΔHfº to calculate ΔHr

º

• Possible because ΔH is a state function: path doesn’t matter, just final minus initial.

• ΔHrº = ΣnΔHf

ºproducts - ΣnΔHf

ºreactants

UNIT4DAY3-VDB


Use ΔHfº to calculate ΔHr

º

• Possible because ΔH is a state function: path doesn’t matter, just final minus initial.

• ΔHrº = ΣnΔHf

ºproducts - ΣnΔHf

ºreactants


Example: Use ΔHfº to calculate ΔHr

º

• Calculate the standard enthalpy of combustion of methanol from data.


Bond enthalpies

• The heat required to break a mole of bonds at constant pressure.

• Calculate the heat of reaction for methanol using bond energies.

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Bond enthalpies

Example: Estimate the ΔHrº for the reaction

CCl3CHCl2(g) + 2HF(g) CCl3CHF2(g) + 2HCl(g)

using bond enthalpy data.



POLL: CLICKER QUESTION 2 FROM ACTIVITY

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WHAT HAVE WE LEARNED TODAY?

The transfer of heat energy into or out of a system at constant pressure is a state function called Enthalpy.

The change in Enthalpy can be determined experimentally using a coffee cup calorimeter at constant pressure.Then change in Enthalpy can be calculated based on a variety of tabulated data:Heats of formation/Other Heats of Reaction/Bond Energies


Learning Outcomes

Write a formation chemical equation for a compound

Calculate change in enthalpy for a reaction based on calorimetry data

Calculate change in enthalpy for a reaction based on tabulated data (Hess’s law, formation data, bond energy data).

UNIT4DAY3-VDB

unit4day3-vdb - gchemctp.cm.utexas.edu/courses/spring2013/ch302/notes/unit4day3-vdb.pdf ·...

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