Top Banner
THERMOCHEMISTRY Chemistry 30 Review Sessions
23

Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Jul 14, 2020

Download

Documents

dariahiddleston
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

THERMOCHEMISTRY

Chemistry 30 Review Sessions

Page 2: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum bullets

� Use Q = mcΔt to analyze heat transfer.� Use calorimetry data to determine the enthalpy

changes in chemical reactions.

Page 3: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

A Bomb Calorimeter Enthalpy Change in Bomb shows up as

Temperature Change in Calorimeter

Calorimeters rely on the principle of energy conservation:

Heat Gained = Heat Lost

Qcal = ΔrH

mcΔt = nΔrHm

Page 4: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Typical Heat Lost=Heat Gained Problem

Wikipedia gives the molar enthalpy of combustion for methane as –890 kJ/mol.

What minimum mass of methane must be burned to warm 4.00 L of water from 22.4ºC to 87.6ºC, assuming no heat losses?

Page 5: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Heat Lost = Heat Gained

Water increased in temperature – heat gained – QBurning Methane (CH4) – enthalpy change - ΔH

Q = ΔHmcΔt= nΔrHm

n = mcΔt/ ΔrHmn = (4.00 kg)(4.19kJ/kg°C)(87.6-22.4°C)/(890kJ/mol)

n = 1.2278 molm = nM

m = (1.2278 mol)(12.01 +4.04 g/mol) = 19.7 g

Page 6: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum Bullet

� design a method to compare the molar enthalpy change when burning two or more fuels

� evaluate the economic and environmental impacts of different fuels by relating carbon dioxide emissions and the heat content of a fuel

� provide examples of personal reliance on the chemical potential energy of matter, such as the use of fossil fuels

Page 7: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Comparing Fuels

Compare propane with methane in terms of a. energy per gram of fuelb. energy per mole of CO2(g) produced

Molar Enthalpies of Combustion propane ΔcHm= -2220 kJ/molmethane ΔcHm= -890.4 kJ/mol

Page 8: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Energy per mass of Fuel

Calculation: molar enthalpy /molar mass= ΔcHm /M

Propane: -2220 kJ/mol/44.11g/mol = - 50.33 kJ/gMethane: -890.4 kJ/mol/16.05g/mol = -55.47 kJ/g

Methane is a little better, i.e. more energy/gram

Page 9: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Comparing Fuels: Energy/CO2 produced

CO2(g) contributes to global warming.Write the balanced equation for 1 mole of fuel

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

Calculate : ΔcHm /n CO2propane ΔcHm /n CO2 = - 2220 kJ/mol/3 = -740 kJ/molmethane ΔcHm /n CO2 = -890.4 kJ/mol/1 = -890.4 kJ/mol

Methane gets you more energy per CO2 - a good thing!

Page 10: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum Bullet

� Explain, in a general way, how stored energy in the chemical bonds of hydrocarbons originated from the sun.

butHydrocarbonsaredecomposedlivingthings!and all living things rely upon photosynthesis…

which just converts light to chemical energy and

Hydrocarboncombustionreleasesenergy,soHCsarestoringenergy,

light comes from the sunso… light from the sun provides the energy which builds organic molecules

Page 11: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum bullet

� Define enthalpy and molar enthalpy� Write balanced equations for chemical

reactions that include energy changes.� Classify endothermic or exothermic,

including : photosynthesis, cellular respiration and hydrocarbon combustion.

� Use and interpret ΔH notation

Page 12: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

One of these ain’t like the other ones, one of these just ain’t the same…

State the molar enthalpy

2 Na(s) + Cl2(g) 2 NaCl(s) ΔfHm= -411.2 kJ/molNaCl

Include the absolute value of the energy in the balanced equation.2 Na(s) + Cl2(g) 2 NaCl(s) + 822.4 kJ

State the enthalpy change at the end of the reaction.

2 Na(s) + Cl2(g) 2 NaCl(s) ΔH= - 822.4 kJ

Page 13: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

What’s the energy change here?

The molar enthalpy of formation of sodium chloridesolid is -411.2 kJ/mol. This reaction is balanced if__i__ kJ of energy is included at position __ii__ .

2 NaCl(s) + x 2 Na(s) + Cl2(s) + yi ii

a 411.2 xb 411.2 yc 822.4 xd 822.4 y

Page 14: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Hess’ Law

� Predict the enthalpy change for chemical equations using standard enthalpies of formation.

� Explain and use Hess’ law to calculate energy changes for a net reaction from a series of reactions.

Page 15: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Solve for ΔrH by summation

SO3(g) + H2O(g) H2SO4(l) ΔH = ?

Calculate the enthalpy of the above reaction using:

H2S(g) + 2O2(g) H2SO4(l) ΔH= -78.5 KJH2S(g) + 2O2(g) SO3(g) + H2O(l) ΔH= -69 KJ

H2O(l) H2O(g) ΔH= +11 KJ

Page 16: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Solve for ΔrH with formation reactions

George Bush has made corn very expensive because he plans to make automobile fuel out of ethanol – which is made from corn.

1. Write the balanced chemical equation for the combustion of liquid ethanol.

2. Calculate the enthalpy change for the balanced reaction.

C2H5OH(l) + 3 O2(g) 3 H2O(g) + 2 CO2(g)

ΔrH = [Sn ΔfHm ]prod – [Sn ΔfHm ]reac

Page 17: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

ΔrH = [Sn ΔfHm ]prod – [Sn ΔfHm ]reac

ΔrH = [Sn ΔfHm ]prod – [Sn ΔfHm ]reac

ΔrH =[ 3 x h20+ 2 x co2 ] – [1 x ethanol]ΔrH =

Page 18: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum Bullet

� Identify that liquid water and carbon dioxide gasare reactants in photosynthesis and products of cellular respiration

� gaseous water and carbon dioxide gas are the products of hydrocarbon combustion in an open system.

6 H2O(l) + 6 CO2(g) + 2802.5 kJ 6 O2(g) + C6H12O6(s)

CH4(g) + 2 O2(g) 2 H2O(g) + CO2(g) + 890 kJ

Page 19: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum Bullet

� Define activation energy as the energy barrier that must be overcome for a chemical reaction to occur.

� draw and interpret enthalpy diagrams for chemical reactions

Page 20: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum Bullet

� Explain the energy changes that occur during chemical reactions, referring to bonds breaking and forming and changes in potential and kinetic energy.

� Analyze and label energy diagrams of a chemical reaction, including reactants, products, enthalpy change and activation energy.

Page 21: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Enthalpy Diagrams

� Use the enthalpy diagram to determine the molar enthalpy of reaction for:

S8(s) and Cl2 (g)

SCl2(g)

SCl4(l)

SCl4(l) SCl2(g) + Cl2(g)

56.1

ΔH (k

J/m

ol)

19.7

Page 22: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

The Effect of a Catalyst1 How does the graph show the enthalpy change is not affected by the catalyst?

2. What is the benefit of the catalyst?

3. Where is the activation energy of the forward reaction.

4. Where is the activation energy of the reverse reaction?

5. Is the forward reaction endo or exo?

Page 23: Chemistry 30 Review ThermoChem - Ms. Mogck's Classroom · Chemistry 30 Review ThermoChem Created Date: 12/6/2016 8:57:30 PM ...

Curriculum Bullet

� Explain that catalysts increase reaction rates by providing alternate pathways for changes, without affecting the net amount of energy involved; e.g., enzymes in living systems.