Bell Work

1
Bell Work

• What does it mean to be exothermic? What about
  endothermic?
• If you were to guess, what do you think standard
  enthalpies of formation are?
• Discuss something that you have encountered in
  your life that is the sum of its parts.

2
Reality Check 12.2

• What amount of energy as heat is needed to raise
  the temperature of 6 moles of NaCl from 54 ?C to
  89 ?C? The molar heat capacity of NaCl is 50.5
  J/mol.K
• What amount of energy as heat is needed to raise
  the temperature of 320 g of silver from 12 ?C to
  76 ?C? The molar heat capacity of silver is 25.3
  J/mol.K
• What is the specific heat of iron if the molar
  heat capacity of iron is 25.1 J/mol.K?

3
Enthalpy and Hesss Law
4
Summary of equations thus far

• qn C ?T, used to calculate heat using the moles,
  molar heat capacity and temperature change
• Molar Mass x cp C, used to calculate the molar
  heat capacity using the specific heat and molar
  mass
• ?H C ?T, used to calculate the molar enthalpy
  change using the molar heat capacity and change
  in temperature if a substance is just being
  heated (no chemical reaction or phase change).
  This gives answer in J/mol. One can get rid of
  mol by multiplying by n.

5
From the homework, you may have realized that ?H
can have a negative number.

• It relates to the fact that energy as heat has
  either entered or left the system.
• If it was positive, it meant that the heating of
  the sample required energy.
• Energy in means endothermic
• So a positive ?H means that the process was
  endothermic.
• The opposite is true if ?H was negative.
• A negative ?H means the process was exothermic
  (energy left the system)

6
Thermodynamics

• The branch of science that is concerned with the
  energy changes that accompany chemical and
  physical changes
• To standardize the enthalpies of reactions,
  scientists have agreed upon a standard
  thermodynamic temperature of 25.00 ?C (298.15 K).

7
Equations and Enthalpy

• As a result, we can incorporate more information,
  like ?H values and temperatures, into chemical
  equations.
• For example
• Fe (s, 300 K) ? Fe (s, 1100K) ?H 20.1 kJ/mol
• H2 (g, 298 K) Br2 (l, 298 K) ? 2 HBr (g, 298K)
  ?H -72.8 kJ/mol

8
How do they get ?H values?

• To get ?H values, scientists use calorimetry.
• It is defined as the measurement of heat-related
  constants (specific heat, latent heat)
• It is an experimental measurement of an enthalpy
  change.
• A calorimeter is utilized a device used to
  measure the heat absorbed or released in a
  chemical or physical change.

9
Hesss Law

• It is stated that the amount of heat released or
  absorbed in a chemical reaction does not depend
  on the number of steps in the reaction.
• Example Phosphorous pentachloride can be made
  two ways.
• Way 1 1 Step
• P4 (s) 10 Cl2 ? 4 PCl5 (g) ?H -1596 kJ

10
Hesss Law

• Way 2 takes two steps
• P4 (s) 6 Cl2 (g) ? 4 PCl3 (g) ?H -1224 kJ
• PCl3 (g) Cl2 (g) ? PCl5 (g) ?H -93 kJ
• Notice that we need to do the second step 4 times
  to use all the PCl3 from step 1.
• So, -1224 4(-93) -1596 kJ
• Comparing the 1 step way from the last slide to
  the 2 step way from this slide
• -1596 kJ -1596 kJ, Hesss Law works

11
Using Hesss Law, standard enthalpies of
formation and some Algebra

• Things to remember
• Standard enthalpy of formation the enthalpy
  change in forming one mole of a substance from
  elements in their standard states (Pg 355)
• When equations are added or subtracted, enthalpy
  changes must be added or subtracted.
• When equations are multiplied by a constant, the
  enthalpy change must be multiplied by the same
  constant. A -1 will flip the equation around.
• Equations will have to be balanced.

12
What does Page 355 (and 833) mean?

• What this table is saying is when a compound is
  formed from the elements that make it up, ?H of
  formation will be the value listed.
• Note When doing these problems, it is acceptable
  to use fractions for coefficients.
• Remember the diatomics and that S by itself is S8
• ?H of elements is zero, so those equations dont
  need to be written down.
• Ca O2 ?
• Al O2 ?

CaO
?H -634.9 kJ/mol
½
2
Al2O3
?H -1676.0 kJ/mol
3/2
13
Steps I like to follow.

• Write down the balanced equation you are working
  toward.
• Write the formation equation for each compound
  with its ?H value. Remember this will be the
  elements going to the compound. Balance the left
  hand side only with coefficients.
• If any compounds have a coefficient in the final
  equation, multiply that equation and ?H by that
  coefficient
• Reverse any equations by multiplying its ?H by -1
  and rewrite those equations.
• Add the equations together and cancel repeating
  compounds and elements that appear on both sides.
  Also add ?H values. Use this as a double check.

14
Now we can look at an example

• We want to look at the enthalpy change when
  carbon and carbon dioxide are reacted together to
  make carbon monoxide. (C CO2 ? 2 CO)
• So, we need to look at the formation of CO and
  CO2 to complete this problem.
• Writing the formation of each of these, we have
• C ½ O2 ? CO ?H -110.5 kJ/mol
• C O2 ? CO2 ?H -393.5 kJ/mol
• With this information, lets finish the problem
  on the overhead.

15
Examples

• Use the Table 2 on page 355 to calculate ?H for
  the decomposition of calcium carbonate into
  calcium oxide and carbon dioxide.
• What enthalpy change accompanies the reaction
• 2Al(s) 3H2O (l) ? Al2O3 (s) 3H2 (g)

16
Homework

• Page 371 26, 27, 28, 29