Heat in Chemical Reactions

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Heat in Chemical Reactions

• Enthalpy

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Measuring Heat Energy

• 1 calorie is amount of heat energy to raise the
  temperature of 1 g water 1 deg. Celsius
• 1 Cal 1000 cal 1 cal 4.184 J
• Convert 15 J to calories.
• How many kJ are stored in a 250. Cal (note BIG
  C) candy bar?

15 J X
1 cal 4.184 J
3.6 cal (2 sf like the measure)
250 Cal X 4.184 kJ 1050 kJ
1 Cal
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Temperature

• Measures average kinetic energy of matter
• Kinetic energy energy of motion
• Temperature actually measures how fast particles
  are moving

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Celsius (oC)

• Metric system based on water
• Normal Freezing point 0 oC
• Normal Boiling point 100 oC

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Kelvin (K)

• SI unit based on Absolute Zero
• Absolute zero temperature where all motion
  stops (lowest possible temperature)
• Absolute zero 0 K - 273 oC

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Remember Temperature Conversions

• oC ? K ADD 273
• Ex. What is the freezing point of water in
  Kelvin? Boiling point?
• K ? oC SUBTRACT 273
• The boiling point of argon is 87 K. What is the
  boiling point of argon in oC?

0 C 273 273 K is freezing pt. 100 C 273
373 K is boiling pt
87 K-273 -186 C is boiling pt. of argon
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What is exothermic? endothermic?

• Initial T Final T ? T
• Low higher
• High lower -

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Endothermic

• Heat is GAINED or ABSORBED by the matter
• ?H (positive value)
• Observe decrease in Temperature because heat
  energy being converted to potential energy stored
  in bonds (surroundings lose heat to reaction
  products)

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Exothermic

• Heat is LOST or RELEASED
• - ?H (neg. value)
• Observe increase in Temperature because potential
  energy stored in bonds being released to the
  surroundings

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Using Stoichiometry to calculate Enthalpy

• All matter has heat energy. Breaking and making
  of bonds during reactions will release or absorb
  heat.
• Enthalpy (?H ) Change in heat during a reaction
  due to breaking and making bonds
• Calculated by ?H Hproducts -
  Hreactants

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2 C6H6 15 O2 ? 12 CO2 6 H2O ?H -98.0
kJ

• Is this reaction endothermic or exothermic?
• Calculate the heat transferred by the combustion
  of 1.75 mol of benzene (C6H6).
• Treat enthalpy like it is another ratio of the
  balanced equation.
• So, just like you see 2 mole benzene makes 6
  moles water, 2 moles benzene contributes to 98 kJ
  heat produced!

Exothermic! Energy is released (neg. value tells
us that the products have less energy than the
reactants).
172 kJ 2
1.75 mol C6H6
X -98.0 kJ 2 mol C6H6
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When heated, ammonium dichromate ((NH4)2Cr2O7)
decomposes into nitrogen, water, and chromium
(III) oxide (?H -315 kJ). Determine how
much heat is transferred for the production of
53.0 g of water.
(neg. means heat is product side)
(NH4)2Cr2O7 ? N2 4 H2 O Cr2O3 315
kJ
1. write a balanced equation
2. Convert g to moles (cant use balanced
equation with grams!)
3. Use stoichiometry (mole ratio from equation)
53.0 g H2O X 1 mol H2O 18.02 g H2O
X 315 kJ 4 mol H2O
232 kJ
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CaO 3 C ? CaC2 CO ?H 464.8 kJ
Positive means endothermic!

• How much heat is transferred when 100.0 g of
  carbon reacts with 50.0 g calcium oxide? Is the
  reaction endothermic or exothermic?

Which quantity limits the reaction? Many ways to
get the answer. Heres one
100.0 g C X 1 mol C X ( 464.8 kJ) 1290.
kJ 12.01 g 3mol C
50.0 g CaO X 1 mol CaO X ( 464.8 kJ) 414
kJ 56.08 g 1 mol CaO
This is less energy- the most possible with
these quantities of reactants.
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Practice!
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Determining ?H for Rxns(end of page/extra
handout)

• We can calculate the enthalpy if we know the
  initial and final energy of the reactants and
  products using
• ?H ?H products ?H reactants
• The total enthalpy change is the enthalpy total
  of the products minus the enthalpy total of the
  reactants.

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Example 1

• How much enthalpy is produced or absorbed during
  the complete combustion of methane?
• CH4 (g) O2(g) ? CO2 (g) H2O (g)

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2
Hf -75 kJ/mol 2X (0 kJ/mol)? -393.5kJ/mol
2X(-242kJ/mol)
?Hreactants -75 0 kJ
?Hproducts -393.5 -484kJ
?H (-877.5kJ) - ( -75kJ) -802.5
kJ THIS IS EXOTHERMIC
1. Balance the equation.
2. Find the enthalpy of formation for each item
its phase substance in the given table.
3. Add the enthalpy of the products, add the
enthalpy of the reactants.
4. Subtract sum of reactants from sum of
products.
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Practice!
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Hesss Law

• If a series of reactions are added together, the
  enthalpy change for the net reaction will be the
  sum of the enthalpy changes for the individual
  steps.
• Whole sum of the parts
• Or, path doesnt matter just the start and the
  end comparison!

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Hesss Law Example

• N2 O2 ? 2 NO ?H1 181 kJ
• 2 NO O2 ? 2 NO2 ?H2 -113 kJ

N2 2 O2 ? 2 NO2 ?H total 68 kJ
Treat like these are algebraic equations If
something (a chemical formula) is the same on
the two sides of the arrows, they cancel out
Then, add the equations (including the enthalpy)
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Rules of Hesss Law

• If the coefficients of a rxn are multiplied by a
  factor, the ?H is multiplied by the same factor.
• If the rxn is reversed, the sign of the ?H is
  reversed.

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Example 2

• The combustion of propene proceeds in 2 steps
• CH2CHCH3 H2 ? CH3CH2CH3 ?H -124 kJ
• CH3CH2CH3 5 O2 ? 3 CO2 4 H2O ?H -2220 kJ
• Calculate the value of ?H for the combustion of
  propene into carbon dioxide and water.

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Calculate the value of ?H for the combustion of
propene into carbon dioxide and water

• CH2CHCH3 H2 ? CH3CH2CH3 ?H -124 kJ
• CH3CH2CH3 5 O2 ? 3 CO2 4 H2O ?H -2220
  kJ

CH2CHCH3 H2 5 O2 ? 3 CO2 4 H2O ?H
-2344 kJ
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Example 3

• From the following enthalpy changes,
• 2 PbO 2 SO2 ? 2 PbS 3 O2 ?H 827.0 kJ
• PbO C ? Pb CO ?H 106.8 kJ
• calculate the value of ?H when PbS reacts to
  form lead in the following reaction
• 2 PbS 3 O2 2 C ? 2 Pb 2 CO 2 SO2
• Is the reaction endothermic or exothermic?

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Example 3

• From the following enthalpy changes,
• 2 PbO 2 SO2 ? 2 PbS 3 O2 ?H 827.0 kJ
• PbO C ? Pb CO ?H 106.8 kJ
• 2 PbS 3 O2 2 C ? 2 Pb 2 CO 2 SO2
• Is the reaction endothermic or exothermic?

2 PbO 2 SO2
2 PbS 3 O2
?H -827.0 kJ
2X ( )
2 PbO 2C ? 2Pb 2CO
?H 213.6kJ
Now that we have constructed the summary
equation, we can add the enthalpy -827.0 kJ
213.6 kJ -614 kJ
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Example 4

• Determine the change in enthalpy for the
  following reaction
• C 2H2 ? CH4
• Use these reaction equations
• C O2 ? CO2 ?H -394 kJ
• H2 ½ O2 ? H2O ?H -286 kJ
• CH4 2 O2 ? CO2 2 H2O ?H -890.3 kJ
• How much heat is produced when 35.0 g of CH4 is
  synthesized? 1.25 x 1025 molecules? 15.0 L?

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Example 4

• C 2H2 ? CH4
• Use these reaction equations
• C O2 ? CO2 ?H -394 kJ
• H2 ½ O2 ? H2O ?H -286 kJ
• CH4 2 O2 ? CO2 2 H2O ?H -890.3 kJ
• How much heat is produced when 35.0 g of CH4 is
  synthesized? 1.25 x 1025 molecules? 15.0 L?

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PRACTICE!
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Calorimetry

• Calorimeter insulated container where a
  reaction takes place and the resultant ?T can be
  measured accurately
• Provides the data necessary to determine the
  heats of reaction (enthalpy)

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Calorimetry Calculations

• Assumes heat released or absorbed by reaction is
  absorbed or released by surroundings
• ?Hrxn ?H surroundings
• ?H m ?T Csp (Csp also can
  be Scap )
• where m mass
• ?T change in temperature (final initial)
• C specific heat capacity amount of energy
  needed to increase the 1 gram of substance by 1oC

Enthalpy here is also known as Q
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Example 1

• How much heat is needed to change the temperature
  of 28.4 g sample of water from 20.0 oC to 35.0
  oC? (specific heat 4.184 J/g oC)

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Example 2

• What is the specific heat of nickel if the
  temperature of a 32.2 g sample of nickel is
  increased by 3.5 oC when 50. J of heat is added?

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Example 3

• When a 13.7 g sample of solid lead(II) nitrate
  dissolves in 85.0 g of water in a calorimeter,
  the temperature drops from 23.4oC to 19.7oC.
  Calculate the ?H (kJ per mole)for the solution
  process
• Pb(NO3)2 (s) ? Pb2 (aq) 2 NO3- (aq)

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Heat Curve shows physical changes caused by
adding heat
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?H during Phase Change

• Heat of fusion (Hfus) energy needed to change
  from solid to liquid
• Heat of vaporization (Hvap) energy needed to
  change from liquid to gas
• No temperature change ? so math is
• ?H m x Hfus OR ?H m x Hvap
• Unique heat values for these for each substance
  for each phase change
• for H2O Hvap is 40.7 kJ/mol H fus is 6.01
  kJ/mol

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Example

• How much energy is needed to boil 165 g H2O if at
  100C? (Hvap 2256 J/g)

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Example

• How many molecules of ice at 0 C can be melted
  with 550. J of heat energy?
• (Hfus 334 J/g)

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Example

• How many joules are required to change
• 15.0 g ice at -10.oC to water at 20.oC?
• Cice 0.50 cal/goC Hfus 6.01 kJ/mol