Heat in Chemical Reactions

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Heat in Chemical
Reactions
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10.1 Chemical Reactions that Involve Heat

• Heat Energy (symbol - q) that is transferred
  from one object to another due to a difference in
  temperature. Measured in Joules (symbol - J)
• 2. Thermochemistry The study of heat changes in
  a chemical reaction.

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3. Types of Chemical Reactions

• a. Exothermic Reactions release heat into their
  surroundings. Heat is a product of the reaction
  and temperature increases. This occurs during
  bond formation.
• Combustion reactions are exothermic burning
  propane
• C3H8 5O2 ? 3CO2 4H2O 2043 kJ

surroundings
Exothermic Reaction
?T
?H -
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Exothermic Reaction
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• b. Endothermic Reactions Heat is absorbed by the
  reactants and stored in the chemical bonds of the
  products. Heat acts as a reactant and
  temperature decreases. This occurs during bond
  breaking.
• Electrolysis of water requires electrical energy.
• 2H2O 572kJ ? 2H2 O2

surroundings
Endothermic Reaction
?T -
?H
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Exothermic/Endothermic Reactions
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10.2 Heat and Enthalpy Changes

• 1. Enthalpy The heat content of a system at
  constant pressure (symbol is H ).
• 2. Enthalpy Change The heat absorbed or released
  during a reaction (symbol is ?H ).
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• 4. When reactions take place at standard
  temperature and pressure, q H.
• 5. Standard Enthalpy Change (?H?) Enthalpy
  change that occurs when reactants in their
  standard states (most stable form) change to
  products in their standard states. STP Standard
  Temperature and Pressure are 0 ?C and 1 atm.
• The ?H? is listed after the equation. If the
  ?H? is positive the reaction is endothermic and
  heat was absorbed. If the ?H? is negative, the
  reaction is exothermic and heat was released.
• 6. The amount of heat absorbed or released in a
  reaction depends upon the number of moles of
  reactants.

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7. Enthalpy Changes in Stoichiometry Problems

• Ex) How much heat will be released if 5.0 g of
  H2O2 decomposes?
• 2H2O2 ? 2H2O O2 ?H? -190 kJ

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• Ex) How much heat is transferred when you eat a
  10. g Jolly rancher which is made of glucose
  (C6H12O6)? It reacts in your body with oxygen
  according to the following equation. If 4.184kJ
  1 Cal, how many Calories are in the Jolly
  Rancher?
• C6H12O6 6O2 ? 6CO2 6H2O ?H? -2803 kJ

C6H12O6 6O2 ? 6CO2 6H2O 2803 kJ
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3. Enthalpy Diagrams 1 2
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1 2
a. Which has a higher enthalpy? Products or Reactants R P
b. Was heat absorbed or released? R A
c. Is this an endothermic or exothermic reaction? Exo Endo
d. Is ?H for this reaction positive or negative? -
e. Would the ?H be on the left or right side of the yield sign? R L
f. Is the reverse reaction exothermic or endothermic? Endo Exo
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g. Rewrite each equation with the heat term in
the reaction as a reactant or product

• 1) C3H8 5O2 ? 3CO2 4H2O 2043 kJ
• 2) C H2O 113kJ ? CO H2

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? change in ?H Hproducts - Hreactants
?H Hproducts Hreactants
Exothermic Reaction - low high
Endothermic Reaction high low
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10.3 Hesss Law - (1802-1850)

• The enthalpy change for a reaction is the sum of
  the enthalpy changes for a series of reactions
  that adds up to the overall reaction.
• This is also called the Law of Heat of Summation
  (S)
• 3. This allows you to determine the enthalpy
  change for a reaction by indirect means when a
  direct method cannot be done.

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4. Steps for using Hesss Law

• 1. Identify the compounds
• 2. Locate the compounds on the Heat of Reaction
  Table.
• 3. Write the reaction from the table so the
  compound is a reactant or product.
• 4. Write appropriate ?H for each sub equation.
• If needed, multiply equation and enthalpy change
  value. (coefficients)
• If you reverse the equation, change the sign of
  the enthalpy change.
• 5. Add the equations to arrive at the desired net
  (original) equation.
• 6. Add ?H (enthalpy changes) of each sub
  equation.

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10.3 Calorimetry

• 1. The Kinetic Theory states that heat results
  from the motion vibration of particles.
• 2. Heat The transfer of kinetic energy from a
  hotter object to a colder object. Heat is
  dependent on composition and amount.
• 3. Temperature is a measure of how hot or cold
  something is specifically it is the measure of
  the average kinetic energy (speed) of the
  particles in an object. It is independent of
  amount.

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Heat

• Temperature is not the same as heat.
• Temperature is a measure of the average kinetic
  energy of the particles in an object.
• A temperature change is a result of a energy
  transfer.
• Julius Sumner Miller - Physics - Heat
  Temperature
• Heat vs. Temperature Animation

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• 4. Calorimetry is the study of heat flow and
  measurement.
• 5. Calorimetry experiments determine the heats of
  reactions (enthalpy changes) by making accurate
  measurements of temperature changes produced in a
  calorimeter.
• 6. A Calorimeter is an insulated device used to
  measure heat absorbed or released in a chemical
  or physical change.

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• 7. Specific Heat (Cp) The amount of heat needed
  to raise 1 g of a substance by 1?C.
• Formula for specific heat
• mmass (substance)
• ?Tchange in temperature of the substance (Tf-Ti)
• Specific Heat of Water 4.184 J/g ºC
• 1 calorie or .001food Calorie

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• 8. Measuring Specific Heat of a Metal
• Ex 1) What is the specific heat of a nickel if
  the temperature of a 32.2 g sample of nickel is
  increased by 3.5ºC when 50. J of heat is added.

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• Ex 2) How much heat is absorbed to be able to
  increase the temperature of a 26.2 g sample of
  aluminum (Cp 0.897 J/gºC) from 25.3 ºC to 65.9
  ºC?

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• 9. Measuring Heat (q) of a Substance Dissolved in
  Water
• You can rearrange this formula to determine
  the heat released or absorbed by the surroundings
  (solution) as the substance dissolves based on
  this assumption
• q reaction -q surroundings
• ?q
• 1) Calculate q for the surroundings (solution)
  and determine qrxn.
• 2) Calculate the moles of solute dissolved in the
  water.
• 3) Calculate ?H

?T Tf - Ti
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• Ex) When a 4.25 g sample of solid NH4NO3
  dissolves in 60.0 g of H2O in a calorimeter,the
  temperature drops from 21.0 ºC to 16.9 ºC.
  Calculate ?H. Rewrite the thermochemical
  equation with the heat term as a reactant or
  product.
• NH4NO3(s) ? NH4(aq) NO3-(aq) ?H
  ?
• qsurmCp?T (64.3g)(4.184J/gC)(-4.1C)
• qsur-1100J
• qrxn -qsur 1100J
• 2.
• 3.

21 kJ
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10. Foods as Fuels

• A. Carbohydrates typically have high enthalpies
  however, the products of their combustion, CO2
  and H2O, have low enthalpies.
• B. Therefore, the combustion of carbohydrates and
  fats, is exothermic.
• C. Sugars and Starches break down to glucose,
  which reacts with O2 in a combustion reaction.
• D. Nutritional information on food labels can be
  gathered using a calorimeter.
• Carbs 4 Cal/g Protein 4 Cal/g Fat 9 Cal/g