HEAT IN CHEMICAL REACTIONS

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HEAT IN CHEMICAL REACTIONS

• Thermodynamics /
• Thermo chemistry
• Unit 8
• Ch 17/18

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Exothermic and Endothermic Reactions

• Heat (q) energy that transfers from one object
  to another because of a temperature difference
  between them.
• Joule The SI unit for energy and heat.
• Bond breaking vs. bond forming.
• Thermo chemistry The study of the changes in
  heat within chemical reactions.

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Exothermic

• Exothermic reactions release heat.
• Heat is a product.
• Combustion reactions.
• Examples
• MREs
• Hot hands

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Endothermic

• Endothermic reactions absorb heat.
• Heat is a reactant.
• Example
• Cold pack (p 525 figure 17.11)

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Enthalpy (?H)

• The enthalpy of a substance is its energy plus a
  small-added term that includes the pressure and
  temperature of the substance.
• When the pressure remains constant, the heat
  absorbed or released during a chemical reaction
  is equal to the enthalpy change for the reaction.
• Heat transfer and the sign of the enthalpy
  change
• Positive Endothermic Heat absorbed
• Negative Exothermic Heat released

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Enthalpy of Reactions ?Hrxn Hproducts -
Hreactants

• The enthalpy change for a reaction is equal to
  the heat absorbed or gained during the reaction.
• Enthalpy change is the enthalpy of the products
  minus the enthalpy of the reactants.
• The amount of heat absorbed / released is
  dependent on the quantity (mole).
• Standard enthalpy change (?Ho) at 1atm and 250C.

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Enthalpy of Reactions Sample Problem

• How much heat will be released if 1.0 g of
  hydrogen peroxide decomposes in a bombardier
  beetle to produce a steam spray.
• 2H2O2 2H2O O2 ?H -190 kJ

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Enthalpy and the Spontaneous Process

• Enthalpy diagrams.
• Spontaneous Process a process that proceeds on
  its own, without any outside intervention.
• The Chemists Questions
• What direction is spontaneous?
• What is the extent of the reaction?
• What is the speed of the reaction?

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• Spontaneous vs. Non-Spontaneous
• Exothermic Endothermic
• No catalyst Catalyst
• Energy of Activation The difference between the
  energy of an activated complex and the energy of
  the reactants of a chemical reaction.
• Demonstrate with an enthalpy diagram.
• Although most spontaneous reactions are
  exothermic, some are endothermic. Why?

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Entropy (?S)Ch 18

• Order vs. Disorder.
• Relate the states of matter and the degree of
  order.
• Entropy A quantitative measure of the disorder,
  or randomness, in the substances involved in a
  reaction.

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Entropy Changes?Srxn Sproducts - Sreactants

• Entropy Increases
• Sproducts gt Sreactants
• Entropy Decreases
• Sproducts lt Sreactants
• Predicting entropy increases
• Gases are formed from liquids or solids.
• Solutions are formed from liquids and solids.
• There are more molecules of gas as products than
  there are as reactants.
• The temperature of a substance is increased.

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Entropy Criterion

• In any spontaneous process, the overall entropy
  of the universe always increases.
• Entropy of the Universe
• ?Suniverse ?Sreaction ?Ssurroundings
• A reaction is spontaneous when ?Suniverse is
  positive.
• Change in entropy of the reaction (see
  predicting).
• Change in entropy of surroundings (endo vs. exo)
• If ?H is (-), then ?Ssurroundings is ().
• If ?H is (), then ?Ssurroundings is (-).

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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.
• ?Hnet ?H1 ?H2
• Two Rules
• If the coefficients of an equation are
  multiplied/divided by a factor, the enthalpy
  change for the reaction is multiplied/divided by
  the same factor.
• If an equation is reversed, the sign of ?H
  changes also.
• Sample problems.

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Practice Problem

• The combustion of sulfur can produce SO2 as well
  as SO3 depending upon the supply of oxygen. From
  the following reactions and their enthalpy
  changes,
• 2SO2(g) O2(g) ? 2SO3(g) H -196kJ
• 2S(s) 3O2(g) ? 2SO3(g) H -790kJ
• calculate the standard enthalpy change for the
  combustion of sulfur to produce SO2.
• S(s) O2(g) ? SO2(g)

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CalorimetryCh 17

• The study of heat flow and heat measurement.
• How do you measure an enthalpy change of a
  reaction?
• Calorimetry experiments determine the enthalpy
  changes of reactions by making accurate
  measurements of temperature changes produced in a
  calorimeter.

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Simple Calorimeter

• See pages 511-512

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Heat and Temperature

• Exothermic reactions release heat to
  surroundings. The size of the temperature
  increase depends on the amount of heat released
  and the heat capacity of the surroundings.
• Heat capacity The amount of heat needed to
  raise the temperature of the object 1oC.
• Heat capacity depends on the objects mass and its
  composition.
• Specific heat (C) The heat capacity of 1 gram
  of a substance.
• C of H2O 4.184 J/g-oC.

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Calculations

• Qsur m x C x (Tf Ti)
• Qrxn -Qsur
• Q m x C x ?T

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Calculations

• Calculate Qsur
• Convert to Qrxn
• Convert grams to moles
• ?H Qrxn / moles

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Entropy and Gibbs Free Energy (G)

• Spontaneity depends on entropy and enthalpy.
• Gibbs proposed a thermodynamic concept to
  simultaneously incorporate the concepts of
  entropy and enthalpy.
• ?G ?H - T?S

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Free Energy Spontaneity

• If ?G is negative, the reaction is spontaneous
  and can proceed on its own.
• If ?G is positive, the reaction is not
  spontaneous and requires a sustained input of
  energy to make it occur.
• If ?G is zero, the reaction is at equilibrium.

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Free Energy and Work

• Spontaneous reactions release free energy that
  can perform work.
• ?G represents the maximum work that a spontaneous
  process can perform.
• ?G for a nonspontaneous reaction is the minimum
  amount of work that must be performed to make a
  reaction occur.

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?G Spontaneity