Thermochemistry: The heat energy of chemical reactions heat

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ThermochemistryThe heat energy of chemical
reactions
heat

• Enthalpy is the amount of ________ transferred
  during a reaction. The symbol for the change in
  enthalpy is ?H.
• An endothermic reaction is one that ___________
  heat from the surroundings. (___ ? H) An
  endothermic reaction feels ______.
• Example--an instant ice pack
• An exothermic process is one that _____________
  heat to the surroundings. (___ ? H) An
  exothermic reaction feels _____.
• Example--burning paper

gains

cold
loses

hot
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ThermochemistryHow to measure heat (Energy)
changes
Calorie (Cal)

• A ____________ is the amount of energy (heat)
  required to raise the temperature of one gram of
  water by one degree Celsius.
• The calorie written on food is actually not one
  calorie in chemistry. It is actually 1
  __________ (or ____calories) and is written with
  a capital C (Calorie) to keep the two separate.
• A ____________ is the SI unit for measuring the
  amount of energy or heat transferred in
  chemistry.
• Write down this conversion factor

kilocalorie
1000
Joule (J)
1 Cal 4.184 J
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McDonalds McChicken
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McDonalds McDouble
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Crunchy Cheetos
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So what do we do with all these Calories?

• Your body will use these Calories as energy to do
  everyday activities but what if you dont use all
  the calories you consume?
• Your body will either use the energy or it will
  store it as fat!
• So people who eat more food than their body can
  use exercise as a way of releasing the extra
  energy. But how much exercise do you really need
  to do
• in order to burn off those extra Calories?

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Exercise!

• Running and walking are not the same!! Even
  though you can run OR walk a mile, there is a
  difference. According to David Swain, a Ph.D. in
  exercise physiology, When you perform a
  continuous exercise, you burn five Calories for
  every liter of oxygen you consume and running in
  general consumes a lot more oxygen than walking.
• Running burns approximately 100 Calories per
  mile.
• SO WHAT DOES THIS MEAN?

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I LOVE FOOD! ?But I hate running! ?

• To run off the food previously mentioned, this is
  approximately how long you would have to RUN!
• 1 McChicken 1.38 MILES
• 1 McDouble 3.10 MILES
• 1 bag of Cheetos 3.30 MILES

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Thermochemical Reactions

• A thermochemical reaction is written as follows
  
• 2S 3O2 ? 2SO3 791.4 kJ
• This equation represents an ___________ reaction
  since the heat is a ________.
• H2 Br2 72.80 kJ ? 2HBr
• This equation represents an ___________ reaction
  since the heat is a __________.

exothermic
product
endothermic
reactant
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Thermochemistry Problems

• Problem 1 How much heat will be released when
  6.5 moles of sulfur reacts with excess oxygen
  according to the following equation? Also, tell
  whether it will be exothermic or endothermic!
• 2S 3O2 ? 2SO3 ?H -791.4 kJ

-791.4 kJ
6.5 mol S
_______________
X

-2572 kJ
2 mol S
Exothermic

• ?H means heat
• is a product!

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Phase Changes Energy
Endothermic melting, evaporating/boiling
sublimation Exothermic freezing, condensation,
deposition
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Reaction Profiles
Endothermic
Exothermic
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Calculations in Thermodynamics

• In order to calculate how much heat is
  transferred by a thermochemical reaction the
  equation we use is
• q mc?T
• q the ______ lost or gained in the process
• m the _____ of the substance
• c the ________ _____ ________
• The specific heat of water is 4.186 Joules/gram
  C
• ?T Change in temperature

heat
mass
specific
heat
capacity
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Calculations Practice

• Example 1 How many Joules would it take to raise
  the temperature of 250 g of ice from -20 C to -5
  C? (The specific heat of ice is 2.108 Joule/gram
  C)

q mc?T
q 250g (2.108 J/g C)( 15 C)
q 7905 Joules
7.9 kJ
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More Practice

• Example 2 How many calories would it take to
  raise the temperature of 100 grams of Ethyl
  Alcohol from 30 C to 50 C? (The specific heat
  of Ethyl alcohol is 0.58 cal/g C)

q mc?T
q 100 g (0.58 cal/g C) (20 C)
q 1160 cal
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Chemical Kinetics
fast

• Kinetics is the study of how _____ chemical
  reactions occur.
• There are 4 important factors which affect rates
  of reactions
• reactant ______________ (more molecules more
  collisions)
• ________________(more motion more energized
  collisions)
• action of catalysts (________________are
  biological catalysts.)
• ___________ area
• For the reaction A ? B there are two ways of
  measuring rate
• (1) the speed at which the reactants
  ____________
• (2) the speed at which the products __________
• In general, rates of reactions _________ as
  concentrations increase since there are more
  _____________ occurring between reactants.

concentration
Temperature
Enzymes
Surface
disappear
appear
increase
collisions
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Catalysts and Reaction Rates
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Rate Law

• The overall concentration dependence of reaction
  rate is given in a rate law or rate expression.
  
• Heres what a general rate law for a reaction
  will look like
• A B ? C
• Rate k Am Bn
• - A B represent the reactant
  concentrations.
• - The exponents m and n are called reaction
  orders.
• - The k is called the rate constant.
• - The overall reaction order is the sum of the
  reaction orders.
• - The overall order of reaction is therefore
• m n .

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Rate Law Example

• Consider the following reaction
• NH4(aq) NO2-(aq) ? N2(g) 2H2O(l)
• Lets say that the following observations from
  several experiments were made
• as NH4 doubles the rate doubles with NO2-
  constant.
• as NO2- doubles the rate doubles with NH4
  constant.
• The rate of this reaction would be expressed
  as.
• Rate kNH4NO2-
• The reaction is said to be first order with
  respect to NH4 and first order with respect
  to NO2-.
• But the overall order of the reaction is said to
  be second order.
• Reaction rates come from experiment data, not
  stoichiometry!

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Order of Reactions

• A reaction is zero order in a reactant if the
  change in concentration of that reactant produces
  no effect.
• A reaction is 1st order if doubling the
  concentration causes the rate to double.
• A reaction is 2nd order if doubling the
  concentration causes a quadruple increase in
  rate.
• -3rd orderdoubling concentration leads to 23
  (or 8 times) the rate.
• Note-- The rate constant, k, does not depend on
  concentration! It will vary with temperature.

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Kinetics
Practice Problem A reaction is first order with
respect to reactant A and second order with
respect to reactant B. What is the rate law
expression? What is the overall order of the
reaction? What will happen to the rate of the
reaction if the concentrations of both reactants
are doubled?
Rate k A B2
3rd order
Rate k 2 22 8 times faster
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KineticsA B C ? Products
Practice Problem A reaction doubles in speed
when reactant A is doubled. The reaction
triples in speed when reactant B is tripled.
The reaction quadruples in speed when reactant
C is doubled. What is the rate law for this
reaction? What is the overall order of the
reaction? What will happen to the rate of the
reaction if the concentrations of A and B are
doubled while C is cut in half?
Rate k A B C2
4th order
Rate k 2 2 1/22 same rate
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Kinetics

• What does the magnitude of the rate constant, k,
  mean?
• The larger the value of k, the faster reaction
  will be.
• Practice Problem Which reaction is the fastest?
• a) k0.0034 M/s b) k34.0 M/s c) k29800
  M/s

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Chemical Equilibrium

• Some reactions are reversible, so as products
  accumulate they can begin to turn back into
  reactants.
• When the rate of the forward reaction
  ____________ the rate of the reverse reaction, it
  is at equilibrium.
• Heres an example
• Each is constantly being formed at the same rate
  that it is being consumed. It is therefore
  called a _____________ equilibrium.
• Chemical equilibrium is the point at which the
  concentrations of all species are
  ________________.

equals
dynamic
constant
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Dynamic Equilibrium
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Changes in Equilibrium

• Le Chateliers Principle If a stress is
  applied to a system that is already at
  equilibrium, the equilibrium will shift to reduce
  the effect of the stress.
• We will now look at changing various things on a
  system at equilibrium.

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Le Chateliers Principle

• Consider this reaction A B
  C D
• Here are some things that one can change in order
  to shift the equilibrium
• (1) Changing the ________________ of the
  Solutions Increasing a concentration will cause
  a shift away from the increasing concentration
  and decreasing a concentration will cause a shift
  toward the decreasing concentration. Pure solids
  and pure liquids dont affect the equilibrium!
  
• (2) Changing the ________________ of the Gases
  Increasing pressure will cause a shift towards
  the side of the reaction with less molecules of
  gas and decreasing pressure will cause a shift
  towards the side of the reaction with more
  molecules of gas.
• (3) Changing the ________________ This is like
  adding heat (energy) and will depend on if the
  reaction is endothermic or exothermic. Endo
  adding heat shifts towards the products (right).
  Exo adding heat shifts towards the reactants
  (left).
• Adding a catalyst ________ _______shift the
  equilibrium. It just makes the reaction achieve
  equilibrium __________.

Concentration
Pressure
Temperature
does not
faster
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Changes in Equilibrium
For the reaction below, predict the direction the
equilibrium will shift given the following
changes. Temperature and volume are held
constant. Fe3(aq) SCN1(aq)
FeSCN2(aq) (colorless)
(red) More Fe3 is added to the reaction.
____________________ - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - NO2 (g) 7 H2 (g) ? 2 NH3 (g)
4 H2O(g) Predict the shift in equilibrium given
the following changes a. addition of ammonia
____________________ b. removal of nitrogen
dioxide ____________________ c. removal of water
vapor ____________________ d. addition of a
catalyst ____________________
Shifts to the right (more red)
Shifts to the left
Shifts to the left
Shifts to the right
No change
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Changes in Equilibrium

• 2S(s) 3O2(g) ?? 2SO3(g) 791.4 kJ
• Increasing the temperature __________
• Decreasing the pressure __________
• Decreasing the volume __________
• d) Adding more sulfur _________

3 gas molecules
2 gas molecules
Shifts to the left
Hint Look at the heat!
Shifts to the left
Hint Look at the molecules of gas on both sides
of the reaction!
Shifts to the right
Hint Decreasing volume increases pressure!
No changeIts a solid.
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Catalysts Changes in Equilibrium
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The Equilibrium Constant

• Consider the following reaction
• There are two reactions going on, forward and
  reverse. The rate of each reaction can be
  expressed separately
• Rateforward kfAaBb
• Ratereverse krCcDd
• At equilibrium, Rateforware Ratereverse
  orkfAaBb krCcDd
• We can rearrange this equation and combine the
  rate constants into a single constant. We end up
  with this
• where Keq is called the equilibrium constant.

Keq productsx/reactantsy
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The Magnitude of Equilibrium Constants

• When Keq is greater than 1, there are more
  products present at equilibrium compared to
  reactants.
• When Keq is less than 1, there are more
  reactants present at equilibrium compared to
  products.

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The Equilibrium Expression

• Only the reactants and products that are aqueous
  or gaseous are shown in an equilibrium
  expression. Pure solids and liquids do not
  change in concentration, so they will not change
  the value of Keq and therefore, they do not
  appear in the equilibrium expression.
• Practice Problems Write the equilibrium
  expression for each reaction shown.
• HCl(aq) ?? H(aq) Cl-(aq)
• 2PCl5(g)??2P(s) 5Cl2(g)
• c)
• d) 2S(s) 3O2(g) ?? 2SO3(g)

Keq HCl-/HCl
Keq Cl25/PCl52
Keq NO22/N2O4
Keq SO32/O23
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Calculating the Equilibrium Constant

• Once you have the equilibrium expression, all you
  have to do to calculate the value of Keq is to
  plug in the equilibrium concentrations (or
  pressures) for each substance and do the math!
• Practice Problem Calculate the value of the
  equilibrium constant given the following data.

Keq NO22/N2O4
Keq 0.5262/0.0429
Keq 6.44