Ch. 17 Notes – Thermodynamics

 

Thermodynamics:  The study of energy changes as they relate to macroscopic quantities such as pressure, volume, temperature, enthalpy and entropy.

 

Measuring Heat Transfer:  In this chapter we will measure the heat transferred in two different scenarios:  1)  The heat transferred when there is a phase change (eg. liquid to gas – PHYSICAL HEAT).  2)  And the heat transferred when molecules are being created or taken apart (eg. the decomposition reaction of water into hydrogen and oxygen – CHEMICAL HEAT).  Heat transfer is measured in kJ or Calories (C).

 

Energy, Temperature and Heat (what is the difference between these three?) (p. 511)

Energy:  The ability to do work or produce heat on a particular MASS of an object.

The unit for Energy is called the Joule.  The non-metric version is called the Calorie.

1 cal = 4.184 J.  One calorie is the amount of energy it takes to raise the temperature of one gram of water one degree Celsius. (one food calorie is 1000 calories – called a Kilocalorie.  But we drop the three zeros)

Temperature:  A measure of the average kinetic energy of molecules in a substance.  This is independent of MASS of the entire system.  Remember: more massive molecules move slower than small ones but have the same KE/temp.                                Energy + Temperature = HEAT  

      Example:  A hot cup of coffee poured from the coffee pot has the same temperature as the coffee pot coffee.  But, the coffee pot has more heat because it has more mass. (FYI:  There is more heat in the Arctic Ocean than there is in a hot cup of coffee – the reason: more mass in the Arctic Ocean.)

Heat:  The amount of thermal energy transferred from one substance to another.  Heat depends upon the MASS of a substance as well as the motion of the molecules (temp/kinetic energy).  Heat always flows from warmer to colder objects.  Ice cubes in a drink absorb the heat from the liquid, the liquid is not cooled by the ice – rather it heats the ice and turns cold

Endothermic:  When heat is absorbed from the surroundings

Exothermic:  When heat is released into the surrounding..

TEMP. IS INDEPENDENT OF MASS.  ENERGY AND HEAT ARE DEPENDENT ON MASS

 

Specific Heat (J/goC) (p. 512):  The amount of heat required to raise one gram of a substance one degree Celsius.  Look at table 17-1 p. 513 to see a list of Specific Heats. Water has a high specific heat and Aluminum has a low one.  Water retains heat well and doesn’t change its temperature too easily.  Aluminum foil loses its heat out of the oven very quickly.  To find the amount of heat transferred, use this equation:

q = cp x m x DT    (q = heat,  cp = specific heat,  m = mass, DT = change in temp)

                        Units:  q = Joules (J)        Cp = J/goC            m = grams       T = oC

Do sample problem 17-1 on p. 513 to calculate specific heat

 

Homework #1(Specific Heat):  General Questions that you can do.  AND #16-24

 

Heat of Vaporization (J/g):  The amount of heat needed to vaporize one gram of liquid into a gas.  Also the amount of heat released when gas condenses to liquid.  Remember freon in your refrigerator.  Compressing a gas into a liquid releases heat (out the back of your fridge).  Expanding a liquid into a gas requires heat, so heat is absorbed from the inside of your fridge and it gets cold in there.

(q = m x heat of vaporization)  There is no DT because there is no temp change with a phase change

 

Heat of Fusion (J/g):  The amount of heat needed to melt one gram of solid into its liquid.  Also the amount of heat released when a liquid turns into a solid.  When water freezes, heat is released. Freezing water is exothermic because melting ice into water requires heat (endothermic). This is why orange tree farmers spray water on freezing fields to protect their crops from frostbite.  Also, water freezes to 0oC so if the temp is below 0oC, then the fruit won’t get that cold.

(q = m x heat of Fusion) There is no DT because there is no temp change with a phase change

 

Enthalpy (symbol: H.  UNITS kJ/mole):  This is the measure of the energy content of a system.   DH is the measure of heat change in a system (so long as the pressure of that system remains constant). DH does depend upon the mass of the substance as well as temperature and pressure, but for all of the problems we will do, you can assume STP conditions. DH is measured in kJ. 

Since we will keep pressure constant, DH = heat change.

     To find DH for a reaction: DH = DHproducts - DHreactants.

 

      When DH is POSITIVE, the reaction is ENDOTHERMIC    (Could this be a general question?)

      When DH is NEGATIVE, the reaction is EXOTHERMIC

 

DH is used to describe the heat of a CHEMICAL reaction (Example:  2H2 + O2 -à 2H2O + heat)

q is used to describe the heat of  a PHYSICAL reaction (Example:  H2O(s) + heat  --à  H2O(l))

 

                Making vs. Breaking bonds:  Remember that breaking a bond between two or more atoms USUALLY requires energy (endothermic).  Creating a bond between two or more atoms USUALLY releases energy (exothermic).  Example:  Breaking water molecules into H2 and O2 requires energy.  But making H2O out of H2 and O2 releases energy.  (2H2 + O2 à 2H2O + 285.8 kJ – This form is:  -DH)

               

                Phase Changes:  Remember that in phase changes, going from Liquid to Gas is Endothermic (you have to heat water to boil it away).  Going from Solid to Liquid is Endothermic (you need to heat an ice cube to melt it). Therefore, going from Gas to Liquid is Exothermic and going from Liquid to Solid is Exothermic.  Phase changes depend upon intermolecular forces (dipole-dipole, hydrogen bonding) and making and breaking bonds depend upon intramolecular forces (the electronegativity difference between two or more atoms).

Solid ---Endoà  Liquid----Endoà Gas

Gas ----Exo-àLiquid ----Exo -à Solid

 

Standard Enthalpy of Formation (Symbol: DHf) p. 517:  The f stands for the formation of one mole of substance.  Standard state is the state the elements are in at Standard Pressure and 25oC.  For example, Oxygen is a diatomic gas at this Temp and Pressre.  Gold is a solid…etc.  The DHf  value for all elements is ZERO!.    Look on p. 902 at table A-14 to see a list of DHof for some common molecules.  Notice that they are negative numbers if they are an exothermic reaction to create them and a positive number if they require and endothermic reaction to create them.

     Example, 285.8 kJ of heat are given off in the creation of one mole of liquid water (exothermic).

     Example, in forming 1 mole of NO, 90.29 kJ of heat is required (endothermic).

 

VERY IMPORTANT:  Exothermic reactions have -DH values.  Endothermic have +DH.  Melting ice is positive.  Boiling water is positive.  Freezing water is negative.  Condensing steam into water is negative.

Do Sample Problem 17-2 p. 521 to calculate the heat of reaction

 

Homework #2 (Heats of Reaction):  General Questions that you can do AND #25-31

 

You need to memorize:  4.0 kcal/gram Carbohydrate,  4.0 kcal/gram protein,  9.0 kcal/gram fat.  Carbs and Proteins have roughly the same caloric content.  Fat is over twice as many calories as carbs or proteins.

 

Entropy (Symbol:  S.  UNITS:  kJ/K mole) p. 526:  This is the measure of order in a system.  A positive DS means there is a lot of disorder.  A negative DS means there is a lot of order.  Most things tend toward disorder – such as ice melting.  Ice melting would have a positive DS.  The expansion of a gas into a larger container would have a positive DS. (Read the 5 examples on p. 556 to get a clearer idea of positive and negative values).   To find the entropy of a system, you would do: DS = DSproducts - DSreactants.  Just like Enthalpy, you must multiply the DS value by the number of moles in the equation.

Predicting a spontaneous reaction:  Spontaneous reactions occur without any assistance.  Ice melting is spontaneous (at certain temperatures and pressures).  A ball rolling up a hill is non-spontaneous.  Spontaneous reactions can be fast or slow (example:  Methane will mix with oxygen to form carbon dioxide and water.  It will do it faster if you light them with a match.  But they are spontaneous even without the match).  Not all reactions are spontaneous.  Most exothermic reactions are spontaneous.  Some endothermic reactions are spontaneous – ice melting is endothermic and spontaneous.  Look at table 17-2 on page 529 to see the factors affecting spontaneity.  You will need to memorize these. 

 

Gibbs Free Energy (Symbol:  G.   UNITS:  kJ/mol) p. 528:  To determine if a reaction is spontaneous or not, you need to look at the effects of Entropy (S), Enthalpy (H) and Temperature (T).  The equation is called the Gibbs Free Energy Equation and it looks like this:  T is in units of Kelvin, DH is in kJ/mol and DS is in kJ/Kmole. DG is in kJ

DG = DH - TDS

     If DG is negative, then the reaction is spontaneous. The more negative DG is, the more product will be produced from the rxn.   DG has absolutely, positively nothing to do with the rate of the reaction itself.  A car will rust spontaneously with a -DG value.  Ice will melt spontaneously at a temp above 0oC with a -DG value.  Cars rust slowly, ice melts relatively quickly.  How much of that car will rust and how much of that ice will melt depend largely on the value of DG, but not the speed at which they do this.  Spontaneous also means: “self sustaining”.  Once the reaction is given enough activation energy to begin, it will keep on going under the heat it creates through its reaction.

 

Example:  H2O(s) + heat  --à  H2O(l)                  DH is +,         DS is +          so DG is – at a high temperature.

                   H2O(l) --à  H2O(s) + heat                   DH is -,          DS is -           so DG is – at a low temperature.

 

    Look at table 17-2 on p. 529 to see how you can predict when a reaction is going to be spontaneous or not.  Notice that the higher the Temp, the lower S can be and still be spontaneous. 

Do problem 17-4 p. 530

Homework #3:  (Entropy and DG):  All of the general questions AND #32-35

 

Rates of RXN

What is the Rate of a Rxn? (see p. 538-540)

     Rate of rxn can be determined by the slope of the line of a “amount produced” vs. “time” graph.  The amount produced can be measured in volume, pressure, color, temperature or concentration.

                                Rate = Amount produced/time = slope of the line

Therefore:  If rate increases, time decreases (and amount produced stays the same)

Therefore:  If rate increases, amount produced increases (and time stays the same)

Factors which affect the rate of a rxn:

     Basically, a rxn occurs when particles (molecules) come in contact with each other. 

Surface area:  The greater the surface area, the greater the reaction rate

     Consider the following:  A cube which is 5cm x 5cm x 5cm has the same volume as two cubes with the dimensions of 3.97cm x 3.97cm x 3.9cm, but the two smaller cubes have a surface area of 189.13cm2 while the larger cube has a surface area of 150cm2.  Same # of molecules, but more exposed on the surface.

Concentration:  As the concentration increases, the reaction rate will increase

Nature of the reactants involved in the rxn:  Some reactants are more reactive than others.  For example, as you move down the alkali metals on the chart, they get more reactive because their outermost electron is farther away from the nucleus of the atom.

Temperature:  If temperature increases, the reaction rate will increase - USUALLY.  Think about

 DG = DH - TDS

Remember that temperature is an average.  Some molecules are moving faster than others,  You cannot say that at 37oC, all molecules are moving at a temp equal to 37oC, some are faster, some are slower.  The average molecule is moving at 370C.

Catalysts:  A catalyst is something which helps molecules get together to make a rxn occur but it itself is not involved directly in the rxn.  MnO2 acts as a catalyst in the breakdown of H2O2 into water and O2 gas.  Protein enzymes in your body allow sucrose to be burned at 37oC (body temp).  Catalytic converters in cars and factories help turn harmful CO and NO into CO2 and N2 gases (less harmful).

Activation energy:

     This is the energy needed to get a rxn going.  In other words, the molecules have to be moving with a certain kinetic energy (KE=1/2 mass * velocity2) AT LEAST to create the products of the rxn (and remember, Kinetic Energy = temperature).