AP Chemistry Thermochemistry thermodynamics: the study of energy
AP Chemistry Thermochemistry thermodynamics: the study of energy and its transformations -- thermochemistry: the subdiscipline involving chemical reactions and energy changes Energy kinetic energy: energy of motion; KE = mv2 -- all particles have KE -- Thermal energy is due to the KE of particles. We measure the average KE of a collection of particles as... temperature. potential energy: stored energy Chemical potential energy is due to electrostatic forces between charged particles.
+ -- related to the specific arrangement of atoms in the substance + SI unit Units of energy are joules (J), kilojoules (kJ), calories (cal), or nutritional calories (Cal or kcal). James Prescott Joule
(1818-1889) -- conversions: 4184 J = 4.184 kJ = 1000 cal = 1 Cal = 1 kcal system: the part of the universe we are studying surroundings: everything else -- In chemistry, a closed system can exchange energy but not matter with its surroundings. -- Usually, energy is transferred to... (1) change an objects state of motion ...or...(2) cause a temperature change Work (w) is done when a force moves through a distance. W=Fd
Heat (q) is an amount of energy transferred from a hotter object to a colder one. Find the kinetic energy of a single dinitrogen monoxide molecule moving at 650 m/s. N2O (laughing gas) KE = mv2 ? 1g 1 kg m = 44 amu 23 6.02 x 10 amu 1000 g
= 7.31 x 1026 kg KE = (7.31 x 1026 kg) (650 m/s)2 = 1.5 x 1020 J First Law of Thermodynamics = Law of Conservation of Energy -- Energy morphs between its various forms, but the total amount remains the same. (pretty much) internal energy (E) of a system: the sum of all the KE and PE of the components of a system (this is impossible for us to know)
-- The change in the internal energy of a system would be found by: DE = Efinal Einitial And for chemistry, this equation would become: DE = Eproducts Ereactants DE is + if E>final Einitial (i.e., system...gains energy ) ENDOTHERMIC DE is if E
The Titanic was propelled by massive steam engines. The internal energy of the water molecules of the steam changed from instant to instant, depending on how much heat they were absorbing and how much work they were doing during a given time interval. absorbed by In endothermic processes, heat is _________ the system. e.g., melting boiling sublimation In exothermic processes, heat is released ________ by the system. e.g., freezing condensation
deposition To go further, we must introduce the concept of enthalpy (H). -- Enthalpy (H) is defined as... H = E + PV where E = systems internal energy P = pressure of the system V = volume of the system Heike Kamerlingh Onnes 18531926 The Dutch physicist and Nobel laureate H.K. Onnes coined the term enthalpy, basing it on the Greek term enthalpein, which means to warm. -- There is much that could be said about enthalpy, but what you need to know is:
If a process occurs at constant pressure, the change in enthalpy of the system equals the heat lost or gained by the system. i.e., DH = Hfinal Hinitial = qP P indicates constant pressure conditions. When DH is +, the system... has gained heat. (ENDO) When DH is , the system... has lost heat. (EXO) Enthalpy is an extensive property, meaning that the amount of material affects its value. In the burning of firewood at constant pressure, the enthalpy change equals the heat released. DH is () and depends on the quantity of wood burned.
enthalpy of reaction: DHrxn = Hproducts Hreactants (also called heat of reaction) For exothermic rxns, the heat content of the reactants is larger than that of the products. 2 H2(g) + O2(g) 2 H2O(g) DH = 483.6 kJ What is the enthalpy change when 178 g of H2O are produced? 1 mol H2O 483.6 kJ 178 g H2O 18 g H2O 2 mol H2O DH = 2390 kJ
The space shuttle was powered by the reaction above. DH for a reaction and its reverse are the opposites of each other. 2 H2(g) + O2(g) 2 H2O(g) 2 H2O(g) Enthalpy/energy is a reactant. (DH = 483.6 kJ) 2 H2(g) + O2(g)(DH = +483.6 kJ) Enthalpy change depends on the states of reactants and products. 2 H2(g) + O2(g)
2 H2(g) + O2(g) 2 H2O(g) (DH = 483.6 kJ) 2 H2O(l) (DH = 571.6 kJ) Calorimetry: the measurement of heat flow -- device used is called a... calorimeter heat capacity of an object: amount of heat needed to raise objects temp. 1 K = 1oC molar heat capacity: amt. of heat needed to raise temp. of 1 mol of a substance 1 K specific heat (capacity): amt. of heat needed to raise temp. of 1 g of a substance 1 K i.e., molar heat capacity = molar mass X specific heat
We calculate the heat a substance loses or gains using: q = m cP DT AND (for within a given state of matter) q = + / m cX (for between two states of matter) where q = heat m = amount of substance cP = substances heat capacity DT = temperature change cX = heat of fusion (s/l) or heat of vaporization (l/g)
Typical Heating Curve Temp. t a e h s/l s ) q ( d g e
v o l/g m e r l e d d a t a he HEAT
) q d (+ What is the enthalpy change when 679 g of water at 27.4oC are converted into water vapor at 121.2oC? cP,g = 36.76 J/mol-K l/g Temp. cf = 333 J/g cv = 40.61 kJ/mol cP,l = 4.18 J/g-K cP,s = 2.077 J/g-K g
l s/l s HEAT Heat liquid q = m cP DT = 679 g (4.18 J/g-K) (100 27.4) = 206 kJ Boil liquid q = +m cX = +37.72 mol (40.61 kJ/mol) = 1532 kJ Heat gas q = m cP DT = 37.72 mol (36.76 J/mol-K) (121.2100) = 29.4 kJ DH = + 1767 kJ With a coffee-cup calorimeter, a reaction is carried out under constant pressure conditions. -- Why is the pressure constant? calorimeter isnt sealed,
atmospheric pressure is constant -- If we assume that no heat is exchanged between the system and the surroundings, then the solution must absorb any heat given off by the reaction. i.e., qabsorbed = qreleased the specific heat of water -- For dilute aqueous solutions, it is a safe assumption that cP = 4.18 J/g-K When 50.0 mL of 0.100 M AgNO3 and 50.0 mL of 0.100 M HCl are mixed in a coffee-cup calorimeter, the mixtures temperature increases from 22.30oC to 23.11oC. Calculate the enthalpy change for the
reaction, per mole of AgNO3. Assume: AgNO3 + HCl AgCl + HNO3 0.05 L, 0.1 M 0.05 L, 0.1 M 0.005 mol 0.005 mol q = m cP DT = 100 (4.18) (23.1122.30) -- mixture cP = cP of H2O -- mixture mass = 100 g 338.58 J DH =
0.005 mol AgNO3 kJ 67.7 mol AgNO3 = 338.58 J (for 0.005 mol AgNO3) Combustion reactions are studied using constantvolume calorimetry. This technique requires a bomb calorimeter. -- The heat capacity of the bomb calorimeter (Ccal) must be known. unit is J/K (or the equivalent) bomb calorimeter
-- Again, we assume that no energy escapes into the surroundings, so that the heat absorbed by the bomb calorimeter equals the heat given off by the reaction. Solve bomb calorimeter problems by unit cancellation. another bomb calorimeter A 0.343-g sample of propane, C3H8, is burned in a bomb calorimeter with a heat capacity of 3.75 kJ/ oC. The temperature of the material in the calorimeter increases from 23.22oC to 27.83oC. Calculate the molar heat of combustion of propane. kJ 3.75 o (27.83oC23.22oC) C 17.29 kJ
0.343 g 44 g 1 mol C3H8 2220 kJ/mol Hesss Law The DHrxns have been calculated and tabulated for many basic reactions. Hesss law allows us to put these simple reactions together like puzzle pieces such that they add up to a more complicated reaction that we are interested in. By adding or subtracting the DHrxns as appropriate, we can determine the DHrxn of the more complicated reaction.
The area of a composite shape can be found by adding/subtracting the areas of simpler shapes. Calculate the heat of reaction for the combustion of sulfur to form sulfur dioxide. 2 SO2(g) + O2(g) 2 SO3(g)(DH = 198.2 kJ) S8(s) + 12 O2(g) 8 SO3(g)(DH = 3161.6 kJ) S8(s) + 8 O2(g) S8(s) + 12 O2(g) 8 SO2(g)
8 SO3(g) (TARGET) (DH = 3161.6 kJ) 82 SO O (DH +792.8 kJ) 2 SO OSO SO SO4O (DH = 198.2 ==+198.2 kJ)kJ)
33(g)2(g) + 82 2(g) 2(g)(DH 2(g) 2(g)2 + 3(g) 2(g) S8(s) + 8 O2(g) need to 8 SO2(g) DHcancel = 2368.8 kJ 5 C + 6 H2C5H12 Calculate DH for the reaction given the following: C5H12 + 8 O2
5 CO2 + 6 H2O (DH = 3535.6 kJ) C + O2 CO2 H2 + O2 5C CO 2O 5H212 ++ 68HO 2 (DH = 393.5 kJ) H2O (DH = 285.8 kJ)
C52H12 + H82O O2 (DH (DH==3535.6 +3535.6kJ) kJ) 5 CO + 6 5 C + 5 O22 6HH2 2 ++ 3OO2 2 5 C + 6 H2 5 CO2 (DH = 393.5 1967.5kJ) kJ)
6 H2HO2O (DH == 1714.8 (DH 285.8kJ) kJ) C5H12 DH = 146.7 kJ Calculate DH for the reaction given the following: C5H12 + 8 O2 5 C + 6 H2C5H12 5 CO2 + 6 H2O (DH = 3535.6 kJ)
12 + 8 kJ 5 C ++ 5 O O22 393.5 kJ 5 CO2 + 1967.5 6HH OO 2 2+ + 3 2 2 6 HH 285.8kJkJ 2O 2O + + 1714.8 5 C + 6 H2
C5H12 + 146.7 kJ DH = 146.7 kJ enthalpy of formation (DHf): the enthalpy change associated with the formation of a compound from its constituent elements -- also called heat of formation When finding the standard enthalpy of formation (DHfo), all substances must be in their standard states. The standard state of a substance has arbitrarily been chosen to be the state of the substance at 25 oC (298 K). If more than one form of the element exists at 298 K, then the standard state is the most stable form, e.g., O2 rather than O3. -- By definition, DHfo for the most stable form of
any element in its standard state is zero. e.g., DHfo for O2(g) or Al(s) or S8(s), etc. is ZERO -- DHf values are for 1 mol of substance, so the units are typically kJ/mol. -- Many DHf values have been tabulated. DHfo for Ni(s) = 0, but is = 0 for Fe2O3. standard enthalpy of a reaction (DHorxn): DHorxn is the change in enthalpy of a reaction when all substances are in their standard states (i.e., at 25oC). -- Using Hesss law, we can easily calculate DHorxn from the DHfo of all R and P.
-- equation: Germain Henri Hess (1802 1850) DHorxn = Sn DHfo(products) Sm DHfo(reactants) where n and m are the coefficients in the balanced equation Approximate the enthalpy change for the combustion of 246 g of liquid methanol. 2 CH3OH(l) + 3 O2(g) 238.6 kJ/mol X2 0 kJ/mol (Look these up. See App. C,
64 g 246 g X = DH = 4910 kJ for 2 mol (i.e., 64 g) of CH3OH Food and Fuel fuel value: the energy released when 1 g of a material is combusted -- measured by calorimetry calor is the Latin word for heat metria is the Greek
word for to measure Food The body runs on glucose, C6H12O6. With food intake, -- When it is in the blood stream, glucose is called blood sugar increases; with physical (or mental) blood sugar. activity, it decreases. Insulin is the hormone that moves glucose from the blood stream into the cells. -- Our bodies produce glucose out of the foods that we consume. Diabetics must closely monitor blood sugar levels and take insulin to keep that level within range.
carbs: 4 kcal/g; quickly broken down into glucose; not much can be stored as carbs fats: 9 kcal/g; broken down slowly; insoluble in water; easily stored for future use proteins: 4 kcal/g; contain nitrogen which ends up as urea, (NH2)2CO after digestion O HNCNH H H Fuel fossil fuels: coal, petroleum, natural gas
-- products of what used to be living things -- nonrenewable coal-burning power plant Future oil supplies are in question. coal gasification: coal is treated with superheated steam to make the gases CH4, H2, and CO coal gasification plant -- most impurities (e.g., sulfur compounds) are easily removed in this process -- the fuel gases can be transported
by pipeline and then burned for fuel Combustion of ANY fuel contributes to the greenhouse effect. Nuclear energy, from the splitting or fusing of atoms, also is nonrenewable. FISSION -- a lot of bang for your buck, but there is the problem of U or Pu daughter released hazardous waste disposal nuclei cooling towers
neutrons containment building Renewable energy sources include: solar solar panels wind wind generators geothermal geothermal plant in Iceland hydroelectric
biomass crops, biowaste biomass plant in Britain Solar heating can be used to generate CO and H2 gases, which could be burned... for energy or reacted together to get the heat back. heat from + CH4 + H2O Sun CO + H2 Solar (or photovoltaic) cells directly convert solar energy into electricity. Problems with solar energy: -- it is dilute -- storing it for later use
-- it fluctuates w/time of day and weather conditions
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