Name: Date: Period: #: UNIT 4 NOTES & EXAMPLE PROBLEMS. W = kg m s 2 m= kg m2. Pressure =

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1 BACKGROUND ON UNITS Acceleration = velocity time Force (F) = mass acceleration = m a F = kg m s 2 = kg m s 2 UNIT 4 NOTES & EXAMPLE PROBLEMS = m s s Work (W)= Force distance = F d = m s 2 = m s-2 W = kg m s 2 m= kg m2 s 2 = joule (J) Pressure = Force unit area = Pressure Volume = kg m s 2 m 2 = kg m s 2 kg m s 2 m3 = kg m2 s 2 = joule (J) SETTING THE TABLE Two systems with different temperatures that are in thermal contact will exchange energy. The quantity of thermal energy transferred from one system to another is called heat. The process of kinetic energy transfer at the particulate scale is referred to in this course as heat transfer, and the spontaneous direction of the transfer is always from a hot to a cold body via collisions of particles, until temperatures are equal. Energy is neither created nor destroyed, but only transformed from one form to another. Energy transfer can occur through either heat exchange or work. Chemical systems undergo three main processes that change their energy: heating/cooling, phase transitions, and chemical reactions. Calorimetry is an experimental technique that is used to determine the heat exchanged/transferred in a chemical system. TOTAL ENERGY E q w positive System gains energy Endothermic Surroundings do work on system Heat gained by system Gas compression negative System loses energy Exothermic System does work on surroundings Heat lost by system Gas expansion 1) A system releases 125 kj of heat while 104 kj of work is done on it. 2) A gas absorbs 28 kj of heat while 53 kj of work is done to compress it. 3) A balloon does 72 kj of work as it expands while releasing 24 kj of heat. 4) While expanding, a balloon does 62 kj of work and absorbs 79 kj of heat. PRESSURE-VOLUME WORK Calculations involving work are limited to that associated with changes in volume of a gas. An example of the transfer of energy between systems through work is the expansion of gas in a steam engine or car piston. Reasoning about this energy transfer can be based on molecular collisions with the piston: The gas is doing work on the piston, and energy is transferred from the gas to the piston. For an ideal gas, work can be calculated using w = PΔV system expands: V = (+/-) system contracts: V = (+/-) Conversion factor for L atm and J: p. 1 Tait, L.

2 5) Calculate the work for the expansion of a gas from 2.0 L to 6.0 L against a pressure of 2.0 atm at constant temperature. POTENTIAL ENERGY DIAGRAMS System vs. Surroundings Exothermic Ereact (greater than/less than) Eprod Net (gain/loss) of energy by the system Feels (warm/cool) to touch because Energy is (absorbed/released) (from/by) the system (to/from) the surroundings Add heat to the reactant or product side of the following equation: A + B C Endothermic Ereact (greater than/less than) Eprod Net (gain/loss) of energy by the system Feels (warm/cool) to touch because Energy is (absorbed/released) (from/by) the system (to/from) the surroundings Add heat to the reactant or product side of the following equation: A + B C ENTHALPY STOICHIOMETRY a) When reversing the rxn, reverse the sign on ΔH b) Enthalpy is an extensive property (dependent on the amount of material present) c) The enthalpy change for a rxn depends on the state of the reactants and products. d) We assume the temperature is 25 C unless otherwise indicated. 2H2(g) + O2(g) 2H2O(g) ΔH = -242 kj/molrxn 6) How many kilojoules of heat are released when 0.750g H2 reacts with an unlimited supply of O2? 7) How many kilojoules of heat are required to decompose 10.0 g of H2O in a gaseous state? p. 2 Tait, L.

3 NaOH(s) H 2O NaOH(aq) ΔH = -43 kj/molrxn 8) Calculate the amount of heat released when 100.g NaOH(s) is dissolved in water. CALORIMETRY 9) What is the specific heat of an unknown substance if 20.0 grams of it absorbs 850. J producing a temperature change from 25.0 C to 90.0 C? 10) Calculate the specific heat of an unknown metal. A 20.0g sample of the metal is heated to C and is transferred to a coffee cup calorimeter containing 75.0g of water at 24.0 C. The specific heat of water is J. The final temperature in the calorimeter is 27.5 C. g 11) When 50.mL of 1.0M HCl and 50.mL of 1.0M NaOH are mixed in a calorimeter, the temperature increases from 21.0 C to 27.5 C. Calculate ΔH in kj mol HCl. Assume no loss of heat by the calorimeter, total volume of the solution is 100.mL, density = 1.00 g J, and specific heat is ml g. 12) When a 3.88 g sample of solid ammonium nitrate dissolves in 60.0 g of water in a coffee-cup calorimeter, the temperature drops from 23.0 C to 18.4 C. Calculate the ΔH (in kj/mol NH4NO3) for the solution process. Assume that the specific heat of the solution is the same as that of pure water. p. 3 Tait, L.

4 HEATING & COOLING CURVES Diagonal: Horizontal: ΔHfusion ΔHsolid ΔHvaporization ΔHcondensation 13) How much energy is required to warm 10.0g of ice at C to 40.0 C? Cp water = 4.18 J g Cp ice = 2.09 J g HESS S LAW 14) Given: B2O3(s) + 3H2O(g) B2H6(g) + 3O2(g) 2H2O(l) 2H2O(g) H2(g) + ½O2(g) H2O(l) 2B(s) + 3H2(g) B2H6(g) ΔH = kj/molrxn ΔH = +88 kj/molrxn ΔH = -286 kj/molrxn ΔH = +36 kj/molrxn Calculate the enthalpy of reaction for: 2B(s) + 3 2O2(g) B2O3(s) p. 4 Tait, L.

5 15) Given: C2H2(g) + 5 2O2(g) 2CO2(g) + H2O(l) ΔH = kj/molrxn C(s) + O2(g) CO2(g) ΔH = kj/molrxn H2(g) + ½O2(g) H2O(l) ΔH = kj/molrxn Calculate ΔHrxn for: 2C(s) + H2(g) C2H2(g) ENTHALPY OF FORMATION 16) Write the rxn for the formation of: a) CH4(g) b) H2O(l) c) C6H12O6(s) 17) Calculate the H rxn for the following reaction: 2C3H6(g) + 9O2(g) 6CO2(g) + 6H2O(l) pg ΔHf data 18) The complete combustion of 1 mol of acetone liberates kj of energy. Calculate the enthalpy of formation of acetone. C3H6O(l) + 4O2(g) 3CO2(g) + 3H2O(l) ΔHrxn = kj/molrxn p. 5 Tait, L.

6 Types of enthalpy: SYMBOL TYPE OF PROCESS DEFINITION OF PROCESS WITH EXAMPLE ΔHrxn Heat of reaction Enthalpy change for any chemical reaction. AgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) ΔHrxn = -68 kj/molrxn ΔHcomb Heat of combustion Enthalpy change for a combustion reaction of one mole of an organic compound. CH4(g) + 2O2(g) CO2(g) + 2H2O(g) ΔHcomb = -802 kj/mol ΔHfus Heat of fusion Heat change when a mole of a solid melts. H2O(s) H2O(l) ΔHfus = +6.0 kj/mol ΔHvap Heat of vaporization Heat change when a mole of a liquid vaporizes. H2O(l) H2O(g) ΔHvap = +44kJ/mol ΔHBDE Bond enthalpy Heat required to break a one mole of a chemical bond. H2(g) H(g) + H(g) ΔHBDE = +436 kj/molrxn ΔHf Heat of formation Heat change when one mole of a substance is formed from its elements. H2(g) + ½O2(g) H2O(l) ΔHf = kj/molrxn ΔHsoln Heat of solution Heat change when one mole of a solute dissolves in a solvent. LiCl(s) Li + (aq) + Cl - (aq) ΔHsoln = kj/mol Ways to calculate enthalpy: 1) Enthalpy stoichiometry (replace half of mole ratio with ΔH check coefficients!) 2) Calorimetry (at constant pressure q = total ΔH) 3) Enthalpy of phase changes (q = nδh(insert phase change here)) 4) Equation manipulations (flip equation = reverse ΔH sign; multiple equation = multiply ΔH) 5) Hess s Law (manipulate and cancel to match goal equation) 6) Enthalpy of formation equation (products reactants; multiply by coefficients) 7) Bond enthalpy (bonds broken + bonds formed) (Second Semester) Also don t forget q = mcδt! Works when there is no phase change occurring, regardless of a chemical reaction. BREAKING ATTRACTIONS ALWAYS REQUIRES ENERGY! Endothermic (+) values p. 6 Tait, L.