CHM2045 Exam 2 Review Spring 2018 Gases

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1 CHM2045 Exam 2 Review Spring 2018 Gases Gases are assumed to behave Ideally What is ideal behavior? o Gas molecules do not exert forces on each other (i.e. no IMFs) o Gases have fully elastic collisions (No net energy loss when they collide with walls and/or each other) o Gas molecules themselves have negligible volume compared to their container How do you facilitate ideal behavior? i.e. when are gasses most ideal? Question 1: Under which of the following conditions would gasses deviate from ideal behavior? I. High temperatures II. Low average kinetic energy III. Low internal pressure IV. High density a. II, and III b. I, II, and III c. II and IV d. II, III, and IV Ideal Gas Law PV = nrt, where P is pressure (atm), V is volume (L), n is moles of gas, T is temperature (K), and R is the gas constant (0.0821) All gas other gas laws can be derived from the Ideal Gas Law o How? 1

2 The Gas Laws Boyle s Law Charles Law Gay-Lessac s Law Avogadro s Law Dalton s Law The gas laws are typically used when comparing the effects of changing conditions Question 2 A weather balloon is initially at a volume of 10 ml and temperature of 298 K at sea level. It is released into the air and once it reaches a certain altitude, the temperature is now 278K and the pressure is 0.76 atm. What is the volume of this balloon at the new altitude? Question 3 Two vessels of different volumes are connected through a closed valve. One of the vessels contains 2L of gas A, and vessel B contains 3L of gas B, and they are both at 758mmHg. When the valve is opened, gas A and B mix and react to for A2B3. What is the final pressure after the reaction has occurred? 2

3 Thermochemistry q and ΔH ARE NOT THE SAME THING ΔH is q/quantity q is measured in KJ/J or Cal/cal ΔH is measured in KJ/mol, J/mol, KJ/g, J/g, etc. o Sometimes they will not include the quantity and simply say the enthalpy change of some reaction if X KJ Question 4 Acetylene, C2H2, burns in air by the following reaction: 2 C2H2(g) + 5 O2(g) à 4 CO2(g) + 2 H2O(g) ΔH = 2510 kj How much heat is released when 3.0 g of C2H2react with 6.0 g of O2? (1) 94.1 kj (2) 145 kj (3) 188 kj (4) 289 kj (5) 477 kj Hess s Law Hess s Law allows us to add enthalpies of individual reactions as we add their respective reactions Hess s Law also tells us that enthalpy is a state function (path independent) When manipulating reactions, the same must be done to their corresponding enthalpies; when multiplying the reaction by a number, you must multiply its enthalpy by the same number when flipping a reaction, its enthalpy must be negated 3

4 Question 5 Calculate the enthalpy for this reaction: 2C(s) + H2(g) ---> C2H2(g) ΔH =??? kj Given the following thermochemical equations: C2H2(g) + 5 2O2(g) ---> 2CO2(g) + H2O(l) ΔH = kj C(s) + O2(g) ---> CO2(g) H2(g) + 1 2O2(g) ---> H2O(l) ΔH = kj ΔH = kj 4

5 Question 6 Calculate the ΔHf of SrCl2(s), given the following thermochemical equations. Sr(s) Sr(g) ΔHvap/sub = kj/mol Sr(g) Sr+(g) + e- IE1 = 549 kj /mol Sr+(g) Sr2+(g) + e- IE2= 1064 kj /mol Cl2(g) 2Cl(g) BE = 243 kj /mol Cl(g) + e- Cl1-(g) EA = -349 kj /mol Sr2+(g) + 2Cl1-(g) SrCl2(s) LE = kJ /mol (1) kj (2) kj (3) kj (4) kj (5) kj Heat Transfer q = mcδt where q is heat energy (J), m is mass (g), C is specific heat (heat capacity) (J/g*K), and T is the difference in temperature (final initial) (can be in kelvin or Celsius since it is a change in temperature. *Pay attention to signs if q < 0, then heat is being released if q > 0, then heat is being absorbed 5

6 When solving problems involving the transfer of heat energy, it is important to be able to identify the system and the surrounding The system is the substance causing the heat transfer The surrounding is the substance being affected by the system o Example. When a hot metal ball is dropped in cold water, the metal ball will be the system as it is releasing heat whereas the water (and maybe the container) will be the surroundings as they are the substance accepting the heat that is leaving the ball Once the system and the surroundings have been identified, the general equation for the heat transfer is q sysetm = q surrounding For example, if water is a surrounding as well as the container, then their combined heat energies will count as qsurrounding When identifying the system and the surroundings, label each of their heat energies in a recognizable way Metal > qmetal solution > qsoln water > qwater container (bomb) > qbomb reaction > qrxn Steps to Solving Heat Transfer Problems 1. Identify the existing q s (qmetal, qsoln, etc.) 2. Determine what the surroundings and the systems are 3. Plug into qsys = - qsurr 4. Write out mcδt for each q (if the information is given) 6

7 Question 7 A g copper rod is heated and then is placed into an insulated vessel containing 1000.g of water at 20.0 C. The final temperature after equilibrium is achieved is 27.5 C. What was the temperature (K) of the copper rod? The heat capacity of copper is J/g C. (1) 573 K (2) 360 K (3) 1285 K (4) 281 K (5) 103 K 7

8 Question 8 13) A ml sample of M HCl is mixed with a 50.0 ml sample of M NaOH in a Styrofoam cup. If both solutions were initially at C, and the enthalpy of the neutralization reaction is 57 kj/mole of H2O formed, what is the final temperature of the mixture? Assume that the solution has a density of 1.00 g/ml and a specific heat of J/g C, and that the Styrofoam cup has an insignificant heat capacity. 1) C 2) 33.8 C 3) C 4) 50 C 5) C 8

9 Kinetics: Reaction Rates Rate Laws Rate laws are determined through experimental data For the reaction A + B > C + D, the rate law is Rate = k [A] x [B] Where x and y are the orders of A and B respectively, and k is the reaction rate constant Finding the Rate Law from a Data Table The data table will typically contain the initial rate, the concentration of each reactant, and the experiment number If you are determining the order of a reactant, look for experiments where all other reactant concentrations stay constant except for the concentration of the reactant under question Question 9 Given the data below, find the rate law for the following reaction at 300 K and the corresponding rate constant. A+B C+D Trial [A]initial (M) [B]initial (M) Rateinitial (M/s)

10 Finding the Rate Law from elementary steps When given a mechanism for a specific reaction, the rate law of the reaction can be determined by finding the rate determining step of the mechanism (aka the slowest step) The mechanism is only as good as its weakest link When given mechanisms with reversible (equilibrium) steps, those steps must be taken into consideration for the rate law as well. If there are any intermediates in the reversible step(s) and the slow step, you must solve for the intermediate and substitute that reaction into the slow reaction Question 10 Which of the rate laws below would be compatible with the following reaction mechanism? Step 1 : A + B D (fast, equilibrium) Step 2 : D + B E + F (slow, rate-determining) Step 3 : F G (fast) (1) rate = k[d][b] (2) rate = k[a][b]2 [D] (3) rate = k[d][b][f] (4) rate = k[a][b]2 (5) rate = k[a][b] 10

11 Kinetics What increases the rate of reaction? Increasing the temperature (average kinetic energy) of the reaction Increasing the surface area of the reactant Increasing the number of collisions between reactants Question 11 Which of the following conditions would hinder the progress of a gaseous reaction? (1) Increasing the temperature (2) Increasing the average kinetic energy (3) Decreasing the pressure of (4) Increasing Volume (5) both (1) and (2) 11

12 Light Chemistry and Photons a photon is a pocket of energy that is released when an electron drops from an excited state of energy to a lower state of energy (we will treat it as if it were an electron) photons move through space in a wave-like manner and thus they have both a frequency and wavelength Light Energy c = λv where c is the speed of light (3.0 x 10 8 m/s), λ is the wavelength (m), and v is the frequency (Hz or 1/s) Light energy (E) is related to frequency through the following equation E = hv Where h is Planck s Constant (6.626 x ) It is vital to understand that E is measured in Joules per individual PHOTON, so if the question asks for joules per MOLE of photons, you must do the appropriate unit conversion Question 12 A certain light bulb consumes 200 J of energy per second. If a light bulb converts all of this energy to 500 nm light, how many photons are produced each second? (1) 5.0 x 10^20 (2) 200 (3) 5.0x10^-7 (4) 5.0x10^-19 (5)

13 Question 13 What is the longest wavelength of light which can photo-dissociate a CO molecule when the bond energy of CO is 1046 kj/mol. (1) 114 nm (2) 265 nm (3) 74.0 nm (4) 318 nm (5) 93.5 nm 13

14 Question 14 Enormous numbers of microwave photons are needed to warm household samples. A bowl of soup containing 400g of water is heated in a microwave oven from 20.0 C to 98.0 C using radiation with wavelength 122 mm. Assuming that the specific heat capacity of the soup is the same as that of water (4.184 J/g C) and no heat loss to the bowl, which choice is closest to the number of photons absorbed? (1) 4.0*10-6 (2) 8.0*10-20 (3) 4.0*106 (4) 8.0*1020 (5) None of the above 14