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  • CHEMISTRY CP Name: _________________________ Per: _____ TEST: _________________

    Unit 7 Review Sheet: Thermochemistry KEY Potential Energy Diagrams: Choose the appropriate letter.

    1. What is the H of the forward reaction? f 2. What is the activation energy of the forward reaction? b 3. What is the H of the reverse reaction? f 4. The forward reaction is (endothermic or exothermic). 5. What is the potential energy of the activated complex? c 6. What is the potential energy of the products? e 7. What is the activation energy of the reverse reaction? d 8. The reverse reaction is (endothermic or exothermic).

    Equilibrium and LeChateliers Principle 1. Using the following reaction at equilibrium answer the following questions:

    N2(g) + 3H2(g) 2NH3(g) + heat a. What direction will the reaction shift if heat is added? LEFT b. What direction will the reaction shift if pressure is increased? RIGHT (4 moles vs. 2 moles) c. What will happen to the concentration of N2 if the temperature decreases? DECREASE d. What will happen to the concentration of NH3 if the more N2 is added? INCREASE e. What direction will the reaction shift if more NH3 is added? LEFT

    2. Explain how the following changes in reaction conditions will affect the position of the equilibrium

    below, and explain your reasoning. A(g) + B(aq) C(s) + heat Hrxn= -453 kJ/mol a. The pressure of A in the reaction chamber is increased. The reaction is pushed toward

    products. b. The temperature of the reaction is increased by 200 C. Because heat can be thought of as

    being a product, the reaction will be pushed toward reactants. c. A catalyst is added to the system. No change. A catalyst doesnt change the equilibrium

    position, it only changes how quickly equilibrium is reached. d. As the reaction progresses, more of compound B is steadily added to the reaction

    chamber. The reaction is pushed toward products. e. Is the forward reaction endothermic or exothermic? Exothermic

    3. Using the following graph of the hypothetical reaction A B to answer the questions that follow.

    a. Identify the product and reactant. A = Reactants and B = Products b. What happens to the number of moles of products and reactants over time? Moles of reactants DECREASE with time and moles of the products INCREASE with time. c. At what time is the number of moles of each at a maximum and a minimum? REACTANTS: Minimum # moles (0.16) at time 60 min Maximum # moles (1.00) at time 0 min PRODUCTS: Minimum # moles (0) at time 0 min Maximum # moles (0.84) at time 60 min d. At what time is the reaction at equilibrium? 24 minutes

  • 4. Know what the rate of a chemical reaction can be measured by: Decrease in concentration of reactants or increase in concentration of products over time.

    5. Know what factors affect the rate of a reaction: temperature, concentration of reactants and products, size of particles, presence of a catalyst.

    6. Know what conditions are true of a chemical reaction at equilibrium: (1) rates of forward and reverse reactions are equal. (2) The concentrations of all substances involved stop changing (3) reactants and products are both present but concentrations are not necessarily equal (4) The system is a dynamic one, still an ongoing inter-conversion of reactants and products.

    Equilibrium Expressions and Constants 1. Write the equilibrium expression (Keq) for the following equations and state whether it is

    homogeneous or heterogeneous. (Remember, dont include solids and liquids):

    a. CaCl2(s) Ca2+(aq) + 2Cl-(aq) 22 ]][[ += ClCaKeq

    b. SO2(g) + O2(g) SO3(g) 2/1




    SOKeq =

    c. S8(s) + 8 O2(g) 8 SO2(g) 8




    OSOKeq =

    d. C(s) + CO2(g) + 2Cl2(g) COCl2 (g) ]][[][



    ClCOCOClKeq =

    2. Consider the following equilibrium: SO3(g) + H2O(g) H2SO4(l) *DO NOT include liquids At equilibrium [SO3] = 0.400M [H2O] = 0.480M [H2SO4] = 0.600M Calculate the value of the equilibrium constant. Keq = 5.21

    3. Consider the following equilibrium: 2SO2 (g) + O2 (g) 2SO3 (g) When 0.600 moles/L of SO2 and 0.600 moles/L of O2 are placed into a 1.00 litre container and allowed to reach equilibrium, the equilibrium [SO3] is to be 0.250 M/L. Calculate the Keq value. Keq = 0.289

    4. If Keq = 1.5 x 10-5 at 35C is the equilibrium constant for the forward reaction what is Keq for the reverse reaction at the same temperature? Keq = 66666.7

    Phase Changes and Phase Diagrams 1. Know the name for all the phase changes and know which phases they occur between (i.e.,

    sublimation when a solid turns to a gas) Melting: solid becomes a liquid Freezing: liquid becomes a solid Boiling/Evaporation: liquid becomes a gas Condensing: gas becomes liquid Sublimation: solid to gas without passing through liquid phase Deposition: gas to solid without passing through liquid phase

    2. Know which phase changes are endothermic and which are exothermic. Endothermic (absorbs heat) Melting, Boiling, Sublimation Exothermic (releases heat) Freezing, Condensing, Deposition

    3. Define the following terms: Critical Point, Triple Point, Phase Boundary, Boiling Point, Melting Point Critical Point: Temperature at which no distinction between liquid and gas exists Triple Point: The temperature and pressure at which all three phases (solid, liquid, gas) coexist Phase Boundary: The distinction (line/curve) used to separate different phases of matter on a phase diagram Boiling Point: The temperature when a liquid will become a gas Melting Point: The temperature when a solid will melt into a liquid

  • 4. Refer to the phase diagram of a pure substance for the following questions:

    a. Write the words solid, liquid, and gas where they correspond to this phase diagram. b. Write the name of the phase change that occurs between points E to C and C to E,

    respectively. E to C: MELTING C to E: FREEZING c. What is the name of point A and what

    is significant about it? A is the triple point where all three phases exist at that specific temp and pressure

    d. What is the state of matter for this substance at a pressure of 0.5 atm and a temperature of 150C? GAS

    e. Describe what happens when this substance moves from point E to F. What is the name of this transition? The substance goes from a solid to a gas; sublimation

    f. What is the approximate temperature at which this substance exists as a gas only? 180 degrees C

    g. What is the approximate normal boiling point (assume P = 1atm)? 150 deg C

    h. At what pressure and temperature can this substance exist in all three states of matter simultaneously? 0 C, 0.40 atm

    i. What is the name for the point of the diagram labelled B? Why is it significant? Critical Point, at this point and above on this phase boundary, there is no distinction between liquid and gas phases. They are combined into a supercritical fluid

    Heat Energy and Specific Heat Calculations 1. Define the following terms: heat, temperature, Law of Conservation of Energy, endothermic,

    exothermic, system, surroundings, heat of fusion, heat of vaporization Heat: is the total amount of energy (both kinetic and potential) possessed by the molecules in a piece of matter. Heat is a measure of energy flow from Hot to Cold ans is measured in Joules. Temperature: is not energy; it is a measure of the average kinetic energy of the particles and is measured in celsius, Kelvin, or Farenheit Law of Conservation of Energy: Energy cannot be created or destroyed; all energy in the world is conserved Endothermic: A reaction which is accompanied by the absorption of energy (positive value of energy) Exothermic: A reaction where energy is released as a product (negative value of energy) System: part of the universe we wish to focus our attention on; usually the reaction we wish to study Surroundings: Everything else in the universe Heat of Fusion: The amout of energy needed to change a gram of substance from solid to liquid Heat of Vaporization: The amount of energy needed to change a gram of substance from liquid to gas

    2. What units are usually used for heat energy? Joules or calories

    Q = m T Cp where Q = heat energy, m = mass, and T = change in temp. Remember, T = (Tf Ti). 3. The temperature of an unknown metal with a mass of 18 g increases from 25C to 40C when the

    metal absorbs 124 J of heat. What is the specific heat of the metal? Cp = 0.46 J/gC

    4. How many joules of heat are lost by 3580 kg granite as it cools from 40C to 10C? Cp=0.795 J/gC. Convert 3580 kg into 3580000 g Then: Q = - 8.54 x 107 J

    5. A 0.3 g piece of copper is heated and fashioned into a bracelet. The amount of energy transferred by heat to the copper is 66,300 J. If the specific heat of copper is 390 J/g 0C, what Is the change of the copper's temperature? Change in Temp = 566.7 degrees C



  • Heat Calculations for Phase Changes and Multistep Problems

    1: 2: Hfus 3: Hvap = = Hfus = Hvap

    1. How much energy is needed to melt 35 grams of ice? Hfus = 334 J/g Q = 11690 J 2. How much energy is needed to boil 150 grams of water? Hvap = 2260 J/g Q = 339000 J 3. The boiling point of ethanol is 78C. How much heat is required to heat 100 grams of ethanol from

    25C to its boiling point and then to turn it into a vapor? Cp = 2.44 J/g C Hvap = 855 J/g Q= 98,432 J 4. FIVE STEP PROBLEM: Calculate the amount of energy required to change 50.0 g of ice at -20.0 C to

    steam at 115.0 C. Please use these values:

    Constants: Heat of fusion = 334 J/g Heat of vaporization = 2260 J/g Specific heat for solid water (ice) = 2.06 J/gC Specific heat for liquid water = 4.18 J/gC Specific heat for gaseous water (steam) = 2.02 J/gC Boiling point of water = 100C Freezing point of water = 0C

    QI: 2060 J Qtotal = 154175 J Q2: 16700 J Q3: 20900 J Q4: 113000 J Q5: 1515 J

    Heating and Cooling Curves (alternative phase diagram) 1. Be able to draw the heating curve for water and label the phases solid, liquid and gas on the

    diagram. See diagram above (1) Solid (3) Liquid (5) Gas 2. Use the diagram below and answer the following questions:

    a. What is the melting temperature of the

    substance? 5 degrees C b. What is the freezing temperature of the

    substance? Not indicated on this curve, but would be 5 degrees C

    c. What is the boiling temperature of the substance? 15 degrees C

    d. What is happening to the molecules at points a, c and e? What is happening to the kinetic energy of the molecules at these points? Temperature is increasing so kinetic energy is increasing

    e. What is happening at point b? Which type of energy is increasing? Phase change (melting) Potential energy is increasing

    f. What is happening at point d? Which type of energy is increasing? Phase change (boiling) Potential energy is increasing

    g. Label solid, liquid and gas on the curve. h. On which point of the curve are the particles

    moving the least? (a) i. Where on the curve is ONLY a liquid present? (c) j. Why doesnt the temperature change at b or d?

    No temp change during phase change




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