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 LeChatelier’s 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 doesn’t 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, don’t include solids and liquids):

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

b. SO2(g) + ½ O2(g) ⇌ SO3(g) 2/1

22

3

]][[][OSO

SOKeq =

c. S8(s) + 8 O2(g) ⇌ 8 SO2(g) 8

2

82

][][

OSOKeq =

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

]][[][

22

2

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 35°C 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 150°C? 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 25°C to 40°C when the

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

4. How many joules of heat are lost by 3580 kg granite as it cools from 40°C to 10°C? Cp=0.795 J/g°C. 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

SOLID LiQUID

GAS

Heat Calculations for Phase Changes and Multistep Problems

𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛  1:𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑖𝑛𝑔  𝐻𝑒𝑎𝑡 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛  2:𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑖𝑛𝑔 ΔHfus 𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛  3:𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑖𝑛𝑔 ΔHvap 𝑄 = 𝑚𝑎𝑠𝑠  ×  ∆𝑇  ×𝐶𝑝                                                                      𝑄 = ΔHfus  ×  𝑚𝑎𝑠𝑠                                                                      𝑄 = ΔHvap  ×𝑚𝑎𝑠𝑠                                              

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 78°C. How much heat is required to heat 100 grams of ethanol from

25°C 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/g°C Specific heat for liquid water = 4.18 J/g°C Specific heat for gaseous water (steam) = 2.02 J/g°C Boiling point of water = 100°C Freezing point of water = 0°C

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 doesn’t the temperature change at b or d?

No temp change during phase change

Solid

Liquid

Gas