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3/16/2009
The Mid-Term Test
• COVERS ALL MATERIAL IN CHAPTERS 4, 5,
and 6, PLUS NECESSARY SKILLS FROM
CHAPTERS 1-3
• The following outline of the test indicates the most
important topics and skills, but is not meant to
indicate that items not listed here will not be covered
• Section A: 8 multiple choice questions (16 marks)
– Chapters 4-6
– Includes 5 calculations
Section B: longer questions (34 marks)
• Writing balanced equations
– Learn Reaction Types
– Learn Solubility Rules
– Learn Activity Series (Na, Zn, H2, Cu, Ag)
• Writing ionic equations and identifying spectator ions
– Learn weak and strong electrolytes and acids/bases
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Oxidation, reduction and oxidation number
Stoichiometry – solutions, gases and heat
Gas Laws and calculations
Heat transfer and Calorimetry
Hess’s Law and Heats of Formation
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3/16/2009
The Mid-Term Test
• UNITS AND SIGNIFICANT FIGURES
• SHOW WORKING
• The periodic table on the back of your lab manual will be
provided, as well as the list of constants inside the back cover.
Review of Chapter 6
• Checklist, Key Equations, Summary and Operational Skills on
pages 194-195
• Types of energy (kinetic, potential, internal, heat)
• ENTHALPY (H) is internal energy that can be released as heat
• Law of conservation of mass
• Identify the SYSTEM (reaction) and SURROUNDINGS
• qsurroundings = -qsystem
• qsystem is negative if heat is transferred from the system to the
surroundings (qsurroundings is positive)
• Calculate heat transferred to/from surroundings from:
• Change in the temperature of the surroundings
• A thermochemical equation
– Exothermic vs. Enothermic reactions (sign of q and
? H; temperature change)
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Review of Chapter 6
• Calorimetry
? H = qreaction/nreaction
or Heat of combustion = qreaction/norganic
compound
• Constant pressure: qreaction = -sm? T
• Bomb: qreaction = -Ccalorimeter? T
or
qreaction = -(Ccalorimeter + mwater swater)? T
• Use thermochemical equation stoichiometry to convert to
? H. E.g. for:
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l) ? H = ? kJ/mol
nreaction = mol C2H6 x (1 mol reaction/2 mol C2H6)
Review of Chapter 6
• Hess’s Law
• For a chemical reaction that can be written as the sum of two or more
steps, the enthalpy change is the sum of the changes for the individual
steps
• You do not need to draw enthalpy diagrams
• Standard Enthalpy of Formation
• ? Hfo is the enthalpy change for formation of 1 mole of a substance in its
standard state from its elements in their standard states
? Ho = ? n? Hfo(products) - ? n? Hfo(reactants)
Applies only when all enthalpies of formation are given
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Applying Stoichiometry
• The thermochemical equation provides a conversion factor to
convert between moles of a substance and heat released or
absorbed.
Moles of substance ⇔ heat release or adsorbed
– Note that the heat is determined by the limiting reagent
• Example
How much ethane would need to be burned to heat 100 g of
water by 10 K?
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)
? H = -3122 kJ/mol
Answer: 2.68x10-3 mol
Calorimetry Example Problem
• Sodium metal reacts with water to produce hydrogen gas and
sodium hydroxide according to the equation:
2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g)
?H = ?
When 0.0300 mol of Na was added to 100.0 g of water, the
temperature of the resulting solution rose from 25.0 oC to 37.9
oC. If the specific heat of the solution was 4.18 J g-1 K-1,
calculate ? H, in kJ, for the reaction as written.
• Answer: -359 kJ
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Gas Example Problem
• A 1.00 L flask at 30.0 oC and 215 kPa contains
a mixture of CO2(g) and Ar(g) in a 1:1 molar
ratio (XAr = 0.500). Calculate the mass of Ar in
the flask.
• ANSWER: 1.70 g
Gases Example Problems
• Calculate the change in pressure (in torr) when 1 mL
of water is added to dry air in a 10.0 L flask at STP
(the vapour pressure of water is 4.6 torr at O oC).
– ANSWER: pressure change = 4.6 torr
• If Xoxygen = 0.20 in the dry air, calculate the partial
pressure of O2 (in atm) after addition of the water.
– ANSWER: Poxygen = 0.20 atm
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Titration Problem
• 20.0 mL of a saturated solution of Ca(OH)2
was neutralized by 8.85 mL of 0.0510 M HCl.
Calculate the solubility of Ca(OH)2 in mol/L.
– Answer: 0.0113 mol/L
Gases Example Problem
• If 5.0 cm3 of gas at 25oC are heated to 250oC
at constant pressure. What is the final volume
of the gas in L?
• Answer: 8.8 x 10-3 L
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