1
Chemistry
Second Semester Final Review
Final Exam Learning Targets Unit 5
• Perform 3-step grams-moles-moles-grams stoichiometry calculations. • Solve molarity problems.
• Solve combined gas law problems. Unit 6
• Calculate the ΔH⁰ for a chemical reaction from experiment data. • Calculate the ΔH⁰ for a chemical reaction from thermodynamic tables. • Calculate the ΔH⁰ for a reaction by summing a series of other reactions. Unit 7
• Calculate the oxidation numbers of chemical materials.
• Use oxidation numbers to identify the oxidized and reduced materials in chemical reactions. • Use a table of half-cell potentials to determine voltages of electrochemical cells.
• Compare the power and energy in electrical systems to heat-producing systems. Unit 8
• Define entropy and explain its relationship to chemical equilibrium. • Define ∆G⁰ and explain its relationship to chemical equilibrium.
• Calculate, compare, and interpret the ∆G⁰ and ∆H⁰ values for chemical reactions. • Explain the variables that affect the rates of chemical reactions using graphs. • Calculate the values of unknown molarities and molar masses using titration data. • Calculate the pH of acid-base mixtures.
Unit 9
• Describe the attractive forces that exist in metals, ionic compounds, and covalent compounds. • Describe the key intermolecular forces that exist in covalent liquids and solids.
2 1. Use the following chemical equation in answering items (a) through (g) on this page.
Mg(s) + HCl(aq) → H2(g) + MgCl2(aq) a) Identify the oxidized and reduced substances in this reaction.
b) Calculate the maximum mass of Mg that would be completely consumed by 35.0 mL of 0.50 M HCl solution.
c) Calculate the volume of dry hydrogen gas at STP that would be produced from the complete consumption of 6.50 g of Mg.
d) Calculate the mass of dry MgCl2 that would be produced from the complete consumption of 6.50 g of Mg.
e) Calculate the ∆G⁰ and ∆H⁰ for the reaction. Note: The ∆H⁰ for MgCl2(aq) is -769 kJ/mole, and the ∆G⁰ value is -717 kJ/mole.
f) Would you expect this reaction to produce an increase in entropy or a decrease in entropy? Explain.
g) Explain why Mg(s) conducts electricity, why MgCl2(aq) conducts electricity, and why MgCl2(s) would not conduct electricity.
3 2. Use the following chemical equation in answering items (a) through (g) on this page.
NaOH(aq) + HCl(aq) → H2O(l) + NaCl(aq) a) Identify the oxidized and reduced substances in this chemical reaction.
b) What volume of 0.0750 M NaOH would be needed to completely neutralize 38.50 mL of 0.0650 M HCl ?
c) What volume of 0.115 M NaOH would be needed to completely neutralize 116.50 mL of 0.0800 M HCl ?
d) An unknown solid acid required 45.5 mL of 0.115 M NaOH for complete titration. What was the molar mass of the acid?
e) A student obtained 25.00 mL of 0.125 M HCl in a flask, and then added 0.125 M NaOH to the acid in a titration. Calculate the pH of the mixture when the following total volumes of 0.100 M NaOH had been delivered: 24.50 mL, 24.75 mL. 25.00 mL.
f) Calculate the ∆G⁰ and ∆H⁰ for the reaction.
g) Would you say that the reaction represented by the equation at the top of the page has a relatively large or relatively small activation energy? Explain.
4 3. Use the following chemical equation in answering items (a) through (g) on this page.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) a) Identify the oxidized and reduced substances in this chemical reaction.
b) The copper (II) sulfate for the reaction was made by dissolving 60.0 g of CuSO4 in enough water to make 250.0 mL of solution. What was the molarity of this solution?
c) What mass of Zn would be consumed by the complete reaction of a Zn strip with the CuSO4 solution described in (b)?
d) What mass of Cu would be produced from the oxidation of 12.50 g of Zn in this reaction?
e) If we made an electrochemical cell from this chemical reaction by using a zinc anode on one side of the cell and aqueous copper (II) ions on the other side, what would be the standard voltage of the cell?
f) Calculate the ∆G⁰ and ∆H⁰ for the reaction. Note: The ∆H⁰ for CuSO4(aq) is -770 kJ/mole, and the ∆G⁰ value is -662 kJ/mole. The ∆H⁰ for MgSO4(aq) is -980 kJ/mole, and the ∆G⁰ value is -875 kJ/mole.
g) In relation to (e), if we replaced the zinc anode in the electrochemical cell with a tin anode, what would be the new standard cell voltage?
5 4. Use the following chemical equation in answering items (a) through (h) on this page.
C25H52(s) + O2(g) → H2O(l) + CO2(g)
a) Identify the oxidized and reduced substances in this chemical reaction, which represents the burning of a candle.
b) Balance the chemical equation.
c) What mass of paraffin would be consumed by its complete reaction with 30.0 L of pure oxygen gas at STP?
d) What mass of paraffin would need to be burned to produce 100.0 L of CO2(g) at STP ?
e) In an experiment, a student burned 0.55 g of wax under a can that contained 150.0 g of water that started at 22 ºC. The water’s final temperature was 55.5 ºC. Use these data to calculate the ∆H⁰ for the reaction at the top of the page in kJ/mole of paraffin.
f) Calculate the ∆G⁰ for the reaction. Note: The ∆G⁰ for paraffin is -39 kJ/mole (estimated).
g) Describe the intermolecular forces that exist in a lump of candle wax.
h) Using a histogram and showing the activation energy on the histogram, explain why heating candle wax to a high temperature increases the rate of the burning of the candle.
6 5. Several of the questions on this page will require you to compare various answers that you have
generated in the first four questions.
a) Rank the reactions from questions 1 through 4 from the highest heat output (ΔHº) to the lowest. Comment on the ranking.
b) For each reaction, compare the ΔH⁰ and ΔG⁰ values. As you do this comparison for all four reactions, do you notice any trends or patterns?
c) In some fashion, you have performed or witnessed all four of these reactions. Rank them according to their speed at room temperature, based on your memory of the reactions.
d) Which of the four reactions are the most useful to humans in terms of the reactions’ energy output? Explain.
e) A student wired together two of the zinc/copper electrochemical cells in problem 3(e) and used this battery to operate a circuit that contained several LED’s. The circuit delivered 0.015 amps of current. What was the power of the circuit? For what length of time would the circuit need to operate In order to generate a total of 1 kJ of energy? For what length of time would the circuit need to operate in order to generate the same amount of energy as the burning of 0.25 g of candle wax?
f) Rank the four reactions from problems 1 – 4 in order of their increasing tendency to favor the products at equilibrium. Stated another way, rank the reactions from the most reversible to the least reversible.
g) Use an Ep–vs.–time graph to explain why the energy output of a reaction, either endothermic or exothermic, is not related to the speed of a reaction.
