NCFE Spring 2013 Released Items & Explanations

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Avice Lane
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1 Correct answer = C Top #= mass number (which is the total number of protons and neutrons), bottom # = atomic number (which is equal to the number of protons only) These isotopes of carbon have the same atomic number, the same number of protons, but a different mass (because there are differing numbers of neutrons) Unit 3 info Correct answer = A boron-11 5 p+ (Periodic Table), 6 n o [11 (mass) - 5 (p+)] Carbon-14 6 p+ (Periodic Table), 8 n o [14 (mass) - 6 (p+)] unit 3 info Correct answer = A See diagram on next page to help. Unit 4 info

2 Correct answer = C Aluminum s symbol is Al. It is #13 on the Periodic Table. It is in Group 13/IIIA/3A, which means there are 3 valence electrons. Unit 4 info Correct answer = D Use Bohr Model for Hydrogen Atom diagram on p. 8 of Reference Tables. Energy (in this case light) is emitted when an electron FALLS from the excited state back to the ground state. (Energy is absorbed to jump to a higher energy level.) Unit 4 info

3 Correct answer = B In a nuclear equation, the top numbers (mass numbers) must equal the same amount on both sides of the arrow. Same with the bottom numbers (atomic numbers). In this particular case, 222 = 4 +? Well, the? must be 218. Unit 16 info

Correct answer = A metallic bonds ( bonds between atoms of the same metal) are produced by overlapping d orbitals that allow free movement of the electrons.

4 Correct answer = A metallic bonds ( bonds between atoms of the same metal) are produced by overlapping d orbitals that allow free movement of the electrons. These freely moving electrons do not belong to any one particular metal atom, so they are referred to as delocalized electrons. Ionic bonds are when electrons are transferred from one atom/element to another. (Covalent bonding is when electrons are shared between two atoms/elements.) Unit 6 info Correct answer = A Aluminum has a +3 oxidation number when it is in a compound. Sulfur has a -2 oxidation number in a compound. Al +3 X -2 Cross out the + and -. Simplify oxidation numbers if possible. (These cannot be simplified.) Then criss-cross the numbers. Oxidation number of first element becomes subscript of second element. Oxidation number of second element becomes subscript of first element. Unit 7 info Correct answer = C ionic cmpds are formed between a metal and a nonmetal. Metals are on the left side of the staircase line on the Periodic Table. Nonmetals are on the right side of the staircase line on the Periodic Table. Sodium = Na = #11 (left side), Fluorine = F = #9 (right side) Unit 6 info

5 Correct answer = B (This wack-a-doodle question gives answers that are all over the place. Usually, questions will only ask you to compare two types of bonds. But I digress ) The more electrons being shared, the stronger the bond. (Triple = strongest, single = weakest) The stronger the bond is, the more attraction there is between the two elements. Stronger attraction means closer together. Stronger bonds have shorter bond lengths. Unit 6 info Correct answer = A Fe has an oxidation number of +3 (you know that because of the Roman numeral in parentheses). Chromate is CrO -2 4 according to the list of polyatomic ions on p. 7 of the Ref Tables. Cross out + and -. Simplify oxidation numbers if possible. (Not possible here.) Criss-cross the numbers. See explanation for #8 for more specifics. Correct answer = C D is not correct because Greek prefixes can only be used with binary molecular compounds (2 nonmetals). There must be a Roman numeral because Pb is in Group 14 (not Group 1 or 2 or an exception like Al +3, Zn +2, Cd +2, or Ag +1 ), so answer A is not correct. To determine the oxidation number of an element in a binary compound, use this formula: (number of atoms of each element multiplied by the oxidation number for each element) = 0 and Let x = oxidation number of the metal PbO 2 (1. x) + (2. -2) = 0 x 4 = 0 x = 4, so lead (IV) oxide is the correct answer Unit 7 info

6 Correct answer = D Unit 6 info Look at chart in Unit 6 Materials that compares Ionic vs. Molecular Substances (See below.) Correct answer = D malleable & conduct heat are properties of metals Unit 2 info

Correct answer = B elements in the same group (column) have similar properties Unit 5 info Correct answer = D strongest attraction for electrons = most wants to gain another electron = highest

7 Correct answer = B elements in the same group (column) have similar properties Unit 5 info Correct answer = D strongest attraction for electrons = most wants to gain another electron = highest electronegativity Highest electronegativity is at the top of Group 17 Unit 5 info Correct answer = A liquid-vapor system in equilibrium means a substance at its boiling point. If energy is removed, the system will increase the amount of liquid. If I added more energy, then more would boil and the amount of vapor would increase. Can also think of this in terms of equilibrium and LeChatelier s principle. I ll use H 2O as my example as we are all familiar with it in its different states of matter. H 2O (l) + heat energy H 2O (g) If energy is lowered, then equilibrium will shift towards the left side. That increases the amount of liquid present. Unit 13 info Correct answer = B As heat energy increases

8 As heat energy decreases Unit 11 info

Correct answer = B melting = converting from solid to a liquid. When substance does not change temperature (instead changes state of matter), the equation used is q = m.

9 Correct answer = B melting = converting from solid to a liquid. When substance does not change temperature (instead changes state of matter), the equation used is q = m. Hf (or Hv) When phase change involves solid liquid, the Hf constant should be used. (See p. 1 of Ref Tables for constant) Unit 11 info Correct answer = B TEMPERATURE is a measure of average kinetic energy. When freezing occurs, temperature does not change, so avg. KE wouldn t change- eliminating C & D as answer choices. So, even though you may not be familiar with the term latent heat, you can tell that the two answers are exactly the same except for the absorbed or released into the air part. To go from a liquid to a solid, heat/energy must be released. Unit 11 info

10 Correct answer = D Unit 11 info

Correct answer = C At STP, one mole of any gas has a volume of 22.4 liters. Unit 10 info 135 L x 1 mole = 6.02 moles You could also solve this with PV=nRT using the correct values 22.

11 Correct answer = C At STP, one mole of any gas has a volume of 22.4 liters. Unit 10 info 135 L x 1 mole = 6.02 moles You could also solve this with PV=nRT using the correct values 22.4 L for P and T for STP. Correct answer = B mixture of gases and/or partial pressure = Dalton s law Unit 10 info Dalton s law = add up pressures to get total -or- pressure of gas in a mixture is proportional to the percentage of the mixture that is that gas In this case, it is an add-them-up-to-get-the-total problem! = x x = 7.32 Correct answer = C Unit 15 info A potential energy diagram involves the energy differences as a chemical reaction proceeds. It does not have anything to do with ionic or covalent bonding or compounds. The reaction is exothermic because the energy before the reaction started (around point W) is higher than the energy after the reaction finished (around point X). Overall, energy was lost so it is an exothermic reaction.

12 Correct answer = A Unit 15 info Increasing the temperature increases the average kinetic energy of the molecules. When the kinetic energy of the molecules increases, the molecules will collide more frequently and with more momentum (energy). Correct answer = C HOWEVER this is the single, most ridiculous question in the history of chemistry. You have only 40 questions that are supposed to cover all that was learned over the course and you waste a question on this nonsense?!? Here s why the official test for the presence of carbon dioxide is running it through lime water and it turning cloudy. However, with the restrictions of a real classroom and 90 days to teach the material, no teacher in their right mind is going to make the lime water solution, then bubble the resultant gas through the lime water. Instead, teachers show the presence of carbon dioxide by extinguishing a match a much more realistic way to show the same material. Besides all of these reasons, it is a stupid question because it is a ridiculously nit-pick detail question. Students are expected to know general information about the subject, not meaningless minutia. Rant over. Maybe not completely over Just for fun, here s the recipe for making lime water. I want someone to show me a general chemistry teacher in the public school system in their right mind who would do this for a simple demonstration You have got to be joking me! On that note, rant over now. For real.

13 Correct answer = B Unit 8 info Balanced eqn= 4 Fe + 3 O 2 2 Fe 2O 3 Correct answer = D Unit 8 info (can be found on p. 6 of Ref Tables) AgNO 3 and NaCl are both compounds (more than two elements). When two compounds react together, it is a double replacement reaction. Correct answer = D Unit 3 info Strangely worded should read more like What is the mass of a 3.25-mole sample of NH 4OH? N= 1 x = moles x g = 114 g H= 5 x = g/mole 1 mole O= 1 x = Correct answer = B Unit 9 info Stoichiometry problem b/c given chemical rxn and amt of one substance, asked for an amt of another substance. 7.2 moles O 2 = x moles MgO x = 14.4 moles MgO 1 2 Given moles & asked for moles, so only step 2 (mole ratio) is necessary.

Correct answer = B Unit 15 info Of temperature, number of particle collisions, concentration, and density (differences between the answer choices), the only one connected to surface area is the

14 Correct answer = B Unit 15 info Of temperature, number of particle collisions, concentration, and density (differences between the answer choices), the only one connected to surface area is the number of particle collisions. Correct answer = A Unit 15 info Enzymes are catalysts. Catalysts speed up chemical reactions (without taking part in the reaction) by lowering the activation energy. Correct answer = A Keq expression set up Unit 13 info [right side] [left side] Coefficients in balanced equation become exponents in Keq expression

15 Correct answer = C Unit 13 info Allowing NH 3 and HCl to escape lowers the concentration of those substances. Lower concentration of substances on the left side of the equation will shift the equilibrium towards the left side. Shifting left means that the concenrations of the substances on the left side increase. Correct answer = D Unit 14 info Arrhenius definition of a base = substance that produces hydroxide (OH - ) ions in solution. Correct answer = C Unit 14 info hydrogen ion concentration = [H + ] ph = -log[h + ] -log[1.0 x 10-7 ] = 7 At a ph of 7, bromocresol green would have turned blue from its yellow color at a lower ph. Congo red is not an option because it is not yellow. Indigo carmine is not correct because it does not turn yellow until about ph 13.

Correct answer = A Unit 12 info Electrolytes (or electrolytic solutions) are solutions made by dissolving ionic compounds in water. Acids & bases are also considered electrolytes.

16 Correct answer = A Unit 12 info Electrolytes (or electrolytic solutions) are solutions made by dissolving ionic compounds in water. Acids & bases are also considered electrolytes. (Nonelectrolytes are either not soluble in water or are not ionic compounds, acids, or bases) Correct answer = B Unit 12 info Find line for NH 4Br. Find 50 g on y-axis. (Red point on graph to the right.) Read where on the x-axis this point is. It is a little past 50, so 54 o C would be best answer.

17 Correct answer = A Unit 12 info Find point where 80 o C on the x-axis meets 50 g on the y-axis. (Point in orange on graph to the left.) This point is exactly on the line for KCl, so the correct answer is a saturated solution of KCl. The lines on the graph tell the maximum amount of solute (amount needed to have a saturated solution) that can be dissolved in 100 g of water at the different temperatures. To be unsaturated, the point given will be below the line on the graph. To be supersaturated, the point given will be above the line on the graph.

18 Correct answer = C Units 6 & 12 info When salt dissolves in water, it separates into its positive and negative ions - Na + and Cl - We know that opposite charges attract each other, so the chlorine (chloride) ion would not be attracted to a negative charge and the sodium ion would not be attracted to a positive charge. That rules out answer choices A and D. So, does oxygen have a partial positive or partial negative charge? Oxygen is more electronegative (wants to gain an electron more than) hydrogen. If oxygen wants to gain an electron more, the electrons spend more time around the O, giving it a partial negative charge.

NCFE Released Items. Correct Answer = D Number of protons = atomic number

NCFE Released Items. Correct Answer = D Number of protons = atomic number Correct Answer = D Number of protons = atomic number Pa = 91 protons U = 92 protons Np = 93 protons Pu = 94 protons Info from Unit 3 Correct Answer = B Valence electrons are the electrons in the HOEL.