Chemistry Review Using OGT questions 2005—2009 retired exam questions

Concept: Conservation of matter and energy. Neither energy or matter can be created or destroyed, but it may be converted from form to form.

During a chemical process, all atoms and all energy that entered the process via the reactants must be accounted for in the ending products. In chemical equations, Reactants  products Balancing chemical equations shows that no new atoms or energy are created and that none are lost.

When methane (CH 4 ) is burned in the presence of oxygen (O 2 ), the two chemicals react together in a process called combustion. Which of these compounds could be a possible product of this combustion reaction? 1.NH 3 2.SO 2 3.H 2 O 4.CS 2

Reactants: CH 4 + O 2  ? What makes each choice correct or incorrect? 1.NH 3 No N atoms in either reactant 2.SO 2 No S atoms in either reactant 3.H 2 O possible—both H & O atoms in reactants 4.CS 2 No S atoms in either reactant During combustion reactions, C & H containing compounds react with O 2 to produce heat (released from bonds broken in the CH 4 ), CO 2, & H 2 O. CH 4 + O 2  CO 2 + H 2 O + heat energy

Concept: density= mass/volume density is a physical property density shows how much mass occupies a certain amount of space Materials will float on top of liquids or gases having higher density, but will sink if the liquids or gases have lower density. Density can change with temperature.

Gertrude cut two bars of different types of soap into four pieces each. She put one piece from each bar into each of four beakers, labeled Beaker W, Beaker X, Beaker Y and Beaker Z. Each beaker contained a different unknown liquid. According to the results shown above, which beaker contained the liquid that was densest? 1.Beaker W 2.Beaker X 3.Beaker Y 4.Beaker Z

Explanation—materials will float on top of liquids or gases having higher density, but will sink if the liquids or gases have lower density. Density = mass/volume Both soaps float in Beaker W, so the liquid In beaker W is more dense Than both soaps. Order of increasing density: Liquid in Y least dense, Liquid in X 2 nd least dense, Liquid in Z 2 nd most dense, Liquid in W most dense

At 25°C, water has a density of 1.0 g/mL and vegetable oil has a density of 0.90 g/mL. How would a substance with a density of 0.95 g/mL behave when placed in both oil and water? 1.Sink in both oil and water 2.Sink in oil and float in water 3.Float in oil and sink in water 4.Float in both oil and water

Substance will float on top of objects more dense than themselves. The object having D= 0.95 g/mL is more dense than oil with D=0.90 g/ml, so it will sink in oil. The object’s less dense than water (D=1.0 g/ml), so it will float on top of water.

Two identical flasks containing different liquids are placed on identical balances. Based only on what you can observe from the picture, what property differs between the two liquids? 1.density 2.volume 3.alkalinity 4.conductivity

Explanation Students are expected to remember that: Density=mass/volume, to know that balances and units of grams show mass, and to know that graduated cylinders and units of ml show volume. alkalinity refers to basic (>7) pH. conductivity is a measure of the ability of a substance to allow electricity to flow through it.

Suppose scientists discovered four new elements (W, X, Y, Z) while studying rock and soil samples brought back from a Mars mission. Which Lewis dot structure represents an element that should be placed in column VIIA (17) of the periodic table? 1.Element W 2.Element X 3.Element Y 4.Element Z

Explanation: Elements in families 1, 2, & 3—18 possess the same numbers of valence electrons as other members of their families. The dots in a Lewis dot structure show only an atom’s valence electrons. Members of family 17 (VIIA) have 7 valence electrons.

Concept: the atomic # of an element shows the number of protons in its atoms. The atomic weight of an element shows its total # neutrons & protons, each having a mass of 1 atomic mass unit (1 amu). In neutral atoms, the number protons and electrons are equal. In ions, metal atoms have lost 1 electron for each + charge or nonmetal atoms have gained 1 electron for each – charge. Fill in this table.

F 9p + 10 average n o 10 e - charge 1- S 16p + 16 average n o 16 e - charge 0 Mg 12p + 12 average n o 11 e - charge 1+ O 8p + 8 average n o 10 e - charge 2-

A neutral atom of silicon has 1.12 electrons 2.13 electrons 3.14 electrons 4.15 electrons

Explanation: The atomic # of silicon is 14, so it has 14 protons. In neutral atoms, protons are equal in number to electrons.

Concept: Only noble gases (family 18 or VIIIA) are stable in elemental form since only these have valence energy level electron octets. Other elements undergo chemical reactions to form either ionic or covalent bonds, thereby achieving valence octets.

Background: metals have low electronegativity nonmetals have higher electronegativity

Metals & nonmetals achieve stable valence shell electron octets by ionic bonding. Metals donate electrons & become + ions. Nonmetals accept electrons & become – ions. Metal Na atoms nonmetal Cl atoms 11e-,11p+, 17e-17p+ Na ions 10 e-, 11p+ Cl ions 18e-, 17p+

Nonmetals share electrons with other nonmetals, forming covalent bonds. By sharing valence electrons (1 each for a single bond ­, 2 each for a double bond =, 3 each for a triple bond =) both atoms achieve valence octets (or for H, a duet of electrons in the 1 st shell forms) H 2 H—H H:H F 2 F—F CO 2 O=C=O F | CF 4 F—C—F | F

Use the partial periodic table to answer the question. In terms of electrons, describe the difference between the formation of a covalent bond in Cl 2 & an ionic bond in NaCl. Respond in the space provided in your Answer Document. (2 points)

Sample answer for 2 points: Na is a metal element and Cl is a nonmetal element, so these two elements react to form an ionic bond. Na, a metal atom, donates its one valence electron to form a 1+ ion, and Cl a nonmetal atom, accepts the one valence electron from Na to form a 1- ion. Since 2 Cl atoms are both nonmetal atoms, they share 1 valence electron each to form a covalently bonded molecule, Cl 2. Both NaCl and Cl 2 formation Allows the atoms to form stable valence energy level octets.

2 The student response describes the difference between the covalent bond in Cl 2 and the ionic bond in NaCl in terms of electrons. OR The student response may provide two correctly labeled drawings (e.g., Lewis Dot diagrams, Bohr model, etc.) OR The student response may provide one correctly labeled drawing (e.g., Lewis Dot diagrams, Bohr model, etc.) of the covalent bond in Cl 2 or the ionic bond in NaCl and a description of the bond not illustrated in the drawing. 1 The student response describes the covalent bond in Cl 2 OR the ionic bond in NaCl. OR The student response may provide one correctly labeled drawing of either the covalent bond in Cl 2 OR the ionic bond in NaCl in terms of electrons. OR The student response may provide two partially correct drawings, but they may contain errors (e.g., some labels may be missing). 0 The student response indicates inadequate or no understanding of the task. The response may provide incorrect or irrelevant information.

What score does each answer earn? 1 0 0

The following graph shows the change in temperature of a sample of H 2 O, which begins as ice, as thermal energy is added. Which region of the graph represents water (H 2 O) in the liquid form only?

Explanation: When a substance reaches its melting/freezing point or its boiling/condensing point, it begins to change state. Energy being absorbed is used for the change in state, not to increase kinetic energy (temperature) of the substance, so temperature remains constant until all of the substance has changed state.

In his investigations of air, Cavendish discovered a bubble of gas that would not combine with nitrogen. His observations went unnoticed until Ramsay performed experiments in which he obtained similar results. Ramsay recalled & repeated Cavendish’s experiments exactly to verify the results. Then, using Kirchhoff’s spectroscopy technique, Ramsay was able to identify the leftover gas as the element he called argon. Upon further investigation, he found the elements neon, krypton and xenon. Based on these results it can be said that: 1.the combined work of Cavendish, Kirchhoff and Ramsay led to the discovery of the noble gases. 2.Kirchhoff’s work was insignificant in the investigations leading to the discovery of argon. 3.Ramsay violated ethical practice in science by repeating Cavendish’s experiments. 4.Cavendish is directly responsible for the discovery of argon, but not neon, krypton or xenon.

Explanation Students are expected to remember that: Xe and Ar are noble gases That science is a process in scientists are expected to repeat/verify or refute, then build upon, the findings of other scientists, and that scientific theories can be discarded or revised if new data (e.g., data obtained using improved technology) contradicts the original theory or if it reveals more information to refine the theory.