Chem 101 Exam 1

What is the difference between a pure substance and a mixture? Give two examples of each.

A pure substance cannot be separated into simpler substances by physical means, such as filtration, chromatography, crystallization, or distillation. A mixture contains two or more pure substances and can be separated into those individual components by selection of the appropriate physical methods.

Would a bond between two atoms of the same element be polar? Why or why not?

No. Two atoms of the same element have identical electronegativities and therefore share electrons in the bond equally.

Which state of matter has both a fixed volume and a fixed shape?

solid

The isotope Be-9 has how many neutrons in its nucleus?

5

What is meant by the statement "Macroscopic observations have molecular causes"? Give two examples.

All natural phenomena in the world we can see are the result of molecular interactions we cannot see. Examples are numerous. Some suggestions are as follows: Ice melting to water A match being struck, producing a flame A shirt fading when exposed to light or too many washings

What is the difference between an isotope and an ion?

An ion is defined by the relative number of protons and electrons, whereas an isotope is defined by the number of protons and neutrons.

Carbon dioxide and water are both composed of a central atom that bonds to two other atoms, yet carbon dioxide is a linear molecule, whereas water is bent. Why?

Carbon dioxide has two double bonds and no lone pairs on the central atom, making it linear. Water has two single bonds and two lone pairs on the central atom, making it bent. The lone pairs on the central oxygen in water cause the bent geometry.

Suppose that when drawing a Lewis structure for a molecule, you end up with a structure in which the central atom lacks an octet. What should you do?

Form a multiple bond by moving a lone pair from a terminal atom to the bonding region between the terminal atom and the central atom.

Match each of the people in column A with their contribution to scientific knowledge in column B. A Galileo Democritus John Dalton Andreas Vesalius Empedocles Joseph Proust Copernicus Ernest Rutherford Thales Antoine Lavoisier Robert Boyle B conservation of mass all things are water Inquisition the nuclear atom Sun-centered universe human anatomy the atomic theory constant composition atomos four basic elements criticized idea of four Greek elements

Galileo Inquisition Democritus atomos John Dalton the atomic theory Andreas Vesalius human anatomy Empedocles four basic elements Joseph Proust constant composition Copernicus Sun-centered universe Ernest Rutherford the nuclear atom Thales all things are water Antoine Lavoisier conservation of mass Robert Boyle criticized idea of four Greek elements

Based on the Lewis structures for hydrogen and helium, explain why buoyant balloons are filled with helium instead of hydrogen gas, even though hydrogen gas is cheaper and more buoyant. H (one dot) He (2 dots)

Helium has a duet and is therefore chemically stable. It will not react with other elements and therefore, should the balloon pop, would not represent a danger to humans. Hydrogen, on the other hand, is one electron short of a duet. Elements that are very close to having an octet, or a duet in the case of hydrogen, are among the most reactive. Hydrogen atoms are so reactive that they react with each other to form hydrogen molecules, but hydrogen molecules themselves are also reactive and combine explosively with oxygen to form water.

We have seen how the shape of water molecules determines the properties of water. If the molecule had a different shape, water would be a different substance. Suppose the shape of water were different in a way that resulted in ice being denser than liquid water. How would a cup of ice water look? Would icebergs exist? What would happen to marine life in winter as ice formed in lakes?

If ice were denser than liquid water, it would sink. Icebergs would not exist. As water froze in a lake or ocean, it would sink to the bottom, probably killing marine life.

What is the atomic theory?

John Dalton, using the laws of Lavoisier and Proust and the data from his own experiments, combined a number of ideas to formulate the atomic theory. Dalton's atomic theory was based on three parts: First, each element is composed of particles called atoms, which can neither be created nor destroyed. Second, all atoms of the same element have the same weight and other properties. These properties are unique characteristics of each element and thus differ from other elements. Third, atoms of different types can combine to form compounds in simple whole number ratios. For example, the compound carbon dioxide is formed from one carbon atom and two oxygen atoms. The numbers and are simple whole numbers.

In Chapter 3, we also learned that fluorine, bromine, and iodine each exist as diatomic molecules in nature. Use Lewis structures to explain why.

Like chlorine, these elements are all in group 7A (the halogens) on the periodic table and therefore have seven valence electrons. Consequently, they form diatomic molecules to attain octets, as shown in their lewis structures

Can a molecule composed of two atoms with different electronegativities be nonpolar?

No. Because the molecule has only one polar bond, there can be no other bonds to cancel its polarity.

Classify each of the following changes as physical or chemical: a. the crushing of salt b. the rusting of iron c. the burning of natural gas in a stove d. the vaporization of gasoline

Physical and chemical changes are distinguished by whether or not a change in composition occurs. a. The crushing of salt is a physical change. There is no change in composition. b. The rusting of iron represents chemical change. Rust results from combination of the iron with the element oxygen to form iron oxide. c. The burning of natural gas in a stove is a chemical change. Burning involves the combination of oxygen in the air with the carbon and hydrogen atoms of the natural gas. The energy released in the reaction provides the all-important heat we use. d. The vaporization of gasoline is a physical change. All changes of state are physical changes: Intermolecular forces may be gained or lost, but there are no changes in the chemical bonds between the atoms of the substances. Gasoline in the vapor or in the liquid is still gasoline, and if it were condensed, it would be indistinguishable from the original liquid.

How are science and art similar? How are they different?

Science and art are similar mostly in their creativity and observations of the world. The difference lies in what they do with their observations and how they are judged. Scientists take their observations and create a model of reality that is judged by experimentation for its validity. Artists observe the world and create a painting or sculpture that is judged by its creativity and workmanship.

Copper is composed of two naturally occurring isotopes: Cu-63 with a mass of 62.94 amu and Cu-65 with a mass of 64.93 amu. Look up the atomic mass of copper and determine which of the two isotopes is more abundant.

The atomic mass of copper is 63.55, so the Cu-63 isotope must be more abundant.

Describe the structure of the atom as explained by Rutherford. How was this picture consistent with the outcome of his gold foil experiment?

The only way to explain the results of the gold foil experiment was to propose a new model of the atom in which most of the atom must be empty space. This structure would allow most of the alpha particles to pass through the gold foil with little or no deflection. However, the atom must also contain a nucleus, a dense, positively charged central core containing most of the mass. In the experiment, whenever an alpha particle came close to a nucleus or hit it head on, it experienced a large repulsive force, causing it to be scattered. Furthermore, because the atom is electrically neutral, it must contain an equal number of negative charges (electrons) and positive charges. The exact identity of the positive charge was later established to be the proton. Rutherford proposed the electrons were outside the nucleus.

A chemist combines 22 grams of sodium with 28 grams of chlorine. A spectacular reaction occurs and produces sodium chloride. After the reaction, the chemist finds that all the chlorine was used up by the reaction, but 4 grams of sodium remained. How many grams of sodium chloride were formed?

The reaction must follow the law of conservation of mass. Therefore the amount of sodium chloride formed must be grams.

Describe the scientific method.

The scientific method involves first making observations of nature, from which patterns are identified. From the patterns, broadly applicable generalizations called scientific laws are established. A theory or model is then constructed to provide an interpretation of the behavior of nature. The theory is then tested by further experiments and modified if necessary.

When did the scientific revolution begin? What events signaled its beginning?

The scientific revolution began in 1543, signaled by the publication of two books. The first book, by Copernicus, announced his Sun-centered universe theory. The second book, by Vesalius, gave an accurate description of human anatomy. The reason these books mark the beginning of the scientific revolution is the methods Copernicus and Vesalius used to learn about the natural world—they both used observation instead of pure reason.

Explain the differences among a solid, a liquid, and a gas, based on the atoms or molecules that compose them.

The three states of matter are distinguished by the strength of the interactions between the molecules relative to their thermal energy. (For the sake of brevity, we will refer here only to molecules, but it should be remembered that substances may be composed of atoms, molecules, or ions.) The relative strength of the molecular interactions decreases in the sequence solid-liquid-gas. In a solid, the interactions are strong enough to prevent free movement of the molecules, and they are locked into rigid arrangements (lattices). In a liquid, the molecular energies are strong enough to loosen the grip of the intermolecular forces sufficiently to allow free movement of molecules, but the intermolecular forces still maintain enough control to keep the molecules together. In a gas, molecular energies have increased to the extent that now all intermolecular shackles are broken and gas molecules behave completely independently. This molecular picture explains the obvious properties of solids, liquids, and gases. Solids are rigid and dense; liquids flow freely yet have a density similar to the solid; gases have a very low density and are confined only by their container.

The burning of gasoline in automobile engines is a chemical reaction. In light of the law of conservation of mass, explain what happens to the gasoline in your car's tank as you drive.

When gasoline is burned in an automobile engine, it does so in the presence of oxygen gas. This chemical reaction of gasoline plus oxygen produces energy, water, carbon dioxide, and carbon monoxide gases. The mass of the products is equal to the mass of the gasoline and oxygen gas that you started with.

Your friend tells you about an article that he read in a tabloid that reported the discovery of a new form of carbon containing eight protons in the nucleus of its atoms. According to the article, this form of carbon spontaneously turns into diamond. How would you respond to your friend?

You should tell your friend that the "form of carbon containing eight protons" was discovered long ago, and it is not carbon at all. We call it oxygen, and it does not form diamonds.

To the best of your knowledge, classify each of the following as an element, a compound, or a mixture. If it is a mixture, classify it as homogeneous or heterogeneous. a. silver coin b. air c. coffee d. soil

a. A silver coin is composed of the element silver. b. Air is a homogeneous mixture of different gases. c. Coffee is a homogeneous mixture of many different substances. d. Soil is a heterogeneous mixture because it may be composed of dirt, sand, and rocks that can be separated into distinct regions with different compositions.

Which statement is best categorized as a law? a. In a chemical reaction, matter is neither created nor destroyed. b. Matter is ultimately composed of tiny indestructible particles called atoms. c. When sodium reacts with chlorine, the mass of the sodium chloride that form equals the sum of the masses of the sodium and chlorine that reacted. d. None of the above

a. In a chemical reaction, matter is neither created nor destroyed.

According to the nuclear theory of the atom: a. Most of the volume of the atom is empty space. b. The nucleus composes most of the volume of the atom. c. Atoms are like solid impenetrable spheres. d. None of the above.

a. Most of the volume of the atom is empty space.

Which statement best describes one of the main goals of alchemy? a. to synthesize new substances for household use b. to make the elixir of life, a potion that would grant immortality c. to determine whether matter is composed of atoms d. to determine how bodies move through space

b. to make the elixir of life, a potion that would grant immortality

Classify each of the following properties as chemical or physical: a. the tendency of dry ice to vaporize directly from a solid to a gas b. the flammability of LP gas (the kind you put in your outdoor grill) c. the boiling point of water d. the smell of cologne

a. Physical property. The change in physical state of a substance does not result in a chemical change. b. Chemical property. The ability of a substance to undergo combustion represents a chemical change. c. Physical property. The change in physical state of a substance does not result in a chemical change. d. Physical property. The smell of a substance is due to its vapor. Conversion of a liquid into vapor is a physical process because there is no change in its composition. Of course, it could be argued that the complex process of detection of the smell involves numerous chemical processes in the body and brain.

Explain the following observations on a molecular level: a. A brightly colored carpet appears faded in an area near a large window. b. A teaspoon of salt dissolves when stirred into water.

a. The bright color of the carpet is the result of particular molecules in the rug. When particles of sunlight—photons—hit the carpet, the bright color molecules are destroyed or altered in some way. b. Because water molecules are attracted to salt molecules, the water breaks up the salt crystals and surrounds the individual salt molecules, a process observed as dissolving.

Determine whether any of the following data sets on chemical reactions are inconsistent with the law of conservation of mass and therefore erroneous. a. 6 grams of hydrogen react with 48 grams of oxygen to form 54 grams of water. b. 10 grams of gasoline react with 4 grams of oxygen to form 9 grams of carbon dioxide and 5 grams of water.

a. This is consistent with the law of conservation of mas b. This is consistent with the law of conservation of mass

According to Rutherford's model of the atom, how many electrons would be found in each of the following atoms? a. sodium, which has 11 protons in its nucleus b. calcium, which has 20 protons in its nucleus

a. To preserve electrical neutrality in the Rutherford atom, the number of electrons will be equal to the number of protons in the nucleus (the atomic number). An atom of sodium contains 11 electrons . b. To preserve electrical neutrality in the Rutherford atom, the number of electrons will be equal to the number of protons in the nucleus (the atomic number). An atom of calcium contains 20 electrons .

Which substance is a liquid mixture? a. lemonade b. brass c. air d. wood

a. lemonade

Classify each of the following as an observation or a law: a. When water boils, small bubbles form in the liquid and rise rapidly to the surface. b. Two grams of hydrogen combine with grams of oxygen to form grams of water. c. Chlorine and sodium readily combine in a chemical reaction that emits much heat and light. d. The properties of elements vary periodically with the mass of their atoms.

a. observation b. represents an observation. It would be the result of one experiment. c. observation d. law. The relationship between element properties and size statement was derived from examination of many observations. It is often referred to as the periodic law.

Which statement is not part of Dalton's atomic theory? a. All matter is composed of indivisible particles called atoms. b. All atoms are alike in mass and other properties. c. Atoms of different element combine to form compounds in simple whole number ratios. d. Atoms have an equal number of protons and neutrons.

d. Atoms have an equal number of protons and neutrons.

Which Greek thinker was among the first to suggest that matter was composed of atoms? a. Plato b. Aristotle c. Empedocles d. Democritus

d. Democritus

Which description best describes the field of chemistry?

the science that investigates the connection between the macroscopic world and the particles (atoms and molecules) that compose it