chemistry

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Name five areas of chemistry research and give examples of the types of problems chemists are trying to address within each area

Organic chemistry (deals with substances containing carbon), Inorganic chemistry (deals with substances not containing carbon) Biochemistry (deals with processes in organisms) Analytical (deals with the composition of substances) Physical (deals with the mechanism, rate, and energy transfer) . The demand for energy requires conservation and production, including the development of alternate energy sources. Chemists contribute to the health and well-being of humans through the development of medicines and materials for replacement body parts. Chemists help to develop more productive crops and more effective, safer ways to protect crops from pests and diseases. Chemists help to identify pollutants and prevent pollution. Chemists analyze materials from locations other than Earth directly or indirectly.

Explain how a hypothesis, a theory, and a scientific law are related to observations

A hypothesis is a proposed explanation for one or more observations. If a hypothesis addresses a broad range of observations, and if it has been tested by repeated experiments, it may be raised to the level of a theory. The same set of observations can lead to both a scientific law and a theory. A scientific law is a concise summary of the observations. A theory provides the explanation.

List and describe the two steps for solving conceptual problems.

The two steps for solving conceptual problems are analyzing and solving. Analyzing involves identifying the relevant concepts. Solving involves applying concepts to the given situation.

What is the difference between a solution and a heterogeneous mixture? Give an example of each.

A solution is a mixture that has a uniform composition. The composition of a heterogeneous mixture is not uniform throughout. All solutions are mixtures, but not all mixtures are solutions. Salt water is a solution; sand in water is a heterogeneous mixture.

Give five examples of heterogeneous mixtures and five examples of homogeneous mixtures.

The examples should follow this rule: A homogeneous mixture is uniform in composition, while a heterogeneous mixture is not. Examples of a homogeneous mixture include air and stainless steel. Examples of a heterogeneous mixture include chicken noodle soup and the mixture of oil and vinegar

Define element and compound. Explain the difference between an element and a compound.

An element is the simplest form of matter that has a unique set of properties. A compound is a substance that contains two or more elements chemically combined in a fixed proportion. An element cannot be broken down into simpler components through chemical reactions. Compounds are substances that can be broken down into simpler substances through chemical reactions.

Describe some of the physical and chemical changes involved in cooking

Physical changes in cooking include the melting of solids, such as butter, and the boiling of liquids, such as water. Examples of physical changes would be the melting of fats and the boiling of water. Chemical changes involve the production of new substances. Examples of chemical changes would be the browning of sugar during candy-making and the release of carbon dioxide during the baking of bread.

Discuss the difference between physical changes and chemical changes. Use an example to illustrate the difference.

Physical changes do not alter the composition of a substance, but chemical changes do. When water is boiled, the resulting gas is still water. However, when water is broken down into hydrogen and oxygen in a chemical reaction, the water no longer exists. The latter is a chemical change; the former a physical change.

What observations by Rutherford led to the hypothesis that atoms are mostly empty space, and that almost all of the mass of the atom is contained in an atomic nucleus?

Rutherford's gold-foil experiment led to this hypothesis. Alpha particles were observed to mostly pass through a gold foil, which suggests that the volume of individual gold atoms consists mainly of empty space. The observation that some alpha particles were scattered at large angles led to the suggestion that the gold atom has a central core, or nucleus, composed of a concentrated mass capable of deflecting the alpha particles.

Explain how the atoms of one element differ from those of another element

Isotopes of the same element have different numbers of neutrons, and therefore, different mass numbers and different atomic masses. Isotopes of the same element have the same number of protons and electrons. The electrons, not the neutrons, are responsible for an element's chemical behavior.

Describe the different principles that govern the building of an electron configuration.

The aufbau principle states that electrons enter the orbitals of lowest energy first. The Pauli exclusion principle states that each orbital can hold only two electrons. Hund's rule states that electrons first enter separate orbitals of the same energy, with each electron having the same spin, before pairing with electrons that have opposite spins.

Describe the trends in the atomic size of elements within groups and across periods in the periodic table. Provide examples

Atomic size increases with increasing atomic number within a group. For example, sodium atoms are larger than lithium atoms, and potassium atoms are larger than sodium atoms. Atomic size decreases with increasing atomic number across a period. For example, lithium atoms are larger than beryllium atoms, and beryllium atoms are larger than boron atoms.

Explain how ions form. Provide examples

Ions form when electrons are transferred among atoms. For example, a group 1A element, such as potassium, tends to transfer one electron to other atoms, causing it to form a net positive charge, in this case K . Such ions with net positive charges are called cations. Ions with net negative charges are anions. Nonmetal atoms, such as chlorine, tend to accept electrons from other atoms. Chlorine tends to gain a single electron, forming the anion Cl .

Positive ions are smaller than the atoms from which they are formed, but negative ions are larger than the atoms from which they are formed. Explain why this is so.

When an electron is added to an atom, the attraction of the nucleus for any one electron decreases and the size of the ion's radius increases. When an electron is removed from an atom, there is an increase in the nuclear attraction experienced by the remaining electrons. Consequently, the remaining electrons are drawn closer to the nucleus.

Describe the trends in electronegativity within groups and across periods in the periodic table. Provide examples

Electronegativity values decrease from top to bottom within a group, and from right to left across a period. For example, rubidium is less electronegative than lithium. Lithium is less electronegative than fluorine.

Explain the octet rule and give an example of how it is used.

The electron configurations of the noble gases are extremely stable. The octet rule states that, in chemical reactions, elements gain or lose electrons to achieve a noble gas configuration. This stable configuration is called an octet because it consists of 8 valence electrons (s p ), 2 from the outermost s orbital and 6 from the outermost p orbital. Oxygen has the electron configuration 1s 2s 2p . When oxygen reacts to form ionic compounds, it completes its octet by gaining two electrons from the element it reacts with. These two electrons add to the p orbital of oxygen, giving it the electron configuration (1s 2s 2p ) of neon.

Explain how atoms (ions) are held together in an ionic bond. Give an example of an ionic compound

In an ionic bond, oppositely charged ions are held together by the electronic force of attraction that exists between oppositely charged particles. In the ionic compound, anions and cations are present in a ratio that causes the total charge on the compound to be zero. Sodium phosphide, Na P, has three sodium ions for each phosphide ion. This ratio insures a zero total charge given the charges on the two individual ions (Na = 1 , P = 3-).

Why must each cation in an ionic solid be surrounded by anions?

In this arrangement, like-charged ions are shielded from each other and electronic repulsion is reduced. Also, the force of attraction between oppositely charged ions is maximized. Each of these events contributes to a lowering of energy and an increase in stability of the ionic compound.

Explain how a pure metal is held together. Include a definition of a metallic bond in your explanation.

A piece of pure metal, such as copper or iron, consists not of metal atoms, but of closely packed cations. The cations are surrounded by mobile valence electrons that are free to drift from one part of the metal to another. Metallic bonds result from the attraction between the free-floating valence electrons and the positively charged metal ions.

What is bond dissociation energy, and how does it affect carbon compounds?

Bond dissociation energy is the energy required to break a single bond. The greater the bond dissociation energy, the more stable the compound. Due in part to the high bond dissociation energy of carbon-carbon bonds, carbon compounds are not very reactive chemically.

Can some atoms exceed the limits of the octet rule in bonding? If so, give an example.

Yes, sulfur and phosphorus can expand the octet. They can have 12 or 10 valence electrons, respectively, when combined with small halogens. In PCl , phosphorus has 10 valence electrons

Explain what is meant by VSEPR theory. Give an example of how VSEPR theory can be applied to predict the shape of a molecule.

VSEPR (valence-shell electron-pair repulsion) theory states that because electron pairs repel, molecules adjust their shapes so that the valence-electron pairs, both bonding and non-bonding, are as far apart as possible. Methane, CH , for example, has four bonding electron pairs and no unshared pairs. The bonding pairs are farthest apart when the angle between the central carbon and each of its attached hydrogens is 109.5 . This is the angle that is observed experimentally.

Explain what a polar molecule is. Provide an example.

A polar molecule is one in which one end of the molecule has a slightly negative electric charge and the other end has a slightly positive electric charge. An example of a polar molecule is water. The oxygen atom in water develops a slightly negative charge and the hydrogen atoms develop slightly positive charges because of the difference in electronegativity between the oxygen and hydrogen atoms

What determines the degree of polarity in a bond? Distinguish between nonpolar covalent, polar covalent, and ionic bonds in terms of relative polarity.

The relative electronegativity of the two bonded atoms determines the polarity of a bond. If the difference in electronegativities between the two atoms is less than 0.4, the bond is nonpolar covalent. If the difference in electronegativities between the two atoms is 0.4 to 1.0, the bond is moderately polar covalent. If the difference in electronegativities between the two atoms is 1.0 to 2.0, the bond is highly polar covalent. If the difference in electronegativities between the two atoms is more than 2.0, the bond is ionic.

What are dispersion forces? How is the strength of dispersion forces related to the number of electrons in a molecule? Give an example of molecules that are attracted to each other by dispersion forces.

Dispersion forces are the weakest of all molecular interactions, and are thought to be caused by the motion of electrons. Generally, the strength of dispersion forces increases as the number of electrons in a molecule increases. Diatomic molecules of halogen elements are an example of molecules whose attraction for one another is caused by dispersion forces.

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