What facts make a circumstantial case against George Trepal in the murder of Peggy Carr and the poisoning of the Carr’s sons? He had been in conflict with his neighbors for years. When asked by murder investigators about why someone would want to harm the Carrs he glibly answered that maybe someone wanted them to move away.
What was the poison used on the Carr family? It was a thallium compound.
What other evidence implicated George Trepal as the culprit in these crimes? Trepal had a prior conviction for manufacturing methamphetamine. One of the chemicals used to make the raw materials for this drug is a thallium compound. Also, he had a bottle of thallium(I) nitrate in his garage, which was discovered by an undercover police officer renting the house.
Section 4.1 Chemical Nature of Evidence: Regions of the Periodic Table
What is a group in the periodic table (also known as a family? It is a column of elements in the periodic table. They undergo similar chemical reactions and have closely related physical properties.
What are some common names for groups on the periodic table? Group 1 is called the Alkali Metals. Group 2 is called the Alkaline Earth Metals. Group 17 is called the Halogens. And group 18 is the Noble Gases. Groups 3 through 12 are the Transition Metals.
Why is there a separate part of the periodic table below the main table? That part of the table shows the ‘inner transition metals’. These elements are also sometimes called the ‘rare earth metals’. Some of them are as common as copper (Ce, for example) in the Earth’s crust so this is a not entirely accurate name. The Lanthanides (the first row with atomic numbers 58 to 71) belong after Lanthanum (La) in the main table. The Actinides (90 - 103) belong after Actinium (Ac). They are not included in the main table only because they would make it awkward to print and work with.
What are periods in the periodic table? They are the rows in the table and the way the elements are arranged into these rows reveals which elements belong in the same group. Each row starts over when the properties of the elements repeat periodically. Hence, the name.
It may be worthwhile to talk over the Worked Examples on pp 96 - 97 which concern identifying elements by group and period.
Section 4.2 Types of Compounds: Covalent Compounds
What is the definition of a chemical compound? A compound is a pure substance which is made of two or more elements chemically bound together.
What are the two main types of compounds? There are covalent compounds and ionic compounds. They have physical and chemical properties related to the type of bonding (covalent or ionic) and different rules for naming them.
How do atoms form a covalent bond? They form a covalent bond by sharing electrons.
What kind of atoms form covalent bonds? Generally, covalent bonding occurs between two atoms of non-metallic elements. Examples include, H2S, CO2, H2O, and NH3.
What two features must the name of a compound have? It must be unique to that compound so that two different compounds are not confused for each other. Also, it must contain enough information to be able to construct the chemical formula of the compound.
Do the subscripts in chemical formulas refer to atoms of the element after the subscript or before? The subscripts in chemical formulas refer to atoms of the element whose symbol appears directly before the subscript. For example, in H2O the two refers to hydrogen so that molecules of water have two atoms of hydrogen and one atom of oxygen each.
How are prefixes used in naming covalent compounds? The Greek numerical prefixes specify the number of atoms of each element found in molecules of the compound. The prefix mono- is never used for the first element in a compound. Vowels are modified appropriately when used with atoms of elements that start with a vowel (tetroxide instead of tetraoxide, for example).
Compounds of this kind are relatively easy to name but doing it well and correctly every time takes practice. Try the Worked Examples on pp 99 and 100.
What is the melting point of a compound? The melting point of a compound is the temperature at which the particles that make up that compound have enough energy to break free from the attractions that hold them in one place in the solid phase. The stronger the attractions between particles, the higher the melting point.
What is the boiling point of a compound? The boiling point of a compound is the temperature at which the particles of the compound have enough energy to form a gas by breaking free from the attractions that hold the particles in contact in the liquid phase. In order to actually enter the gas phase, the particles must create a gas pressure equal to or greater than the gas pressure on the liquid. This is the reason water boils at lower temperatures at high elevation locations like Boulder, CO.
What is typical for the melting points and boiling points of covalent compounds? Typically, covalent compounds have low melting points and boiling points. This is because though the atoms in molecules are tightly bound to other atoms within the molecule, the molecules themselves are only weakly attracted to one another. For example, the covalent compound hydrogen fluoride (HF) has a melting point of –35°C. By contrast, the ionic compound sodium fluoride (NaF) has a melting point of 995°C.
Section 4.3 Types of Compounds: Ionic Compounds
How do atoms form an ionic bond? Atoms form an ionic bond when oppositely charged ions attract one another due to their opposite charges.
What kind of atoms form ionic bonds? Ionic bonds form between atoms of metallic elements with atoms of non-metallic elements.
What is a cation? A cation is an ion with a positive charge. It is formed by the loss of one or more electrons from an atom. Elements that form cations are metallic elements. Non-metallic elements never form cations.
What is an anion? An anion is an ion with a negative charge. It is formed by the gain of one or more electrons from an atom. Elements that form anions are non-metallic elements. Metallic elements never form anions.
Are ions limited to single atoms? No. Ions can have two or more atoms. When they do they are called polyatomic ions. They are still called cations and anions (depending on their charge) but do not become ions by virtue of gaining or losing electrons.
How are individual ions named? Cations are named by simply adding the word ion after the name of the element. Anions are named by adding the suffix -ide and then adding the word ion. Take a look at Worked Exmaple 5 starting on page 101.
How can the ionic charge of an element be predicted using the periodic table? First, see the Group
Activity: Atomic Structure II, which explains this in detail and gives reasons for the observed pattern. Next, memorize the pattern. Group 1: +1 cations, Group 2: +2 cations, Group 11: +1 cation (Ag only), Group 12: +2 cations (Zn and Cd only), Group 13: +3 cations (B and Al only), Group 15: -3 anions, Group 16: -2 anions, Group 17, -1 anions, and Group 18 never forms ions.
How are the formulas of ionic compounds determined? The compound’s formula must contain the lowest-terms ratio of atoms so that the ionic charges balance and add up to zero. The compounds are always neutral overall. See the Activity:
Chemical Formulas and Compounds for details.
Take a look at Worked Examples 7 and 8 on pp. 104 and 105. The only way to get good at the skill of naming compounds is to practice!
How can elements be named that can have different ionic charges in different compounds? Various transition metals can have more than one ionic charge. For example, copper ions can be either Cu+ or Cu2+. The Cu+ ions are called copper(I) ions and the Cu2+ ions are called copper(II) ions. The Roman numeral in the name of the ion refers to the ionic charge on the ion. Since metals ions always have positive charges, I means +1, II means +2, III mean +3, and so on. See Table 4.2 on page 106 to see a list of common cations of elements with multiple possible charges.
What are polyatomic ions? Polyatomic ions are ions made up of two or more atoms covalently bound to one another. They are essentially molecules that carry an overall ionic charge. Common ions can be found on the ions reference sheet handed out in class and in Table 4.3 on page 107 in the text. Knowing the names and formulas of some of the common polyatomic ions is valuable when it comes to naming compounds that contain them. When more than one polyatomic ion is necessary to make an ionic compound then the polyatomic ion is put in parentheses. For example, calcium phosphate is made of the ions Ca2+ and PO43- and the chemical formula of the compound is written: Ca3(PO4)2.
What is a crystal lattice and how is it important to understanding why chemists do not talk about ionic molecules? The crystal lattice refers to the 3-D repeating structure of ions in samples of ionic compounds. There is an illustration at right. See also, fig. 4.4 on page 108. Ions in the crystal lattice are held in place by the opposite charges of the atoms. It is this lattice which makes crystals of ionic compounds hard and brittle and which gives them their high melting and boiling points. There is no such thing as an ionic molecule because atoms in, for example, NaCl are not bound in a one-to-one ratio as molecules CO are. Instead, each sodium ion has six chloride neighbors and each chloride ion has six sodium ion neighbors.
How can you qualitatively compare the melting points of different ionic compounds? Ionic compounds have melting points that vary based on the size of the charges on the ions. For larger charges there are stronger attractions between ions. For example, CaO has a higher melting point than NaCl because calcium ions have a +2 charge and oxide ions have a -2 charge while sodium and chloride ions are only +1 and -1.
I strongly recommend you take the time to carefully work through and commit to memory the steps for naming all kinds of compounds found on page 110. It is especially critical not to confuse ionic compound naming rules with covalent compound naming rules.
Section 4.4 Common Names and Diatomic Elements
Some terms used to name substances mean different things to lay people and to chemists. What does the word salt mean to a chemist? In everyday speech, salt refers to table salt, NaCl. To a chemist, the word salt refers to a whole class of compounds. Technically, salts are compounds that result from the reaction of an acid and a base. More simply, salts are any combination of pairs of ions other than the hydrogen ion (H+) and the hydroxide ion (OH–). So KBr, LiCl, and NaI are all also properly called salts.
What does the word alcohol mean to a chemist? Normally, people say alcohol when they talk about the kind of alcohol found in alcoholic beverages such as beer, wine and liquor. To a chemist, that is only one kind of alcohol properly known as ethanol. To chemists, alcohols are a whole class of related organic (that is, carbon-based) compounds that all contain a carbon atom covalently bound to an OH group.
What is the systematic name of water? It is dihydrogen monoxide. Using such a name would be more confusing than helpful. For this reason a number of compounds are known by common names rather than systematic ones. Examples include ammonia (NH3, not nitrogen trihydride), methane (CH4, not carbon tetrahydride), and nitric acid (HNO3, not hydrogen nitrate). Other examples can be found on the ions reference sheet. See especially the common acids.
Some elements occur naturally as diatomic molecules (with covalent bonds, naturally). What elements are these? Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), and Iodine (I2). Memorize these elements’ formulas.
Section 4.5 Basics of Chemical Reactions
What is a chemical reaction? A chemical reaction is a process in which chemicals change from one substance to another.
What are reactants and products? Reactants are the substances which react with one another in a chemical reaction. In chemical equations they appear on the left of the arrow. Products are the compounds or elements that form as a result of the chemical reaction. They always appear to the right of the arrow in a chemical equation.
What are some example chemical reactions? Here are a two: C + O2CO2 Elemental carbon reacts with oxygen to form carbon dioxide. This is the reaction for burning charcoal.
4KNO3(s) + 7C(s) + S(s)
3CO2(g) + 3CO(g) + 2N2(g) + K2CO3(s) + K2S(s) The mixture of solids that make up the reactants in this equation are commonly known as gunpowder. In this chemical equation the state of matter is clearly shown in parentheses after each chemical formula (s-solid, l-liquid, g-gas).
How does a chemical equation express the law of conservation of mass? Mass is conserved in chemical and physical changes: the total mass of substances before and after the change is the same. This is true because the same number of atoms exist before and after the change. In a chemical equation the same number of each type of atom appear both before and after the arrow. For example, in the burning of gunpowder there are 4 atoms of nitrogen among the reactants and four among the products.
What do the coefficients mean in a chemical equation? They show the number of formula units of the substance they precede. In the burning of gunpowder there are 4 units of KNO3 for every 7 C atoms and for every single atom of S. These coefficients make it possible to balance a chemical reaction. Balanced chemical reactions show the same number of each type of atom on both the reactant and products sides.
Why can’t the subscripts in the formulas of chemicals be changed in order to balance a chemical equation? If the subscripts are changed then that changes the identity of the substance.
There is a systematic process for balancing chemical equations. I have written it out comprehensively in the document linked to above. I will quote here the information given on pp 113 - 114 about balancing chemical equations. Note that this process requires a bit more thought than it might seem at first. Some equations are easy but for others there is no simple way to balance them. This task borders on being non-algorithmic. Algorithms are sets of instructions for completing a task. The instructions on my page and those below are more like a general procedure than a detailed step-by-step process. The following is a direct quote:
List each element found in the reactants in a column beneath the reactant side of the equation. If oxygen and hydrogen are present, always place them at the bottom of the list.
Copy the identical list of elements in a column beneath the product side.
Record the number of atoms of each element found on the reactant side. Repeat this process on the product side.
Starting at the top of the list, attempt to balance the equation by placing coefficients in front of compounds to increase the number of atoms. Note: the coefficient modifies the number of all atoms in a compound; therefore, in your list, readjust the number of each atom in the compound.
Repeat until all elements are balanced in the equation.
See the worked examples and practice problems on pp. 114 - 115.
Section 4.7 Mathematics of Chemical Reactions: Mole Calculations
Chemical equations are recipes that relate information about the number of molecules necessary to carry out a particular reaction. Molecules are too small to measure individually on a lab balance. How are manageable quantities of atoms and molecules made measurable? By using the mole.
What is the mole? The mole (mol) is the number of atoms it takes to have a mass in grams of an element equal to the atomic mass. This number is called Avogadro’s number (NA) and equals 6.022 × 1023 per mole. Avogadro’s number is no different in concept from a dozen. A mole contain 6.022 × 1023 items and a dozen contains 12.
If you measure 12.011 g of carbon on a lab balance then how many atoms of carbon do you have? You have one mole of carbon atoms: 6.022 × 1023. 1 mole = 12.011 g = 6.022 × 1023 carbon atoms. 12.011 grams of carbon is the molar mass of carbon. The molar mass of a substance is the mass of it that contains one mole of formula units of that substance.
How are the molar masses of compounds and molecules calculated? They are calculated by adding up the molar masses of the elements in them. Subscripts alter the number of each element in the compound and so should multiply the molar mass of the element when you find the molar mass. For example, the molar mass of Cl2 is 70.906 g/mol and the molar mass of SF6 is 146.053 g/mol. (Try these yourself to make sure this makes sense.)
Do the worked example and practice exercises on pp 116 - 117.
Section 4.8 Mathematics of Chemical Reactions: Stoichiometry Calculations
Section 4.9 Types of Reactions
Section 4.10 Mathematics of Chemical Reactions: Limiting Reactants and Theoretical Yields