EXPERIMENT 3 Nomenclature and Formula Writing

Transcription

1 Noble Gases EXPERIMENT 3 Nomenclature and Formula Writing Outcomes After completing this experiment, the student should be able to: 1. Recognize anions and cations and balance charges to write formulas for compounds. 2. Interpret chemical formulas. 3. Identify types of chemical compounds, and names of cations and anions. 4. Name inorganic compounds when formula is given and write the formula of a compound when the name is given. Introduction Writing Chemical Formulas The chemical formula for a compound is the short hand definition of a chemical substance. From a chemical formula one can identify the elements present in the chemical substance, the ratio in which they are combined and the number of atoms of each element present in the compound. Elements in the periodic table are also classified as s, p and dblock elements based on the distribution of electrons into s, p or dorbitals. The elements belonging to s and pblock elements are often referred to as main group elements, whereas the dblock elements are referred to as transition elements. Metal atoms during chemical reactions loose electrons to form cations whereas nonmetals gain electrons to form anions with the exception of hydrogen. Hydrogen atom can either loose or gain electrons during chemical reactions, and thus, can form a cation as well as an anion. The main group elements generally loose or gain a fixed number of electrons, and thus, form ions with fixed charges (Figure 3.1). Figure 3.1. Main group elements and charges of the ions they form. Sblock IA pblock H + IIA IIIA IVA VA VIA VIIA Li + Be 2+ Transition elements (dblock) N 3 O 2 F Na + Mg 2+ Al 3+ P 3 S 2 Cl K + Ca 2+ Ga 3+ Rb + Sr 2+ In 3+ Se 2 Br Te 2 I Cs + Ba 2+ Fr + Ra 2+ In Summary: 13

2 Group IA, IIA and IIIA form ions with +1, +2 and +3 charges respectively. Nonmetals are in the upper right hand portion of the periodic table. o Group VIIA often forms ions with a 1 charge. o Group VIA often forms ions with a 2 charge. o Group VA often forms ions with a 3 charge. On the other hand, the transition metals and some pblock elements (posttransition metals) are capable of loosing multiple electrons and form cations with multiple charges (Figure 3.2) Figure 3.2. Ions of some transition metals and posttransition metals. IA IIA IIIA IVA Fe IIIB IVB VB VIB VIIB [ VIIIB ] IB IIB Sc 3+ Ti 2+ Cr 2+ Mn Co 2+ Ni Cu + Zn 2+ Ti 4+ Cr 3+ Mn 3+ Fe 3+ Co 3+ Ni 2+ Cu 2+ Y 3+ Ag + Cd 2+ Sn 2+ Ni 3+ Sn 4+ La 3+ Au + Hg + Pb 2+ Au 3+ Hg 2+ Pb 4+ Naming of various cations and anions listed in Figures 3.1 and 3.2 has been shown below in Tables A list of polyatomic ions and their naming are shown in the Table 3.4. Types of compounds: Depending on the type of elements and/or groups of atoms (polyatomic ions) present, there are at least 5 major categories of compounds (described below). 1. Binary ionic compounds are composed of charged ions that are held together by a strong electrostatic force, and essentially means that only two different elements are involved. Binary ionic compounds are of two types. i. Type 1 binary ionic compounds Are made up of two elements and are composed of metallic positive ions (cations) with fixed charges and nonmetal negative ions (anions). The metals that from type1 binary ionic compounds include elements from groups 1A, 2A; and Al, Ga and In from group 3A, as well as elements Zn, Cd and Ag from transition metals (Table 3.2). Various nonmetals found in ionic compounds and their common charges are shown in Table 3.3, and Figure 3.2. Example: Calcium chloride CaCl 2. ii. Type 2 binary ionic compounds Are also made up of two elements and are composed of metallic positive ions (cations) and nonmetal negative ions (anions). All metals except those which from type I binary ionic compounds, form type II compounds. These metals can have multiple charges (shown in Table 3.3 and Figure 3.2). Examples: Iron(II) chloride FeCl 2. (Iron has +2 state) Iron(III) chloride FeCl 3 (Iron has +3 state) 2. Compounds with polyatomic ions: These are ionic compounds that contain a cation such as an ammonium or a metallic ion, and polyatomic anion. The most common polyatomic ions are shown in Table 3.4. Examples: Sodium phosphate Na 3 PO 4 Ammonium nitrate NH 4 NO 3 14

3 Cobalt(II) sulfate CoSO 4 3. Molecular compounds: In these compounds the elements are held together by covalent bonds or shared pairs of electrons. Molecular compounds are also referred to as Type 3 binary compounds. They are made up of only nonmetals. See Figure 3.1 for the location of nonmetals in the periodic table. Examples: Hydrogen chloride HCl Dinitrogen pentoxide N 2 O 5 4. Binary acids: Acids: Many compounds that contain hydrogen are often referred to as acids. Acids when dissolved in water release hydrogen ions (H + ) into solution. When a substance is dissolved is water, it is referred to as an aqueous solution and is designated by the symbol (aq) written after the formula of the compound. In the case of acids, the designation (aq) shows that the ions are present in aqueous (water) solution. Binary acids are compounds formed between hydrogen ion (H + ) and other nonmetal aniona (e.g., F, Cl, Br, I and S 2 ) and dissolved in water. Examples: Hydrochloric acid HCl(aq) Hydrosulfuric acid H 2 S(aq). 5. Oxyacids: These are compounds formed between hydrogen (H + ) and polyatomic anions containing oxygen. Examples: Sulfuric acid H 2 SO 4 (aq) Nitric acid HNO 3.(aq) Rules for writing chemical formulas: The following are simple rules used for writing chemical formulas. Rule 1: Write the symbol for the metallic element first. Then, write the symbol for the nonmetallic element. Rule 2: Write the combining capacities for the elements (charges). Rule 3: Crisscross the combining capacity numbers (Remember the compounds are neutral, therefore the sum of positive and negative charges must equal to zero). Use subscripts to indicate the number of atoms each element the compound has in it (if the element has only one atom, then no subscript is needed for that element) (See examples below). Rule 4: Writing formulas for compounds that contain polyatomic ions: The rules above also apply when writing formulas of compounds containing polyatomic ions, except that in order to indicate more than one polyatomic ion in the formula, put parentheses around the polyatomic ion before writing the subscript (See example below). Examples: Write the formulas for calcium chloride, sodium oxide and ammonium sulfate. 15

4 Table 3.1: List of type I metals and the charges on the cations they form Metal (symbol) Ion Cation Name Metal Ion Cation Name Lithium (Li) Li + Lithium Strontium (Sr) Sr 2+ Strontium Sodium (Na) Na + Sodium Barium (Ba) Ba 2+ Barium Potassium (K) K + Potassium Aluminum (Al) Al 3+ Aluminum Rubidium (Rb) Rb + Rubidium Gallium (Ga) Ga 3+ Gallium Cesium (Cs) Cs + Cesium Indium (In) In 3+ Indium Beryllium (Be) Be 2+ Beryllium Zinc (Zn) Zn 2+ Zinc Magnesium(Mg) Mg 2+ Magnesium Cadmium (Cd) Cd 2+ Cadmium Calcium (Ca) Ca 2+ Calcium Silver (Ag) Ag + Silver Table 3.2: List of common nonmetals and the charges on the anions they form Nonmetal Symbol for Ion Base Name Anion name Fluorine (F) F Fluor Fluoride Chlorine (Cl) Cl Chlor Chloride Bromine (Br) Br Brom Bromide Iodine (I) I Iod Iodide Oxygen (O) O 2 Ox Oxide Sulfur (S) S 2 Sulf Sulfide Nitrogen (N) N 3 Nitr Nitride Phosphorus (P) P 3 Phosph Phosphide Table 3.3: Some metals that form cations with multiple charges type II metals Metal Ion Name Metal Ion Name Chromium Cr 2+ Chromium(II) or Chromous Tin (Sn) Sn 2+ Tin(II) or Stanous (Cr) Cr 3+ Chromium(III) or Chromic Sn 4+ Tin(IV) or Stanic Iron (Fe) Fe 2+ Iron (II) or Ferrous Mercury (Hg) Hg + Mercury (I) or Mercurous Fe 3+ Iron (III) or Ferric Hg 2+ Mercury (II) or Mercuric Cobalt (Co) Co 2+ Cobalt (II) or Cobaltous Lead (Pb) Pb 2+ Lead(II) or Plumbous Co 3+ Cobalt (III) Cobaltic Pb 4+ Lead(IV) or Plumbic Copper (Cu) Cu + Copper (I) or Cuprous Gold (Au) Au + Gold(I) or Aurous Cu 2+ Copper (II) or Cupric Au 3+ Gold(III) or Auric Note: In classical style of naming compounds the name of the cation with lesser charge ends with a suffix ous and higher charge ends with a suffix ic. The classical naming system is not commonly used, however, texbooks in the areas of biological and health science continue to use this system and student must be aware of it. Table 3.4: Common polyatomic anions, their formulas and charges Name Formula Name Formula Ammonium + NH 4 Acetate CH 3 COO Hydroxide OH Cyanide CN Nitrate NO 3 Nitrite NO 2 Carbonate CO 3 2 Hydrogen sulfate (bisulfate) HSO 4 16

5 Hypochlorite ClO Hydrogen phosphate HPO 4 2 Chlorite ClO 2 Chlorate ClO 3 Perchlorate ClO 4 Sulfate SO 4 2 Sulfite SO 3 2 Peroxide O Hydrogen carbonate (bicarbonate) HCO 3 Phosphate PO 4 Note: Ammonium ion is the only polyatomic ion with the positive charge. Nomenclature The elements shown in the periodic table can combine with each other and form many compounds. Chemists have identified and/or created millions of compounds. A systematic method of naming these chemical compounds is therefore very important to distinguish them from each other and this process is referred to as nomenclature. Nomenclature in chemistry was devised by International Union of Pure and Applied Chemistry (IUPAC). As per the IUPAC method of naming compounds, every chemical compound can be given a name that not only identifies it uniquely but also defines its chemical structure. Naming inorganic compounds with examples: As you learned in chemical formula writing experiment, inorganic compounds may be divided into five categories shown below. The procedures used to name the compounds in each of the categories with examples are described below. 1. Binary ionic compounds a. Naming Type 1 binary compounds: Type 1 binary ionic compounds are made up of two elements and are composed of metallic cations and nonmetal anions. Type 1 metal cations exhibit fixed charges. The metals that form type1 binary ionic compounds include elements from group 1A (except hydrogen), 2A, elements Al, Ga and In from group 3A, and elements Zn, Cd and Ag from transition metals (Table 3.1). The various nonmetals and their common charges in ionic compounds are shown in Table 3.2. Rules: 1. Order the names for ions first the cation, then the anion. 2. Name of cation: Same as the name of element from the periodic table (Table 3.3). 3. Name of the anion: Derived from the root of the element's name plus the suffix "ide." 4. Combine the names of cation and anion to generate the name of the compound. Summary: Example: Write the name for: KCl Step 1: In the above example, the cation derived from potassium and anion derived from chlorine. Step 2: Cation = potassium (K + ). Anion = chloride (Cl ).. (Root = chlor plus the ending "ide" = chloride). Step 3: The name of the compound is = Potassium chloride. b. Naming Type 2 binary ionic compounds: Type 2 binary ionic compounds are made up of two elements and are composed of metallic cations with variable charges and nonmetal anions. Some of the metals that form type 2 binary ionic compounds are shown in Table 3.3s. Rules 1. Order the names for ions first the cation, then the anion. 2. Determine the charge on cation based on the type of anion and subscripts. 17

6 3. Name of cation with its charge in parenthesis as Roman numeral: Name of cation is same as the name of element from the periodic table. 4. Name of the anion: Derived from the root of the element's name plus the suffix "ide." 5. Combine the names of cation with Roman numeral and anion to generate the name of the compound. Summary: Example: Write the name for: PbCl 4 Step 1: In the above example, the cation is derived from lead (Pb) and anion is derived from chlorine. Step 2: Lead (Pb) is a transition element and can have multiple charges and the charge on Pb has to be represented by a Roman numeral. Here is how to determine its value: 1. Multiply the charge of the anion (Cl ) by its subscript. Ignore the fact that it is negative. In this example it is: 1 x 4 = Divide this result by the subscript of the cation (Pb). This is the value of the Roman numeral to use. In this example, it is 4 1 = The value of the Roman numeral equals the positive charge on the cation in this formula. Since the result of step #2 is 4, we then use Lead (IV) for the name. Note: There is no space between the name and the parenthesis. Step 3: Cation = Lead(IV) (Pb 4+ ); Anion = chloride (Cl ); (Root = chlor plus the ending "ide" = chloride). Step 4: The correct name of the example is Lead(IV) chloride. Classical method of naming type2 binary ionic compounds: In classical style of naming type two binary ionic compounds, the name of the cation with lesser charge ends with a suffix ous and higher charge ends with a suffix ic. Although, the classical naming system is not commonly used in chemistry anymore, the texbooks in the areas of biological and health science continue to use it. Therefore the students must be aware of this system of naming compounds as well. Example: Write the name for: PbCl 4 as per the classical system of naming compounds. The steps used in this type of naming is same as above except for the following: In the above example the cation is Pb 4+ [Lead(IV)] and anion is Cl (chloride) From Table 5.4, it is apparent that the Lead has two cations, Pb 2+ (Plumbus) and Pb 4+ (Plumbic). Therefore, as per the classical system of naming PbCl 4 is named as Plumbic chloride. 2. Naming molecular compounds: In these compounds the elements are held together by covalent bond(s) or shared pair(s) of electrons. Molecular compounds are also referred to as Type 3 binary compounds. They are made up of only nonmetals. Rules: 1. Look at first element and name it (the element to the left side retains its name). 2. Derive the name for the second element by adding ide to the root of elemental name. 3. Use Greek prefixes for number of atoms: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca, and write the name. Summary: Example: Write the name of the following formula: N 2 O 5 18

7 Step 1: Look at first element and name it. The first element is = Nitrogen. Step 2: Look at second element. Use root of its full name (Ox plus the ending "ide." The anion = oxide. Step 3: Determine the Greek prefixes for number of atom based on subscripts. In this case di for nitrogen and penta for oxide. Step 4: Write the name of the compound using Greek prefixes (step 3) and names written in steps 1 and 2. The name of the compound N 2 O 5 = Dinitrogen pentoxide. Note: If the subscript for the first element is mono the Greek prefix is ignored for the first element but not for the second element. 3. Compounds with polyatomic ions: These are ionic compounds that contain a cation such as an ammonium or a metallic ion, and polyatomic anions. The most common polyatomic ions are shown in Table 5.5. Rules: 1. Order the names for ions first the cation, then the polyatomic anion. 2. Name of cation: Same as the name of element from the periodic table. 3. If the cation is a transition element, the name of cation should be followed by Roman numeral indicating the charge in parenthesis. 4. Name of the anion: same as name of polyatomic ion. 5. Combine the names of cation and anion to generate the name of the compound. Summary: a. If the metal present in the compound is one of the metals shown in Table 5.2. b. If the metals present in the compound is one of the metals shown in Table 5.4 Example 1: Write the name of Na 3 PO 4. Step 1: Names of ions: Cation sodium (Na + ) and polyatomic anion Phosphate (PO 4 3 ). Step 2: Cation is not from a transition metal. Step 3: Name of compound: Sodium phosphate. Example 2: Write the name of Fe(NO 3 ) 3 Step 1: Names of ions: Cation Iron (Fe 3+ ) and polyatomic anion Nitrate (NO 3 ). Step 2: Cation is a transition metal and carries a charge of 3+ Iron(III). Step 3: Name of compound: Iron(III) nitrate. Classical method of naming compounds: The procedure described under type 2 binary ionic compounds also applies under example 2. Accordingly, classical name of Fe(NO 3 ) 3 is Ferric nitrate. 4. Binary acids: These are compounds formed between hydrogen (H + ) and other nonmetals (e.g., F, Cl, Br, I and S) and dissolved in water. Writing names of binary acids is as follows Rules: 1. Check to make sure the compound begins with hydrogen (all acids must begin with a hydrogen) and the formula ends with (aq). 2. To be a binary acid, the second element must be a nonmetal. 3. To name a binary acid the hydrogen is named hydro 4. The second element is named with its root name followed by a suffix ic and the word acid. Summary: 19

8 Example: Write the name of: HBr(aq). Step 1: The compound starts with hydrogen and the formula ends with (aq), therefore it is an acid and the name must start with hydro. Step 2: The second element in the compound is Br, which is a nonmetal, therefore a binary acid. Step 3: The root name for Br is brom and with the suffix ic added it is named as bromic acid) Step 4: Complete name = Hydrobromic acid. 5. Oxyacids: These are compounds formed between hydrogen (H + ) and polyatomic ions containing oxygen. Rules: 1. Check to make sure the compound begins with hydrogen (all acids must begin with hydrogen). 2. To be an oxyacid, the compound must contain a polyatomic ion containing oxygen. 3. Only polyatomic ion is used in naming oxyacids. 4. Name the polyatomic ion changing the ending as follows: If the polyatomic ion ends with ate change its ending to ic If the polyatomic ion ends with ite change its ending to ous The name is followed by the word acid. Summary: If the oxyanion name ends with ate : Example 1: Name the following compound: H 2 SO 4 (aq). Step 1: The compound contains hydrogen and sulfate (a polyatomic ion) Step 2: Name of polyatomic ion is Sulfate. As per rules, its name is modified by replacing ate by ic, i.e., Sulfate Sulfuric. Step 3: The name should end with the word acid. Step 4: Therefore the name of H 2 SO 4 is Sulfuric acid. If the oxyanion name ends with ite : Example 2: Name the following compound: H 2 SO 3 (aq). Explanation: The difference between this and the above example is that the polyatomic ion is SO 3 2 sulfite. Therefore as per rules, its name is modified by replacing ite by ous. Therefore the name of H 2 SO 3 is Sulfurous acid. Writing formulas of inorganic compounds from names Many a times you are given names of compounds and asked to write formulas. The rules for writing formulas from names are somewhat simpler. Shown below are rules and some examples of how to write formulas of compounds from their names. Rules: Rule 1: Rule 2: First write the name of the compound, and identify the names and symbols for each of the ions. Look up the charge (oxidation numbers) of the elements or polyatomic ions involved (Tables ), and write them as superscripts to the right of the elemental symbols. 20

9 Rule 3: Use the correct combination of ions to produce a compound with a net charge of zero. Use subscripts if there are >two atoms of the same element and/or parenthesis and subscripts if there are two polyatomic ions in the formula. Experimental Procedure 1. This experiment has different parts. Each part corresponds to a type of compound(s), formed based on the classification described in the introduction. 2. Each table has a column of cations (more metallic elements) and a row of anions (more nonmetallic elements). 3. Combine these cations and anions (metallic and nonmetallic elements) appropriately to determine the formula of the compound (use the examples below as guide lines to arrive at the correct formula). 4. Fillin the appropriate box with the correct formula. Example: Write the formulas of the compounds formed between the cations and anions given in the table below: Cation/anion Cl SO 4 2 Ba 2+ H + Fe 3+ Correct answer: Cation/anion Cl SO 4 2 Ba 2+ BaCl 2 BaSO 4 H + HCl H 2 SO 4 Fe 3+ FeCl 3 Fe 2 (SO 4 ) 3 5. The molecular compounds are formed due to sharing of electrons between nonmetals. When writing formulas for molecular compounds, it is important to remember that the more metallic element (element located to the left in the periodic table as compared to the element it is combining with) is written first and then the nonmetallic element. The subscript number or the number of atoms present in the formula is determined by the number of electrons they share. The number of electrons they share can be determined from the group (column) they belong to in the periodic table. For Example, carbon belongs to group IVa and it can share 4 electron with other elements, whereas chlorine belongs to group VIIa and it can only share one electron. Accordingly, carbon (C) when combined with chlorine (Cl) will result in CCl 4. Example: Write the formulas of the molecular compounds formed between the elements given in the table below: Element C F O 21

10 Correct answer: Element C F O CO 2 OF 2 6. Naming compounds when the formulas are given: The rules and stepbystep procedures used to name compounds (with examples) in each of 5 categories of compounds are described in the introduction to this experiment. Follow the directions described above and write the names of compounds from their formulas listed in the laboratory data sheet. 7. Writing formulas of compounds when the names are given: In this part of the experiment you are given names of compounds and asked to write formulas. The rules and procedures for writing formulas from names (with examples) are also described in the introduction above. Follow the directions described above and write the formulas of compounds from their names listed in the laboratory data sheet. 22

11 Experimental General Chemistry 1 Experiment 3: Nomenclature and Formula Writing Laboratory Data Sheet: Complete the following tables Name: Section: Type 1 Binary Ionic Compounds Practice Ion Cl Br O 2 S 2 N 3 Na + Be 2+ Ca 2+ Al 3+ Test Zn 2+ Ag + In 3+ Ba 2+ Practice Type 2 Binary Ionic Compounds Ion Cl Br O 2 S 2 N 3 Cu + Cu 2+ Pb 2+ Test Pb 4+ Fe 2+ Fe 3+ Cd 2+ 23

12 Practice Ionic Compounds with Polyatomic Ions Ion OH CO 3 2 NO 3 SO 4 2 PO 4 3 Na + Ca 2+ Cu + Test NH 4 + Fe 2+ Al 3+ Test Molecular Compounds Element F Cl Br I H B C Test Binary Acids Ion Cl S 2 Br I F H + Test Oxyacids Ion SO 4 2 CO 3 2 PO 4 3 NO 3 ClO 3 HCO 3 CN CH 3 COO H + Practice Type 1 Binary Ionic Compounds Given Formula, Write the Name Given Name, Write the Formula Li 3 N MgS KI Lithium fluoride Magnesium phosphide Potassium oxide 24

13 Test ZnF 2 Al 2 O 3 BeS Na 2 S Calcium oxide Zinc sulfide Aluminum nitride Silver bromide Practice Type 2 Binary Ionic Compounds Given Formula, Write the Name Given Name, Write the Formula FeCl 3 CuO Cr 2 O 3 CrCl 2 Cobalt(II) iodide Cobalt(III) oxide Copper(II) sulfide Tin(IV) chloride Test Pb 3 N 2 SnCl 4 CoI 3 HgS FeCl 2 Iron(II) oxide Chromium(II) sulfide Chromium(III) chloride Nickel(II) oxide Lead(II) chloride Practice Ionic Compounds with Polyatomic Ions Given Formula, Write the Name Given Name, Write the Formula CuSO 4 Sn(CO 3 ) 2 KNO 3 MgSO 4 Potassium hydroxide Magnesium phosphate Iron(III) carbonate Copper(II) nitrate 25

14 Test Fe(OH) 3 NH 4 Cl CuNO 2 KClO 3 Ammonium hydroxide Sodium phosphate Cobalt(II) sulfate Sodium hydrogen carbonate Practice Molecular Compounds Given Formula, Write the Name Given Name, Write the Formula SiO 2 SO 2 N 2 S 3 Carbon tetrachloride Boron triiodide Sulfur hexafluoride Test B 2 O 3 IF 5 HI Dinitrogen pentaoxide Hydrogen chloride Dinitrogen monoxide Practice Binary Acids Given Formula, Write the Name Given Name, Write the Formula HF HClO 3 (aq) H 2 SO 3 (aq) H 2 CO 3 Hydrobromic acid Selenous acid Nitric acid Chloric acid Test HI HNO 2 (aq) H 3 PO 4 (aq) H 2 SO 4 Sulfuric acid Perchloric acid Hypochlorous acid Carbonic acid 26