PORTMORE COMMUNITY COLLEGE DEPARTMENT OF PURE AND APPLIED SCENCES CHEMISTRY REVISION EXERCISE. Part One

Transcription

1 1 Enclosed in this handout are: PORTMORE COMMUNITY COLLEGE DEPARTMENT OF PURE AND APPLIED SCENCES CHEMISTRY REVISION EXERCISE Part One NAMING AND WRITING CHEMICAL FORMUALE OF INORGANIC COMPOUNDS NOTES NAMING AND WRITING CHEMICAL FORMUALE OF INORGANIC COMPOUNDS WORKSHEET Part Two NAMING, WRITING AND BALANCING MOLECULAR EQUATIONS NOTES NAMING, WRITING AND BALANCING MOLECULAR EQUATIONS WORKSHEET Part Three WRITING IONIC EQUATIONS (PREDICTING PRECIPITATION REACTIONS NOTES WRITING IONIC EQUATIONS (PREDICTING PRECIPITATION REACTIONS WORKSHEET

2 2 PART ONE NAMING AND WRITING CHEMICAL FORMUALE OF INORGANIC (IONIC AND COVALENT) COMPOUNDS At the end of this chapter, you should be able to: 1. interpret the information conveyed by chemical formulas. 2. derive the formulas for various compounds. 3. apply the rules for determining the names of compounds. Chemical formulas are the shorthand that chemists used to describe compounds, both simple and complex. Chemists used the symbol of an element to write chemical formulas. Essentially, a chemical formula is a type of notation made with numbers and chemical symbols. It has two purposes: (i) to indicate the composition of a compound and (ii) to indicate the number of atoms in one molecule of an element. Recall the definition of a molecule - A molecule may a single atom, a group of two or more atoms of different elements that have combined to form a compound. During chemical reactions, the atoms in molecules actually rearrange themselves to form different groups. How do we write the names and formulas of the cations and anions, and polyatomic ions to write formulas of compounds? To write a correct formula for a compound, we must know or be given the ionic charges of the cations and anions. Consider: One atom molecule of the element helium, a gas at room temperature. Each molecule consists of a single atom. One atom molecules are called monoatomic molecules. In addition to helium, the other gaseous elements that are monoatomic are neon, argon, krypton, xenon and radon. There also exist a molecule of hydrogen which consists of two hydrogen atoms. Two-atom molecules are called diatomic molecules. There are five elements that are gaseous at room temperature and are diatomic molecules: hydrogen, oxygen, nitrogen, chlorine and fluorine. One element bromine is a diatomic liquid and iodine a diatomic solid. Some molecules of the elements phosphorous and Sulphur are both solids at room temperature. A molecule of phosphorous is made of four (4) atoms (P 4 ), and a molecule of sulphur, eight (8) atoms (S 8 ). Ionic compounds may be consist of ions formed from single atoms or ions (polyatomic ions) formed from small group of atoms bonded together as a single unit called radicals, e.g. ammonium, NH 4 +, sulphate, SO Since we use different methods in naming covalent (molecular) compounds and ionic compounds, the first step in naming and writing the formula of a compound is to determine which of the two (2) compounds classes it belongs. This can be done as follows: The only exception we will see to the above flow chart is when we see the polyatomic ion (radical) ammonium ion (NH 4 + ) combined with any anion; in these cases the compound is ionic even though the compound is composed of non-metals only. Once it is deteremined that the compound is ionic or covalent, the student can then give the name of the compound and write the formula using the criss-cross method. This method is a shortcut to figuring the necessary ratio between two ions. TYPES OF CHEMICAL BONDING COVALENT IONIC COMPOUNDS COMPOUNDS Non-metals + Nonmetals Metals + Non-metals Sharing of electrons Metals + polyatomic ions (radicals)

3 3 YOU ARE REQUIRED TO MEMORIZE THESE IONS THROUGH DRILL AND PRACTICE! TABLE OF SELECTED MONO- AND POLYATOMIC IONS Common Cations Monovalent Divalent Trivalent Hydrogen, H + Magnesium, Mg 2+ Iron(III), Fe 3+ Lithium, Li + Calcium, Ca 2+ Aluminum, Al 3+ Sodium, Na + Barium, Ba 2+ Chromium, Cr 3+ Potassium, K + Iron(II), Fe 2+ Coppe(I), Cu + Copper(II), Cu 2+ Silver, Ag + Zinc, Zn 2+ + Ammonium, NH 4 Tin(II), Sn 2+ Lead(II), Pb 2+ Common Anions Monovalent Divalent Trivalent Fluoride, F - Oxide, O 2- Nitride, N 3- Chloride, Cl - Sulphide, S 2-3- Phosphate, PO 4 Bromide, Br - 2- Carbonate, CO 3 Iodide, I- 2- Sulphite, SO 3 Hydride, H - 2- Sulphate, SO 4 Hydroxide, OH - 2- Dichromate(VI), Cr 2 O 7 - Nitrite, NO 2 2- Chromate, CrO 4 - Nitrate, NO 3 2- Peroxide, O 2 - Manganate(VII), MnO 4 Hydrogen sulfide, HS - - Hydrogen sulphate, HSO 4 - Hydrogen (bi) carbonate, HCO 3 2- Hydrogen phosphate, HPO 4 Ethanoate (acetate), CH 3 COO - Key - polyatomic ions (also called radicals or complex ions - which are groups of atoms that share electrons and exist in aqueous solution as a unit. Italicized and unbold - some elements have more than one oxidation number or states. Italicized and bold - hydrogen polyatomics Highlighted - hydrogen can behave like a metal and non-metal Underlined - pay close attention! COMMON METALS IONS WITH MORE THAN ONE OXIDATION NUMBER FORMULA (written as oxidation #) STOCK NAME CHEMICAL NAME Cu +1 Copper(I) ion Cuprous ion Cu +2 Copper(II) ion Cupric ion Fe +2 Iron(II) ion Ferrous ion Fe +3 Iron(III) ion Ferric ion Pb +2 Lead(II) ion Plumbous ion Pb +3 Lead(III) ion Plumbic ion Pb +4 Lead (IV) For students to find out. Sn +2 Tin(II) ion Stannous ion Sn +4 Tin(IV) ion Stannic ion

4 4 WRITING FORMULAE OF IONIC COMPOUNDS Metal-Non-Metal and Metal-Polyatomic Ions Formula of Ionic Substances (Applying the criss-cross method) SIX STEPS FOR WRITING AN IONIC COMPOUND FORMULA 1. Write the symbols for the two elements in the given compound. 2. Write the valence [or charge on the ions for each element (or groups of elements)] as superscripts. (N.B. Charges are written as n- or n+.[refer to the Table of Ions] 3. Assign the oxidation number or states (written as -n or +n, = opposite of the charge denotation) to the ions. 4. Criss-cross the superscripts, i.e. the oxidation numbers, so they become subscripts. 5. Remove the positive and negative signs. 6. Reduce when possible (to the lowest term). Place brackets around radicals if more than one is present. Always place the metal or ammonium ion first in the formula. N.B. When writing formulae of ionic compounds, the sum of the positive charges must equal the sum of the negative charges, since both types of atom or radical forming the compound must lose or gain the same number of electrons. To form the compound sodium nitrate: Ions Na + and NO 3 -. The charges cancel out (+1-1) = 0. Formula NaNO 3. Regarding potassium sulphate: K + and SO 4 2- ; -ve charges = -2, + ve charge = +1 The whole aim of the exercise regarding the formation of compounds is to balance the charges so that they are cancelled out to zero. When naming anions, those formed from single atoms are named after the element, with ending -ide, eg. S 2- = sulphide. If oxygen is present in a radical, its name is derived from the name of the element combined with oxygen and the ending -ite or ate, e.g. SO 3 2- = sulphite; SO 4 2- = sulphate. Chuck off on these. Write the chemical formulae for the following ionic compounds: 1. Sodium chloride 2. Copper(II) bromide 3. Lithium oxide 4. Zinc carbonate 5. Barium hydroxide 6. Calcium phosphate 7. Ammonium chloride 8. Ammonium suphite

5 5 CHEMICAL FORMULA OF COMPOUNDS WRITING Fill in the symbols and charges of the ions and then write correct chemical formulas and names in the corresponding blocks. IONS Nitrite (NO 2 - ) Chromate Sulfide Sulfite Dichromate Ammonia NH 4 + Copper (I) Potassium Calcium Copper(II) Iron(II) Lead(II) Magnesium Aluminum Iron(III) Chromium Mercury(I) Zinc Barium Mercury(II)

6 6 WRITING FORMULAE OF COVALENT COMPOUNDS Non-Metal-Non-Metal Formula of Molecular Substances NB. NON-METAL FORMULAS FOR MOLECULAR SUBSTANCES A system of prefixes is used to distinguish between these compounds. mono - one (1) di - two (2) tri - three (3) penta - five (5) hexa - six (6) hepta - seven (7) octa - eight (8) nona - nine (9) deca - ten (10) NAME SYMBOLS FORMULA Carbon monoxide C O CO Carbon dioxide C O CO 2 Carbon tetrachloride C Cl Dinitrogent monoxide N O Dinitrogen pentafluoride N F Water (alternative name) Tetraphosphorous decaoxide Idoine hexafluoride Tetra arsenic decoxide Binary Acids contain hydrogen and a nonmetallic element are an aqueous solution of the pure compound have prefix = hydra; suffix = ic Fill in the blank. Hydrofluoric acid Hydrochloric acid HFaq) HBr(aq) HI(aq) Find out the chemical formula for bleach.

7 7 Hydrated compounds** Ionic compounds that produce water when decomposed on heating. Name as ionic compound Indicate number of water molecules using previous prefixes of di, tri etc. and the ending hydrate. Copper (II) sulphate pentahydrate = CuSO 4. 5H 2 O Zinc chloride hexahydrate = ZnCl 2. 6H 2 O

8 8 WRITING AND NAMING CHEMICAL FORMUALE OF INORGANIC (IONIC AND COVALENT) COMPOUNDS WORKSHEET 1. Write chemical formulas for the compounds in each box. The names are found by finding the intersection between the cations and anions. Example: The first box is the intersection between the zinc cation and the chloride anion, so you should write ZnCl 2, as shown. zinc iron (II) iron (III) gallium silver lead (IV) chloride ZnCl 2 acetate nitrate oxide nitride sulfate Write the formulas for the following compounds: 1) copper (II) chloride 2) lithium acetate 3) vanadium (III) selenide 4) manganese (IV) nitride 5) beryllium oxide 6) sodium sulfate 7) aluminum arsenide 8) potassium permanganate 9) chromium (VI) cyanide 10) tin (II) sulfite 11) vanadium (V) fluoride 12) ammonium nitrate Name the following ionic compounds: 1) NaBr 2) CaO 3) Li 2 S 4) MgBr 2 5) Be(OH) 2 Write the formulas for the following ionic compounds: 6) potassium iodide

9 9 7) magnesium oxide 8) aluminum chloride 9) sodium nitrate 10) calcium carbonate 11) lithium sulfate 12) beryllium phosphide 13) magnesium hydroxide 14) sodium phosphate 15) aluminum carbonate 16) calcium chloride 17) sodium cyanide 18) aluminum oxide 19) magnesium acetate 20) ammonium chloride Write the names of the following covalent compounds: 21) SO 3 22) N 2 S 23) PH 3 24) BF 3 25) P 2 Br 4 26) CO 27) SiO 2 28) SF 6 29) NH 3 30) NO 2 Write the formulas of the following covalent compounds: 31) nitrogen trichloride 32) boron carbide 33) dinitrogen trioxide 34) phosphorus pentafluoride 35) methane 36) sulfur dibromide 37) diboron tetrahydride 38) oxygen difluoride 39) carbon disulfide 40) nitrogen monoxide

10 10 PART TWO NAMING, WRITING AND BALANCING MOLECULAR EQUATIONS PLEASE REFER TO THE SUPPLEMENTAL HANDOUT!

11 11 PART THREE WRITING IONIC EQUATIONS (PREDICTING PRECIPITATION REACTIONS) Previous knowledge Resources needed Naming and writing chemical formula Table of Cations and Anions Writing balanced chemical equations Solubility of Compounds in Water Chart Insofar, all the equations you have been introduced to are called molecular equations by chemists. Recall that a molecular equation is a chemical equation in which the reactants and products are written as if they were molecular substances even though they actually exist in solution as ions. A molecular equation is useful in describing the actual reactant and product substances, but it does not give an ionic theory of the chemical reaction. Often we need rewrite the molecular equation as ionic equation. An ionic equation is typically used to show what happens to the ions that take part in a chemical reaction. All aqueous double displacement reactions follow the general pattern: AX + BY AY (ppt) + BX where A and b are cations and X and Y are anions. For the reaction to result in a net change in the solution, ions must be removed from solution. It is customary to simplify metathesis reaction equations by expressing them in net ionic form. This involves: 1. Write the complete net ionic equation by expressing all soluble (aqueous) ionic compounds as ion. 2. Removing the spectator ions that appear on both sides of the reaction equation. 3. Remember that insoluble solids, liquids, gases do not exist as ions in solution which are free moving (and therefore should not be expressed as ions in the complete and net ionic equation. Key terms: Metathesis or precipitation or double displacement reaction - involves mixing two solutions to achieve a precipitate (ppt). Spectator ion - is an ion in an ionic equation that does not take part in the reaction. They can be cancelled out from both sides of the equation to express the essential reaction that occurs. (Net) ionic equation (shows all molecules ionized) - one in which the spectator ions have been cancelled. Essentially, it shows the actual reaction that occurs at the ionic level. They help us to focus on the heart of the chemical change in a particular reaction.

12 12 ION NO 3 - GENERAL SOLUBILITY RULE All nitrates are soluble - C 2 H 3 O 2 All acetates are soluble (AgC 2 H 3 O 2 only moderately) All chlorides, bromides and iodides are soluble except Ag +, Pb 2+ and Hg 2+. (PbCl 2 is slightly soluble in cold Cl -, Br -, I - water and moderately soluble in hot water. All sulphates are soluble except those of Ba 2+, Pb 2+, Ca SO 4 and Sr 2+ All carbonates and phosphates are insoluble except those CO and PO 4 of Na +, K + and NH + 4 (Many phosphates are soluble.) All hydroxides are insoluble except those of Na + and K +. OH - Hydroxides of Ba 2+ and Ca 2+ are slightly soluble. All sulfides are insoluble except Na +, K + and NH + 4 and those of alkaline earth: Mg 2+, Ca 2+, Sr 2+ and Ba 2+. S 2- (Sulfides of Al 3+ and Cr 3+ hydrolyze and precipitate as the corresponding hydroxide. All salts of Na +, K + and NH + 4 are soluble except several Na +, K + + and NH 4 uncommon ones. Moderately/sparingly/partially/slightly Task: Assemble the cations and anions per block to form the relevant compound. Indicate using the key below the table, whether the compound is soluble or partially soluble or insoluble. Key soluble (S) or insoluble (I) or partially soluble (PS)

13 13 RULES FOR WRITING IONIC EQUATIONS The key to writing net ionic equations is the ability to recognize monoatomic and polyatomic ions and the solubility rules. State (Phase) Symbols in Chemical Equations Symbol (s) (l) (g) (aq) Meaning Indicates that the substance is in a solid state. Indicates that the substance is in a liquid state. Indicates that the substance is in a gaseous state. Indicates that the substance is dissolved in water. 1. Write the balanced molecular equation. 2. Assign the state symbols of each substance in the molecular equation by determining their solubilities (Use solubility rule chart). 3. Replace the formula of each substance in the molecular equation with formulas of its ions of ONLY THOSE PRESENT IN WATER (aqueous form). (In doing so, be careful to keep the coefficients in the balanced molecular equation.) Also, ensure that you IGNORE any solids, liquids or gases when splitting the compounds into its original ions. (TOTAL IONIC EQUATION) 4. Cancel the ions (spectator ions) that appear on both sides of the equation. Spectator ions do not undergo no chemical change during a chemical reaction. 5. NET IONIC EQUATION produced. YOU MUST ALWAYS ASSIGN THE STATE SYMBOLS TO THE SUBSTANCES IN AN ONIC EQUATION.

14 14 Example One: Let's first start with a complete chemical equation and see how the net ionic equation is derived. For example, take the reaction of lead(ii) nitrate with hydrochloric acid to form lead(ii) chloride and nitric acid, shown below: Pb(NO 3 ) 2 (aq) + 2 HCl (aq) PbCl 2 (s) + 2 HNO 3 (aq) This complete equation may be rewritten in ionic form by using mainly the solubility rules. Both hydrochloric acid and nitric acid are strong acids (they are on the list of exceptions) and will therefore be dissociated. The lead(ii) chloride, however is insoluble - remember that all halides are soluble except the silver, lead, and mercury(i) halides. The above equation written in its dissociated ( ionic ) form is: Pb 2+ (aq) + 2 NO 3 (aq) + 2 H + (aq) + 2 Cl (aq) PbCl 2 (s) + 2 H + (aq) + 2 NO 3 (aq) Notice that the stoichiometry of the balanced equation must be maintained. Pb(NO 3 ) 2 is dissociated into Pb 2+ and 2 NO 3, and the 2 HCl molecules shown in the reactants are dissociated into 2 H + and 2 Cl. At this point, one may cancel out those ions which have not participated in the reaction. Notice how the nitrate ions and hydrogen ions remain unchanged on both sides of the equation. Ions which don t change during the chemical reaction are called spectator ions and can be removed from the equation without destroying the equality (as long as they are removed in exactly the same number from both sides!). Pb 2+ (aq) + 2 NO 3 (aq) + 2 H + (aq) + 2 Cl (aq) PbCl 2 (s) + 2 H + (aq) + 2 NO 3 (aq) What remains is the net ionic equation, showing only those chemical species participating in the chemical process: Pb 2+ (aq) + 2 Cl (aq) PbCl 2 (s) It is also possible to predict the net ionic equation given only the reactants. Example Two: Suppose you had to determine the net ionic equation resulting from the mixing of solutions of barium bromide and sodium sulfate: BaBr 2 (aq) + Na 2 SO 4 (aq)? One way to approach this problem is to determine what ions are in solution. Since both reactants are salts, they will be fully dissociated into Ba 2+, Br, Na +, and SO We know that barium bromide is soluble, but will sodium ions or sulfate ions combine with barium ions to form an insoluble compound? Barium ions and sodium ions, both being positive in charge, will repel each other, so no compound is expected to form between them. Similarly, since bromide and sulfate ions both have a negative charge, we would expect no compound to form from this combination. On the other hand, sulfate ions and barium ions could easily form barium sulfate. Now it is just a matter of consulting the solubility rules to see if barium sulfate is soluble or insoluble. The solubility rule for sulfates is that they are all soluble except for those of strontium, barium, and lead; silver sulfate and calcium sulfate are partially soluble. As you can see from these rules, barium sulfate will be insoluble. The sodium ions must therefore combine with bromide ions to form sodium bromide. According to the solubility rules, sodium bromide should be soluble all sodium salts are soluble, as are most halides. Now we can write a complete balanced equation: BaBr 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + 2 NaBr (aq) As before, the above equation can be rewritten in ionic form, showing the soluble species as ions in solution: Ba 2+ (aq) + 2 Br (aq) + 2 Na + (aq) + SO 4 2- (aq) BaSO 4 (s) + 2 Na + (aq) + 2 Br (aq)

15 15 Next, cross out the spectator ions: Ba 2+ (aq) + 2 Br (aq) + 2 Na + (aq) + SO 4 2- (aq) BaSO 4 (s) + 2 Na + (aq) + 2 Br (aq) What remains is the balanced, net ionic equation: Ba 2+ (aq) + SO 4 2- (aq) BaSO 4 (s) Example 3: magnesium chloride is mixed with one of copper(ii) nitrate: MgCl 2 (aq) + Cu(NO 3 ) 2 (aq)? Summary Molecular and Ionic Equations Worksheet Instructions: Write the molecular, total-ionic and net-ionic equation for the following. 1. Potassium phosphate and calcium nitrate Molecular equation: Total-ionic equation: ` Net-ionic equation: 2. Barium chloride and sodium sulphate Molecular equation: Total-ionic equation: ` Net-ionic equation:

16 16 3. Sodium acetate and potassium nitrate Molecular equation: Total-ionic equation: ` Net-ionic equation: 4. Ammonium bromide and sodium hydroxide Molecular equation: Total-ionic equation: ` Net-ionic equation: 5. Zinc and hydrochloric acid Molecular equation: Total-ionic equation: ` Net-ionic equation: 6. Sulphuric acid and potassium hydroxide Molecular equation: Total-ionic equation: ` Net-ionic equation: 7. Calcium carbonate and hydrobromic acid Molecular equation: Total-ionic equation: ` Net-ionic equation: 8. Calcium iodide and potassium carbonate Molecular equation: Total-ionic equation: ` Net-ionic equation: