From Writing Formulas to Balancing Equations A Tutorial

Transcription

1 Chemistry Revised 2013 Name From Writing Formulas to Balancing Equations A Tutorial Period Oxidation Numbers. The Oxidation numbers written as s _ tell whether an element or group of elements lost or gained electrons to achieve a stable ion. Atoms are neutral or have a charge of. Metal ions form by losing outer, valence _ therefore they have excess charge. Their Oxidation number is a positive number corresponding to the number of electrons. Example: Al 3+ means aluminum lost three electrons. Losing an electron leaves the hydrogen _ positive (H 1+ ). Nonmetal ions form by gaining _ and have excess negative charge (Cl 1- ). Give an example of an ion with a positive 2 Oxidation number. Give an example of an element that gains 2 valence electrons. Ionic bonds usually form between metal ions (like Al 3+ ) and nonmetal ions (like Cl 1- ) when one atom (Al) loses electrons to another atom (Cl) forming two ions (Al 3+, Cl 1- ). The transfer of outer electrons results in a great difference of charge. The difference in charge is like a magnetic attraction between opposites. Ionic bonds are usually made between West coast and East coast elements (Zig zag line is like the Mississippi). Give and example of a likely ionic pair of elements (metal) and (nonmetal) Covalent bonds exist largely between nonmetals and other nonmetals, and especially between two atoms of the same element. Electrons are shared back and forth between atoms in a covalent bond because space is available for the electrons to be shared and neither atom has a strong enough pull on the electrons to keep them for very long. Oxygen forms O 2, an example of an element that is bonded covalently, and CO 2 is a covalently bonded compound. Some bonds have partial ionic character and partial covalent character and can be considered in between covalent and ionic- but more on that when you are older. Representative Particles. What do we call the smallest representative unit? Particles of most elements are called _. Units of ionic compounds are referred to as formula units. This special label helps to remind us that ionic compounds (like NaCl) exist in a crystal lattice (like a grain of salt) when in the solid state, or they exist as a mixture of Na +1 and Cl -1 ions in water forming an aqueous solution. There are no NaCl particles two atoms big. The formula simply tells the proportion in which we expect to find them. Since compounds obey the Law of Definite Proportions, there is only one correct proportion or ratio for the elements in a compound. (Water is H 2 O and never HO 2 for example.) However, covalently bonded compounds can exist as intact particles, or units of their formula. Chemists call these units molecules. Diatomic gases are two atoms big, hence the term diatomic and these molecules are easily remembered as the HONCLBRIF gases: H 2, O 2, N 2, Cl 2, Br 2, I 2, F 2. The HONCLBRIF elements must be grouped in two atom molecules whenever they are present as a gas. Representative Particles represent the smallest unit of an Element or Compound Elements (excluding HONClBrI F) Covalent Compounds and exclusively the elements: H, O, N, Cl, Br, I, F, when they exist as gases Ionic Compounds Atoms Molecule Formula Unit Cu, S, P, Ag CO 2, H 2 O, H 2, O 2, N 2, Cl 2, Br 2, I 2, F 2 NaCl Mg(NO 3 ) 2

2 Common Mono and Polyatomic Single Charge Ions Ion name & symbol for +1 ions Ion name & symbol for -1 ions Ammonium, NH4 +1 (Memorize) *Copper I, Cu +1 Hydrogen, H +1 Potassium, K +1 Silver, Ag +1 Sodium, Na +1 * Roman numeral required to distinguish from a +2 or + 3 ion Bromide, Br -1 Chloride, Cl -1 Fluoride, F -1 Hydride, H -1 Hydroxide, OH -1 Iodide, I -1 Nitrate, NO3-1 To write a formula of an ionic compound the order of ions is very important. First write the positive ion then the negative ion. For example KCl is potassium chloride and CuOH is copper I hydroxide. Use the chart to do problems 1-3. Please observe polyatomic ions are given special names like hydroxide instead of hydrogen oxygen. 1. Try writing the formula for silver iodide. 2. Now write the logical name for NaNO If hydrogen gas H 2 reacts with a molecule of HCl is formed. 4. If chlorine gas is written Cl 2, then oxygen gas is written:. Formulas between ions of different values (like Na +1 and O -2 ) combine in simple whole number ratios (two Na +1 plus one O -2 as in Na 2 O). The ratio fixes the problem of the charges being out of proportion. When the charges between ions are balanced the charges add up to zero; a formula correctly describes how ions of opposite charge combine in nature. (Ions of the same charge repel each other). Here are some examples from the Oxidation Chart that contains ions with a double charge: Names, symbols for +2 ions Barium, Ba +2 Calcium, Ca +2 *Copper II, Cu +2 *Iron II, Fe +2 Magnesium, Mg +2 Names, symbols for -2 ions Carbonate, CO3-2 Oxide, O -2 Sulfate, SO4-2 Sulfite, SO3-2 Sulfide, S -2 If a barium ion combined with an oxide ion the formula would be BaO. However the formula of barium chloride is written BaCl 2. The subscript 2 means: Two chloride ions combine with a single barium ion to make the formula BaCl 2. Adding the charge on the ions we get: (-1) = 0. The net charge is zero. The formula is balanced when the opposite charges are equal in value. Or we say a positive two cancels a negative two. Parentheses are used to group ions with two or more atoms together when more than one is needed to balance the charge. For example barium hydroxide is written Ba(OH) 2. The subscript two means two (-1) hydroxide ions combine with one (+2) barium ion. Write the formula of the following compounds. Remember to include the Roman numeral in the name if there is more than one value of the positive ion. Use the Oxidation Chart. 5. magnesium sulfide 6. magnesium chloride 7. iron II oxide 8. copper I carbonate

3 How do you write the name if you just have the formula? Chemists write it the same as before. Use the Oxidation _ to look up the ion names. Single, negative ions are given the same as the element name except they have an ide ending to show they are ions instead of atoms. To choose which Roman numeral is used with a positive ion, first look up the charge on the negative ion. For example in Cu 2 O, if oxide is -2, then copper must be Cu +1 if two are needed in the formula. The correct name then is copper I oxide. The name of CuO must be copper II oxide since oxide has a -2 charge and therefore only one Cu +2 ion is needed to balance the charge. 9. NH 4 Cl 10. Ca(OH) Ag 2 SO FeO From Formulas to Equations Students practice combining ions and elements using an Oxidation Chart. Chemists, who combine the elements in chemical reactions to make compounds, created the Oxidation chart by analyzing the proportion in which the elements combine. They found the elements combine in simple whole number ratios of their atomic mass. For example chemists found sodium metal will react vigorously with chlorine gas to make sodium chloride. The chemist s way of writing this reaction is an equation. To show sodium metal is in the solid state they use (s) next to the symbol for sodium (Na). The chlorine s gaseous state is designated (g).. The product NaCl is in the crystal lattice or grains of salt, also a solid: (s). The arrow show the reactants produced products. The coefficient, 2, is written to show that the equation represents correctly the proportion of reactants to products to achieve the Law of Conservation of Matter: Matter is not created or destroyed in a chemical reaction. (The atoms are simply rearranged.) Reactants: Products: 2 Na (s) + Cl 2(g) 2NaCl (s) Since chlorine is diatomic, two sodium atoms are needed to react with the molecule of chlorine gas to make two sodium chloride formula units. Use the following sentences and unbalanced equations to write the products of the suggested chemical reactions: 13. Sodium metal combines with solid iodine to produce solid sodium iodide. (Don t forget the Oxidation numbers) Na (s) + I 2(s) (s) 14. Magnesium metal combines with solid iodine to produce solid magnesium iodide. (Don t forget the Oxidation numbers) Mg (s) + I 2(s) (s) 15. Potassium metal combines with oxygen gas to produce solid potassium oxide. (Don t forget the Oxidation numbers) K (s) + O 2(s) (s) 16. Barium metal combines with oxygen gas to produce solid barium oxide. (Don t forget the Oxidation numbers) Ba (s) + O 2(s) (s) There are five types of chemical reactions. Problems are examples of combination reactions. Two elements combine in a combination reaction so the reactants are always elements. Decomposition reactions always produce elements as products. For example Al 2 O 3 decomposes to produce O 2 gas and aluminum metal. Aluminum oxide is a complex formula. Two ions of aluminum (Al +3 ) combine with three ions of oxide (O -2 ) to make Al 2 O 3. When aluminum oxide decomposes the elements are produced: aluminum metal and oxygen, a diatomic gas. Al 2 O 3. (s) Al (s) + O 2(s) Law of Conservation of Mass: Since atoms are not created or destroyed in a chemical reaction the arrow in an equation is read like an equal sign. To balance the equation numbers called coefficients are added. Observe: 2Al 2 O 3. (s) 4Al (s) + 3O 2(g) Two formula units of aluminum oxide decompose to make four atoms of aluminum and three molecules of oxygen gas. Now this equation is correctly describing the reaction.

4 For now let s write some unbalanced or skeletal equations. After some practice we will introduce the steps to balancing equations. First, practice writing the products of these decomposition reactions: 17. CS 2(g) (s) + (g) 18. Cu 2 O (s) (s) + (g) 19. MgCl 2(g) (s) + (g) 20. Fe 2 O 3. (s) (s) + (g) Names, symbols for +3 ions Aluminum, Al +3 Chromium III, Cr +3 Cobalt III, Co +3 Iron III, Fe +3 Names, symbols for -3 ions Nitride, N -3 Phosphate, PO4-3 Phosphite, PO3-3 Phosphide, P -3 Another type of chemical reaction involves a metal replacing a metal in a compound or a nonmetal replacing a nonmetal in a compound. Both are examples of a single replacement reaction. If the single element is more reactive, then it can replace an element of its kind. If an element can replace another element it is considered more reactive than the element it replaces. For example aluminum can replace copper in copper II chloride, but copper can not replace aluminum in aluminum chloride. Al (s) + CuCl 2(aq) AlCl 3(aq) + Cu (s) Cu + AlCl 3(aq) No reaction Why does aluminum chloride have three chloride ions and copper II chloride only has two chloride ions in their respective formulas? (Because aluminum forms an Al +3 ion and copper II forms a Cu +2 ion) The equation between aluminum and copper II chloride is an example showing why it is important to use Oxidation numbers to write formulas. After the correct formulas are written the equation can be balanced using coefficients as follows: 2Al (s) + 3CuCl 2(aq) 2AlCl 3(aq) + 3Cu (s) Sometimes the element doing the replacing is a nonmetal like chlorine gas (Cl 2 ). Cl 2(g) + AlI 3(s) AlCl 3(s) + I 2(s) Diatomic gases, like chlorine and iodine, must combine according to their Oxidation number and their reactivity. Chlorine is more reactive than iodine. 21. Balance the following equation using coefficients. Cl 2(g) + AlI 3 AlCl 3 + I 2(s) 1. Complete an Cl = Al = Cl = I = atom inventory. I = Al = 2. Add coefficients to make each side of equation obey the law of conservation. 3. Revise atom inventory to reflect the new number of atoms on each side of the equation.

5 Name Period Date Use the Oxidation chart to complete the following unbalanced single replacement reactions. Remember to switch the elements according to their kind (metals with metals and nonmetals with nonmetals). Turn this page in 22. AgNO 3 (aq) + Mg (s) (aq) + (s) 23. Cu(NO 3 ) 2 (aq) + Ca (s) (aq) + (s) 24. KCl (aq) + F 2(g) (aq) + (g) Now use coefficients to balance the following equations: 25. AgNO 3 (aq) + Ca (s) Ca(NO 3 ) 2(aq) + Ag (s) 26. Fe(NO 3 ) 3 (aq) + Mg (s) Mg(NO 3 ) 2(aq) + Fe (s) 27. KI (aq) + F 2(g) KF (aq) + I 2(g) Complete equations by writing the correct formula, then try balancing these combination reactions: 28. H 2 + O Al (s) + O 2(g) 30. Ag (s) + Cl 2(g) 31. Fe (s) + O 2(g) Fe 2 O 3(s)

6 Answers: 1. Try writing the formula for silver iodide AgI. 2. No write the logical name for NaNO 3 sodium nitrate 3. If hydrogen gas H 2 reacts with Cl 2 a molecule of HCl is formed. 4. If chlorine gas is written Cl 2, then oxygen gas is written: O 2 5. magnesium sulfide MgS 6. magnesium chloride MgCl 2 7. iron II oxide FeO 8. copper I carbonate Cu 2 CO 3 9. NH 4 Cl ammonium chloride 10. Ca(OH) 2 calcium hydroxide 11. Ag 2 SO 4 silver sulfate 12. FeO iron II oxide 13. Sodium metal combines with solid iodine to produce solid sodium iodide. Na (s) + I 2(s) NaI (s) 14. Magnesium metal combines with solid iodine to produce solid magneisum iodide. Mg (s) + I 2(s) MgI 2 (s) 15. Potassium metal combines with oxygen gas to produce solid potassium oxide. K (s) + O 2(s) K 2 O (s) 16. Barium metal combines with oxygen gas to produce solid barium oxide. (Don t forget the Oxidation numbers) Ba (s) + O 2(s) BaO (s) 17. CS 2(g) C (s) + S (g) 18. Cu 2 O (s) Cu (s) + O 2(g) 19. MgCl 2(g) Mg (s) + Cl 2(g) 20. Fe 2 O 3. (s) Fe (s) + O 2(g) Cl 2(g) + 2 AlI 3 2 AlCl 3 +3 I 2(s) 22. AgNO 3 (aq) + Mg (s) Mg(NO 3 ) 2(aq) + Ag (s) 23. Cu(NO 3 ) 2 (aq) + Ca (s) Ca(NO 3 ) 2(aq) + Cu (s) 24. KCl (aq) + F 2(g) KF (aq) + Cl 2(g) AgNO 3 (aq) + 1Ca (s) 1 Ca(NO 3 ) 2(aq) + 2 Ag (s) Fe(NO 3 ) 3 (aq) + 3 Mg (s) 3 Mg(NO 3 ) 2(aq) + 2 Fe (s ) KI (aq) + 1 F 2(g) 2 KF (aq) + 1 I 2(g)