Notes On Equations

UNIT 6 - WRITING AND BALANCING CHEMICAL EQUATIONS Before you can write or balance a chemical equation, you must first be able to count the atoms which are present. A subscript multiplies only what is immediately in front of it. A coefficient placed in front of a molecule or a formula unit multiplies every atom in the formula by that number Example 6-1 How many oxygen atoms are there in a formula unit of iron (III) dichromate: (b) in 5 formula units of iron (III) dichromate? Example 6-2 How many total atoms are there in 6 formula units of BaCl2 • 2H2O Example 6-3 How many elements are present in 3 formula units of aluminum bicarbonate? A chemical equation is exactly what it says it is--an equality between the reactants (which are substances written on the left side of the equation) and the products (which are substances written on the right side). An arrow pointing to the right serves as the = sign and is read "yields" or “produces”.

HOW TO WRITE A BALANCED CHEMICAL EQUATION (1) write a correct formula for each of the reactants; put a plus sign between the reactants because a plus sign means “reacts with” or “is added to” (2) draw the yield arrow pointing to the right (3) write a correct formula for each of the products, putting a plus sign between them also. On the products side, the plus sign means "as well as" or "in addition to" (4) balance the metals (except hydrogen) FIRST, adding coefficients (not subscripts) where necessary (5) balance the polyatomics next (IF THEY STAY TOGETHER-- that is, if there are the same polyatomics on each side of the equation) by adding a coefficient in front of the entire compound (not in the middle of the compound) (6) if the polyatomic comes apart OR if there is no polyatomic present, balance the non-metals except oxygen next by adding coefficients where necessary (7) balance the hydrogens and the oxygens (which were not part of the polyatomics) LAST

There are four physical state symbols which are used as subscripts immediately following substances whose physical states are known or given. They are (s) - solid, which is used for solids or precipitates (l) - liquid, which is used only for "true" liquids such as elements which are liquids at room temperature such as mercury and bromine; also for water; also for molten substances (melted); (g) - gas, which is used for gases or vapors. (aq) - aqueous solution, which means hat the compound is dissolved in water, making a solution Example 6-4 Write a balanced equation for the following reactions (a) a solid piece of zinc reacts with hydrochloric acid to produce a solution of zinc chloride and hydrogen gas as the products

(b) phosphoric acid reacts with a solution of sodium hydroxide to produce a solution of sodium phosphate and water as the only products

(c) chlorine gas is bubbled through a solution of lithium iodide, and the products are found to be a solution of lithium chloride and solid di-iodine.

(d) solutions of silver nitrate and barium chloride are mixed, and the products are a precipitate of silver chloride and a solution of barium nitrate

A few hints on balancing equations involving “unusual” circumstances and how NOT to panic while doing it. 1. sometimes there are an even number of oxygens on the right side and an odd number on the left side of the equation. What do I do now, Coach? EXAMPLE: Potassium chlorate (heated) yields potassium chloride and oxygen gas

(Hint: When all of the atoms of the element you are struggling to balance are in ONE place on the left side and all are in ONE place on the right side, find the LCM between the atoms to place your coefficients. 2. if balancing an equation involving a hydrocarbon reacting with oxygen (burning), follow these steps since there is no metal to start with: (a) balance the carbons 1st, (b) balance the hydrogens 2nd (c) count the number of oxygens on the right side of the equation and decide how best to “deal with” balancing it, remembering these possibilities. EXAMPLE: Methane plus oxygen yields carbon dioxide plus water

3. sometimes you can balance a hydrocarbon reaction by using fractional coefficients. For now, I want you to use the LOWEST SET OF WHOLE NUMBER COEFFICIENTS., so you must multiply all reactants and products by the lowest number which will make all whole numbers (the reciprocal of the fraction) EXAMPLE: Butane plus oxygen yields carbon dioxide and water

Hint: Right now we are interpreting the coefficients in a balanced equation to mean “numbers of molecules” or “numbers of formula units”, and there is no such thing as ½ molecule or ½ formula unit. 4. What if you have an odd number of oxygen on the left side and an even number of oxygens on the right side and all of the oxygens are NOT in one place on either side of the equation? EXAMPLE: Methyl alcohol (liquid) plus oxygen gas yields carbon dioxide and water (Hint: when balancing the oxygens, (and they are not all in the same compound either on the right nor the left sides of the equation, do not even think about adding a coefficient in front of the methyl alcohol because then you would have to “correct” the carbons and hydrogens on the right side, which would, in turn, mess up the number of oxygens you counted on the right side, and you go around forever in a loop. A better plan is to count the number of oxygens on the right side; then decide to COUNT the oxygen inside the alcohol as ONE oxygen atom and add a coefficient in front of the oxygen gas to bring the number of oxygens up to the number on the right side...

HOW TO PREDICT THE PRODUCTS OF A CHEMICAL REACTION: To be able to predict the products of a reaction, you must first be able to recognize what type of reaction it is. There are five specific types of reactions which you need to be able to recognize: (1) SYNTHESIS - means "putting together"; characterized by having two pure elements as its reactants and there will be only ONE product formed which is a compound between these two elements; LIKE A MARRIAGE; A + B → AB (Hint: be sure to write the METAL first in the compound, even if it is listed as the 2nd reactant on the left side) Example 6-5 Write a balanced equation for the reaction which will take place when potassium reacts with chlorine gas.

(b) Write a balanced equation for the reaction which will take place when aluminum is allowed to react with oxygen gas.

(c) If iron (III) nitride is the only product formed in this reaction, write a balanced equation for the reaction.

(2) DECOMPOSITION - means "breaking apart"; characterized by having only ONE reactant which simply comes apart into its elements (the products); there must be some type of energy which causes this reaction to happen, and this energy is usually written OVER the yield arrow; LIKE A DIVORCE; CD → D + C (Hint: it does not matter which of the elements you write first as products) Example 6-6 Write a equation for the decomposition of lead (II) oxide by heat.

(b) Write a balanced equation for the decomposition (by electricity) of water

This is also a decomposition reaction since it has only ONE reactant, but you would not be able to predict the products, so I will give them to you. (c) Write a balanced equation for the decomposition (by heat) of lithium chlorate into lithium chloride and oxygen gas

(3) SINGLE REPLACEMENT - characterized by having an element and a compound as reactants and the products will be another element and another compound. There are really two types of single replacement reactions--one in which the positive ion of the compound is replaced by the “free” element and one in which the negative ion of the compound is replaced by the “free” element.; LIKE A LOVE TRIANGLE; A + BC →AC + B or in other cases XY + Z → XZ + Y (Hint: it does not matter whether the element or the compound is written first, both on the reactant side and on the product side) Example 6-7 Write a balanced equation for magnesium is added to a solution of tin (IV) nitrate

(b) lithium is added to a solution of barium hydroxide

(c) magnesium chloride and hydrogen gas are the PRODUCTS of this reaction

(d) liquid bromine is poured into a solution of sodium iodide.

(e) fluorine gas bubbles through a solution of strontium bromide

(4) DOUBLE REPLACEMENT - characterized by having two compounds as reactants and two different compounds will be the products; LIKE DO-SI-DO AND CHANGE PARTNERS. (Hint: it does not matter which compound is written first on both the product and the reactant sides of the equation) Example 6-8

Write a balanced equation for solutions of barium chlorate and silver nitrate being mixed

(b) Write a balanced equation for the reaction of potassium cyanide solution plus tin (II) fluoride crystals

(c) calcium phosphate and aluminum bromide are the PRODUCTS of this reaction

(d) hydrochloric acid is mixed with a solution of calcium hydroxide

(5) COMBUSTION - literally means "burning", but we will take it to mean that a hydrocarbon or an oxyhydrocarbon reacts with oxygen. The products are always the same—carbon dioxide and water, no matter which pure hydrocarbon or oxyhydrocarbon is burned. REMEMBER TO ALWAYS WRITE OXYGEN AS A REACTANT IN A COMBUSTION REACTION. Example 6-9 Write a balanced equation for the combustion of propane.

(b) Write a balanced equation for the burning of octane

(c) What is the balanced equation when methyl alcohol (CH3OH) is burned?

(d) cyclobutane reacts with oxygen

We have been assuming that if you can write an equation, IT WILL OCCUR, and this is not necessarily true. It is difficult to predict whether equations will actually take place or not, so we will assume that ALL REACTIONS EXCEPT SINGLE REPLACEMENT REACTIONS WILL TAKE PLACE as written. Whether or not a single replacement reaction will take place will be based on the ACTIVITY SERIES of metals and on metals shown below. It is not necessary to memorize it because it will be provided for you on all quizzes and exams, but you must know how to use it AND YOU MUST REMEMBER TO USE IT!

Activity Series of Metals Li K Ba Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb Sb Cu Hg Ag Pt Au The last five metals will only replace each other if more active than the metal in the compound. The middle set of metals will (1) replace each other if more active (2) will replace the hydrogen from acids. The first six metals will (1) replace each other if more active (2) will replace the hydrogen from acids (3) will replace the hydrogen from water

The Activity Series for Non-metals will be confined to the halogens (Group VIIA) and can be read directly off the Periodic Table

IF A REACTION DOES NOT TAKE PLACE, WRITE THE REACTANTS CORRECTLY, DRAW THE YIELD ARROW AND WRITE "NO REACTION."

Example 6-10 Write a correct balanced equation for each of the following: (a) a piece of zinc is added to a solution of silver nitrate.

(b) tin (II) chloride solution is poured over magnesium

(c) iron (III) nitrate solution plus copper

(d) copper (II) chloride solution plus aluminum

(e) sodium chloride solution plus calcium

(f) silver is dropped into hydrochloric acid

(g) magnesium is added to phosphoric acid

(h) sodium plus water

(i) calcium fluoride plus iodine

(j) water is poured over gold

(k) sulfuric acid plus lithium

To determine which of the physical states should be placed after every reactant and every product, you must know the following solubility rules for ionic compounds in water. SOLUBILITY RULES FOR IONIC COMPOUNDS (1) all compounds whose cation is Group IA or ammonium are soluble, no matter what the anion is. (2) all nitrates, chlorates, perchlorates, acetates and nitrates are soluble, no matter what the cation is. (3) all chlorides, bromides and iodides are soluble except silver, lead (II) and mercury (I) (4) all sulfates are soluble except silver, lead (II) and mercury (I), calcium, strontium and barium (5) everything else in INSOLUBLE unless it begins with Groups IA or ammonium Example 6-11 Predict whether the following ionic compounds are soluble in water: (a) iron (III) nitrate (b) sodium iodide (c) ammonium phosphate (d) mercury (I) chlorate (e) barium sulfate (f) zinc nitrite (g) calcium sulfate (h) lead (II) perchlorate (i) potassium bromide (j) ammonium sulfide (k) cadmium fluoride (l) tin (II) iodide (m) cesium phosphite (n) strontium cyanide (o) silver nitrite (p) silver bromide (q) lead (IV) sulfite (r) copper (I) chloride

HOW TO WRITE NET IONIC EQUATIONS 1. Write a molecular equation for the reaction given. It is not necessary to balance it. 2. Break apart into ions (ionize) all strong acids, strong bases, and soluble salts which are present as reactants unless the context indicates that they are present as a solid crystal. (b) Ionize all strong, strong bases and soluble salts which form as products There are only 7 strong acids, and it is easier to just memorize them and know that everything else is weak. These are HCl, HBr, HI, HNO3, H2SO4, HClO3 and HClO4 The strong bases are the hydroxides of Group I and II A except hydrogen, magnesium and beryllium. All other hydroxides (including ammonium hydroxide) are weak. Soluble salts are broken apart into their ions. The soluble ionic salts are determined from memorizing the solubility rules. 3. Do NOT ionize molecular compounds such as gases, water, or organic molecules. Leave them together as a molecule. 4. Remove all spectators (species which occur unchanged in any way from the reactant side to the product side of the equation). Unchanged means oxidation number as well as physical state. Sometimes nothing is removed; sometimes everything is removed and there is no reaction. 5. That’s it!!

Example 6-12 Write net ionic equations for the following reactions 1. sulfuric acid is poured into a solution of calcium hydroxide

2. a solution of acetic acid is poured into ammonium hydroxide

3. solutions of silver nitrate and hydrochloric acid are mixed.

4. phosphoric acid solution is mixed with barium hydroxide solution

5. lead (II) acetate solution is stirred into sodium chlorate solution

6. hydrochloric acid is poured over sodium carbonate crystals

7. acetic acid acid neutralizes a solution of potassium hydroxide.

8. solutions of ammonium chloride and sodium hydroxide are mixed

9. solid potassium chlorate (when heated) decomposes into potassium chloride and oxygen gas.

10. sodium chloride solution is mixed with mercury (II) acetate solution

11. sulfurous acid (H2SO3) is poured into calcium chloride solution

Important Dates for this Unit Mon, Jan 26 Tue, Jan 27 Wed, Jan 28 Fri, Jan 30 Mon, Feb 2

Quiz 1: Practice 1 Quiz 2: Practice 2 Quiz 3: Practice 3 Quiz 4: Practice 4 Quiz 5: Practice 5 & 6

Mon, Feb 2

Review Session (4:15-6:30)

Tue, Feb 3

Exam