CHEMICAL REACTIONS. Discuss breaking/forming bonds 10/29/2012. Products Reactants

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1 CHEMICAL REACTIONS - Page 321 Products Reactants Reactants: Zn + I 2 Product: Zn I 2 Discuss breaking/forming bonds Chemical Reaction or Change process in which one or more substances are converted into one or more new substances. Reactant - starting substance; on the left side of the reaction Product substance that is produced; on the right side of the reaction Reactants --> Products --> is read yields In a chemical reaction the reactant's bonds break and the atoms are rearranged to form the products. Chemical reactions may occur spontaneously which means without adding energy or non-spontaneously which means you must add energy for the reaction to occur. Word Equation a chemical equation using ( formulas words (not Ex. Calcium reacts with oxygen to produce calcium oxide Formula Equation a chemical reaction ( names using formulas (not Ex. Convert the above word equation to a formula equation: 1

2 WORD EQN: Silver nitrate reacts with copper to form copper (II) nitrate and silver FORMULA EQN: WORD EQN: hydrogen perioxide (H 2 O 2 ) will decompose into water and oxygen FORMULA EQN: BALANCING EQUATIONS: Law of Conservation of Matter Atoms are neither created nor destroyed in any chemical or physical process. So...the number of atoms you start with (reactants) will be the same number of atoms you end up with (products)...the only difference is the way in which they are all bonded together. Coefficients whole numbers placed in front of a formula to indicate how many units of that formula there are. Balancing Examples: Chemical Equations Their Job: Depict the kind of reactants and products and their relative amounts in a reaction. ( s ) 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 The numbers in the front are called stoichiometric coefficients. The letters (s), (g), and (l) are the physical states of compounds. 2

3 Introduction Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction Parts of a Reaction Equation Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A + sign separates molecules on the same side The arrow is read as yields Example C + O 2 CO 2 This reads carbon plus oxygen react to yield carbon dioxide Chemical Equations The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O 2 CO 2, contains the same information as the English sentence but has quantitative meaning as well. Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 Symbols Used in Equations Solid ( l ) Liquid Gas ( aq ) Aqueous solution Catalyst H 2 SO 4 ( ) Escaping gas ( ) Change of temperature Balancing Equations When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ( covalent ionic or sharing for 3

4 Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound. Chemical Equations ( s ) 4 Al(s) + 3 O 2 (g) ---> 2Al 2 O 3 This equation means 4 Al atoms + 3 O 2 molecules ---produces---> 2 molecules of Al 2 O 3 AND/OR 4 moles of Al + 3 moles of O 2 ---produces---> 2 moles of Al 2 O 3 Steps to Balancing Equations There are four basic steps to balancing a chemical equation 1. Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. 4. Check your answer to see if: The numbers of atoms on both sides of the equatio are now balanced. The coefficients are in the lowest possible whole ( reduced ) number ratios. Some Suggestions to Help You Some of Mr. Rapp s Helpful Hints for balancing equations: Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an ( element (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units Balancing Equations ( O(l 2 H 2 (g) + O 2 (g) ---> 2 H 2 What Happened to the Other Oxygen Atom????? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule. Balancing Equations ( s ) 2 Al(s) + 3 Br 2 (l) ---> Al 2 Br 6 4

5 Balancing Equations Balancing Equations Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphate C 3 H 8 (g) + O 2 (g) ---- > ( O(g CO 2 (g) + H 2 B 4 H 10 (g) + O 2 (g) ---- > B 2 O 3 (g) + H 2 O(g) > Na 3 PO 4 + Fe 2 O Na 2 O + FePO 4 1. Synthesis two or more reactants come together to form a single product. 2. Decomposition a single reactant is broken down into two or more products. 5

6 3. Single Replacement an uncombined element displaces an element that is part of a compound. 4. Double Replacement atoms or ions from two different compounds replace each other. 5. Combustion when a hydrocarbon burns in oxygen to produce carbon dioxide and water. Sometimes the physical state of reactants and products are indicated as follows: (s) solid (l) liquid (g) gas ( water (aq) aqueous (means dissolved in Sometimes reaction specifications or conditions are written above the arrow. Examples: 6

7 1. Synthesis (you will only be responsible for ( synthesis ionic Ex1 2. Decomposition (you will only be ( decomposition responsible for elemental Ex. 1 Ex 2 Ex. 2 Ex 3 Ex Single Replacement (you will only be ( replacement responsible for metal Use the activity series to decide whether or not the single replacement reaction will occur or not. Metals will only replace metals below them on the activity series. Ex. 1 Ex. 4 Ex. 5 Ex. 6 Ex. 2 Ex. 7 Ex Double Replacement A double replacement reaction occurs between two ionic compounds that are dissolved in water aka ionic solution. Remember that ionic compounds break into ions when dissolved in water. Double replacement reactions are sometimes called precipitation reactions because a precipitate is formed. A double replacement reaction occurs ONLY IF one of the products is insoluble. The insoluble product is the precipitate because it falls out of solution and makes the solution cloudy and will eventually settle on the bottom. Use the solubility rules to determine whether or not the ionic products are soluble or insoluble. 7

8 Ex. 1 Ex. 2 Types: Synthesis Example C + O 2 C + O O O C O Ex. 3 Ex. 4 A + B AB Types: Decomposition NaCl Types: Decomposition Example 2HgO Cl Na Cl + Na O Hg O Hg Hg Hg + O O AB A + B AB A + B Types: Single displacement Zn + CuCl 2 Types: Double displacement MgO + CaS Cl Cu Cl + Zn Cl Zn Cl + Cu Mg O + Ca S Mg S + Ca O AB + C AC + B AB + CD AD + CB 8

9 #1 - Combination Reactions There are millions of reactions. Can t remember them all Fall into several categories. We will learn 5 major types. Will be able to predict the products. For some, we will be able to predict whether they will happen at all. Will recognize them by the reactants Combine - put together 2 substances combine to make one compound. Ca +O 2 CaO SO 3 + H 2 O H 2 SO 4 We can predict the products if they are two elements. Mg + N 2 Write and balance Ca + Cl 2 Fe + O 2 iron (II) oxide Al + O 2 Remember that the first step is to write the correct formulas Then balance by using coefficients only #2 - Decomposition Reactions decompose = fall apart one reactant falls apart into two or more elements or compounds. NaCl --> Na + Cl 2 CaCO 3 --> CaO + CO 2 Note that energy is usually required to decompose #2 - Decomposition Reactions Can predict the products if it is a binary compound Made up of only two elements Falls apart into its elements H 2 O --> HgO --> #2 - Decomposition Reactions If the compound has more than two elements you must be given one of the products The other product will be from the missing pieces NiCO 3 CO 2 +? H 2 CO 3 (aq) CO 2 +? 9

10 #3 - Single Replacement One element replaces another Reactants must be an element and a compound. Products will be a different element and a different compound. Na + KCl K + NaCl F 2 + LiCl LiF + Cl 2 #3 Single Replacement Metals replace other metals (and ( hydrogen K + AlN Zn + HCl Think of water as HOH Metals replace one of the H, combine with hydroxide. Na + HOH #3 Single Replacement We can tell whether a reaction will happen Some chemicals are more active than others More active replaces less active There is a list on page called the Activity Series of Metals Higher on the list replaces lower. #3 Single Replacement Note the * concerning Hydrogen H can be replaced in acids by everything higher Li, K, Ba, Ca, & Na replace H from acids and water Fe + CuSO 4 Pb + KCl Al + HCl #3 - Single Replacement What does it mean that Hg and Ag are on the bottom of the list? Nonmetals can replace other nonmetals Limited to F 2, Cl 2, Br 2, I 2 (halogens) Higher replaces lower. F 2 + HCl Br 2 + KCl A double replacement reaction occurs between TWO IONIC compounds that are DISSOLVED in WATER...aka IONIC SOLUTIONS. Remember that IONIC compounds break into IONS when dissolved in water. Double replacement reactions are sometimes called PRECIPITATION reactions because a PRECIPITATE is formed. A double replacement reaction occurs ONLY IF one of the products is INSOLUBLE. 10

11 The INSOLUBLE product is the PRECIPITATE because it FALLS out of solution and makes the solution CLOUDY and will eventually SETTLE on the bottom. Use the SOLUBILITY RULES to determine whether or not the ionic products are SOLUBLE or INSOLUBLE. #4 - Double Replacement Two things replace each other. Reactants must be two ionic compounds or acids. Usually in aqueous solution NaOH + FeCl 3 The positive ions change place. NaOH + FeCl 3 Fe +3 OH - + Na +1 Cl -1 NaOH + FeCl 3 Fe(OH) 3 + NaCl #4 - Double Replacement Complete and balance Has certain driving forces Will only happen if one of the products: doesn t dissolve in water and forms a solid (a precipitate ), or is a gas that bubbles out, or is a covalent compound (usually water). assume all of the following reactions take place: CaCl 2 + NaOH CuCl 2 + K 2 S KOH + Fe(NO 3 ) 3 (NH 4 ) 2 SO 4 + BaF 2 How to recognize which type Look at the reactants: E + E = Combination C = Decomposition E + C C + C = Single replacement = Double replacement Examples H 2 + O 2 H 2 O Zn + H 2 SO 4 HgO KBr +Cl 2 AgNO 3 + NaCl Mg(OH) 2 + H 2 SO 3 11

12 #5 - Combustion Means add oxygen A compound composed of only C, H, and maybe O is reacted with oxygen If the combustion is complete, the products will be CO 2 and H 2 O. If the combustion is incomplete, the products will be CO (possibly just C) and H 2 O. Examples ( complete C 4 H 10 + O 2 (assume ( incomplete ) C 4 H 10 + O 2 ( complete ) C 6 H 12 O 6 + O 2 ( incomplete ) C 8 H 8 +O 2 An equation... Reactions Describes a reaction Must be balanced in order to follow the Law of Conservation of Mass Can only be balanced by changing the coefficients. Has special symbols to indicate physical state, and if a catalyst or energy is required. Come in 5 major types. Can tell what type they are by the reactants. Single Replacement happens based on the activity series Double Replacement happens if the product is a solid, water, or a gas. 12