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Slide 2 Slide 3 Effects of chemical reactions: Reactants Products Chemical reactions rearrange the atoms in the reactants to form new products. The identities and properties of the products are completely different from that of the reactants! Production of gases and color changes are signs (evidence) of chemical reactions. Slide 4 Energy and Reactions Energy must be added to break bonds. Energy is released when bonds are formed. Chemical energy is CONSERVED in chemical reactions. Slide 5 Exo- vs. Endo- EXOTHERMIC REACTIONS: release energy More energy is released as the products form bonds than energy is absorbed to break the bonds in the reactants. ENDOTHERMIC REACTIONS: absorb energy More energy is absorbed to break the bonds in the reactants, than energy is released to bonds form in the products. Slide 6 Chemical Equations Chemical equations are used to represent or describe chemical reactions. For example when hydrogen (H 2 ) burns, it reacts with oxygen (O 2 ) in the air to form water. We write the chemical equation for this reaction as follows: 2H 2 + O 2 > 2H 2 O Slide 7 Chemical Equations An equation shows Formulas of reactants Formulas of products Molar ratios of all compounds in the reaction. Slide 8 Chemical Equations Here, we read the (+) sign as reacts with and the arrow (>) as produces or yields. 2H 2 + O 2 2H 2 O mole ratio = 2:1:2 ReactantsProducts Slide 9 To show physical states of each substance: (s) or solid (l) liquid (g) or gas (aq) aqueous ( dissolved in water) Slide 10 To show physical states of each substance: Consider the reaction of iron (a solid) with oxygen (a gas) to form iron(III) oxide, or rust (a solid). Fe (s) + O 2 (g) Fe 2 O 3 (s)(unbalanced) Slide 11 Coefficients & Subscripts COEFFICIENTS: numbers in front of reactant or product that represents the number of moles SUBSCRIPTS: lower, smaller numbers that define the formula of a compound 2H 2 SO 4 Coefficient Subscript Slide 12 H 2 OOne molecule of water 2H 2 OTwo molecules of water H 2 O 2 One molecule of hydrogen peroxide Slide 13 During a chemical reaction, atoms are rearranged, not created nor destroyed! Chemical equations must be balanced to show the relative amounts of all substances. Balanced: each side of the equations has the same number of atoms of each element. CH 4 + O 2 > H 2 O + CO 2 Unbalanced CH 4 + 2O 2 > 2H 2 O + CO 2 Balanced Slide 14 In order to balance Reactants Products Write correct formulas for all reactants and products Count the number of atoms of each element in reactants & products. Balance one at a time using coefficients. Check for balance Are the coefficients in the lowest possible ratio? Slide 15 Balancing Equations NOTE: When balancing equations, you may change coefficients as much as you need to, but you may never change subscripts because you cant change what substances are involved. Slide 16 Fe (s) + O 2 (g) Fe 2 O 3 (s)(unbalanced) 4Fe (s) + 3O 2 (g) 2Fe 2 O 3 (s)(balanced) Slide 17 Sample Problem 1 Water is decomposed (broken down) to form the gaseous products hydrogen (H 2 ) and oxygen (O 2 ). Write the balanced equation. H 2 O H 2 + O 2 2H + 1O 2H + 2O O is not balanced 2H 2 O 2H 2 + O 2 4H + 2O The equation is balanced! Slide 18 Sample Problem 2 Chlorine gas (Cl 2 ) reacts with potassium bromide (KBr) to form potassium chloride and bromine (Br 2 ). Write the balanced equation. Cl 2 + KBr KCl + Br 2 2Cl + 1K + 1Br 1Cl + 1K +2Br Cl and Br are not balanced Cl 2 + 2KBr 2KCl + Br 2 2Cl + 2K + 2Br 2Cl + 2K +2Br The equation is balanced! Slide 19 Balancing equations involves a great deal of trial and error at first, but there are some tricks Slide 20 For example.. Sodium metal reacts with water to produce sodium hydroxide and hydrogen gas. Na + H 2 O > NaOH + H 2 Note that on the product side (right side) there are an odd number of hydrogens (3). On the reactant side (left side) there is an even number (2). This implies there must be an even coefficient in front of the NaOH. Lets start with the even number two! Slide 21 _Na + _H 2 O > 2NaOH + _H 2 Now lets balance sodium; we need a 2 in front of the Na 2Na + _H 2 O > 2NaOH + _H 2 Now consider hydrogen 2Na + 2H 2 O > 2NaOH + H 2 Slide 22 2Na + 2H 2 O > 2NaOH + (1)H 2 Check to see if it balances 2 Na on the left2 Na on the right 4 hydrogen 2 + 2 = 4 hydrogen2 oxygen the equation is balanced. Slide 23 Examples CuCl 2 (aq) + Al(s) Cu(s) +AlCl 3 (aq) 3CuCl 2 (aq) + 2Al(s) 3Cu(s) +2AlCl 3 (aq) (3:2:3:2) Slide 24 Examples Propane (C 3 H 8 ) burns in oxygen (O 2 ) to form carbon dioxide and water. C 3 H 8 + O 2 CO 2 + H 2 O Balance C then H then O C 3 H 8 + 5O 2 3CO 2 + 4H 2 O (1:5:3:4) Slide 25 Examples Pentane (C 5 H 12 ) burns in oxygen (O 2 ) to form carbon dioxide and water. C 5 H 12 + O 2 CO 2 + H 2 O Balance C then H then O C 5 H 12 + 8O 2 5CO 2 + 6H 2 O (1:8:5:6) Slide 26 Examples Silver nitrate reacts with copper to produce silver and copper(II) nitrate. AgNO 3 + Cu Ag + Cu(NO 3 ) 2 2AgNO 3 + Cu 2Ag + Cu(NO 3 ) 2 (2:1:2:1) Slide 27 Examples Phosphorus reacts with oxygen gas to produce diphosphorus pentoxide. P + O 2 P 2 O 5 4P + 5O 2 2P 2 O 5 (4:5:2) Slide 28 Examples C 7 H 14 + O 2 CO 2 + H 2 O Balance C then H then O C 7 H 14 + 10O 2 7CO 2 + 7H 2 O 2C 7 H 14 + 21O 2 14CO 2 + 14H 2 O (2:21:14:14) Slide 29 Slide 30 Types of Chemical Reactions Synthesis / Combination Decomposition Single Replacement Double Replacement Combustion Slide 31 Synthesis / Combination Reactions Definition: Reaction where two or more substances react to form a single substance. A + B AB Examples : 2K(s) + Cl 2 (g) 2KCl(s) SO 2 (g) + H 2 O (l) H 2 SO 3 (aq) Slide 32 Decomposition Reactions Definition: Reaction where a single compound is broken down into two or more products. AB A + B Examples : 2H 2 O (l) 2H 2 (g) + O 2 (g) CaCO 3 CaO + CO 2 Slide 33 Single-Replacement Reactions Definition: Reaction where atoms of one element replace atoms of a second element in a compound. XA + B BA + X Note: A reactive metal will replace any metal listed below it in the activity series. Generally, nonmetal replacement is limited to the halogens. The activity of the halogens decreases as you go down Group 7A of the periodic table. See handout. Examples : 2AgNO 3 + Mg Mg(NO 3 ) 2 +2Ag Mg+LiNO 3 no reaction Slide 34 Li K Ca Na Mg Al Zn Fe Pb (H)* Cu Hg Ag Increasing Activity Any element will replace any element below it. Activity Series *Metals from Li to Na will replace H from acids and water; from Mg to Pb they will replace H from acids only Slide 35 For Example Ca + MgO CaO + Mg The Ca will replace the Mg because Ca is more active than Mg. That is to say: Ca is above Mg on the activity list. Slide 36 Double-Replacement Reactions Definition: Reaction that involves an exchange of positive ions between two compounds. XA + BY BA + XY Note: These reactions generally take place between two ionic compounds in aqueous solution, and are often characterized by one of the products coming out of solution in some way. Examples: 2NaCN(aq)+H 2 SO 4 (aq) 2HCN(g)+Na 2 SO 4 (aq) Na 2 S(aq)+Cd(NO 3 ) 2 (aq) CdS(s)+2NaNO 3 (aq) Slide 37 Combustion Reactions Definition: Reaction where an element or compound reacts with oxygen, often producing energy in the form of heat and light. Examples: CH 4 +2O 2 CO 2 +2H 2 O + heat + light 2Mg(s)+O 2 (g) 2MgO(s) Slide 38 Combustion of Hydrocarbons If the reactant is a hydrocarbon, the products are always carbon dioxide and water. CH 4 + 2O 2 CO 2 + 2H 2 O Slide 39 Ionic Equations When a soluble substance is dissolved in water, the substance often breaks into ions. This solution is said to be an aqueous solution. Pb(NO 3 ) 2 (aq) Pb 2+ + 2NO 3 - NaI (aq) Na + + I - An aqueous solution is ions dissolved in water Slide 40 Ionic Equations Consider the reaction Pb(NO 3 ) 2 (aq) + NaI(aq) PbI 2 (s) + NaNO 3 (aq) What is really going on is Pb 2+ + NO 3 - + Na + + I - PbI 2 (s) + Na + + NO 3 - Note that the Na + ion and the NO 3 - ion are not reacting. They are said to be spectator ions. Slide 41 Net Ionic Equations It is often useful to write an equation showing only the species that are actually reacting. This is called a net ionic equation. It does not show the spectator ions. Pb 2+ + NO 3 - + Na + + 2I - PbI 2 (s) + Na + + NO 3 - becomes. Pb 2+ + 2I - PbI 2 (s)