2 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.
3 Energy and Reactions Energy must be added to break bonds. Energy is released when bonds are formed.Chemical energy is CONSERVED in chemical reactions.
4 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 energyMore energy is absorbed to break the bonds in the reactants, than energy is released to bonds form in the products.
5 Chemical Equations 2H2 + O2 —> 2H2O Chemical equations are used to represent or describe chemical reactions.For example when hydrogen (H2) burns, it reacts with oxygen (O2) in the air to form water.We write the chemical equation for this reaction as follows:2H2 + O2 —> 2H2O
6 Chemical Equations An equation shows… Formulas of reactants Formulas of productsMolar ratios of all compounds in the reaction.
7 Chemical Equations 2H2 + O2 → 2H2O mole ratio = 2:1:2 Here, we read the (+) sign as “reacts with” and the arrow (—>) as “produces” or “yields”.2H2 + O2 → 2H2Omole ratio = 2:1:2ReactantsProducts
8 To show physical states of each substance: (s) or solid(l) liquid(g) or gas(aq) aqueous (dissolved in water)
9 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) + O2(g) Fe2O3(s) (unbalanced)
10 Coefficients & Subscripts COEFFICIENTS: numbers in front of reactant or product that represents the number of molesSUBSCRIPTS: lower, smaller numbers that define the formula of a compound2H2SO4CoefficientSubscript
11 H2O One molecule of water 2H2O Two molecules of waterH2O2 One molecule of hydrogen peroxide
12 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.CH4 + O2 —> H2O + CO2 UnbalancedCH4 + 2O2 —> 2H2O + CO2 Balanced
13 In order to balance… Reactants Products Write correct formulas for all reactants and productsCount the number of atoms of each element in reactants & products.Balance one at a time using coefficients.Check for balanceAre the coefficients in the lowest possible ratio?
14 Balancing EquationsNOTE: When balancing equations, you may change coefficients as much as you need to, but you may never change subscripts because you can’t change what substances are involved.
16 Sample Problem 1 H2O H2 + O2 2H2O 2H2 + O2 Water is decomposed (broken down) to form the gaseous products hydrogen (H2) and oxygen (O2). Write the balanced equation.H2O H2 + O22H + 1O 2H + 2OO is not balanced2H2O 2H2 + O24H + 2O 4H + 2OThe equation is balanced!
17 Sample Problem 2 Cl2 + KBr KCl + Br2 Cl2 + 2KBr 2KCl + Br2 Chlorine gas (Cl2) reacts with potassium bromide (KBr) to form potassium chloride and bromine (Br2). Write the balanced equation.Cl2 + KBr KCl + Br22Cl + 1K + 1Br 1Cl + 1K +2BrCl and Br are not balancedCl2 + 2KBr 2KCl + Br22Cl + 2K + 2Br 2Cl + 2K +2Br The equation is balanced!
18 Balancing equations involves a great deal of “trial and error” at first, but there are some tricks…
19 Na + H2O —> NaOH + H2 Let’s start with the even number two! For example…..Sodium metal reacts with water to produce sodium hydroxide and hydrogen gas.Na + H2O —> NaOH + H2Note 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.Let’s start with the even number two!
20 _Na + _H2O —> 2NaOH + _H2Now lets balance sodium; we need a 2 in front of the Na…2Na + _H2O —> 2NaOH + _H2Now consider hydrogen…2Na + 2H2O —> 2NaOH + H2
21 the equation is balanced. 2Na + 2H2O —> 2NaOH + (1)H2Check to see if it balances…2 Na on the left 2 Na on the right4 hydrogen = 4 hydrogen2 oxygen 2 oxygenthe equation is balanced.
29 Types of Chemical Reactions Synthesis / CombinationDecompositionSingle ReplacementDouble ReplacementCombustion
30 Synthesis / Combination Reactions Definition: Reaction where two or more substances react to form a single substance.A + B ABExamples:2K(s) + Cl2(g) 2KCl(s)SO2(g) + H2O(l) H2SO3(aq)
31 Decomposition Reactions Definition: Reaction where a single compound is broken down into two or more products.AB A + BExamples:2H2O(l) 2H2(g) + O2(g)CaCO3 CaO + CO2
32 Single-Replacement Reactions Definition: Reaction where atoms of one element replace atoms of a second element in a compound.XA + B BA + XNote: 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:2AgNO3 + Mg Mg(NO3)2+2AgMg+LiNO3 no reaction
33 Any element will replace any element below it. LiKCaNaMgAlZnFePb(H)*CuHgAgActivity SeriesIncreasing ActivityAny element will replace any element below it.*Metals from Li to Na will replace H from acids and water; from Mg to Pb they will replace H from acids only
34 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.For
35 Double-Replacement Reactions Definition: Reaction that involves an exchange of positive ions between two compounds.XA + BY BA + XYNote: 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)+H2SO4(aq) 2HCN(g)+Na2SO4(aq)Na2S(aq)+Cd(NO3)2(aq) CdS(s)+2NaNO3(aq)
36 CH4+2O2 CO2+2H2O + heat + light Combustion ReactionsDefinition: Reaction where an element or compound reacts with oxygen, often producing energy in the form of heat and light.Examples:CH4+2O2 CO2+2H2O + heat + light2Mg(s)+O2(g) 2MgO(s)
37 Combustion of Hydrocarbons If the reactant is a hydrocarbon, the products are always carbon dioxide and water.CH4 + 2O2 CO H2O
38 Ionic Equations An aqueous solution is ions dissolved in water When a soluble substance is dissolved in water, the substance often breaks into ions. This solution is said to be an aqueous solution.Pb(NO3)2(aq) Pb2+ + 2NO3-NaI(aq) Na+ + I-An aqueous solution is ions dissolved in water
39 Ionic Equations Consider the reaction… Pb(NO3)2(aq) + NaI(aq) PbI2(s) + NaNO3(aq)What is really going on is…Pb2+ + NO3- + Na+ + I- PbI2(s) + Na+ + NO3-Note that the Na+ ion and the NO3- ion are not reacting. They are said to be spectator ions.
40 Net Ionic EquationsIt 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.Pb2+ + NO3- + Na+ + 2I- PbI2(s) + Na+ + NO3-becomes….Pb I- PbI2(s)