Chapter 11 Chemical Reactions

Chemical Reactions

1Empirical formulaThe simplest ratio between the atoms.The formulas for ionic compounds must be written as empirical formulas.For molecular substances sometimes the ratio between the atoms in a molecule is not in the simplest ratio. When the molecular and empirical formula are different then the molecular formula is a multiple of the empirical formula.DO NOW Which compound contains a greater % of Oxygen. Justify your answer with a calculation Li2O, CaO, PbO2

All chemical reactions have two parts:Reactants = the substances you start withProducts = the substances you end up withThe reactants will turn into the products.Reactants Products4

ReactantsProductsHow to Describe a ReactionA reaction can be described several ways:#1. In a sentence every item is a word Copper reacts with chlorine to form copper (II) chloride.#2. In a word equation some symbols usedCopper + chlorine copper (II) chloride#3. In a chemical equation only chemical equations are usedCu + Cl2 CuCl2

6Symbols in Equations (s) after the formula = solid: Fe(s) (g) after the formula = gas: CO2(g) (l) after the formula = liquid: H2O(l)(aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) is a salt water solution

7Symbols used in equationsthe arrow separates the reactants from the products (arrow points to products)Read as: reacts to form or yields or producesThe plus sign + means and used after a product indicates a gas has been produced: H2 used after a product indicates a solid has been produced: PbI28Symbols used in equations double arrow indicates a reversible reaction (more later) shows that heat is supplied to the reaction is used to indicate a catalyst is supplied (in this case, platinum is the catalyst)

9What is a catalyst? A substance that speeds up a reaction, without being changed or used up by the reaction. Enzymes are biological or protein catalysts in your body.10 The Skeleton EquationAll chemical equations are a description of the reaction.A skeleton equation uses formulas and symbols to describe a reactionbut doesnt indicate how many; this means they are NOT balanced11Write a skeleton equation for:Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.

Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water. 12Write a skeleton equation for:Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.FeS(s) + HCl(g) FeCl2(s) + H2S(g) Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

HNO3(aq) + Na2CO3(g) CO2(g) + H2O(l)

13Now, read these equations: Fe(s) + O2(g) Fe2O3(s)

Cu(s) + AgNO3(aq) Ag(s) + Cu(NO3)2(aq)

NO2(g) N2(g) + O2(g)

14ANSWERS TO WORKSHEET3232223132

24. Crystal is a hydrate. Heating removes the water of hydration25 Hydrated crystal is blueAnhydrous crystal is white26 The anhydrous compound is pure. The hydrated crystal contains 50% water so it is more expensive (you are paying for water in the bottle!!!) Balanced Chemical EquationsAccording to the Law of Conservation of Mass: atoms arent created or destroyed in a chemical reaction, they are just rearranged.All the atoms we start with in the reactants we must end up with in the products (meaning: balanced!)A balanced equation has the same number of each element on both sides of the equation.16Rules for balancing:Assemble the correct formulas for all the reactants and products, using + and Count the number of atoms of each type appearing on both sidesBalance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last)Double-Check to make sure it is balanced.17Never change a subscript to balance an equation (You can only change coefficients)If you change the subscript (formula) you are describing a different chemical.H2O is a different compound than H2O2Never put a coefficient in the middle of a formula; they must go only in the front 2NaCl is okay, but Na2Cl is not.Never18Practice Balancing Examples_AgNO3 + _Cu _Cu(NO3)2 + _Ag_Mg + _N2 _Mg3N2 _P + _O2 _P4O10_Na + _H2O _H2 + _NaOH_CH4 + _O2 _CO2 + _H2O19Practice Balancing Examples2AgNO3 + _Cu _Cu(NO3)2 + 2Ag3Mg + _N2 _Mg3N2 4P + 5O2 _P4O102Na + 2H2O _H2 + 2NaOH_CH4 + 2O2 _CO2 + 2H2O20Types of ReactionsThere are probably millions of reactions.We cant remember them all, but luckily they will fall into several categories.We will learn: a) the 5 major types.We will be able to: b) predict the products.For some, we will be able to: c) predict whether or not they will happen at all.How? We recognize them by their reactants21Synthesis ReactionAB+ABAThe general equation is A + B ABalso called Combination Reactions2 substances combine to make one compound #1 Synthesis Reactionsalso called Combination Reactions2 substances combine to make one compound the general equation is : A + B ABCa + O2 CaOelement + elementSO2 + O2 SO3compound + elementCO2 + H2O H2CO3compound + compoundWe can predict the products, especially if the reactants are two elements.Mg + N2 _______Mg3N2 23Synthesis ReactionHCl(g) + NH3(g) NH4Cl(s)

Complete and balance:Ca + Cl2 Fe + O2 (assume iron (II) oxide is the product)Al + O2 Remember that the first step is to write the correct formulas you can still change the subscripts at this point, but not later while balancing!Then balance by changing the coefficients only25The general equation is : AB A + B

A reaction where a more complex molecule breaks down to form two or more simpler productsDecomposition ReactionAB+ABA2NH4NO3(s) 4H2O(g) + 2N2(g) + O2(g) + energyDecomposition Reaction

Timothy McVeigh bombing, 1995

Regular building demolition with ammonium nitrate explosives#2 - Decomposition Reactionsone reactant breaks apart into two or more elements or compounds.the general equation is : AB A + BH2O H2 + O2CaCO3 CaO + CO2CuSO45H2O CuSO4 + 5H2O2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g)Note that energy (heat, sunlight, electricity, etc.) is usually required

28#2 - Decomposition ReactionsWe can predict the products if it is a binary compound (which means it is made up of only two elements)

It breaks apart into the elements:H2OHgO

mercury (II) oxidecinnabar

mercury29Single Displacement ReactionA+BCB+BACAA reaction where an element displaces another element in a compound, producing a new compound and an elementA metal will replace a cation (metal or H)A non-metal will replace an anion (non-metal)A + BC AC + B

Zn(s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)zinc metal and hydrochloric acid react to form zinc chloride and hydrogen gas in this single-displacement reaction.#3 - Single Displacement ReactionsOne element replaces anotherthe reaction follows the form of: compound + element compound + elementReactants must be an element and a compound.Products will be a different element and a different compound.

Na + KCl K + NaCl (cations switched)F2 + LiCl LiF + Cl2 (anions switched) 32#3 Single Displacement ReactionsMetals will replace other metals (and they can also replace hydrogen)Zn(s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)Cu(s) + 2AgNO3 2Ag(s) + Cu(NO3)2

Think of water as: HOHMetals replace the first H, and then combines with the hydroxide (OH).2Na (s) + 2H2O(l) 2 NaOH(aq) + H2(g)

33#3 Single Displacement ReactionsWe can even tell whether or not a single displacement reaction will happen:More active element replaces less active The Activity Series of Metals lists metals (and hydrogen) in order of activity.Elements higher on the list replaces those lower on the list.

34Single Displacement ReactionHg(NO3)2(aq) + 2Ag(s) Hg(s) + 2AgNO3(aq) Will this reaction occur? Both of these reactions do not occur. H2(g) + ZnCl2(aq) 2HCl(aq) + Zn(s) What about this reaction? The Activity Series of Halogens Fluorine Chlorine Bromine IodineHalogens can replace other halogens in compounds, provided they are above the halogen they are trying to replace in the periodic table.2NaCl(s) + F2(g) 2NaF(s) + Cl2(g)MgCl2(s) + Br2(g) ???No Reaction!???Higher ActivityLower Activity#3 Single Replacement Reactions Practice: Fe + CuSO4 Pb + KCl Al + HCl 37Double Displacement ReactionAD + BC AC + BD+BCB+ACAADABDBTwo compounds switch parts to make two new compoundsthe general equation is : AB + CD AD + CBDouble Displacement Reaction

sodium chloride and silver fluoride react to form sodium fluoride and silver chloride in this double displacement reaction#4 - Double Replacement ReactionsTwo compounds switch parts to make two new compoundsthe reaction is: compound + compound compound + compound

NaOH + FeCl3 The positive ions change place.NaOH + FeCl3 Fe+3 OH- + Na+1 Cl-1= NaOH + FeCl3 Fe(OH)3 + NaCl40Double Displacement Reaction

Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)#4 - Double Replacement ReactionsHave certain driving forces, or reasonsWill only happen if one of the products:a) doesnt dissolve in water and forms a solid (a precipitate), or b) is a gas that bubbles out, orc) is a molecular compound (which will usually be water).42Complete and balance:assume all of the following reactions actually take place:CaCl2 + NaOH CuCl2 + K2S KOH + Fe(NO3)3 (NH4)2SO4 + BaF2 NaOH(aq) + HCl(aq) H2O(l) + NaCl(aq)

43How to recognize which type?Look at the reactants:E + E OR C + C =SynthesisC =DecompositionE + C =Single displacementC + C =Double displacement44Practice Examples:H2 + O2 H2O Zn + H2SO4 HgO KBr + Cl2 AgNO3 + NaCl Mg(OH)2 + H2SO3 45Combustion ReactionfuelO2+A reaction of a fuel with oxygen, releasing energy in the form of heat and/or light XyOz + energy Combustion Reaction

Combustion ReactionElement + O2 oxide + energy

2Mg(s) + O2(g) 2MgO(s) + energy

P4(s) + 5O2(g) P4O10(g) + energy#5 Combustion ReactionsCombustion is a fast reaction of a substance with oxygen to make compounds called oxides.the general equation is : fuel + oxygen oxides + energythe three things that must be present for combustion to happen are: fuel oxygen spark / heat49Combustion Reaction Examples:What is the main purpose for which fuels are burned around the world?The following equations show what happens when different carbon-based fuels are burned.C(s) + O2(g) CO2(g) + energyCH4(g) + 2O2(g) CO2(g) + 2H2O + energyethanol CH3CH2OH(l) + O2(g) CO2(g) + 3H2O(l)C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(g) + energy50Combustion of other FuelsSome fuels do not contain carbon.The products are oxides of each element in the fuels.Mg(s) + O2(g) MgO(s)Cu(s) + O2(g) CuO(s)H2(g) + O2(g) H2O(l) P4(s) + 5O2(g) P4O10(g)S(s) + O2(g) SO2(g)

As you see, many combustion reactions may also be classified as synthesis.SUMMARY: An equation...Describes a reactionMust be balanced in order to follow the Law of Conservation of MassCan only be balanced by changing the coefficients.Has special symbols to indicate the physical state, if a catalyst or energy is required, etc. 52ReactionsCome in 5 major types.We can tell what type they are by looking at the reactants.Single Replacement happens based on the Activity SeriesDouble Replacement happens if one product is: 1) a precipitate (an insoluble solid), 2) water (a molecular compound), or 3) a gas.53