Chapter 6 Chemical Reactions and Change

In a chemical change, reacting substances form new substances with different compositions and properties; a chemical reaction takes place In a chemical reaction,old bonds are broken and new bonds formed; atoms in the reactants are rearranged to form one or more different substances

A chemical equation gives the chemical formulas of the reactants on the left of the arrow and the products on the right. Since matter in a chemical reaction is conserved, the number of atoms you begin with must equal the number oand type you end up with.

In a balanced chemical equation, there must be the same number of each type of atom on the reactant side and on the product side numbers called coefficients are used in front of one or more formulas. Al + S Al 2 S 3 Not balanced 2Al + 3S Al 2 S 3 Balanced 2Al = 2Al 3S =3S Coefficients that can be changed 2Al + 3S 1Al 2 S 3 Coefficients that can’t be changed 2Al + 3S Al 2 S 3 Balance the following reaction: P 4 (s) + Br 2 (l) = PBr 3 (g) P 4 (s) 4PBr 3 (g) P 4 (s) + 6Br 2 (l) = 4PBr 3 (g) Al(s) + Fe 2 O 3 (s) = Fe(s) + Al 2 O 3 (s) 2Al(s) Al 2 O 3 (s) 2Al(s) + Fe 2 O 3 (s) = 2Fe(s) + Al 2 O 3 (s) Thermite reaction

Burning of natural gas, CH4 CH 4 + O 2 = CO 2 + H 2 O CH 4 + O 2 = CO 2 + 2H 2 O Burning of butane, C 4 H 10 C 4 H 10 + O 2 = CO 2 + H 2 O C 4 H 10 + O 2 = 4CO 2 + 5H 2 O C 4 H /2O 2 = 4CO 2 + 5H 2 O 2C 4 H O 2 = 8CO H 2 O

Magnesium phoshate, Mg 3 (PO 4 ) 2, is insoluble while sodium phosphate, Na 3 PO 4, and magnesium chloride, MgCl 2, are water soluble Na 3 PO 4 (aq) + MgCl 2 (aq) = Mg 3 (PO 4 ) 2 (s) + NaCl(aq) 6 NaCl (aq)

Reactions of some elements with oxygen Mg(s) + O 2 (g) = MgO (s) 2Mg + O 2 = 2MgO Fe(s) + O 2 (g) = Fe 2 O 3 (s) 2Fe + 3/2O 2 = Fe 2 O 3 4Fe + 3O 2 = 2Fe 2 O 3 P 4 (s, white phosphorus) + O 2 (g) = P 4 O 10 (s) P 4 + 5O 2 = P 4 O 10 (s) Reactions of some metals with water Na(s) + H 2 O (l) = NaOH(aq) + H 2 (g) Na + 3/2H 2 O = NaOH + H 2 2Na + 3H 2 O = 2NaOH + 2H 2 K(s) + H 2 O (l) = KOH(aq) + H 2 (g) K + 3/2H 2 O = KOH + H 2 2K + 3H 2 O = 2KOH + 2H 2 Note that when Mg and Fe reacted with oxygen, they lost electrons and became positively charged in the process; the loss of electrons is called oxidation. The oxygen gained electrons and this is called reduction. All elements in their elemental state are neutral and are assigned an oxidation state of zero. Similarly, when Na and K reacted with H 2 O they became positively charged. In this case the hydrogen in water, H +1 was reduced to its elemental state H 2 0

Quantities Definition of terms. 1 ream of paper = 500 sheets 1 case = 24 cans 1 dozen donuts = 12 donuts Definition of a new quantity, the mol 1 mol = 6.03*10 23 Why choose this number? Several lectures ago we discussed the mass of various particles mass of a proton(p)1.672* g mass of a neutron(n)1.674* g mass of an electron(e)9.1* g Consider the element H; it consists of a proton and electron; the mass of the elctrom is small in comparison so lets ignore it. What is the mass of 1 mol of H atoms? The mass of 1 mol of H atoms: (6.03*10 23) *(1.67* g) = 10.0*10 -1 = 1g

What is the mass of one 12 C 6 atom? C = 6 p + 6 n = 12(1.67* ) g mass of a proton(p)1.672* g mass of a neutron(n)1.674* g The mass of 1 mol of 12 C 6 atoms? 12 (6.03*10 23 )* (1.67* g) = 12*10.0*10 -1 = 12g mass of a proton ≈ mass of a neutron ≈ 1.67* g What is the mass of 1 mol of 12 C 6 atoms? The mass of 1 mol of y M x atoms? y (6.03*10 23 )* (1.67* g) = y*10.0*10 -1 = yg where y = atomic mass of M The atomic mass of any element is also numerically equal to the mass in grams of 6.03*10 23 atoms of that element

How many O atoms are in 1 mole of aspirin, C 9 H 8 O 4 ? 4 oxygen atoms/aspirin* 6.03*10 23 = 24*10 23 How many mols of O are in 1 mole of aspirin, C 9 H 8 O 4 ? 4 moles

What is the mass in grams of a mol of aspirin, C 9 H 8 O 4 ? 9*12+8*1+4*16= 180 g/mol 180 is also referred to as the molar mass

To calculate the molar mass simply add up the (atomic masses of each element times the number of times they appear) in the molecule 6*12+12*1+6*16 = 180

The Law of Conservation of Mass indicates that in an ordinary chemical reaction, matter cannot be created or destroyed no change in total mass occurs in a reaction mass of products is equal to mass of reactants ReactantsProducts 2 moles of Ag + 1 mole of S = 1 mole Ag 2 S 2 (107.9 g) + 1(32.1 g) = 1 (247.9 g) Ag = 215.8g S = 32.1g Total = 247.9

Consider the following equation: 4Fe(s) + 3O 2 (g) = 2Fe 2 O 3 (s) 4 atoms of iron react with 3 molecules of oxygen to produce 2 molecules of iron(III) oxide This equation can be read in “moles” by placing the words “moles of” between each coefficient and formula. 4 moles of Fe + 3 moles of O 2 equals 2 moles of Fe 2 O 3

Energy Changes in Chemical Reactions Ever seen water go up hill on it own?

Why do chemical reactions occur? Consider the reaction of paper which is mainly cellulose, (C 6 H 10 O 5 ) n with oxygen Does a reaction occur? What are the products? Why doesn’t paper react spontaneously with oxygen? or does it? The driving force for most chemical reaction is the conversion of potential energy to kinetic energy (ie the release of heat) similar to water going over a dam

Far less common is the situation illustrated in the photo and described by the diagram on the right Identify each of the following reactions as exothermic or endothermic A. N 2 (g) + 3H 2 (g) =2NH 3 (g) + 22 kcal B. CaCO 3 (s) kcal =CaO(s) + CO 2 (g) C. 2SO 2 (g) + O 2 (g) =2SO 3 (g) + heat A and C are exothermic; B is endothermic and it proceeds because the CO 2 is driven off by heating and escapes into the environment