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 sidenumbers called coefficients are used in front of one or more formulas.

Al + S Al2S3 Not balanced

2Al + 3S Al2S3 Balanced 2Al = 2Al

3S = 3S

Coefficients that can be changed

2Al + 3S 1Al2S3

Coefficients that can’t be changed

2Al + 3S Al2S3

Balance the following reaction:

P4(s) + Br2(l) = PBr3(g)

P4(s) 4PBr3(g)

P4(s) + 6Br2(l) = 4PBr3(g)

Al(s) + Fe2O3(s) = Fe(s) + Al2O3(s)

2Al(s) Al2O3(s)

2Al(s) + Fe2O3(s) = 2Fe(s) + Al2O3(s)

Thermite reaction

Burning of natural gas, CH4

CH4 + O2 = CO2 + H2O

CH4 + O2 = CO2 + 2H2O

Burning of butane, C4H10

C4H10 + O2 = CO2 + H2O

C4H10 + O2 = 4CO2 + 5H2O

C4H10 + 13/2O2 = 4CO2 + 5H2O

2C4H10 + 13O2 = 8CO2 + 10H2O

Magnesium phoshate, Mg3(PO4)2, is insoluble while sodium phosphate, Na3PO4, and magnesium chloride, MgCl2, are water soluble

Na3PO4(aq) + MgCl2(aq) = Mg3(PO4)2(s) + NaCl(aq)

6 NaCl (aq)

Reactions of some elements with oxygen

Mg(s) + O2 (g) = MgO (s)

2Mg + O2 = 2MgO

Fe(s) + O2(g) = Fe2O3 (s)

2Fe + 3/2O2 = Fe2O3

4Fe + 3O2 = 2Fe2O3

P4 (s, white phosphorus) + O2 (g) = P4O10(s)

P4 + 5O2 = P4O10 (s)

Reactions of some metals with water

Na(s) + H2O (l) = NaOH(aq) + H2 (g)

Na + 3/2H2O = NaOH + H2

2Na + 3H2O = 2NaOH + 2H2

K(s) + H2O (l) = KOH(aq) + H2 (g)

K + 3/2H2O = KOH + H2

2K + 3H2O = 2KOH + 2H2

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 H2O they became positively charged. In this case the hydrogen in water, H+1 was reduced to its elemental state H2

0

Quantities

Definition of terms.1 ream of paper = 500 sheets1 case = 24 cans1 dozen donuts = 12 donuts

Definition of a new quantity, the mol

1 mol = 6.03*1023

Why choose this number?

Several lectures ago we discussed the mass of various particles

mass of a proton (p) 1.672*10-24 gmass of a neutron (n) 1.674*10-24 gmass of an electron (e) 9.1*10-28 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*1023)*(1.67*10-24g) = 10.0*10-1 = 1g

What is the mass of one 12C6 atom?

C = 6 p + 6 n = 12(1.67*10-24) g

mass of a proton (p) 1.672*10-24 gmass of a neutron (n) 1.674*10-24 g

The mass of 1 mol of 12C6 atoms? 12 (6.03*1023)* (1.67*10-24g) = 12*10.0*10-1 = 12g

mass of a proton ≈ mass of a neutron ≈ 1.67*10-24 g

What is the mass of 1 mol of 12C6 atoms?

The mass of 1 mol of yMx atoms? y (6.03*1023)* (1.67*10-24g) = y*10.0*10-1 = ygwhere y = atomic mass of M

The atomic mass of any element is also numerically equal to the mass in grams of 6.03*1023 atoms of that element

How many O atoms are in 1 mole of aspirin, C9H8O4?

4 oxygen atoms/aspirin* 6.03*1023 = 24*1023

How many mols of O are in 1 mole of aspirin, C9H8O4?

4 moles

What is the mass in grams of a mol

of aspirin, C9H8O4?

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 destroyedno change in total mass occurs in a reactionmass of products is equal to mass of reactants

Reactants Products 2 moles of Ag + 1 mole of S = 1 mole Ag2S

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) + 3O2(g) = 2Fe2O3(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 O2 equals 2 moles of Fe2O3

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, (C6H10O5)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. N2(g) + 3H2(g) = 2NH3(g) + 22 kcalB. CaCO3(s) + 133 kcal = CaO(s) + CO2(g)C. 2SO2(g) + O2(g) = 2SO3(g) + heat

A and C are exothermic; B is endothermic and it proceeds because the CO2 is driven off by heating and escapes into the environment