Atoms are neither created nor destroyed. Chemical Reactions So, molecules are collections of atoms linked together (or bonded) in a specific way. In a chemical reaction these bonds are the only things that change – in other words, atoms simply change their partners. Atoms are neither created nor destroyed. We write chemical reactions in the following way: 2 H2 (g)+ O2(g) → 2 H2O(g) reactants → products The equation must balance – i.e., there must be the same number of atoms of each element before and after the arrow.

Balancing Reactions For example, look at the combustion of butane: Most simple reactions can be balanced “by inspection” (which may, of course, involve the “trial and error” approach). For example, look at the combustion of butane: C4H10 + O2 → CO2 + H2O Balance C 1 C4H10 + O2 → 4 CO2 + H2O Balance H 1 C4H10 + O2 → 4 CO2 + 5 H2O Balance O 1 C4H10 +13/2 O2 → 4 CO2 + 5 H2O Finally, 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O

The Stoichiometric Equation The final, balanced equation, in this case: 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O is known as the stoichiometric equation. It contains important information on how many moles of each reactant react to yield how many moles of each product. Recall that a mole is just a number collection. So, this balanced chemical reaction also tells us how many molecules of each reactant react to yield how many molecules of each product.

Balancing Complex Equations As well as the inspection method, there are mathematical procedures by which almost any equation can be balanced (providing you have the will power). Such a procedure is described in Oxtoby but it is sufficient for you know that these methods exist.

More on Chemical Equations “yields” 2 C4H10 (g) + 13 O2 (g) → 8 CO2 (g) + 10 H2O (g) Stoichiometric Coefficients State of Matter (g) Gaseous (s) Solid (l) Liquid (aq) Aqueous Solution Stanford Chemistry Summer Session 2004 Chem 31

Reminder: Molecules are collections of atoms bonded in specific ratios. 1 mole is 6.022 x 1023 atoms (or molecules or ions, etc.,) The molar mass of a substance is the mass of one mole. An empirical formula for a molecule gives the simplest ratio of atoms of different elements. The molecular formula gives the exact number of atoms of each element in the molecule. The relative molecular mass of a molecule (RMM) is the sum of the relative atomic masses (RAM) of its constituent atoms. In a chemical reaction, bonds are broken and new bonds are formed – but atoms are neither created nor destroyed. A stoichiometric equation gives the exact number of molecules (moles) reacting or produced in a reaction.

More on Chemical Reactions Consider the combustion of ethanol: C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (l) How much CO2 & H2O are produced upon burning 100 g ethanol? Each mole of ethanol burned yields 2 mol CO2 and 3 mol H2O 100 g ethanol is 100 g / RMMethanol RMMethanol =[ (2 x 12) + (6 x 1) + 16] g mol-1 = 46 g mol-1 Hence 100 g of ethanol ≡ 2.17 mol of ethanol We produce 4.34 mol CO2 (191 g) and 6.52 mol H2O (117 g)

C2H5OH (g)+ 3 O2 (g) → 2 CO2 (g) + 3 H2O (l) But….. ..the above assumes that we actually have enough O2 to combust all 100 g of the ethanol. How much CO2 is produced by burning 100 g ethanol in 100 g O2? The 2.17 moles ethanol require 6.51 moles O2 to fully combust. 100 g O2 is 100 g / (2 x 16 g mol-1) = 3.125 mol O2 Hence only 3.125 / 3 = 1.04 mol of ethanol can fully burn (yielding 2.08 mol, or 91.52 g of CO2). The yield of CO2 – indeed the extent to which the reaction can proceed - is determined by the amount of O2 present. The O2 is called the limiting reagent. C2H5OH (g)+ 3 O2 (g) → 2 CO2 (g) + 3 H2O (l)