Chemical Equations Formulae, names, equations, moles and stoichiometry

Learning objectives  Apply conservation of matter to balance chemical equations  Apply the mole concept to chemical reactions  Use chemical equations to predict molar quantities of reactants and products

The chemical equation aA + bB = cC + dD The Law of Conservation of Matter states that matter is neither created nor destroyed All the atoms on the left must be the same as those on the right Reactant side Product side coefficient ELEMENT or COMPOUND

Chemical book-keeping  The key to writing correct equations is to ask the question, “Have I gained or lost any atoms?”  Another thing is to put down the correct formula for each reactant or product

 In the reaction of hydrogen with oxygen to produce water, the reactants are the elements H 2 and O 2, and the product is H 2 O  Count the atoms: 4 H and 2 O 4 H and 2 O The big number multiplies every atom after it The subscript only multiplies the atom before it

Balance the equations  A method of trial and error method  CH 4 + O 2 = CO 2 + H 2 O

Try these  C 3 H 8 + O 2 = CO 2 + H 2 O  N 2 + H 2 = NH 3

Working with equations  Predict how much product is obtained from given amount of reactant  Predict how much reactant is needed to give required amount of product  Predict how much of one reactant is required to give optimum result with given amount of another reactant

Relating moles, masses and molecules

The roadmap  Equations are in moles, but we measure in grams  Three conversions required: 1.Must convert grams A to moles A using molar mass 2.Use coefficients in equation to get moles B from moles A 3.Convert moles B to grams B using molar mass

Types of problems: Moles A → moles B aA + bB = cC + dD  Mole:mole ratio:  a mol A ≡ b mol B

Moles A → mass B 1.Convert moles A → moles B: Mole:mole ratio: 2.Convert moles B → mass B using molar mass B

Mass A → mass B 1.Mass A → moles A using molar mass A 2.Moles A → moles B using mole:mole ratio 3.Moles B → mass B using molar mass B

Summary of stoichiometry problems  Maximum of three conversions required 1.Must convert grams A to moles A using molar mass 2.Use coefficients in equation to get moles B from moles A 3.Convert moles B to grams B using molar mass  Maximum of three pieces of information required 1.Molar mass of given substance (maybe) 2.Molar mass of target substance (maybe) 3.Balanced chemical equation (always)

 Molar mass Cl 2 = 35.5 x 2 = 71.0 g/mol  Molar mass NaOH = = g/mol

Work this example  CH 4 + 2O 2 = CO 2 + 2H 2 O  What mass of CO 2 is produced by the complete combustion of 16 g of CH 4  Atomic weight H = 1, C = 12, O = g