Chapter 7 - Stoichiometry

7.1 Chemical Reactions and the Carbon Cycle

Photosynthesis: set of chemical reactions driven by energy from sunlight which use the reactants of carbon dioxide and water and produce various products depending on the organism involved.

Chemical reactions are balanced, meaning the number of atoms of each element are the same on both sides. (g) is for gases, (l) is for liquids and (aq) is for aqueous.

Law of Conservation of Mass: sum of masses of reactants always equals the masses of products

7.2 Writing Balanced Chemical Equations

Combination reactions: two or more reactants combine to form a single product

Combustion reactions: fuel reacts with O2 to release energy

• total number of atoms (in turn moles) must remain constant and identity of atoms does not change, but mole ratio and differ

Four Step Formula for Writing Equations

1. preliminary expression containing a single particle of each reactant and product, including reaction arrows and phase symbols

2. check if expression is balanced

3. choose element that appears in only one reactant and product to balance first

4. choose coefficients for the other substances to make number of atoms on the same of both sides

7.3 Stoichiometric Calculations

Stoichiometry: mole rations of all reactions and products in a chemical reaction

• proper calculations and unit conversions require balanced equations

7.4 Limiting Reactants and Percent Yield

Limiting Reactant: amount of product is limited by the quantity of X initially available

• extra product is described as being “in excess”

• to determine the limiting reactant, we calculate the amount of product form if each reactant were completely consumed for each respective reactant. the reaction that produces less product is the limiting reactant

Theoretical Yield: maximum amount of product that could be obtained from given quantities of reactants

Actual Yield: often less than theoretical yield because reactants form undesirable products or are slow enough to where not all reactants react or reaction reaches equilibrium

Percent Yield: (actual yield)/(theoretical yield) x 100%

7.5 Percent Composition and Empirical Formulas

Percent Composition: percentage by mass of constituent elements of a compound with respect to its total mass

Empirical Formula: lowest whole-number ratio of component elements

• assume we have 100 g of compound and use percent composition to become mass of each element in grams, then convert to moles. divide all values by the smallest mole value to get the mole ratio

7.6 Comparing Empirical and Molecular Formulas

(molecular formula) = (empirical formula)n where n is a positiver integer greater than or equal to one.

• n can be solved for by dividing the molecular mass by the mass of one empirical formula unit

mass spectrometry: determines molecular mass of compounds and measures isotopic abundances

• mass spectrum shows fragments of an ionized molecule according to their mass to charge ratio: peak in molecular-ion spectra is m, the molecular mass of the compound

7.7 Combustion Analysis

Combustion Analysis: complete burning of an unknown substance into known substances whose masses are then used to calculate original composition