III. EL DERECHO COMUNITARIO Y LOS LÍMITES MEDIOAMBIENTALES
3. Las prohibiciones ambientales en la legislación comunitaria
3.1. Normas sobre la contaminación de las aguas
• Processes in which substances are changed into one or more new substances
• Represented by chemical equations:
Reactants Products 2H2 + 1 O2 2H2O
2 molecules + 1 molecule 2 molecules 2 moles + 1 mole 2 moles
4.04 g + 32.00 g 36.04 g 36.04 g reactants 36.04 g products
FOLLOWS THE LAW OF CONSERVATION OF MASS Balancing Chemical Equations
Some important points:
• Use correct chemical formulas
• Adjust only the coefficients, NOT the subscripts
•
Balance elemental forms ( e.g. Ar, Cu, Na, O2, N2, I2, S8…) and H and O last.• Use the simplest possible set of whole no. coefficients
Stoichiometry- The quantitative study of reactants and products in a chemical reaction
Mole Method - The stoichiometric coefficients in a chemical equation can be interpreted as the number of moles of each substance.
Steps:
• Write correct chemical formulas and balance the equation.
• Convert the quantities into moles.
• Use the mole ratios to calculate moles of the required substance.
• Convert calculated moles to whatever units required.
Three types of calculation:
he Mole
In 1971, at the 14th meeting of the General Conference of Weights and Measures, scientists agreed to adopt the mole as the unit of an amount of substance
The mole (abbreviated mol) is the amount of substance that contains the same number of elementary particles as the number of atoms in exactly 12 grams of C-12.
Ways of expressing the mole:
1. by number of particles (use Avogrado’s number, 6.02 x 1023 particles per mole) 2. by mass (use molar mass)
3. by volume (use molar volume, 22.4 L at STP) Interconversions
Mass Mole No. of particles
÷ MM x MM
x 6.02 x 1023
÷ 6.02 x 1023
The molar mass is the mass in grams of 1 mole of a substance. The molar mass is numerically equal to the atomic mass (or atomic weight) of an atom or the formula mass of a molecule, a compound or a polyatomic ion.
Formula and Composition
The percentage composition of a compound is a list of the percentages by weight of the elements in the compound. The percentage by weight of an element in a compound is numerically equal to the number of grams of the element that are present in 100 g of the compound
Ex. What is the percentage composition of quick lime, CaO?
Ans. 71.5% Ca, 28.5% O
Empirical Formula- is the formula with lowest possible whole number subscripts to represent the composition of the compound. It can be determined from the % composition data.
Ex. Barium carbonate, a white powder used in paints, enamels and ceramic, has the following composition: Ba, 69.58%; C, 6.090% and O, 24.03%. Determine its empirical formula
Ans. BaCO3
Molecular Formula- gives the actual composition or the actual number of atoms of each element present in one molecule or one formula unit of the compound
Ex. Molecular formula of glucose: C6H12O6
Empirical Formula of glucose: CH2O Stoichiometry of Reactions
Chemical Stoichiometry- is the quantitative relationship of the amounts of reactants used and amounts of products formed in a reaction. This mass relationship is expressed in the balanced equation for the reaction.
Percent yield- portion of the theoretical yield of product that is actually obtained in the reaction
%yield= (actual amt of product obtained/ theoretical amt) x 100
Theoretical Yield - the amount of product that would result if all the LR reacted.
- Maximum obtainable yield
Actual Yield - The amount of product actually obtained from a reaction - Always less than theoretical yield
Limiting reactant- reactant that is completely consumed in the reaction. It also determines the amount of products that can be formed.
Excess reactant- reactant that is not completely used up in a chemical reaction TIES THAT CHEMISTRY BIND
Chemical Bonds- net forces of attractions that hold atoms together Properties:
Bond energy – amount of energy that must be supplied to separate the atoms that make a bond
Bond length – distance between 2 nuclei of 2 covalently bonded atoms
Bond order – number of bonds between atoms Types of Chemical Bonds
a. covalent bond- pair of electrons that is shared by two atoms of nonmetals; represented by Lewis structure or electron dot formula
Types of Covalent Bonds:
Single bond - two atoms held by one e- pair Double bond – two atoms held by 2 e- pairs Triple bond – two atoms held by 3 e- pairs
• Higher Bond order, shorter Bond length, higher Bond energy
Polar covalent bond – one atom is more electronegative than the other atom; unequal sharing of electrons; the more electronegative atom is partially negative and the less electronegative atom is partially positive.
Nonpolar covalent bond – equal sharing of electrons
Coordinate Covalent Bond – the electrons being shared comes from a single atom
b. ionic bond or electrovalent bond– It is the transefer of electrons from a metal to a nonmetal, i.e., the metal loses an electron while the nonmetal gains an electron converting them intro charged ions.
- attraction between cations and anions
c. metallic bond- the attraction between the cations in the lattice and the “sea of delocalized electrons” moving within the lattice
Lewis Structure-one or a combination of Lewis symbols to represent a single atom (neutral or charged), a molecule or a polyatomic ion.
- based on Octet Rule
Octet rule- the observed tendency of atoms of the main block elements to lose, gain or share electrons in order to acquire an octet of electrons in their outermost main energy level It is more appropriately called Noble Gas Rule
Electron Pairs could either be
Lone pairs – pairs of electrons localized on an atom
Bonding pairs – those found in the space between the atoms Drawing Lewis Structures
1. Sum the valence electrons from all atoms (total # of e-’s)
Total electrons = sum of the valence electrons of all atoms – charge 2. Determine the central atom and draw the skeletal structure.
Cental atom is the most metallic atom or the least electronegative.
3. Use a pair of e-’s to form a bond between each pair of bound atoms.
4. Distribute remaining electrons to the terminal atoms to satisfy octet.
5. If there are still available electrons, put them on the central atom to satisfy octet.
6. If the central atom does not satisfy octet, move electron pair (lone pair) from the terminal atoms towards the central atom to form multiple bonds.
STRICT FOLLOWERS of OCTET: C, N, O, F and H (2 electrons)
7. Check the Lewis structure. H and F are always terminal atoms and joined by a single bond.
HYPERVALENT ATOM – atom that could accommodate more than the octet due to low-lying d-orbitals.
RESONANCE - The use of two or more Lewis Structures to represent a particular molecule or ion.
- Can be written for molecules/ions having a double or a triple bond and single bond(s).
Resonance Structures- one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure.
- The true structure is the average or the “hybrid” of the resonance structures.
FORMAL CHARGE- Used to evaluate non-equivalent Lewis structures (different from resonance structures)
= no. of valence electron in the free state – no. of nonbonding electrons – no. of bonds