Unit 4 – Environmental Chemistry

Acids and Bases

Definitions  There are many different definitions of what makes acids and bases  We will be using the Arrhenius Definition

Arrhenius Acid  Produces a hydrogen ion, H + ion, or hydronium ion (H 3 O + ) when dissolved in water. The general formula is:  HA is a polar covalent molecule that contains a proton (H + ) that easily dissociates and bonds with a water molecule.  H = Hydrogen  A = Anything that is negatively charged HA (aq) + H 2 O (l)  A - (aq) + H 3 O + (aq)

Examples of Arrhenius Acids NameFormula Reaction in Water (Dissociation Reaction) Hydrofluoric AcidHF (aq) HF (aq) + H 2 O (l)  F - (aq) + H 3 O + (aq) Hydrochloric AcidHCl (aq) Hydrobromic Acid Hydroiodic Acid

Examples of Arrhenius Acids – Polyatomic Ions NameFormula Reaction in Water (Dissociation Reaction) Nitric AcidHNO 3(aq) HNO 3(aq) + H 2 O (l)  NO 3 - (aq) + H 3 O + (aq) Carbonic Acid Sulfuric AcidH 2 SO 4(aq) Phosphoric Acid

Arrhenius Base  Produces a hydroxide ion (OH - ) when dissolve in water. There are two ways this can happen 1. A hydroxide ion dissociates from an ionic compound: BOH (aq)  B + (aq) + OH - (aq) 2. A covalent molecule WITH A LONE PAIR (i.e. a Nitrogen atom) steals a proton from a water molecule B (aq) + H 2 O (l)  BH + (aq) + OH - (aq)

Examples of Bases – Type 1 (Ionic Hydroxide) NameFormulaReaction in Water Lithium HydroxideLiOHLiOH (s)  Li + (aq) + OH - (aq) Sodium HydroxideNaOH Magnesium HydroxideMg(OH) 2 Beryllium Hydroxide Calcium Hydroxide

Examples of Bases – Type 2 (Nitrogenous) NameFormulaReaction in Water AmmoniaNH 3 NH 3 + H 2 O  NH 4 + (aq) + OH - (aq) Hydrazine (dinitrogen tetrahydride) N2H4N2H4

The pH Scale

 The pH of a solution is a measure of how acidic or how basic something  The pH scale is a scale ranging from 0-14, 0 being the most acidic and 14 being the most basic.  If a solution is neither acidic nor basic, it is said to be neutral. Neutral solutions have a pH of 7. Pure water has a pH of 7

Water  Water is not strictly H 2 O molecules, it also contains H 3 O + and OH - ions at low concentrations due to ionization H 2 O + H 2 O  H 3 O + + OH -  Pure water has these ions in equal concentrations of: [H 3 O + ] = [OH - ] = 1×10 -7 mol/L  pH (power of hydronium ions) is a measure of H 3 O + ions within a solution.

 pH is a (negative) logarithmic scale… each number differs by a power of a 10  That is: pH = 1 is 1000 times more acid than pH 4

Strong and Weak Acids

 The strength of an acid referes to its ability to dissociate in a solution.\  Consider: HCl  Write the dissociation reaction  In this case, 100% of the HCl will dissociate in water. The concentration of H 3 O + ions will be equal to the initial amount of HCl present  There is no HCl left at the end of the reaction

 Some strong acids:  HCl  HNO 3  H 2 SO 4  HBr  HI  HClO 4

 BUT, some acids (like acetic acid, vinegar) do not dissociate very well (i.e. most CARBOXYLIC ACIDS). RXN:  In acetic acid the dissociation of acetic acid is only about 0.5%  Other weak acids include HNO, HCN, HF

 In a similar fashion, there are strong and weak bases  Some strong bases:  LiOH, NaOH, KOH, Ca(OH) 2, Ba(OH) 2  Some weak bases:  Include all amines (organic bases)  NH 3, CH 3 NH 2, (CH 3 ) 2 NH

Neutralization Reactions

Acid-Base Indicators  An indicator is a substance that changes colour depending on whether it is in an acid or a base. You only need a drop for dramatic colour changes

Acid-Base Neutralization  Neutralization reactions are a type of double displacement reaction Acid + Base  Salt + Water Example: Hydrochloric Acid and Potassium Hydroxide Neutralization reactions are important for many chemical reactions, including in environmental chemistry and throughout human systems

Acid Rain

 Normal rain is slightly acidic (pH = 5.6) due to natural levels of CO 2 in the air  CO 2 reacts with water to yield dilute carbonic acid  Normal occurrences of CO 2 include: respiration, forest fires, volcanic eruptions  Nitrogen oxides from lightning strikes and plant decay also decrease pH by creating nitric acid (strong) and nitrous acid (weak) when they react with water  Sulfur oxides from volcanic eruptions also make sulfuric acid (strong) and sulfurous acid (weak) when they react with water

 Acid precipitation: any form of natural precipitation (rain, snow, etc.) that has an unusually high acidity (pH < 5.6)  Acid Deposition: acid forming pollutants, including acid precipitation as well as dust, and other dry particulate matter  Acid rain is harmful to the environment: makes waters acidic and cause them to absorb aluminum that makes its way from soil into lakes and streams; damages trees and wildlife; damages building materials (particularly concrete) and sculptures

Hard vs. Soft Water

Drinking Water  Undesirable materials need to be removed from water before it is fit for consumption  Colloidal clay, microscopic organisms, chemicals which cause tastes or odours, and acidic substances  Large filtration process: sendimentation, sand filter beds, chlorine to kill bacteria, small amounts of sulfur dioxide, aluminum sulphate, carbon slurries, and lime slurries

Hard and Soft Water  Compounds dissolve in water with a variety of consequences  Water which readily gives a lather with soap (not detergents) is described as soft water  Dissolved substances make water hard  Wastes soap a causes soap scum – insoluble calcium and magnesium salts formed by a reaction of soap molecules and calcium and magnesium ions  Usually from chalk and limestone (naturally); or Epsom salts (magnesium sulphates)  Hard water can be made soft by removing the dissolved calcium and magnesium ions (precipitation or other types of reactions); also sometimes through boiling  Hard water is not always bad, contains dissolved calcium, etc., which is useful in bone development

Plastics and the Environment

Atmospheric Pollutants