Molecules, Bonding, Acids and Bases

Chemical Bonds molecule A group of atoms bonded to one another form a molecule. compound If the molecule has more than one type of element present it is a compound. Examples: O 2 (two atoms of oxygen) is a molecule H2O (two atoms of Hydrogen and one oxygen) is a molecule and a compound bonds Different types of bonds hold molecules and compounds together.

Chemical Formulas Formulas are short hand way to tell you what is in a molecule They include: The symbol of each element in the molecule The number of atoms of each element in the molecule written as a subscript after the symbol Example: H 2 SO 4 (sulfuric acid) is made up of 2 Hydrogen, one Sulfur and 4 Oxygen atoms

Chemical Formulas Types of Formulas Molecular formulas Only show the symbols of elements and their proportions Examples: Water Methane Carbon dioxide H 2 O CH 4 CO 2

Chemical Formulas Structural formulas show the relative positions and number of bonds between the elements in a compound Examples: Water Methane Carbon dioxide H H O H C H H O H C O

Composition of Matter - Compounds Chemical Bonds A chemical bond is the “glue” that holds the elements together in compounds. The types studied this semester are: Ionic, Covalent, Hydrogen The valence electrons of atoms determine how an element will react and bond

Types of Chemical Bonds – Ionic Bonds Electrons can be lost or gained by atoms to fulfill the Octet Rule. ions When atoms lose or gain electrons, they become charged atoms, or ions – Atoms that lose an electron become Positive – Atoms that gain electrons become Negative Ionic bonds Ionic bonds are formed when positive and negative ions are attracted to one another and bond together forming a molecule Usually between a metal and non metal that are far apart horizontally on the periodic table. salt A salt is a dry solid composed of atoms connected by ionic bonds.

Example: Sodium (one valence electron) Loses the electron Becomes positive (+) Chlorine (seven valence electrons) Gains the electron Becomes negative (-) Opposites Attract! Each outer shell has eight electrons Types of Chemical Bonds – Ionic Bonds

Types of Bonds – Covalent Bonds shared Formed when electrons are shared among elements in a compound. Atoms can share one, two or three electrons based on their needs. (Octet rule) Usually formed between two non metals or a non metal and hydrogen Models or diagrams can be drawn to show the shared electrons

Types of Chemical Bonds

Chemical Equations Short hand way to show how atoms bond and react with each other Short hand way to show how atoms bond and react with each other Equations Equations show the: Reactants - chemicals that are reacting (at the beginning of the equation) Products - chemicals that are made (at the end of the equation) amount of each chemical involved – using coefficients before the molecular formula The reactants and products are separated with an arrow Example: Two Water molecules produce two hydrogen and one oxygen molecule 2H 2 O 2H 2 + O 2 Reactants Products

Chemical Reactions reactants Identify the reactants. products Identify the products. 6 CO H 2 O  C 6 H 12 O O 2 carbon dioxide wateroxygenglucose The chemical reaction for photosynthesis

Water, Acids, Bases and pH Dissociation Dissociation occurs when an ionic compound separates into its ions. Example: Salt (NaCl) breaks apart into Na+ and Cl- Na + Cl -

Acids and Bases When water dissociates, it releases an equal number of ions. – Hydrogen ions (H + ) – Hydroxide ions (OH - ) H – O – H H + + OH -

Water, Acids, Bases and pH Acids Acids will dissociate in water to release a hydrogen ion (H+) Examples: HClH 2 SO 4 HC 3 H 5 O 3 (Notice the H at the front of the formula)

Water, Acids, Bases and pH Bases will dissociate in water and release OH- (hydroxyl) ions They have a low concentration of H+ Examples: NaOH KOH (notice they end in –OH)

pH and the pH scale pH The pH is a mathematical way of indicating the number of H + ions in a solution. The lower the number, the higher the concentration. - pH of 1 is more acidic than a pH of 5 pH scale The pH scale is used to express acidity or basicity (alkalinity).

Water, Acids, Bases and pH Acids have a low pH (less than 7) and bases have a high pH (more than 7) Strong Neutral Strong Acid Base The farther away from neutral, the stronger the acid or base.

Water, Acids, Bases and pH Extremely acidic: lemon=2.5; vinegar=3.0; stomach acid=2.0 Very strongly acid: tomatoes=4.5 Strongly acid: carrots=5.0; asparagus=5.5; cabbage=5.3 Moderately acid: potatoes=5.6 Slightly acid: salmon=6.2; cow's milk=6.5 Neutral: saliva=6.6–7.3; blood=7.3; shrimp=7.0 Slightly alkaline: eggs=7.6–7.8 Moderately alkaline: sea water=8.2; sodium bicarbonate=8.4 Strongly alkaline: borax=9.0 Very strongly alkaline: milk of magnesia=10.5, ammonia=11.1;

Water, Acids, Bases and pH Mixing an acid with a base a chemical reaction will change the pH of the mixture Hydrogen ions will react with the hydroxyl ions and form water H + + OH - HOH If the ions are in equal amounts, the mixture becomes neutral (pH 7) neutralization This is called a neutralization reaction. The remaining ions form a salt.

Buffers and pH It is very important for the body to maintain the proper pH in different parts of the body. The pH is kept in a narrow range to maintain health. One way the body does this is by producing Buffers. buffer A buffer is a chemical or combination of chemicals that keeps a pH within a given range. Buffers resist changes in pH by bonding to extra H + or OH - in the solution.

Buffers and pH