Ch. 4 Chemical Reactions Homework: 4.17, 4.18, 4.30, 4.33, 4.43, 4.45, 4.49, 4.55, 4.60, 4.62, 4.63, 4.73, 4.78, 4.100

Chemical Reactions In a chemical change, also known as a chemical reaction, one or more of the starting materials, called reactants, are converted into one or more products There are 3 aspects of a chemical reaction: Mass relationship Types of Reactions Heat gains and loses

Background for Mass relationships Formula Weight- the sum of the atomic weights in amu of all atoms in the compound’s formula Formula weight applies to ionic compounds, and covalently bonded molecules Molecular Weight- means the same thing but only applies to molecules

The mole Mole- the amount of substance that contains as many atoms, molecules, or ions as there are atoms in 12 grams of carbon 12. A mole always contains the same number of formula units That number, known as Avogadro’s Number, is 6.02 x 1023

Molar Mass Molar Mass- the mass of one mole of any substance is the formula weight expressed in grams. To convert between grams and moles, we use the Molar Mass as the conversion factor:

Writing Chemical Equations We use the formulas for the reactants and products We use an arrow to show the direction of the reaction We also designate the state of all reactants and products!! (s)=solid, (l)=liquid, (g)=gas, (aq)=aqueous (aqueous means the substance is dissolved in water)

Example: Chemical Equations must be balanced!!!! This means there must be the same number of atoms on the left as on the right To balance equations, we put coefficients in front of the formulas until we have the same number of each type of atom on both sides.

Guidelines for balancing Eq. Begin with atoms that appear in only one compound on the left and right If an atom occurs as a free element, balance this element last You can only change coefficients, NOT FORMULAS!!!!! A balanced equation is always written with the lowest possible set of coefficients.

Examples

Stoichiometry Stoichiometry- the study of mass relationships in chemical reactions The balanced equation gives us the molar ratios of the reactants and products. We can use this information to calculate the mass of starting material needed to produce a certain amount of product or vice versa.

Basic Process= Example=

Limiting Reagent The limiting reagent is the reactant that will be used up first. Example-

Percent Yield Actual Yield- the mass of a product formed in a chemical reaction that is physical carried out Theoretical Yield- the mass of product that SHOULD form according to the stoichiometry of the balanced equation Percent Yield- The actual yield divided by the theoretical yield time 100%

Combination Example A student combusted 6.0 grams of C3H8 in the presence of 30 grams of O2. 7.2 grams of water was captured. What was the percent yield for the reaction?

Ionic Reaction in Aqueous Solutions When positive and negative ions are placed in water, the ions are separated This is called dissociation Ions in water react with each other in solution only when one of the following can happen:

Types of Reactions Two ions form a solid that is insoluble in water 2 ions form a gas that escapes from the reaction mixture as bubbles An acid neutralizes a base One of the ions can oxidize the other

Example Say we mix a solution of NaCl with a solution of AgNO3

Ion Solubility Rules All compounds containing Na+, K+, or NH4+ are soluble in water All nitrates(NO3-) and acetates(CH3COO-) are soluble in water Most Chlorides(Cl-), and Sulfates(SO42-) are soluble in water. EXCEPTIONS: AgCl, BaSO4, PbSO4 Most Carbonates(CO32-), phosphates(PO43-), sulfides(S2-), and hydroxides(OH-) are insoluble in water. EXCEPTIONS: LiOH, NaOH, KOH, NH4OH

Example Will a reaction occur if a solution of Na2CO3 and a solution of LiCl are mixed together?

Oxidation Reduction Reactions Oxidation- is the loss of electrons Reduction- is the gain of electrons And Oxidation/Reduction reaction, aka Redox reaction, involves the transfer of electrons from one species to another.

Example

These reactions are not independent of one another Something can’t gain electrons from nowhere!!!

Organic Definitions In organic Chemistry, it is very hard to figure out what lost electrons and what gained them, so we use different definitions: Oxidation- a reaction in which there is an increase in the number of bonds to Oxygen and/or a decrease in the number of bonds to Hydrogen. Reduction- a reaction in which there is an increase in the number of bonds to Hydrogen and/or a decrease in the number of bonds to Oxygen.

Categories of Redox Reactions Combustion -What ever is burned is oxidized by O2 Respiration -O2 oxidizes carbon containing molecules to produce CO2 and water, much like a combustion but much slower and lower temperature Rusting -Iron is oxidized to Iron Oxide

4) Bleaching -Most bleaching involves oxidation 5) Batteries - The reaction in the battery is a redox reaction.

Heats of Reactions Heat of Reaction- the heat given off or absorbed by a chemical reaction If a reaction gives off heat, it is exothermic If a reaction absorbs heat, it is endothermic Heat given off in a combustion reaction is called heat of combustion.