CH104Exam3.pdf CH 104
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This 9 page Study Guide was uploaded by Regan Dougherty on Tuesday October 27, 2015. The Study Guide belongs to CH 104 at University of Alabama - Tuscaloosa taught by Stephen Woski in Summer 2015. Since its upload, it has received 114 views. For similar materials see Introductory Chemistry in Chemistry at University of Alabama - Tuscaloosa.
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Date Created: 10/27/15
exam date 11315 CH 104 Exam 3 Study Guide This exam covers chapters 5 and 6 Chapter 5 Chemical Reactions General Features of Physical and Chemical Changes A physical change alters the physical state of a substance without changing its composition A chemical change chemical reaction converts on substance into another Things are changing at the molecular level Chemical reactions involve breaking bonds in the starting materials REACTANTS and forming new bonds in the PRODUCTS A chemical reaction may be accompanied by a visual change two colorless reactants can form a colored product a gas may be given off two liquid reactants may yield a solid product heat may be produced oxygen may be produced causing bubbles of oxygen to appear as foam Writing Chemical Equations CHEMICAL EQUATION an expression that uses chemical formulas and other symbols to illustrate what reactants constitute the starting materials in a reaction and what products are formed reactants on left and products on right separated by a reaction arrow COEFFICIENTS are the numbers written in front of any formula They show the number of molecules of a given element or compound that react or are formed When a formula contains a subscript multiply its coefficient by the subscript to give the total number of atoms of a given type in that formula exam date 11315 Law of Conservation of Matter Atoms cannot be created or destroyed in a chemical reaction Coefficients are used to balance an equation making the number of atoms of each element the same on either side of a chemical equation Oxidation and Reduction General Features of OxidationReduction Reactions OXIDATION is the loss of electrons from an atom The charge increasesbecomes less negativebecomes more positive REDUCTION is the gain of electrons by an atom The charge decreasesbecomes less positivebecomes more negative Oxidation and Reduction are opposite processes and both occur together in a single reaction called an oxidationreduction reaction or a REDOX reaction A redox reaction involves the transfer of electrons from one element to another HALF REACTION an equation written for an individual oxidation or reduction that shows how many electrons are gained or lost A compound that gains electrons is reduced while causing another compound to be oxidized is called an OXIDIZING AGENT A compound that loses electrons is oxidized while causing another compound to be reduced is called a REDUCING AGENT Metals lose electrons and nonmetals gain electrons Cations tend to gain electrons and anions tend to lose electrons The Mole and Avogadro s number A MOLE defines a quantity that contains 602 x 1023 items usually atoms molecules or ions Avogadro s number 602 x 1023 To multiply two numbers in scientific notation multiply the coefficients together and add the exponents in the powers of 10 exam date 11315 To divide two numbers in scientific notation divide the coefficients and subtract the exponents in the powers of 10 Mass to Mole Conversions FORMULA WEIGHT the sum of the atomic weights of all the atoms in a compound reported in atomic mass units amu To Calculate Formula Weight Write the correct formula and determine the number of atoms of each element from the subscripts Multiply the number of atoms of each element by the atomic weight and add the results atomic weight average mass of an element found as a decimal under each element on the periodic table The term MOLECULAR WEIGHT may be used in place of formula weight for covalent compounds since they are composed of molecules not ions Molar Mass The MOLAR MASS is the mass of one mole of any substance reported in grams per mole The value of molar mass of an element in the periodic table in grams per mole is the same as the value of its atomic weight in amu Mole Calculations in Chemical Equations A balanced chemical equation also tells us the number of moles of each reactant that combine and the number of moles of each product formed Coefficients are used to form mole ratios which can serve as conversion factors Mass Calculations in Chemical Equations The number of grams of a substance and the number of moles it contains are related by the molar mass Percent Yield THEORETICAL YIELD the amount of product expected from a given amount of reactant based in the coefficients in the balanced chemical equation exam date 11315 calculated from coefficients of the balanced equation The amount of product formed is usually less than the maximum amount predicted Sometimes side reactions occur between the reactants The ACTUAL YIELD is the amount of product isolated from a reaction It is determined by weighing the product obtained on a balance Calculating Percent Yield PERCENT YIELD the the amount of product actually formed in a particular reaction divided by the theoretical yield multiplied by 100 Limiting Reactants LIMITING REACTANT the reactant that is completely used up in a reaction The limiting reactant controls how much of a product is produced The number of moles of limiting reactant determines the number of moles of product that can form To determine limiting reactant divide the number of moles you have of a substance by the coefficient do this for all of the reactants The smaller number is the limiting reactant Chapter 6 Energy Changes Reaction Rates and Equilibrium Energy ENERGY the capacity to do work Potential energy stored energy Kinetic energy energy of motion LAW OF CONSERVATION OF ENERGY The total energy of the universe does not change Energy cannot be created or destroyed A compound with lower potential energy is more stable than a compound with higher potential energy Reactions that form products having lower potential energy than the reactants are favored exam date 11315 The Units of Energy CALORIE a unit of energy that equals the amount of energy needed to raise the temperature of 1 gram of water by 1 degree C equals 4184 J 1 kca 1000 ca JOULE a unit of measurement for energy 1 kJ 1000 J 1kcal 4184 kJ Energy Changes in Reactions Chemical bonds store potential energy Breaking a bond requires energy Energy is released when bonds are formed HEAT OF REACTION the energy absorbed or released in any reaction and symbolized by AH also called the ENTHALPY CHANGE AH energy of products energy of reactants When energy is absorbed the reaction is said to be endothermic and AH is positive When energy is released the reaction is said to be exothermic and AH is negative Bond Dissociation Energy BOND DISSOCIATION ENERGY is the energy needed to break a covalent bond by equally dividing the electrons between the two atoms in the bond ALWAYS a positive number Breaking a covalent bond is always ENDOTHERMIC Forming a bond is exothermic and AH is negative The stronger the bond the higher its bond dissociation energy Bond dissociation energies generally decrease down a column of the periodic table AH indicates the relative strength of bonds broken and formed in a reaction exam date 11315 Negative AH means that more energy is released in forming bonds than is needed to break bonds The bonds formed in the products are stronger than the bonds broken in the reactants EXOTHERMIC In an exothermic reaction the products are lower in energy than the reactants Vice versa for ENDOTHERMIC reactions AH Exothermic Reactions heat is released AH change in heat is negative The bonds formed in the products are stronger than the bonds broken in the reactants Products are lower in energy than the reactants Endothermic Reactions Heatenergy must be added for the reaction to take place Heat is a reactant AH is positive The bonds broken in the reactants are stronger than the bonds formed in the products The products are higher in energy than the reactants Energy Diagrams In order for two molecules to react they must collide Only collisions that have sufficient energy and proper orientation lead to a reac on Even exothermic reactions require energy to begin ENERGY DIAGRAM a schematic representation of the energy changes in a reaction which plots energy on the vertical axis and the progress of the reaction the reaction coordinate on the horizontal axis exam date 11315 TRANSITION STATE the unstable intermediate located at the top of the energy hill in an energy diagram The difference in energy between the reactants and the transition state is called the energy of activation Ea ENERGY OF ACTIVATION minimum amount of energy needed for a reaction to occur energy barrier The magnitude of energy of activation determines the REACTION RATE how fast the reaction occurs When the energy of activation is high few molecules have enough energy to cross the energy barrier and the reaction is slow Vice versa when the energy of activation is low AH is the difference in energy between the reactants and the products Reaction Rates How Concentration and Temperature Affect Reaction Rate Increasing the concentration of reactants increases the number of collisions so the reaction rate increases Increasing the temperature increases the reaction rate This is because an increase in temperature increases kinetic energy Catalysts CATALYST a substance that increases the rate of a reaction but is recovered unchanged at the end of the reaction Catalysts accelerate a reaction by lowering the energy of activation The energy of the reactants and products does not change AH does not change ENZYMES are biological catalysts held together in a very specific 3D shape The reagent binds to the ACTIVE SITE of the enzyme Equilibrium exam date 11315 A REVERSIBLE REACTION can occur in either direction from reactants to products or from products to reactants The forward reaction proceeds from left to right and the reverse reaction proceeds from right to left When the rate of the forward reaction equals the rate of the reverse reaction the net concentrations of all species do not change and the system is at EQUILIBRIUM The concentrations of all reactants and products do not change However the reaction has not stopped EQUILIBRIUM CONSTANT K a characteristic value for a reaction at a given temperature and equal to the ratio of the concentrations of the products multiplied together to the concentrations of the reactants multiplied together K concentration of products concentration of reactants concentration molarity molesliters The coefficient in the equation is used as an exponent when calculating K When K gt 1 the concentration of the products is larger than the concentration of the reactants equilibrium lies to the right and equilibrium favors the products When K lt 1 the concentration of the reactants is larger than the concentration of the products equilibrium lies to the left and equilibrium favors the reactants K1 gt equilibrium Equilibrium favors the products when when they are lower in energy than the reactants AH is negative Le Chatelier s Principle If a chemical system is disturbed or stressed the system will react in the direction that counteracts the disturbance or relieves the stress Concentration Change Increase in concentration favors the opposite side of the reaction exam date 11315 Decrease in concentration favors the side of the reaction where the concentration was decreased Temperature Changes Endothermic energy is a reactant Increase temperature gt adding more energyheat Increasing temperature means you are adding more reactant so it favors the products Decrease temperature gt removing energyheat reactant side is favored Exothermic reaction energy is a product Increase temperature adding product gt reaction favors reactants Decrease temperature removing product gt reaction favors products A change in pressure is another stressdisturbance but we will not cover this
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