Chapter 5 Notes PDF

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Notes on the lecture and on the mandatory book readings. Make a great study guide when the test comes around. ...

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Chapter 5 Wednesday, October 4, 2017 4:58 PM

5.1 Introduction to Chemical Reactions General Features of Physical and Chemical Changes  A physical change alters the physical state of a substance without changing its composition o Melting and boiling point and changes in state o No bonds broken  A chemical change - chemical reaction - converts one substance into another  Chemical reactions involve breaking bonds in the starting materials, called reactants, and forming new bonds in the products  A chemical reaction may be accompanied by a visual change: color, gas given off, two liquids may yield a solid product, heat given off Writing Chemical Formulas  A chemical equation is 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 o Written with reactants on the left of the reaction arrow and products on the right  Coefficients: numbers written in front of any chemical formula to show the number of molecules of a given element or compound that react or are formed (cannot be fractions) o When a formulas contains a subscript and a coefficient, multiply hem to give the total number of atoms  Law of conservation of matter: atoms cannot be created or destroyed in a chemical reaction o Although bonds are broken and formed in reactions, the number of atoms of each element in the reactants must be the same as the number of atoms of each type in the products  Coefficients are used to balance the equation  If heat is needed for a reaction to occur, change will be written over the arrow  The physical states of reactants and products are sometimes indicated next to each formula solids (s), liquids (l), or gas (g) or aqueous solution (aq) if dissolved in solvent 5.2 Balancing Chemical Equations  Balanced chemical equation: ow much of each reactant we must combine to give the desired product  An equation must be balanced by adding coefficients in front of some formulas so that the number of atoms of each element is equal on both sides of the equation  How to balance a chemical reaction o Write the equation with the correct formulas o Balance the equation with coefficients one element at a time o Check to make sure that the smallest set of whole numbers is used 5.3 Types of Reactions  Many reactions fall into a few major categories: combustion, decomposition, single replacement and double replacement Combination and Decomposition Reactions  Combination reaction: the joining of 2 or more reactants to form a single product o Reactant A combines with reactant B to form product AB o The reactants may be elements or compounds, but the product is always a compound  Decomposition reaction: the conversion of a single reactant to two or more products o Reactant AB forms products A and B

o The products may be elements or compounds but the reactant is always a compound Combination reactions form only one product, whereas decomposition reactions begin with only one reactant Replacement Reactions  Replacement reaction: an element in a reactant is replaced by another element o Characterized as single replacement or double replacement o Always 2 reactants and 2 products in replacement reactions  Single replacement: a reaction in which one element replaces another element in a compound to form a different compound and element as products o Reactants A and BC form products B and AC o An element and product are the reactants, and an element and compound are the products  Double replacement: a reaction in which 2 compounds exchange "parts" - atoms/ions - to form 2 new compounds o Reactants AB and CD form products AD and CB o 2 compounds are reactants and 2 compounds are products 5.4 Oxidation and Reduction  Oxidation and reduction reactions constitute a class of chemical reactions that involves electron transfer General Features of Oxidation-Reduction Reactions  Oxidation: the loss of electrons from an atom  Reduction: gain of electrons by an atom  Oxidation and reduction are opposite processes, and both occur together in a single reaction called an oxidation-reduction or redox reaction o 2 components: one that's oxidized and one that’s reduced o Involves transfer of electrons from one element to another  Each of these processes can be written as individual reactions called half reactions to emphasize which electrons are gained and lost o Gain of electrons = reduction o Loss of electrons = oxidation  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  Tricks to deciding which element/ion gains electrons and which one loses electrons o When considering neutral atoms, metals lose electrons, and nonmetals gain electrons o When considering ions, cations gain electrons and anions lose electrons  Batteries consist of metal and cation undergo a redox reaction when electrons are transferred from metal to cation, electric current results  Oxidation results in the gain of oxygen atoms or loss of hydrogen atoms  Reduction results in the loss of oxygen atoms and gain of hydrogen atoms Focus on Heath and Medicine  A pacemaker is powered by a small, long-lasting battery that generates an electrical impulse by a redox reaction o Most pacemakers today use LiI battery. Each neutral lithium atom is oxidized by losing one electron, each I2 molecule reduced by gaining 2 electrons 5.5 The Mole and Avogadro's Number 

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Mole: defines a quantity contains 6.02 x 10^23 items - usually atoms, ions, molecules Avogadro's number: definition of mole based off the number of atoms contained in exactly 12 g of carbon-12 isotope o One mole always contains a Avogadro's number of particles o One mole of C atoms = 6.02 x 10^23 C atoms o One mole of H2O molecules = 6.02 x 10^23 H2O molecules  We can use Avogadro's number as a conversion factor to relate the number of moles of a substance to number of atoms and molecules it contains  To multiply 2 numbers in scientific notification, multiply the coefficients together and add the exponents in the powers of 10  To divide 2 numbers in scientific notation, divide the coefficients and subtract the exponents in the powers of 10 5.6 Mass to Mole Conversions  Atomic weight: is the average mass of an element (in amus)  Formula weight: the sum of the atomic weights of all the atoms in a compound, reported in atoms (molecular weight) o Add weights of each element Molar Mass  Molar mass: the mass of one mole of any substance, reported in grams per mole  The value of the molar mass of an element is the same as the value of the atomic weight Relating Grams to Moles  The molar mass is a very useful quantity because it relates the number of moles to the number of grams in a substrate o 1 mole = molar mass of compound Relating Grams to Number of Atoms or Molecules  Since molar mass gives the number of grams in a mole and a mole contains 6.02 x 10^23 molecules, we can use molar mass to show the relationship between grams and number of molecules (or atoms) 5.7 Mole Calculations in Chemical Equations  A balanced chemical equation tells us the number of moles and molecules of each reactant that combines and the number of moles and molecules of each product  Coefficients are used to form mole ration, which can serve as conversion factors  Use the mole ration from the coefficients in the balanced equation to convert the number of moles of one compound into the number of moles of another compound 5.8 Mass Calculations in Chemical Equations  We utilize a balance to measure the number of grams of a compound used and the number of grams of a product formed  The number of grams of a substance and the number of moles it contains are related by molar mass Converting Grams of Reactant to Grams of Product  The coefficients in chemical equations tells us the ratio of the number of molecules or moles that are involved in a chemical reaction 5.9 Percent Yield  In determining the number of molecules or grams of a product, we assumed each reaction gives the max amount of product from given amount of reactant  Theoretical yield: the amount of product expected from a given amount of reactant based on the coefficients in the balanced chemical equation

o However, the amount of product formed is less than the max amount predicted  Side reactions: undesired, occur between reactions, energy lost  Actual yield: amount of product isolated from a reaction Calculating Percent Yield  Percent yield: the amount of product actually formed in a particular reaction o {Actual yield (g) / theoretical yield (g)} x 100 Calculating Percent Yield from Grams of Reactant  Convert the number of grams of reactant to the number of moles of reactant using the reactant's molar mass  Convert the number of moles of reactant to the number of moles of product using a mole to mole conversion factor  Convert the number of moles of product to the number of grams of product using the product's molar mass  Use the theoretical yield and actual yield to calculate the percent yield Focus on Health and Medicine - The Importance of Percent Yield in Pharmaceutical Industry  Most widely used drugs are synthesized in the laboratory  Rarely is a drug prepare in a single step  To determine the overall percent yield in a synthesis that has more than one step, multiply the percent yield of each step  Pharmaceutical companies are faced with the task of developing drugs that have the desired physiological effects, which are prepared by reactions that give high yields of the desired compounds 5.10 Limiting Reactions  Occurs when there is an excess of one molecule  Limiting reactant: the reactant that is completely used up in a reaction o Label one reactant as the original quantity and one as the unknown quantity o Write out the conversion factors that relate the number of molecules of reactants to the number of products o Calculate the number of moles of the second reactant needed for complete reaction o Analyze the 2 possible outcomes Using the Limiting Reactant to Determine how much Product is Formed  The number of moles (not grams) of limiting reactant determines the number of moles (not grams) of product o Determine the limiting reactant o Convert the number of moles of limiting reactant to the number of moles of product using mole to mole conversion factor Determining the Limiting Reactant Using the number of grams  Convert the number of grams of each reactant to the number of moles (using molar mass)  Determine the limiting reactant...