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CHAPTER 3 AP CHEMISTRY

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AMU Atomic masses come from the carbon-12 scale Mass of carbon-12 is exactly 12 amu Nitrogen-14 has an amu of 14.0031 this is about 14/12 times heavier than carbon-12. C-12 is 12 times heavier than H-1 Isotopic abundance was proven in

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ATOMIC MASS The atomic mass of an element is the weighted average of the masses of its constituent isotope. –Atomic = (mass Y 1 )(fraction Y 1 ) + (mass Y 2 )(fraction Y 2 ) + etc. Mass Mass The average atomic mass is the weighted average of all the isotopes –Average = (% Y 1 )(amu Y 1 ) + (% Y 2 )(amu Y 2 ) Mass of Y 100 Mass of Y 100

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MOLE Unit of Measurement - like dozen or peck What the mass relations in chemical formulas tell you –The atom ratio in the compound –The mole ratio of the elements in the compound –The mass ratio

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MASS OF AN ATOM Avogadro’s number –Number of particles –One mole contains 6.022 X 10 23 particles Mass of one mole of pure substance is called the molar mass. This is equal to the average atomic mass Avogadro’s number is the number of atoms in a sample of an element with a mass in grams numerically equal to its atomic mass Standard molar volume of gas one mole is the same as 22.4 L at STP

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FORMULA MASS Sum of atomic mass in the formula of a substance

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PERCENT COMPOSITION Sodium carbonate is used in the manufacture of glass and soap. What are the mass percents of sodium, carbon, and oxygen in sodium carbonate? The subscripts in a formula represents atom ratios in which the different elements are combined. Also the mole ratio. For example –1 molecule of urea H 2 NCONH 2

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PERCENT COMPOSITION Percent composition = mass of element X 100 Mass of total compound Mass of total compound

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MOLECULAR FORMULA In order to obtain the molecular formula, we need to know the molar mass Molar Mass = small whole number Simplest Formula mass Molar Mass = small whole number Simplest Formula mass

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REACTIONS Writing and balancing equations In a balanced equation you must conserve mass (atoms) In a balanced equation you must conserve mass (atoms) First you write a “skeleton” equation First you write a “skeleton” equation CS 2 + O 2 ---> CO 2 + SO 2 CS 2 + O 2 ---> CO 2 + SO 2 Indicate the physical state Indicate the physical state CS 2 (l) + O 2 (g) ---> CO 2 (g) + SO 2 (g) CS 2 (l) + O 2 (g) ---> CO 2 (g) + SO 2 (g) Balance the equation Balance the equation CS 2 (l) + 3O 2 (g) ---> CO 2 (g) + 2SO 2 (g) CS 2 (l) + 3O 2 (g) ---> CO 2 (g) + 2SO 2 (g)

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BALANCE AN EQUATION 1) Determine the type of reaction. What are the reactants, products and physical states involved 2) Write a unbalanced equation 3) Balance equation (start with the most complicated molecule). Find the right coefficient to give the same number of each type of atom of both sides. DO NOT CHANGE THE IDENTITES (NH 4 ) 2 Cr 2 O 7 (s)-------> Cr 2 O 3 (s) + N 2 (g) + H 2 O(g)

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INDICATIONS FOR A REACTION Formation of light or heat Production of a gas Formation of a precipitation When a solid (or solution turns cloudy) is formed from two clear solutions Sometimes a color change can indicate a change Hydrogen gas + oxygen gas ---> water REACTANTS PRODUCT Law of conservation of mass –A–A–A–Atoms cannot be created or destroyed. Total mass of the reactants = total mass of product Equations must have correct formulas Balancing atoms can ONLY be done with coefficients, that are placed in front of the formula

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TYPES OF REACTIONS COMBUSTION –Hydrocarbons with oxygen making carbon dioxide and water COMBINATION OR COMPOSITION –Formation of a single product DECOMPOSITION –A single reactant is broken down SINGLE REPLACEMENT –One element replaces another DOUBLE REPLACEMENT –Atoms or ions exchange partners Solubility –Not all ionic compounds are soluble in water

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REACTIONS WHEN COMPOUNDS ARE IN WATER (aq) THEY ARE ALWAYS IN THERE ION FORM EXCEPT WEAK ACIDS MgCl2(aq) --> Mg2+(aq) + 2Cl-(aq) If a compound is a aqueous solution, and there is an ion in the reactants which remains as that ion as a product, we call this a SPECTATOR ION. It does not do anything in the reaction and can be removed Net ionic equations –A–A–A–Any ionic compound in water should be written as ions –B–B–B–Balance the reaction –R–R–R–Remove all spectator ions

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COEFFICIENTS Number of each reactants used and products formed can tell you the following information Number of moles of each reactant and product Volume of each gas used or formed at standard pressure and temperature Avogadro's hypothesis states that at STP all gasses have 6.02 X 1023 particles at the same volume

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MOLE MASSPARTICLES VOLUME of gases at STP Molar mass 6.02 X 10 23 particles/ 1mole molar volume 22.4L/1 mole

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MOLE TO MOLE REACTION Reactions involving masses (in grams) of reactants and products can be achieved by converting grams into moles

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LIMITING REACTANT AND THEORETICAL YIELD When a reaction goes to completion without loss of product, then the amount produced is called the theoretical yield Method to determine limiting reactant Method to determine limiting reactant –Calculate the amount of product that would be formed if the first reactant was consumed –Repeat step 1 for second reactant –The theoretical yield of the product is the smaller of the two calculations

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LIMITING REACTANTS When one reactant has been completely used to form the product, and you still have some of the other reactant left over.

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PERCENT YIELD Actual yield__ X 100 = % yield Theoretical yield

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