CHEMICAL REACTIONS a chemical change produces one or more new substances there is a change in the composition of one or more substances old bonds are broken and new bonds formed atoms in the reactants are rearranged to form one or more different substances REACTANT ATOMS PRODUCT ATOMS BALANCING CHEMICAL EQUATIONS mass conversion law has to be fulfilled ATOMS CANNOT BE CREATED OR DESTROYED during a chemical reaction instead they combine in different ways to make a chemical compound Chemical equations can be used to describe what is going on during reactions represents both the atomic level picture and a big scale molar picture describes how many formula units of different chemicals react a Formula Unit is the smallest unit of a substance could be an atom ion ionic formula or molecule BALANCING CHEMICAL EQUATIONS STRATEGY 1 write unbalanced equation using correct chemical formula of all reactants and products 2 find suitable coefficients numbers placed before each number 3 if necessary reduce numbers to smallest whole number values 4 check your answer by counting how many atoms of each kind are on both sides of the equation EX THESE ARE CURRENTLY UNBALANCED H2 O2 H2O Na Cl2 NaCl CH4 O2 CO2 H2O REPRESENTING CHEMISTRY ON DIFFERENT LEVELS microscopic 2 molecules of hydrogen gas react with 1 molecules of oxygen to yield 2 molecules of liquid water macroscopic 2 mole of hydrogen gas react with 1 mole of oxygen gas to yield 2 moles of liquid water CHEMICAL ARITHMETIC STOICHIOMETRY ratio is given by coefficients in balanced equation stoichiometry tells us in what ratios chemicals react however we cannot weigh chemicals by number of moles need to use mass instead convert using molecular mass or molar mass being able to convert between moles and grams is CRUCIAL MOLECULAR MASS sum of all atomic masses of all atoms in a molecule FORMULA MASS sum of all atomic masses of all atoms in a formula unit of any compound whether molecular or ionic REMEMBER ONE MOLE OF A SUBSTANCE IS THE AMOUNT WHOSE MASS IN GRAMS IS NUMERICALLY EQUAL TO THE SUBSTANCE S MOLECULAR OR FORMULA MASS EX HCl 1 0 amu 35 5 amu 36 5 amu C2H4 2 12 0 amu 4 1 0 amu 28 0 amu HCl 1 mole 36 5 grams 6 022 x 10 23 molecules 36 5g Grams of compound Moles of compound Mole Ratio Grams of product compound YIELD OF CHEMICAL REACTIONS so far we have assumed that every atom or molecule undergoes the chemical reactions we wrote down would be an ideal case scenario this is called a reaction goes to completion in real life most reactions do not only form the desired products unreacted material side products the amount of product formed is called the YIELD of a reaction
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