Unit 4 (Day 1) Ms. Kondra & Mr. Olnhoff

 Statement conveying information about a chemical reaction.  Two ways of describing chemical equations:  Word equations  Formula equations

I. Word Equations  Verbally describe chemical reactions.  Have the following pattern: reactant 1 (state) + reactant 2 (state)  product 1 (state) + product 2 (state)

I. Reactants  Are the substances with which you begin the reaction.  The names of the reactants are listed before the arrow in a word equation.  A plus sign is used between the reactants when there is more than one reactant. II. Products  Are the substances made as a result of the reaction.  The names of the products are listed after the arrow in a word equation.  A plus sign is used between the products when there is more than one product.

 Behind the name of each reactant and product is the state of the substance in brackets using the short form:  Solid (s)  Liquid (l)  Gas (g)  Aqueous (aq) **dissolved in water or solution form

 Involve the breaking and forming of bonds to create new and different substances.  Since bonds can’t be observed directly, we instead look for evidence that a new substance formed (5 different ways)  Formation of a precipitate  Gas produced/bubbles  Color change  Energy change  Not reversible

 (Out of a solution)  A new substance is formed with a much lower solubility than the original.  It will not stay dissolved so a solid appears.

 A new substance is formed which is a gas.  May see bubbles.

 A new substance is formed which is a different color than the original.  The energy of the electrons influences the color of substances.

 (heat and/or light is absorbed)  New substances are formed which contain a different amount of chemical energy than the reactants.  If the new bonds have less energy than the original, then some energy must be released; called EXOTHERMIC.  If the new bonds have more energy than the original, then some energy must have been absorbed; called ENDOTHERMIC.

 Energy is released  Written on products side  Ex. Water formation hydrogen (g) + oxygen (g)  water (l) + heat

 Energy is absorbed  Written on reactants side  Ex. Oxygen formation water (l) + heat  hydrogen (g) + oxygen (g)

 Reaction happens and cannot be undone.  Ex. Toasting bread, explosions

 Methane burns in air (oxygen), giving off carbon dioxide and water vapour.  A. Word equation: Methane + oxygen  carbon dioxide + water {reactants}{products}

 Ex. 1: Magnesium metal burns in oxygen gas with a bright white light to make a white powder called magnesium oxide.  Ex. 2: A solution of sodium iodide is added to a solution of potassium nitrate to make a potassium iodide precipitate and a sodium nitrate solution.  Ex. 3: Gaseous hydrogen and gaseous oxygen react explosively to form water vapor.

 B. Molecular or Formula equation Instead of a verbal description of the chemical reaction, the words are exchanged for chemical symbols. Shorthand method of representing chemical reactions. May include state of each species (s, l, g, aq). All chemical equations should be written in a BALANCED form. In a chemical reaction, the mass of the products MUST equal the mass of the reactants. CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (g)

 The equation must represent fact.  The equation must be represented with the correct formulas and symbols.  The equation must obey the Law of Conservation of Mass  Diatomic elements must be shown as such.  If the phase of a substance is given, it must be included in the equation in abbreviated form (s, l, g, aq)

 We need to follow the Law of Conservation of Mass when balancing equations:  “In any chemical reaction, there is no detectable difference between the total mass of the reactants and the total mass of the products” 

 No new atoms are added.  The atoms of the reactants rearrange to form the products. Example  Co(NO 3 ) 2 (aq) + NaOH (aq) --> Co(OH) 2(s) + NaNO 3(aq) g213.34g

 To balance chemical equations we use ratios.  Coefficients in front of each formula must be used when balancing chemical equations.  These apply to each element in that particular formula.  You cannot change any of the subscripts of any formulas or you will change the compound involved.  To understand which coefficients to use we must be able to count atoms  If there is no coefficient in a formula we assume its coefficient is ‘1’.

 Balance 1 type of atom/ion at a time  Balance the atom/ion in the greatest abundance first  Balance H and O last  Examples  __H 2 + __O 2  __H 2 O  __Na + __Cl 2  __NaCl  __Zn + __HCl  __ZnCl 2 + __H 2  __Cu + __AgNO 3  __Ag + __Cu(NO 3 ) 2

 Handout: Word equations practice 1  Textbook: P. 133 #1-5