Science 1206 Unit 3: Chemical Reactions Page 1 of 15

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1 Science 1206 Unit 3: Chemical Reactions Page 1 of 15 Introduction to Chemical Reactions Notes Part II TEXT p (word equations) There are many chemical reactions too many to count in fact! Like many other areas in science, chemical reactions have to be grouped to stay organized. You know how we scientist types are about organization! To group them we must recognize patterns. We have already grouped other things in chemistry: Ionic vs. molecular - Acid vs. base Being able to recognize the patterns in chemical reactions will help us to: Understand the reaction Predict the products of similar reactions Word Equations - Used to easily represent a chemical reaction using words and symbols. Generally: OR Read as: all reactants all products reactant 1 + reactant 2 product 1 + product 2 reactant one plus reactant two goes to product one plus product two Examples: Zinc and lead (II) nitrate react to form zinc nitrate and lead. Aluminum bromide and chlorine gas react to form aluminum chloride and bromine gas.

2 Science 1206 Unit 3: Chemical Reactions Page 2 of 15 Writing a Chemical Equation If we are given a word equation THEN we are able to write a chemical equation by replacing the words with chemical formulas State of matter (this is where all your naming practice comes in handy!) - We must also include the state for each compound in the reaction; - It goes in brackets after each chemical formula - Most of the time this information will be given in the word problem. - We also need to know how to use a solubility table. Solid (s) all solids including metals (before they combine to make compounds, but remember mercury is the exception it s a liquid at room temperature) and precipitates (Precipitation is the creation of a solid from a solution. When the reaction occurs in a liquid solution, the solid formed is called the 'precipitate'. The chemical that causes the solid to form is called the 'precipitant'.) Liquid (l) for pure liquids such as water Gas (g) gaseous substances like many molecular compounds Aqueous (aq) compounds dissolved in water (solutions) including acids SOLUBILITY TABLES: - Contains general solubility guidelines for ionic compounds. Can be used in just 2 easy steps! Step 1: Look for the anion (negative ion) of the compound in the top of the columns. Step 2: Look down the column to locate the cation (positive ion) and determine if it is in the - high solubility row - write (aq) after the compound in the chemical equation - low solubility row - write (s) after the compound in the chemical equation. o HINT: this is generally the precipitate in the products!

3 Science 1206 Unit 3: Chemical Reactions Page 3 of 15 TRY: Potassium chloride Silver sulfate Ammonium fluoride Copper (II) acetate Copper (I) acetate Examples Be sure to write the chemical formula and correct states for each: Solutions of sodium phosphate and calcium chloride are mixed and react to form a precipitate of calcium phosphate and sodium chloride. Copper metal and sulfuric acid react to form copper (II) sulfate, water and sulfur dioxide. Extra Work - Worksheet Writing Chemical Equations (Unbalanced) - Questions on Pg. 219 # s 2 and 3

4 Science 1206 Unit 3: Chemical Reactions Page 4 of 15 The Law of Conservation of Mass TEXT p (conserving mass) Think about some of the chemical reactions you are familiar with like: A nail becoming rusty rust is added - nail gains mass Wood burning in your woodstove all that s left is a few ashes - mass is lost In fact, it seems like mass is gained or lost in many reactions. However: Through much experimentation, scientists have discovered that mass is NOT gained or lost in any chemical reaction. Rather, it is conserved. Every Single Time! That is: Mass of the reactants = the mass of the products ALWAYS!! This has become known as: The Law of Conservation of Mass - In a chemical reaction the total mass of the reactants is always equal to the total mass of the products A woodstove example

5 Science 1206 Unit 3: Chemical Reactions Page 5 of 15 And getting a little more technical This allows us to solve problems like: Questions p. 223 # s 4 and 5

6 Science 1206 Unit 3: Chemical Reactions Page 6 of 15 Balancing Chemical Equations TEXT p (balancing chemical equations) According to the Conservation of Mass, what does this tell us about the number and type of atoms in the reactants versus the number and type of atoms in the products? They MUST be the same exactly the same ALWAYS During the chemical reaction the atoms become rearranged to form new products. Consider the reaction: H2 + O2 H2O The atoms are not balanced. How do we balance this equation? What could we do to make it balanced? Because the oxygen atoms are not equal on the product side, we add another water molecule. Now the hydrogen atoms are not equal on the reactant side, so we add another hydrogen molecule. Look what we have: - The numbers that we are placing in front of each compound to balance the equation are called coefficients - They do NOT change the chemical formula

7 Science 1206 Unit 3: Chemical Reactions Page 7 of 15 Let s try this: CH4 + O2 CO2 + H2O Check your answer: Hints for balancing: It doesn t matter which side of the chemical equation you start on. It usually helps to start with an atom that there is the most of. Gradually work your way back and forth from reactants to rpoducts side until all atoms are balanced. When balancing combustion reactions (hydrocarbon and oxygen) balance in the order of C-H-O. Examples: 1) N2 + H2 NH3 2) KClO3 KCl + O2

8 Science 1206 Unit 3: Chemical Reactions Page 8 of 15 3) C3H8 + O2 CO2 + H2O 4) Pb(OH)2 + HCl H2O + PbCl2 5) AlBr3 + K2SO4 KBr + Al2(SO4)3

9 Science 1206 Unit 3: Chemical Reactions Page 9 of 15 Types of Chemical Reactions TEXT p. 230 (combustion) p (synthesis and decomposition) p (single/double replacement) - All members of a chemical family react in a similar way there are definite patterns in reactions - Knowledge of patterns is useful because: o We will have a better understanding of chemical reactions o We can predict products of unknown reactions There are 5 types of chemical reactions that we will be studying: 1. Hydrocarbon Combustion - rapid reaction of a substance with oxygen to produce oxides - Often called burning - In complete combustion, the hydrocarbon will always react with oxygen to produce carbon dioxide and water. Example: BASE FORMULA: Hydrocarbon + oxygen carbon dioxide + water Balancing tip: Remember to balance in the order of C-H-O Butane + oxygen carbon dioxide + water vapor 2 C4H O2 8 CO H2O

10 Science 1206 Unit 3: Chemical Reactions Page 10 of Synthesis - also called combination or formation reactions - combination of smaller atoms or compounds to form 1 larger compound (product) - Often elements combine to form compounds BASE FORMULA: A + B AB Examples: 1. Hydrogen + oxygen water 2H2 + O2 2H2O 2. Hydrogen chloride + ammonia ammonium chloride HCl + NH3 NH4Cl 3. Decomposition - The splitting of one compound (reactant) into elements or smaller compounds BASE FORMULA: AB A + B Example Water hydrogen + oxygen 2 H2O 2 H2 + O2

11 Science 1206 Unit 3: Chemical Reactions Page 11 of Single Replacement - Also called single displacement - Involves one element and one compound as reactants - One element replaces (displaces) the other like element in the compound o Metal element replaces the metal element in the compound o Non-metal element replaces the non-metal element in the compound BASE FORMULA Z + AB A + ZB (where Z and A are metals) Y + AB B + AY (where Y and B are non-metals) Example: 1. 2 FeI2 + Pb PbI4 + 2Fe (metal replaces metal) 2. FeI2 + Cl2 FeCl2 + I2(non-metal replaces non-metal)

12 Science 1206 Unit 3: Chemical Reactions Page 12 of Double Replacement - Also called double displacement - Like elements in different compounds replace (displace) each other BASE FORMULA: AB + XY AY + XB EXAMPLES: o Where A and X are metals o Where B and Y are non-metals 1. HCl (aq)+ NaOH (aq) NaCl (s)+h2o (l) 2. CaCO3 (aq)+2 HCl (aq) CaCl2(s) + H2CO3(aq) NOTE: Neutralization reactions between acids and bases are double replacement reactions.

13 Science 1206 Unit 3: Chemical Reactions Page 13 of 15 To Summarize: AND: Hydrocarbon Combustion Occurs when burning takes place. Hydrocarbon + oxygen carbon dioxide + water CH4 + O2 CO2 + H2O

14 Science 1206 Unit 3: Chemical Reactions Page 14 of 15 Chemical Reactions and Energy TEXT: p. 281 (exothermic and endothermic reactions) Some chemical reactions seem to speed up as they progress - A forest fire burns slowly at first and then may burn out of control. What Happens: - Temperature speeds up reactions rates because it increases the motion of particles which in turn increases the collisions between particles that result in a chemical change. - That means that the heat energy released by the forest fire actually works to increase the rate of combustion. Other reactions release energy (heat): - Water freezing - Rubbing alcohol evaporating - Dynamite exploding Chemical changes that release energy: Exothermic Chemical changes that require an input of energy: Endothermic Exothermic Reactions Example: gasoline + oxygen carbon dioxide + water + heat energy Some physical changes can be exothermic Example: sulfuric acid (l) sulfuric acid (aq) + heat An energy diagram can be used to graph the energy changes in an exothermic reaction. A + B -> C + D + Heat

15 Science 1206 Unit 3: Chemical Reactions Page 15 of 15 Endothermic Reactions Some reactions require heat energy on a continuous basis for the change to take place - Heat can be absorbed from the surroundings - These changes do not speed up because surrounding temperature drops as heat is absorbed. Example: water (s) + heat energy water (l) Energy diagram for an endothermic reaction Heat + A + B -> C + D