Chemical Equations and Chemical Reactions

Chemical Equations Chemical Equations and Chemical Reactions Chemical equations are concise representations of chemical reactions. Chemical Equations Symbols Used in Chemical Equations The formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right). Reactant A + Reactant B Product A + Product B The word equation for the above reaction would be: Reactant A reacts with Reactant B to form Product A and Product B. Skeleton Equations A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. Iron reacts with oxygen in the air to produce iron (III) oxide. Here is the word equation for rusting: Iron(s) + oxygen(g) iron oxide(s) The skeleton equation is: Fe + O 2 Fe 2 O 3 Write the formulas of the reactants to the left of the yields sign (arrow) and the formulas of the products to the right. You need to remember your diatomics! Word Equations Methane gas reacts with oxygen gas to produce steam and carbon dioxide. CH 4 gas O 2 gas H 2O gas CO 2 gas The skeleton equation is not balanced: CH 4 (g) + O 2 (g) H 2 O (g) + CO 2 (g) The chemical equation is balanced with coefficients: CH 4 (g) + 2 O 2 (g) 2 H 2 O (g) + CO 2 (g) 1

1 In the reaction CH 4 (g) + O 2 (g) H 2O (g) + CO 2 (g) the products are: 2 In the reaction CH 4 (g) + O 2 (g) H 2O (g) + CO 2 (g) the products are: A B C D oxygen and water carbon dioxide and water oxygen and methane methane and carbon dioxide A B C D E solids liquids gases dissolved in water (aqueous) cannot be determined Word Equations to Chemical Equations Solid potasium chlorate decomposes in air to produce solid potassium chloride and oxygen gas. The "word" equation is: potasium chlorate (s) potassium chloride (s) + oxygen (g) Word Equations to Chemical Equations Write the word equation, then the skeleton equation for this reaction: Aluminum sulfate reacts with calcium chloride to produce calcium sulfate and aluminum chloride The unbalanced "skeleton" equation is: KClO 3(s) KCl (s) + O 2(g) Law of Conservation of Mass We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal amount of matter exists both before and after the experiment. Upon this principle, the whole art of performing chemical experiments depends. Antoine Lavoisier, 1789 Balancing Chemical Equations To write a balanced chemical equation, first write the skeleton equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass. This is a balanced equation for making a bicycle. The numbers are called coefficients small whole numbers that are placed in front of the formulas in an equation in order to balance it. 2

Balancing Chemical Equations CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2O (g) Balancing Chemical Equations CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Reactants appear on the left side of the equation. Products appear on the right side of the equation. The states of the reactants and products are written in parentheses to the right of each compound. Coefficients are inserted to balance the equation. 1's typically are not written. Subscripts and Coefficients 3 How many oxygen atoms are in one formula unit of calcium nitrate? (First, write the formula for calcium nitrate.) Subscripts tell the number of atoms of each element in a molecule. Coefficients tell the number of representative particles (atoms, molecules, or formula units). 4 How many nitrogen atoms are in one formula unit of ammonium sulfate? Balancing Chemical Equations H 2 + Cl 2 HCl H 2 + Cl 2 2HCl unbalanced balanced 2 2 1 2 2 1 2 2 3

Balancing Chemical Equations Write a balanced chemical equation for this reaction. (First write a skeleton equation, then balance it) chlorine + sodium bromide bromine + sodium chloride 5 When the following equation is balanced, the coefficients are Na + O 2 Na 2O Enter ### on your responder (112, etc) 6 When the following equation is balanced, the coefficients are 7 When the following equation is balanced, the coefficients are HgO Hg + O 2 Al + ZnCl 2 Zn + AlCl 3 8 When the following equation is balanced, the coefficients are 9 When the following equation is balanced, the coefficients are NaCl + CaI 2 NaI + CaCl 2 NH 3 + O 2 NO 2 + H 2O 4

10 When the following equation is balanced, the coefficients are Al(NO 3) 3 + Na 2S Al 2S 3 + NaNO 3 Reaction Types There are 5 major types of chemical reactions that we will study this year. Combination Decomposition Combustion Single replacement Double replacement Combination Reactions 11 Which of the following is a combination reaction? Two or more substances react to form one product. A H 2O H 2 +O 2 B Na + Cl 2 NaCl C AgNO 3 + BaSO 4 Ag 2 SO 4 + Ba(NO 3 ) 2 D C 3 H 8 + O 2 H 2 O + CO 2 Examples: 2 Mg (s) + O 2(g) 2 MgO (s) N 2(g) + 3 H 2(g) 2 NH 3(g) C 3H 6(g) + Br 2(l) C 3H 6Br 2(l) One substance breaks down into two or more substances. Examples: Decomposition Reactions 12 Which of the following is a decomposition reaction? A H 2 O H 2 +O 2 B Na + Cl 2 NaCl C AgNO 3 + BaSO 4 Ag 2 SO 4 + Ba(NO 3 ) 2 D C 3 H 8 + O 2 H 2 O + CO 2 CaCO 3 (s) CaO (s) + CO 2 (g) 2 KClO 3 (s) 2 KCl (s) + 3 O 2 (g) 2 NaN 3 (s) 2 Na (s) + 3 N 2 (g) 5

These are generally rapid reactions that produce a flame. Most often involve hydrocarbons reacting with oxygen in the air. O 2 is always one of the reactants. Combustion Reactions Examples: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2O (g) C 3H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2O (g) Combustion Reactions Oxygen is always a reactant in combustion reactions. Examples: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2O (g) C 3H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2O (g) However, other elements may also undergo combustion: Mg (s) + O 2 (g) MgO (s) N 2 (g) + 2 O 2 (g) 2 NO 2 (g) These last two may also be classified as combination reactions. 13 Which of the following is a combustion reaction? A H 2O H 2 +O 2 B Na + Cl 2 NaCl C AgNO 3 + BaSO 4 Ag 2 SO 4 + Ba(NO 3 ) 2 D C 3 H 8 + O 2 H 2 O + CO 2 Combustion Reactions Combustion reactions are easily identified by oxygen as a reactant. However, the products of a combustion reaction may vary depending on how much oxygen is available. Complete combustion products are carbon dioxide and water Incomplete combustion products are carbon monoxide and water Single Replacement Reactions Single replacement reactions occur between an element and a compound. Usually, a pure metal will displace the metal from the compound. Single Replacement Reactions Single replacement reactions can also involve nonmetals, specifically, halogens. In these reactions, one halogen will displace the halogen from the compound. F 2 + NaCl Cl 2 + NaF M + XY X + MY 6

14 Which of the following is a single replacement reaction? Single Replacement Reactions A H 2O H 2 +O 2 B Na + Cl 2 NaCl C AgNO 3 + Ba Ag + Ba(NO 3 ) 2 D C 3 H 8 + O 2 H 2 O + CO 2 In this reaction, copper metal displaces silver ions in the compound. The reverse reaction, however, does not occur. Cu (s) + 2 AgNO 3 (aq) Cu(NO 3)) 2 (aq) + 2 Ag (s) Activity Series Single replacement reactions only occur if the pure metal has a higher reactivity than the metal in the compound. The reactivity/activity of the metals is provided in a table, with the more reactive metals higher on the list. If a pure metal is higher on the list than the metal compound it's to react with, the reaction will takes place. If the pure metal is below the metal compound, the reaction will not occur. Activity Series Note that hydrogen appears on this list. Hydrogen can be displaced from water and from some acids by metals that are above it on the Activity Series. Such reactions yield hydrogen gas as a product. Activity Series Lithium metal will replace calcium metal from its compound such as CaCl 2 since Li is more reactive than Ca. Li + CaCl 2 Ca + 2 LiCl yes! But the reverse reaction will not occur. Ca + LiCl NO! Activity Series You have a copy of the activity series on the back of your periodic table. Remember, Lithium (Li) is the most reactive and will replace all other metals. Gold (Au) is the least reactive and will not replace any other metals (on the list). Ca is less active than Li so it cannot displace Li. 7

15 Which of these reactions will NOT occur? Activity Series A B C D Ag + NaCl AgCl + Na PbF2 + Fe Al + FeCl2 There is an Activity Series for the halogens, (Group 17) F fluorine, most reactive Cl chlorine Br bromine I iodine, least reactive Note that this series matches the placement of the elements on the Periodic Table. Activity Series Let's practice finding which reactions will occur and which will not. Use your reactivity list as a guide. http://www.chem.iastate.edu/group/greenbowe/sections/projectfolder/flashfiles/ redox/home.html Double replacement reactions usually occur between two ionic compounds. They also occur between acids and bases, which will be discussed later. Generally, the cations switch places as shown below: AB + CD AD + CB 16 Which of the following is a double replacement reaction? A H 2 O H 2 +O 2 B Na + Cl 2 NaCl C AgNO 3 + BaCl 2 AgCl + Ba(NO 3 ) 2 D C 3 H 8 + O 2 H 2 O + CO 2 Recall that the cation is written first in an ionic compound. 8

Double Replacement Reaction The positive ions (metal ions) change their partners. AgNO 3 + KCl AgCl + KNO 3(( Double replacement reactions will NOT occur if both products are water soluble. PP Pb(NO) 3 + KI PbI 2 + KNO 3 AB + CD AD + CB So, if AD and CB are both aqueous, then there is no reaction. Double replacement reactions occur ONLY if one of the products is either: a solid precipitate a gas water Most of the double replacement reactions we will discuss form a solid precipitate. Double Replacement Reaction In double replacement reaction, two ionic compounds in an aqueous solution (water) react together. The driving force in this reaction is that one of the product is either a gas or a pure soild (insoluble). The insoluble solid will precipitate out and settle inside the reaction vessel. Double Replacement Reaction Both of these reactions occur because one of the products is a solid precipitate. AgNO 3(aq) + KCl(aq) AgCl(s) + KNO 3(aq) PP Pb(NO) 3(aq) + KI(aq) PbI 2(s) + KNO 3(aq) An insoluble (solid) product is called a precipitate. AB + CD AD + CB A reference table called a Solubility Chart is used to determine whether a compound is soluble or not. 9

Solubility Chart Solubility Chart This reference table gives solubilities for many ionic compounds. In general, all compounds containing alkali metals and the ammonium ion are always soluble. Note that S on this chart means that the compound is soluble. When you write a chemical equation, it will be (aq) aqueous. Insoluble compounds are shown with the letter I. When you write a chemical equation, it will be (s) solid. Predict the products of this reaction: calcium fluoride + sodium sulfate CaF 2 (aq) + Na 2 SO 4 (aq) Now, put in the phases of the products and balance the equation. Predict the products of this reaction: potassium phosphate + magnesium chloride K 3PO 4 (aq) + MgCl 2 (aq) Now, put in the phases of the products and balance the equation. 17 One of the products of a reaction between silver nitrate and potassium carbonate will be: A B C D silver potassium potassium carbonate nitrate carbonate potassium nitrate So far, the single replacement and double replacement reactions we have examined show all ions reacting. In truth, only some of the ions actually take part in the reaction. To more accurately show what species are actually reacting, we write (1) a complete ionic equation and (2) a net ionic equation. 10

The complete ionic equation shows all aqueous substances (i.e. strong acids, strong bases, and soluble ionic compounds) dissociated into their ions. For example: NaCl (aq) would be written as Na + (aq) + Cl (aq) The equation between silver nitrate and potassium chloride would normally be shown as AgNO 3 (aq) + KCl(aq) AgCl (s) + KNO 3 (aq) However, when all the aqueous species are dissociated into their respective ions, it becomes: Ag + (aq) + NO 3 (aq) + K + (aq) + Cl (aq) AgCl (s) + K + (aq) + NO 3 (aq) This is called the complete ionic equation. Note that the insoluble product does not dissociate. Ag (aq) + NO 3 (aq) + K + (aq) + Cl (aq) AgCl (s) + K + (aq) + NO3 (aq) As this example shows, certain ions are present both before and after the reaction. Because they play no significant role in the reaction, they are known as "spectator ions." A spectator ion is an ion that appears on both sides of an equation and is not directly involved in the reaction. The net ionic equation is an equation for a reaction in solution that shows only those particles that are directly involved in the chemical change. To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. In other words, cancel out the spectator ions. Ag + (aq) + NO 3 (aq) + K + (aq) + Cl (aq) AgCl (s) + K + (aq) + NO 3 (aq) Ag + (aq) + NO 3 (aq) + K + (aq) + Cl (aq) AgCl (s) + K + (aq) + NO 3 (aq) After cancelling out the spectator ions, the only things left in the equation are those things that change, or react, during the course of the reaction. Ag + (aq) + Cl (aq) AgCl (s) This is called the net ionic equation. Write a balanced molecular equation. Dissociate any reactants or products that are aqueous. All solids, liquids, and gases stay intact and do not dissociate. Cross out anything that remains unchanged from the left side to the right side of the equation. Write the net ionic equation with the species that remain. 11

Write a balanced molecular equation. Pb(NO 3) 2 (aq)+ 2 KI (aq) PbI 2 (s)+ 2 KNO 3 (aq) Dissociate any reactants or products that are aqueous. Pb 2+ (aq) + 2NO 3 (aq)+ 2K + (aq)+ 2I (aq) PbI 2 (s)+ 2K + (aq)+ 2NO 3 (aq) Cross out anything that remains unchanged from the left side to the right side of the equation. Pb 2+ (aq) + 2NO 3 (aq)+ 2K + (aq)+ 2I (aq) PbI 2 (s)+ 2K + (aq)+ 2NO 3 (aq) Practice writing the balanced equation, complete ionic equation and net ionic equation for these reactions. 1. calcium nitrate (aq) and potassium carbonate (aq) 2. silver nitrate (aq) and sodium phosphate (aq) Write the net ionic equation with the species that remain. Pb 2+ (aq) + 2 I (aq) > PbI 2 (s) 12