Chapter 11 Chemical Reactions

Word Equations  Reactants  Products  Law of conservation of mass  Iron + oxygen  iron(III) oxide  Hydrogen peroxide  water + oxygen  Methane + oxygen  carbon dioxide + water

Chemical Equations  Fe + O 2  Fe 2 O 3  Skeleton equations show reactants and products but do not give relative amounts  Fe (s) + O 2(g)  Fe 2 O 3(s)  Physical state can be indicated by (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution

Chemical Equations  What is a catalyst?  A catalyst is a species that speeds up the rate of a reaction, but is not consumed.

Balancing Chemical Equations  In a balanced equation, each side must have the same number of atoms of each element.  C (s) + O 2(g)  CO 2  Already balanced!  H 2(g) + O 2(g)  H 2 O (l)  Not balanced. What can we do to balance it?

 Creating a bicycle:  Frame + wheel + handlebar + pedal  bicycle  F + W + H + P  FW 2 HP 2  However, this does not tell us the quantities of the reactants.  The balanced equation would be:  F + 2W + H + 2P  FW 2 HP 2 Balancing chemical equations

Rules for Balancing Equations 1. Determine the correct formulas for all reactants and products 2. Write reactants on the left and products on the right (skeleton equation) 3. Count the number of atoms of each element in the reactants and products

Rules for Balancing Equations 4. Balance the elements 1 at a time using coefficients. It is best to begin with elements that appear only once on each side of the equation. Do not change subscripts! 5. Check each atom or ion to be sure that it is balanced 6. Make sure that the coefficients are in the lowest possible ratio that balances

Examples Problem: AgNO 3(aq) + Cu (s)  Cu(NO 3 ) 2(aq) + Ag (s) Answer: 2AgNO 3(aq) + Cu (s)  Cu(NO) 3(aq) + 2Ag (s)

TYPES OF Chapter 11

Reaction Types The five general types of reaction are Combination Decomposition Single-replacement Double-replacement Combustion

Combination Reactions Two or more substances form a single new substance An example is 2Mg + O 2  2MgO We can represent combination reactions as A + B  AB Complete and balance the following combination reaction: Be + O 2  Answer: 2Be + O 2  2BeO

Decomposition Reactions A single compound breaks down into two or more simpler products An example is 2HgO  2Hg + O 2 We can represent combination reactions as AB  A + B Complete and balance the following decomposition reaction: HI  Answer: 2HI  H 2 + I 2

Single Replacement One element replaces another element from a compound An example is 2K + 2H 2 O  2KOH + H 2 We can represent single replacement reactions as A + BC  AC + B Complete and balance the following single replacement reaction: Zn + Ag(NO 3 )  Answer: Zn + 2Ag(NO 3 )  2Ag + Zn(NO 3 ) 2

Double Replacement  How would we show the general equation for a double replacement? (Use A,B,C, and D)  Double replacements consist of ionic compounds (charges must equal zero)  Therefore: AB + CD  AD + CB

Double Replacement  An example of a double replacement reaction: K 2 CO 3 + BaCl 2  2KCl + BaCO 3  Notice that the cations and anions exchange places

Combustion  Finally, a combustion reaction is easy to identify because it usually involves a hydrocarbon reacting with oxygen to form water and carbon dioxide.  Example: CH 4 + 2O 2  CO 2 + 2H 2 O  What is CH 4 ?  Why does oxygen exist as a diatomic molecule?

Predicting Products What are some clues that might help you to predict the products of reactions?

PRACTICE  Complete and balance the reaction: Al + Cl 2   What type of reaction is it?  Do the same for the following Mg + Cl 2  C 2 H 2 + O 2 

Reactions in Aqueous Solution 70 % Earth’s surface is water 66 % Human body is water Chemistry in water is very important.

Net Ionic Equations  When the same ionic compounds dissolve in water, they dissociate into ions.  For example, NaCl becomes NA + and CL -  AgNO 3 becomes Ag + and NO 3 -

Complete Ionic Equation  A complete ionic equation shows dissolved ionic compounds as dissociated free ions.  Ag + (aq) + NO - 3(aq) + Na + (aq) + Cl - (aq)  AgCl (5) + Na + (aq) + Cl - (aq)

…continued  Ions that appear unchanged on both side of the equation may be cancelled out.  So, Ag + (aq) + Cl - (aq)  AgCl (5)  The new equation is the net ionic equation  Na + (aq) + NO - 3 (aq) are known as spectator ions.

Examples  Pb (s) + AgNO 3(aq)  Ag (s) +Pb(NO 3 ) 2(aq)  NO - 3(aq) is the spectator ion (doesn’t change)  Net ionic equation:  Pb (s) + Ag (aq)  Ag (s) + Pb 2+ (aq)  Balance the equation with respect to mass & charge.  Pb (s) + 2Ag + (aq)  2Ag (s) +Pb 2+ (aq)

Predicting Precipitates  You can predict the formation of a precipitate by using solubility rules.