Chemical Equations

Coefficient  subscript = # atoms Chemical Equations (Shorthand communication for a great deal of information) 1. Write the unbalanced equation making sure each chemical formula is correctly written. 2. Count atoms on each side. 3. Add coefficients to make #’s equal. Coefficient  subscript = # atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

Law of Conservation of Mass In a chemical reaction, mass (or matter) is not created or destroyed. Atoms can only rearrange to form new substances “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

Chemical Equations Are concise representations of chemical reactions

Anatomy of a Chemical Equation CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Anatomy of a Chemical Equation CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Reactants appear on the left side of the equation.

Anatomy of a Chemical Equation CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Products appear on the right side of the equation.

Anatomy of a Chemical Equation CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) The states of the reactants and products are written in parentheses to the right of each compound.

Anatomy of a Chemical Equation CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Coefficients are inserted to balance the equation.

Subscripts and Coefficients Give Different Information Subscripts tell the number of atoms of each element in a molecule

Subscripts and Coefficients Give Different Information Subscripts tell the number of atoms of each element in a molecule Coefficients tell the number of molecules, formula units or atoms

Chemical Equations 2 H2 + O2 2 H2O N2O5(g) + H2O 2 HNO3 Must have equal numbers of atoms of each element on each side of the equation = BALANCED EQUATION NOTE The coefficients in front of the formula for a compound refers to the number of particles (intact) involved while a subscript refers to the ratio of atoms within the particle 2 H2 + O2 2 H2O 4 hydrogen 4 hydrogen 2 oxygen 2 oxygen N2O5(g) + H2O 2 HNO3 2 nitrogen 2 nitrogen 6 oxygen 6 oxygen 2 hydrogen 2 hydrogen

Never change subscripts in formulas when balancing chemical reactions! Chemical Equations Chemical Equations balancing equations often requires some trial and error of coefficients NOTE NOTE Never change subscripts in formulas when balancing chemical reactions! Changing subscripts change the identity of compounds; coefficients change amounts PCl3(l) + 3 H2O(l) H3PO3(aq) + 3 HCl 6 hydrogen 6 hydrogen 3 oxygen 3 oxygen 1 phosphorus 1 phosphorus 3 chloride 3 chlorine C6H12(l) + 9 O2(g) 6 CO2(g) + 6 H2O(l) 6 carbon 6 carbon 18 oxygen 18 oxygen 12 hydrogen 12 hydrogen

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: C2H4 + O2 CO2 + H2O

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: C2H4 + O2 CO2 + H2O C C H H O O

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: C2H4 + O2 CO2 + H2O C 2 C 1 H 4 H 2 O 2 O 3

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: C2H4 + O2 2 CO2 + H2O C 2 C (1)2= 2 H 4 H 2 O 2 O 3 5 Chapt. 3.1

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: C2H4 + O2 2 CO2 + 2H2O C 2 C (1)2= 2 H 4 H (2)2 = 4 O 2 O 3 5 6 Chapt. 3.1

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: C2H4 + 3O2 2 CO2 + 2H2O C 2 C (1)2= 2 H 4 H (2)2 = 4 O(2)3 = 6 O 3 5 6

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: Al + HCl AlCl3 + H2

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: Al + HCl AlCl3 + H2 Al Al H H Cl Cl

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: Al + HCl AlCl3 + H2 Al 1 Al 1 H 1 H 2 Cl 1 Cl 3

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: Al + 3HCl AlCl3 + H2 Al 1 Al 1 H(1)3 = 3 H 2 Cl(1)3 = 3 Cl 3

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: Al + 6HCl AlCl3 + 3H2 Al 1 Al 1 H(1)6 = 6 H (2)3 = 6 Cl(1)6 = 6 Cl 3

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: Al + 6HCl 2AlCl3 + 3H2 Al 1 Al (1)2 = 2 H(1)6 = 6 H (2)3 = 6 Cl(1)6 = 6 Cl (3)2 = 6

Chemical Equations Sample exercise: Balance the following equations by providing the missing coefficients: 2Al + 6HCl 2AlCl3 + 3H2 Al(1)2 = 2 Al (1)2 = 2 H(1)6 = 6 H (2)3 = 6 Cl(1)6 = 6 Cl (3)2 = 6

Chemical Reactivity Chemical Reactions The course of a chemical reaction can often be predicted by recognizing general patterns of reactivity through similar reactions previously observed. Elements in same family (column of table) have similar reactions. The periodic table is helpful in predicting products of reactions. Atoms like to assume electron configurations of the Noble Gases.

Example, if you know that Chemical Reactivity Chemical Reactions Example, if you know that 2Li + 2H20 2LiOH + H2 then you should be able to predict the products from the reaction of Na, K and the other members of group 1 (alkali metals) with water. Thus a general reaction would be; 2 M + 2 H2O 2 MOH + H2

Types of Chemical Reactions: Direct Combination/Synthesis Reactions Decomposition/Analysis Reactions Single Replacement Reactions Double Replacement Reactions Combustion Reactions

Direct Combination or Synthesis the combination of 2 or more substances to form a compound only one product forms A + B  AB ex: 2P + 3Br2  2PBr3

Decomposition or Analysis a compound breaks down into 2 or more simpler substances only one reactant AB  A + B ex: 2H2O2  2H2O + O2

A + BC  AC + B ex: Zn + 2HCl  ZnCl2 + H2 Single Replacement one element replaces another in a compound metal replaces metal (+) OR nonmetal replaces nonmetal (-) A + BC  AC + B ex: Zn + 2HCl  ZnCl2 + H2

ex: 2KOH + CuSO4  K2SO4 + Cu(OH)2 Double Replacement ions in two compounds “change partners” cation(+) of one compound combines with anion(-) of the other AB + CD  AD + CB ex: 2KOH + CuSO4  K2SO4 + Cu(OH)2

Combustion - means add oxygen the burning of a substance in O2 to produce heat Often releases energy in the form of heat and light If the combustion is complete, the products will be CO2 and H2O. If the combustion is incomplete, the products will be CO and H2O. CxHy+O2  CO2+H2O ex: CH4 + 2O2  CO2 + 2H2O

Direct Combination/Synthesis Reactions - when two or more elements or compound(s) react to form a product (opposite of decomposition reactions) A + B C often energy (heat) is released 2 Na(s) + 3 N2(g) 2 NaN3(s) (ex. Air Bag Inflator)

Decomposition/Analysis Reactions when one compound reacts to form two or more elements or compounds (opposite of synthesis reactions) C A + B often energy (heat) is required 2 NaN3(s) 2 Na(s) + 3 N2(g) (ex. Air Bag Inflator)

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: Solid mercury (II) sulfide decomposes into its component elements when heated.

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: Solid mercury (II) sulfide decomposes into its component elements when heated. Hg+2 S-2 HgS  Hg + S

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: The surface of aluminum metal undergoes a combination reaction with oxygen in air.

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: The surface of aluminum metal undergoes a combination reaction with oxygen in air. Al+3 O-2 Al + O2  Al2O3

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: The surface of aluminum metal undergoes a combination reaction with oxygen in air. Al+3 O-2 4Al + 3O2  2Al2O3

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: Si2H6 burns when exposed to air.

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: Si2H6 burns when exposed to air. Hint, Si is in the same group as C, and therefore reacts similarly. Si2H6 + O2  SiO2 + H2O

Chemical Reactivity Sample Exercise: Write balanced chemical equations for the following reactions: Si2H6 burns when exposed to air. Hint, Si is in the same group as C, and therefore reacts similarly. 2Si2H6 + 7O2  4SiO2 + 6H2O

Single Replacement Reactions - a free element becomes an ion and an ion in solution becomes a neutral atom A + BC  B + AC Cl2(g) + 2NaBr(aq)  Br2(l) + 2NaCl(aq)

Double Replacement Reactions when ionic “partners” switch AB + CD AD + BC (often in aqueous solutions) Ag(NO3) + KCl AgCl(s) + KNO3 BaCl2 + Na2SO4 BaSO4(s) + 2NaCl