CHEMICAL REACTIONS Reactants: Zn + I2 Product: Zn I2

Introduction Chemical reactions occur when bonds between atoms are formed or broken Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Chemical reactions are written in symbolic form How can you tell a chemical reaction is happening? Color, heat/light, bubbles

Energy is in the bonds! Endothermic rxns – bonds made and energy is stored Exothermic rxns – bonds broken and energy is released

Parts of a Reaction Equation reactants (molecules on left of arrow) products (molecules on right of arrow). “+” sign separates molecules on same side “” is read as “yields” Ex: carbon plus oxygen react to yield carbon dioxide C + O2  CO2

Other symbols Coefficients - The numbers in front Show relative amounts of reactants & products Shows the “recipe” 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s) Letters (s), (g), and (l) are the physical states of compounds.

Chemical Equations Lavoisier, 1788 Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.

Balancing Equations When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

Steps to Balancing Equations DO NOT CHANGE THE FORMULAS! Always start on the left. Compare the number of atoms for each element on either side of the reaction Add or change COEFFICIENTS to balance. Every time you make a change, go back to the beginning. If polyatomic ions are present on both sides, treat them as one unit. Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios. (reduced)

Balancing Equations 2 2 ___ H2(g) + ___ O2(g) ---> ___ H2O(l) What Happened to the Other Oxygen Atom????? This equation is not balanced! What coefficients need to be added to balance this equation?

2 3 ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

Balancing Equations 5 ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g) 3 4 2 11 ____B4H10(g) + _____ O2(g) ----> ___ B2O3(g) + _____ H2O(g) 5.5 4 10 2 5

Balancing Equations Na3PO4 + Fe2O3 ----> Na2O + FePO4 Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na3PO4 + Fe2O3 ----> Na2O + FePO4 2 3 2

Which Is Balanced? Now Try These: Mg + O2  MgO 2Mg + O2  2MgO a) P4 + O2  P4O10 b) Li + H2O  H2 + LiOH c) Bi(NO3)3 + K2S  Bi2S3 + KNO3 d) C2H6 + O2  CO2 + H2O 5 2 2 2 2 3 6 3.5 2 3 C2H6 + O2  CO2 + H2O 2 7 4 6

Balance these skeleton equations: a) Mg + 2HCl  MgCl2 + H2 b) 3Ca + N2  Ca3N2 c) NH4NO3  N2O + 2H2O d) 2BiCl3 + 3H2S  Bi2S3 + 6HCl e) 2C4H10 + 13O2  8CO2 + 10H2O f) 6O2 + C6H12O6  6CO2 + 6H2O g) 3NO2 + H2O  2HNO3 + NO h) Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4 i) Al4C3 + 12H2O  3CH4 + 4Al(OH)3

Here are some more to balance: a) 2KNO3  2KNO2 + O2 b) 2Pb(NO3)2  2PbO + 4NO2 + O2 c) P4 + 6I2  4PI3 d) 3MgO + 2H3PO4  Mg3(PO4)2 + 3H2O e) Br2 + 2KI  I2 + 2KBr f) Ca(OH)2 + 2HNO3  Ca(NO3)2 + 2H2O g) Bi2O3 + 3H2  2Bi + 3H2O h) 3Fe + 2O2  Fe3O4 i) 2CaO + 5C  2CaC2 + CO2

a) 2Li + 2H2O ® H2 + 2LiOH b) P4 + 5O2 ® P4O10 c) 2C2H6 + 7O2 ® 4CO2 + 6H2O d) CS2 + 3O2 ® CO2 + 2SO2 e) 2AsCl3 + 3H2S ® As2S3 + 6HCl f) 3AgNO3 + FeCl3 ® 3AgCl + Fe(NO3)3 g) 2KClO3 ® 2KCl + 3O2 h) 2SO2 + O2 ® 2SO3