Unit 9 Chemical Equations Part I-A

Chemical Equations Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. REACTANTS  PRODUCTS

Showing Phases in Chemical Equations Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s) Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H 2 O(l) Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl 2 (g) H 2 O(s) H 2 O(l) H 2 O(g)

Additional Symbols Used in Chemical Equations “Yields”; indicates result of reaction Used to indicate a reversible reaction A reactant or product in the solid state; also used to indicate a precipitate Alternative to (s), but used only to indicate a precipitate A reactant or product in the liquid state A reactant or product in an aqueous solution (dissolved in water) A reactant or product in the gaseous state (s)(s) (l)(l) (aq) (g)(g)

Additional Symbols Used in Chemical Equations Alternative to (g), but used only to indicate a gaseous product Reactants are heated Pressure at which reaction is carried out, in this case 2 atm Pressure at which reaction is carried out exceeds normal atmospheric pressure Temperature at which reaction is carried out, in this case 0 o C Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction 2 atm pressure 0 o C MnO 2 

Describing a Chemical Reaction Indications of a Chemical Reaction – Evolution of heat, light, and/or sound – Production of a gas – Formation of a precipitate – Color change

Chemical Equations reactantsproducts Depicts the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients stoichiometric coefficients. The letters (s), (g), and (l) are the physical states of compounds. aluminum oxide

Chemical Equations Because the same atoms are present in a reaction at the beginning (reactants) and at the end (products), the amount of matter in a system does not change. Law of Conservation of Mass The Law of Conservation of Mass Kotz web ChemicalFactory 100% 80% 20%

Chemical Equations conservation of mass Because of the principle of the conservation of mass, all equations must be balanced. There must be the same number of atoms of the same kind on both sides. Lavoisier, 1788

? Visualizing a Chemical Reaction Na + Cl 2 NaCl ___ mole Cl 2 ___ mole NaCl___ mole Na

Balancing Chemical Equations Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product What is the relationship between conservation of mass and the fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation? Determine whether the following equation is balanced. 2 Na + H 2 O  2 NaOH + H 2 2 Na + 2 H 2 O  2 NaOH + H 2

Balancing Chemical Equations Write a word equation for the reaction. Write the correct formulas for all reactants and products. Determine the coefficients that make the equation balance.

Balancing Chemical Equations Other examples NO(g) + O 2 (g)  NO 2 (g)is it balanced? Is this balanced? NO(g) + O(g)  NO 2 (g) Is this OK? NO(g) + ½ O 2 (g)  NO 2 (g) Is this balanced? Is this OK?

Balancing Chemical Equations An important point to remember 2 NO(g) + O 2 (g)  2NO 2 (g) The 2 to the left of NO(g) and NO 2 (g) refers to the number of molecules present in the balanced equation. It is a “multiplier” for every atom in the molecule. The subscript 2 in O 2 (g) and NO 2 (g) refers to the number of atoms of this type that are present in each molecules (or ionic compound).

(NH 4 ) 3 PO 4 + Mg(OH) 2  Mg 3 (PO 4 ) 2 NH 4 OH ? ammonium phosphate magnesium hydroxide magnesium phosphate ammonium hydroxide NH 4 1+ OH Now you try… AlCl 3 + Li 2 CO 3  Al 2 (CO 3 ) 3 + LiCl 3 2 6

1) Write a word equation for the reaction. Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. chlorine + sodium bromide  bromine + sodium chloride Cl 2 + NaBr  Br 2 + NaCl Cl NaBr  Br NaCl

1) Write a word equation for the reaction. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. aluminum sulfate + calcium chloride  calcium sulfate Al 2 (SO 4 ) 3 + CaCl 2  CaSO 4 + AlCl 3 Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate. Al 2 (SO 4 ) CaCl 2  3 CaSO AlCl 3 + aluminum chloride ? ?

CH O 2  CO H 2 O Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms