Unit 3: Balancing Equations, Types of Reactions Chapter 8

Key Concepts Chemical Reactions: Reactants yield Products Balancing Chemical Equations (coefficients) Writing Equations (subscripts + coefficients) Types of Reactions: Synthesis Decomposition Combustion Single-Replacement Double-Replacement (precipitation)

Chemical Reactions Chemical Reaction: substances change into new substances with different physical and chemical properties Reactants: go into a chemical reaction Products: come out of a chemical reaction

Chemical Equations Chemical Equation: shows the balanced formulas of the reactants and products Reactants Products Chemical Equation According to the law of conservation of mass, the amount of reactants is equal to the amount of products. [yield]

Chemical Equation Symbols Meaning (s), (l), (g) The substance is in a solid, liquid, or gaseous state (aq) The substance is in an aqueous solution (it’s dissolved in water) Indicates the results of a reaction; “yields” Reversible reaction where the result is a mix of both products and reactants heat The reaction requires heat to occur

Subscripts & Coefficients Subscript: the number of atoms in a molecule Coefficient: the number of molecules

Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g) Chemical Equations In the following equation, label the reactants, products, subscripts, coefficients, and phases Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g) REACTANTS PRODUCTS Green = Phases Orange = Coefficients Purple = Subscripts Calcium + Water = Calcium Hydroxide + Hydrogen gas One significant application of calcium hydroxide is as a flocculant, in water and sewage treatment.

Balancing Equations (coefficients) Ex. ___ C + ___ O2  ___ CO ___ H2 + ___ N2  ___ NH3 Tip: start with something that appears only once on each side ___ CH4 + ___ O2  ___ CO2 + ___ H2O Tip: start with C or H because they only appear once on each side 2 C + 1 O2  2 CO 3 H2 + 1 N2  2 NH3 1 CH4 + 2 O2  1 CO2 + 2 H2O

Writing Chemical Equations Write Skeleton Equation Subscripts Elements  check Br2I2N2Cl2H2O2F2 Ionic Compounds  Balance Charges Coefficients Balance Equation Ionic Compounds = Balance Charges

Writing Equations (subscripts + coefficients) Ex. hydrogen was combined with oxygen to yield water Ex. nitrogen and hydrogen yield ammonia (NH3) Ex. aluminum bromide plus chlorine yield aluminum chloride plus bromine 2 H2 + O2  2 H2O N2 + 3 H2  2 NH3 2 AlBr3 + 3 Cl2  2 AlCl3 + 3 Br2

Types of Reactions Synthesis Reactions Decomposition Reactions Combustion Reactions Single-Replacement Reactions Double-Replacement (precipitation) Reactions

Synthesis Reactions A + B  AB Synthesis: simple substances are combined to make a more complex substance A + B  AB

Decomposition Reactions Decomposition: a complex substance breaks apart into simpler substances Many explosives show violent decomposition AB  A + B

Combustion Reactions A + O2  AXOY CXHY + O2  CO2 + H2O Combustion: a substance burns in oxygen to form oxides Combustion with a hydrocarbon will always produce CO2 + H2O A + O2  AXOY CXHY + O2  CO2 + H2O

Single-Replacement Reactions Single-Replacement: a metal or a nonmetal element reacts with an ionic compound A + XY  X + AY

Double-Replacement Reactions Double-Replacement (precipitation): two ionic compounds react to produce at least one insoluble product AB + XY  XB + AY