Chapter 8 Chemical Reactions

What is a Chemical Reaction? A process by which one or more substances change into one or more new substances whose chemical and physical properties differ from the original substances.

Evidence? What do you see? Fizzing, smoke, color change, precipitate… What do you hear? Fizzing, popping, hissing… What do you smell? Smoke, good, bad… What do you feel? Heat, Cold…

Chemical vs. Physical Chemical Physical New substance is formed Different chemical properties Different physical properties Change of state Same chemical properties Physical properties such as density, boiling point, melting point remain the same

Reactants and Products Reactants are what reacts… what is found on the left side of the equation. Products are what is produced…what is found on the right side of the equation.

Energy Changes In an endothermic reaction, energy is considered to be a reactant Dinitrogen tetroxide + energy  nitrogen dioxide energy N2O4 --------- 2 NO2

Energy In an Exothermic reaction, energy is considered to be a product of the reaction. Methane + oxygen  carbon dioxide + water + energy CH4 + 2 O2 ---- CO2 + 2 H2O + energy

Equation and Reaction Info Equations are like recipes Tells you how much of each reactant to use Tells you what state the reactant should be in Indicates relative amounts of reactants and products

Equations show Physical States A correctly written equation will indicate the state that each reactant and product will be found in. (s) = solid (l) = liquid (g) = gas (aq) = aqueous (dissolved in water)

Additionally... Reaction conditions can be indicated above or below the “yields” arrow Δ = heat “heat” = heat (temp not specified) “Chem. Symbol” = name of chemical used as a catalyst in the reaction (speeds up the reaction)  = reversible reaction in which products can reform into reactants (equation may reach equilibrium)

Balancing Chemical Equations Conservation of Mass – In an ordinary chemical reaction, mass cannot be created or destroyed Reactions rearrange atoms – Reactants and Products are made of the same kinds of atoms – just rearranged In order to show Conservation of Mass, we must have the same number of each type of atom on each side…The equation must be Balanced!

NEVER CHANGE A SUBSCRIPT!!! The Numbers: Coefficient – The number in front of the compound – indicates the number of moles of the compound. Multiplies the number of atoms of each element in the formula 2 H2O = ? 4 CaSO4 = ? Subscript – Indicates the number of atoms of a particular element within a compound. Ca(ClO3)2 = ? (NH4)3PO4 = ? NEVER CHANGE A SUBSCRIPT!!!

Steps for Balancing Identify the Reactants and the Products Count the atoms Insert coefficients Verify results Fe2O3 + H2  Fe + H2O

P4 + O2  P2O5 C3H8 + O2  CO2 + H2O Ca2Si + Cl2  CaCl2 + SiCl4 Try These: P4 + O2  P2O5 C3H8 + O2  CO2 + H2O Ca2Si + Cl2  CaCl2 + SiCl4

Or with words: Silicon reacts with carbon dioxide to form silicon carbide, SiC, and silicon dioxide. Ammonia reacts with oxygen to produce nitrogen monoxide and water vapor.

Polyatomic Ions Can be balanced as a unit if they do exist on both sides of the equation Al + HAsO3  H2 + Al(AsO3)3

Try These: HgCl2 + AgNO3  Hg(NO3)2 + AgCl Al + Hg(CH3COO)2  Al(CH3COO)3 + Hg Calcium phosphate and water are produced when calcium hydroxide reacts with phosphoric acid.