Chemical Reactions & Equations +

Evidence of Chemical Reactions Release of a gas Example: bubbles formed when magnesium and hydrochloric acid were mixed Color change Example: color went from clear to yellowish orange when potassium iodide was added to hydrogen peroxide Formation of a precipitate Example: cloudiness occurred when CO2 gas passed through limewater Change in temperature, light, sound, smell Example: temperature increased when hydrochloric acid and sodium carbonate were mixed

Endothermic or Exothermic? CaO (s) + H2O(l) Ca(OH)2(aq) + 82 kJ Since energy is a product, this reaction is exothermic. This reaction will generate enough heat to fry an egg! NH4NO3(s) + H2O(l) + energy NH4NO3(aq) Since energy is a reactant, this reaction is endothermic. This is the cooling reaction of a cold pack.

Chemical Equations Reactants Products C3H8 O2 + + CO2 H2O energy substances before the change C3H8 O2 + + Products substances after the change CO2 H2O energy Propane and oxygen react to produce carbon dioxide, water, and energy. This reaction is not yet balanced.

Symbols in Equations + (s) (l) (g) (aq) D N.R. “plus” or “and” indicates multiple reactants or products “yields” or “produces” separates reactants & products, arrow points in the direction of the reaction (s) “solid” (l) (g) indicate phase of a reactant or product “liquid” “gas” (aq) “aqueous” dissolved in water (not the same as liquid (l)) indicates reaction is reversible D over the arrow, indicates heating is necessary N.R. “no reaction”

Balancing Equations When balanced, an equation accounts for the conservation of: mass charge energy Balance the numbers of atoms of each element on both sides of the equation by determining the coefficients of the compounds. DO NOT CHANGE SUBSCRIPTS!

An Example: Forming Water H2 (g) + O2 (g) H2O (l) On left: 2 H and 2 O On right: 2 H and 1 O Therefore, not balanced We could use 1/2 O2 on the left but that is not done, we always want whole number coefficients. Instead we double the coefficients of H2 and H2O 2H2 (g) + O2 (g) 2H2O (l) There are now 4 H’s and 2 O’s on each side.

Rules for Balancing Equations 6H2O coefficient subscript Write the formulas for all reactants and products Don’t change formulas (subscripts), only coefficients. Balance elements that only occur once on each side of the equation first To determine the number of atoms of an element, multiply the coefficient by the subscript for the element. Tips: for reactions involving oxygen, balance it last polyatomic ions can be balanced as a unit

Balancing Example Write the equation for burning ethylene (C2H4) to produce carbon dioxide and water. C2H4 + O2 CO2 + H2O C and H both appear once on each side of the equation whereas O appears in both compounds on the right hand side. Multiply CO2 by 2 to balance C Multiply H2O by 2 to balance H C2H4 + O2 2CO2 + 2H2O There are now 6 O’s on the right so multiply O2 by 3 to finish. C2H4 + 3O2 2CO2 + 2H2O

Balancing Practice #1 Unbalanced equation: K + F2 KF Balance F: Balance K: 2K + F2 2KF

Balancing Practice #2 Unbalanced: Notice Ca is fine: Balance O: Ca + H2O Ca(OH)2 + H2 Notice Ca is fine: Ca + H2O Ca(OH)2 + H2 Balance O: Ca + 2H2O Ca(OH)2 + H2 Hydrogen is already taken care of so we’re done.

Balancing Practice #3 Cl: N: O: H: 10 H on each side, so we’re done! NH4Cl + Ca(OH)2 NH3 + H2O + CaCl2 2NH4Cl + Ca(OH)2 NH3 + H2O + CaCl2 Cl: 2NH4Cl + Ca(OH)2 2NH3 + H2O + CaCl2 N: 2NH4Cl + Ca(OH)2 2NH3 + 2H2O + CaCl2 O: H: 2NH4Cl + Ca(OH)2 2NH3 + 2H2O + CaCl2 10 H on each side, so we’re done!

5 Types of Chemical Reactions Combustion Synthesis Decomposition Single Replacement Double Replacement Some reactions fall into multiple categories.

Combustion Reactions A substance reacts with oxygen to create products containing oxygen. Heat may be required to initiate. Exothermic. hydrocarbon + O2 (g) CO2 (g) + H2O(g) Hydrocarbons (C, H, O) - heat homes, cook food, and power transportation. Always get CO2(g) and H2O(g). 2Mg(s) + O2(g) 2MgO(s) Metals produce metal oxides (like rust and tarnishes).

Practice Combustion Write the balanced equation for the combustion of heptane gas (C7H16) Write out the reactants and products: C7H16(g) + O2(g) CO2(g) + H2O(g) + heat Balance the equation (ignore heat): C7H16(g) + 11 O2(g) 7 CO2(g) + 8 H2O(g)

Synthesis Reactions The combining of two or more substances to form a more complex substance. general form A + B AB Note: A & B can be elements or compounds 2Na + Cl2 2NaCl sodium chloride Examples: 2Mg + O2 2MgO magnesium oxide

Decomposition Reactions Compound breaks down into two or more simpler substances (elements or compounds). general form AB A + B NH4NO3(s) N2O(g) + 2H2O(g) Examples: 2H2O(l) 2H2(g) + O2(g) electricity

Single Replacement Reactions One element takes the place of another element. Metals replace metals. Non-metals replace non-metals. Not reversible. A + BC BA + C general forms A + BC AC + B Cl2(aq) + 2KBr(aq) 2KCl(aq) + Br2(aq) Example:

Double Replacement Reactions When solutions of two soluble ionic compounds are mixed, an insoluble product (precipitate) forms. general form AB + XY AY + XB Remember “inny outy” rule Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq) Example:

More on Double Replacement When 2 ionic solutions are mixed there are several events that could happen: Nothing, everything is soluble A precipitate forms (possibly two) A gas is formed (like FeS(s) + H2SO4(aq) --> FeSO4(aq) + H2S(g) A covalent molecule forms (like H+ + OH- --> H2O) Ions that remain in solution are called spectator ions because they don’t participate in the reaction.

Practice - Identifying Reaction Types Ca(s) + CuSO4(aq) CaSO4(aq) + Cu(s) 2AlCl3(l) 2Al(l) + 3Cl2(g) ZnSO4(aq) + SrCl2(aq) ZnCl2(aq) + SrSO4(s) 4Na(s) + O2(g) 2Na2O (s) single replacement decomposition double replacement synthesis, combustion