Chemical Reactions

All chemical reactions involve changes in substances Reactants – starting substances Products – new substances formed Reactants  Products

Example: Iron + Oxygen  Iron (III) oxide (Rust)

In all chemical reactions: Bonds are broken and new bonds form Law of Conservation of matter is supported Atoms are not created or destroyed – just rearranged During an ordinary chemical change, there is no detectable increase or decrease in the quantity of matter

Symbols in Chemical Reactions + used to separate two products or reactants  yields (s) subscript – represents a solid (l) subscript – represents a liquid (aq) subscript – represents an aqueous solution (g) subscript – represents a gas

Symbols (cont.) replaces (s) to represent a precipitate replaces (g) to represent a gas

Symbols (cont.) Fe indicates the use of a catalyst (in this case iron) heat or  indicates that heat is supplied to the reaction

Examples: Zn (s) + 2HCl (aq)  H 2 + ZnCl 2(aq) O 2(g) + 2HO 2H 2 O 2(l) KI O 2(g) + 2H 2 O (l) Pb(NO 3 ) 2 (aq) + 2KI (aq)  PbI 2(s) + 2KNO 3 (aq)

Chemical Equations Use formulas to represent elements and compounds Atoms must balance on both sides of the equation Atom inventories are helpful

Rules for Balancing Chemical Equations Determine the correct formulas for all reactants and products in the reaction Reactants go on the left side of the equation Products on the right side of the equation Count the atoms of each element in the reactants and products Balance the elements one at a time by changing the coefficients (NOT subscripts)

Balancing Equations (cont.) Tricks that help Save uncombined elements for last Use least common multiple to balance elements that have odd and even atoms on opposite sides Remember polyatomic ions often remain intact in a reaction count them as a single part Write water as HOH in acid/base reactions

Balancing Equations (cont.) Check each atom or polyatomic ion to be sure that both sides are equal Make sure coefficients are in the lowest possible ratio

Now lets practice! Potassium chlorate is heated and breaks down into potassium chloride and oxygen gas – Write the balanced equation for this reaction. Hydrochloric acid reacts with zinc to produces hydrogen gas and zinc chloride solution, write the chemical equation for this reaction.

6 Types of Chemical Reactions Synthesis (combination) Decomposition Single Replacement Double Replacement Combustion Acid/Base

Synthesis Reactions Two or more substances react to form a single substance Two or more simple compounds combine to form a more complicated one

Examples of Synthesis Reactions 2Mg (s) + O 2(g)  2MgO (s) 8Fe (s) + S 8(s)  8 FeS (s)

Decomposition Reactions A single compound is broken down into two or more simpler substances a complex molecule breaks down to make simpler ones

Examples of Decomposition Reactions 2 H 2 O (l)  2H 2(g) + O 2(g) 2KClO 3(s)  2KCl (s) + 3O 2(g) CaCO 3(s)  CaO (s) + CO 2(g)

Single Replacement Reactions Atoms of an element replace the atoms of a second element in a compound Element 1+ Compound 1  Element 2 + Compound 2 net/stwbwk05/05flashchem/avreaction/ avreaction.html net/stwbwk05/05flashchem/avreaction/ avreaction.html

Examples of Single Replacement Reactions 2Al (s) + 3NiSO 4(aq)  Al 2 (SO 4 ) 3(aq) + 3Ni (s) 2Fe (s) + 6HBr (aq)  2FeBr 3 (aq) + 3H 2(g) Cl 2(g) + 2KI (aq)  2KCl (aq) + I 2(s)

Double Replacement Reactions Involves the exchange of the positive ions between two compounds Also called metathesis net/stwbwk03/03hchem/nfreaction/nfre action.html net/stwbwk03/03hchem/nfreaction/nfre action.html

Examples of Double Replacement Reactions CaCl 2 (aq) + K 2 CO 3 (aq)  CaCO 3 (s) + 2KCl (aq) 3AgNO 3 (aq) + FeCl 3 (aq)  3AgCl (s) + Fe(NO 3 ) 3 (aq )

Combustion Reactions Oxygen reacts with another substance usually producing energy in the form of heat and/or light

Examples of Combustion Reactions CH 4(g) + 2O 2(g) ---> CO 2(g) + 2H 2 O (g) + heat 2C 2 H 6(g) + 7O 2(g) ---> 4CO 2(g) + 6H 2 O (g) + heat C 6 H 12 O 6(s) + 6O 2(g) ---> 6CO 2(g) + 6H 2 O (g) + heat 4C 2 H 5 OH(l) + 13O 2(g) ---> 8CO 2(g) + 10H 2 O (g) + heat

Acid/Base Reactions acid with a base produces a salt plus water The cation in the salt comes from the base; the anion comes from the acid

Examples of Acid/Base Reactions 2HCl (aq) + Ca(OH) 2 (aq)  CaCl 2 (aq) + 2H 2 O (l) Acid + Base  Salt + Water 3H 2 SO 4(aq) + 2Fe(OH) 3(s)  Fe 2 (SO 4 ) 3(aq) + 6H 2 O (l) Acid + Base  Salt + Water