Chemical Reactions

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) H2 + ZnCl2(aq)

2H2O2(l) KI O O2(g)2(g) + 2H + 2H2OO(l)

Pb(NO3)2 (aq) + 2KI(aq) PbI2(s) + 2KNO3 (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) + O2(g) 2MgO(s)

8Fe(s) + S8(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 H2O (l) 2H2(g) + O2(g)

2KClO3(s) 2KCl(s) + 3O2(g)

CaCO3(s) CaO(s) + CO2(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

http://www.marymount.k12.ny.us/marynet/stwbwk05/05flashchem/avreaction/avreaction.html

Examples of Single Replacement Reactions

2Al(s) + 3NiSO4(aq) Al2(SO4)3(aq) + 3Ni(s)

2Fe(s) + 6HBr (aq) 2FeBr3 (aq) + 3H2(g)

Cl2(g) + 2KI(aq) 2KCl(aq) + I2(s)

Double Replacement Reactions

Involves the exchange of the positive ions between two compounds

Also called metathesis

http://www.marymount.k12.ny.us/marynet/stwbwk03/03hchem/nfreaction/nfreaction.html

Examples of Double Replacement Reactions

CaCl2 (aq) + K2CO3 (aq) CaCO3 (s) + 2KCl(aq)

3AgNO3 (aq) + FeCl3 (aq) 3AgCl(s) + Fe(NO3)3

(aq)

Combustion Reactions Oxygen reacts with another

substance usually producing energy in the form of heat and/or light

Examples of Combustion Reactions

CH4(g) + 2O2(g) ---> CO2(g) + 2H2O(g) + heat

2C2H6(g) + 7O2(g) ---> 4CO2(g) + 6H2O(g) + heat

C6H12O6(s) + 6O2(g) ---> 6CO2(g) + 6H2O(g) + heat

4C2H5OH(l) + 13O2(g) ---> 8CO2(g) + 10H2O(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) CaCl2 (aq) + 2H2O (l)

Acid + Base Salt + Water

3H2SO4(aq) + 2Fe(OH)3(s) Fe2(SO4)3(aq)+ 6H2O(l)

Acid + Base Salt + Water