Chemical Reactions

Chemical Reactions Reactions involve chemical changes in matter resulting in new substances Reactions involve rearrangement and exchange of atoms to produce new substances Reactants ® Products

Chemical reaction = the process by which one or more substances are changed into one or more different substances. Elements are not changed during a reaction

Evidence of a chemical reaction/change:1. Release of a gas2. Unexpected color change3. Changes in heat or light4. Formation of a precipitate5. Production of an odor(Sometimes there isn’t any evidence!)

Chemical Equations Chemical changes are classified and represented by balanced chemical equations These are a shorthand way to describe the changes that substances undergo.

Chemical Equations

Provide information about the reaction:◦Formulas of reactants and products◦States of reactants and products◦Relative numbers of reactant and product molecules that are required

◦Can be used to determine mass of reactants used and of products that can be made

Writing equations Use proper formulas for each reactant and product Proper equation should be balanced - obey the Law of Conservation of Mass◦all elements on reactants side also on product side

◦equal numbers of atoms of each element on reactant side as on product side

Symbols used in equations Symbols used after chemical formula to indicate state◦(g) = gas; (l) = liquid; (s) = solid◦(aq) = aqueous, dissolved in water

Ions in solution (aq) Ionic compounds dissolved in water are often written with (aq);

Example: NaCl (s) NaCl(aq) When dissolved, ionic compounds are split in ions: NaCl (s) Na+(aq) + Cl-(aq)

Writing Balanced Equations

Steps:1. Determine the reactants and products.2. Write the reactants on the left side, and the

products on the right.3. Balance the equation by placing coefficients

where needed (NEVER change subscripts in a formula in order to balance an equation).

4. Use the lowest whole number coefficients possible.

Example

___H2(g) + ___O2(g) ___H2O(l)

2 H2(g) + O2(g) 2 H2O(l)

Practice

__SnS2(s) + __O2(g) __SnO2(s) + __SO2(g)

__Al(s) + __HCl(aq) __AlCl3(aq) + __H2(g)

PRACTICE

__C2H6(g) + __ O2(g) __ CO2(g) + __ H2O(g)

PRACTICE

Example – Recognizing Reactants and Products When iron metal (steel wool) burns in air

it produces a red compound iron (III) oxide◦ Burning in air means reacting with O2◦ Metals are solids, except for Hg which is

liquid◦ Ionic compounds are solid unless they

are dissolved in water (aq)

write the equation in words◦ identify the state of each chemical

iron(s) + oxygen(g) ® iron(III) oxide(s)

write the equation in formulas◦ identify diatomic elements◦ identify polyatomic ions – count as 1 “unit”

when balancing IF it does not change◦ determine formulas

4 Fe(s) + 3 O2(g) ® 2 Fe2O3 (s)

Practice For each of the following, write (including states) and balance the chemical equation:1. Aqueous aluminum nitrate reacts with

aqueous sodium hydroxide to produce aluminum hydroxide as a precipitate and aqueous sodium nitrate.

2. Iron reacts with sulfuric acid (H2SO4) to produce aqueous iron(III) sulfate and hydrogen.

Practice 3. Ethyne (C2H2) gas reacts with oxygen to produce carbon dioxide and water.

4. Magnesium reacts with nitrogen to produce magnesium nitride.

Classifying Chemical Reactions

- Reactions can be classified into 5 types.- Recognizing these types can be used to

predict the products or reactants of a reaction.

- Some reactions fit into more than 1 class

1. Single Replacement One element or ion replaces another element in a compound.

2 Al (s) + 3 CuCl2 (aq) 2 AlCl3 (aq) + 3 Cu (s)

The Activity Series of Metals

Activity series I: Fe(s) + CuCl2(aq) FeCl2(aq) + Cu(s)

II: 2Ag(s) + CuCl2(aq) 2AgCl(aq) + Cu(s) Iron will replace copper, but silver will not. So reaction I will occur, but reaction II will not.

2. Double Replacement Positive and negative portions of two compounds interchange.

Pb(NO3)2(aq) + 2 KI(aq) PbI2(s) + 2 KNO3(aq)

3. Decomposition Substance breaks down into simpler substances; starts with 1 compound and giving 2 (or more) compounds. 2 H2O2(l) 2 H2O(l) + O2(g)

4. Combination (synthesis) Two (or more) substances combine to form one new compound.

2 Mg(s) + O2(g) 2 MgO(s)

5. Combustion Oxidation (reaction with oxygen) of an organic compound (containing C and H) normally forming carbon dioxide and water.

C2H5OH(l) + 3 O2(g) 2 CO2(g) + 3 H2O(l)

Practice For each of the following, write (including states) and balance the chemical equation and give the reaction type:

Nitrogen reacts with oxygen to produce dinitrogen pentoxide gas

Zinc metal reacts with aqueous copper(II) sulfate.

Aqueous silver nitrate reacts with aqueous potassium chloride.

Potassium metal reacts with chlorine.

Practice Ethane (C2H6) gas burns in the presence of oxygen.

Aluminum metal reacts with (aqueous) hydrochloric acid (HCl).

Water reacts when subjected to electricity (produces oxygen and hydrogen gas).

Aqueous sodium carbonate reacts with (aqueous) nitric acid (HNO3).

Precipitation reactions Double replacement reactions often (but not always) form a precipitate

Pb(NO3)2 (aq) + 2 KI (aq) PbI2 (s) + 2 KNO3 (aq)

These reactions are known as precipitation reactions

Ionic equations Precipitation reaction in ions:

Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) + 2 I-(aq)

PbI2(s) + 2 K+(aq) + 2 NO3-(aq)

Such an ionic equation can be simplified by omitting the non reacting ions, giving the net ionic equation.

K+ and NO3- are not reacting and therefore are called

spectator ions.

Net ionic equation Equation (in water) showing only the reacting ions:

Pb2+(aq) + 2 I-(aq) PbI2(s)

PbI2 is a precipitate (so not in ionic form anymore).

Acid Base reaction A double replacement reaction between an acid and a base is called an acid-base reaction.

The reaction products are a salt and water. HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

As ionic equation: H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)

H2O(l) + Na+(aq) + Cl-(aq)

So the net ionic equation is: H+(aq) + OH-(aq) H2O(l)

Gas forming reactions Reactions in which a gas is formed.

Mg(s) + 2 HCl(aq) H2(g) + MgCl2(aq)