Chemical Reactions:An Introduction

Indicators of a Chemical Reaction• A color change

• A solid forms (precipitate)

• A gas forms

• The temperature changes (hot OR cold)

Chemical EquationsReactants – what goes INTO the rxn

Products – what comes OUT OF the rxn

Reactant A + Reactant B Product A

Law of Conservation of MassIn a chemical reaction, atoms are neither

created or destroyed

All atoms in the reactants MUST be accounted for in the products.

Physical StatesAfter each reactant and product is a symbol

representing the physical state of the element or compound

(s) – solid(l) – liquid(g) – gas(aq) – aqueous (dissolved in water)

Balancing Chemical EquationsDone by trial and error

1. Balance elements that only appear once2. Keep polyatomic ions together if possible

3. ***NEVER EVER EVER EVER CHANGE A SUBSCRIPT!!!***

4. Balance hydrogen and oxygen last

Diatomic Molecules• 7 elements exist in nature as diatomic

molecules (2 atoms)– Hydrogen (H2)– Oxygen (O2)– Nitrogen (N2)– Fluorine (F2)– Chlorine (Cl2)– Bromine (Br2)– Iodine (I2)

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

Fe2O3(s) + HNO3(aq) Fe(NO3)3(aq) + H2O(l)

H2S(g) + Pb(NO3)2(aq) PbS(s) + HNO3(aq)

Types of Reactions

Objectives1. Give general equations for types of

reactions2. Classify reactions3. List 3 types of synthesis and 6

decomposition reactions4. List 4 types of single-replacement and 3

types of double-replacement reactions5. Predict products of reactions given the

reactants

Synthesis ReactionsGeneral Formula:

A + X AX

Synthesis with OxygenWith metals form metal oxides

Ex – Mg(s) + O2(g) MgO(s)

K(s) + O2(g) K2O(s)

2Fe(s) + O2(g) 2FeO(s)

4Fe(s) + 3O2(g) 2Fe2O3

S8(s) + 8O2(g) 8SO2(g)

C(s) + O2(g) CO2(g)

Synthesis with SulfurWith metals produce metal sulfides

Ex- 16Rb(s) + S8(s) 8Rb2S(s)

8Ba(s) + S8(s) 8BaS(s)

Metals with HalogensGroup 1: M + X2 2MX

Ex – Na(s) + Cl2(g) 2NaCl(s)

Group 2: M + X2 MX2

Ex – Mg(s) + F2(g) MgF2(s)

Metal Oxides with WaterGroup 1 & 2 form hydroxides

Ex – K2O(s) + H2O(l) 2KOH(aq)

CaO(s) + H2O(l) Ca(OH)2(l)

Non-metal Oxide with WaterForm oxyacids

Ex – SO2(g) + H2O(l) H2SO3(aq)

P2O5(s) + 3H2O(l) 2H3PO4(aq)

Decomposition ReactionsAX A + X

Decomposition of Binary CompoundsBreaks down into its elements

(g)O (g)2H O(l)2H 22yelectricit

2

Process called electrolysis

Decomposition of Metal CarbonatesForm metal oxides and carbon dioxide

(g)CO CaO(s) (s)CaCO 23

Decomposition of Metal HydroxidesForm metal oxides and water

O(g)H CaO(s) (s)Ca(OH) 22

Decomposition of AcidsBreak down into non-metal oxides and water

O(g)H (g)CO (aq)COH 2232

O(l)H (g)SO (aq)SOH 2342

Single ReplacementGeneral Formula

A + BX AX + B

Metal Replaces Another MetalAluminum is more reactive than lead

(aq))2Al(NO 3Pb(s) (aq))Pb(NO 2Al(s)

33

23

Replacement of Hydrogen in Water by a MetalMore Active Metals

(g)H 2NaOH(aq) O(l)2H 2Na(s) 22

Less Active Metals

(g)4H (s)OFe O(g)4H 3Fe(s) 2432

Replacement of Hydrogen in an Acid by a MetalMetals more active than hydrogen

(aq)MgCl (g)H 2HCl(aq) Mg(s) 22

Replacement of HalogensEach halogen can replace the halogen below it

on the periodic table

reaction no 2NaCl(aq) (g)I(g)Cl 2KF(aq) 2KCl(aq) (g)F

(g)Br 2KCl(aq) 2KBr(aq) (g)Cl

2

22

22

Double ReplacementAX + BY AY + BX

• Formation of a precipitate• Formation of a gas• Formation of water

Formation of a Precipitate

An insoluble product forms

(aq)2KNO (s)PbI (aq))Pb(NO KI(aq)2 3223

Formation of a GasInsoluble gas forms

Example

(aq)FeCl S(g)H 2HCl(aq) FeS(s) 22

Formation of WaterWater forms during reaction

Combustion ReactionSubstance reacts with oxygen to release heat

and light

Products are often carbon dioxide and water

Neutralization ReactionHA + BOH AB + HOH

Usually these are acid-base reactions

Products include salt and water

Activity Series

Objectives1. Explain the significance of an activity series

2. Use an activity series to predict if a reaction will take place

Metals vs. NonmetalsGreater activity of a metal indicates how easily

it loses electrons

Greater activity of a nonmetal indicates how easily it gains electrons

In a single-replacement reaction, if an element with lower activity is to be replaced, the reaction will take place.