Chemical Reactions

SCH3U - Unit 2

Chemical ChangeChemical Change = any change in which a new substance is formed

Evidence of Chemical Change:Change in colourChange in odourFormation of gas/solidRelease/absorption of heat

Collision-Reaction Theory A theory stating that chemical reactions

involve collisions and rearrangements of atoms or groups of atoms and that the outcome of collisions depends on the energy and orientation of the collisions No reaction occurs if:

Molecules don’t have enough energy Molecules don’t collide in the right orientation

Orientation The molecules must be in a certain 3-D

arrangement to allow a reaction e.g. CH2=CH2 + HCl -> CH3CH2Cl

Energy Required A reaction doesn’t occur unless the particles

collide with a certain minimum energy called the activation energy of the reaction

Activation energy is the minimum energy required before a reaction can occur. You can show this on an energy profile for the reaction. For a simple over-all exothermic reaction, the energy profile looks like this:

Activation Energy Profile

Chemical Equations Chemical Equation = a

representation of a chemical reaction that indicates the: Chemical formulas Relative number of entities States of matter of the reactants and

productsReactants Products

Chemical EquationsIn general: Reactant A + Reactant B Product C

Reactant = is a chemical that is used up in a chemical reaction

Product = is a product that is created during a chemical reaction.

Chemical Formulas A chemical formula uses subscripts to indicates

the number of atoms in a compound

Example: H2O Has 2 atoms of H And 1 atom of O

Example: C6H12O6 Has 6 atoms of C Has 12 atoms of H And 6 atoms of O

Relative # of Entities Coefficient = a whole number

indicating the ratio of molecules of each substance involved in a chemical reaction The large number on the left side of a

molecule’s formula

Example: Mg + 2 Cl MgCl2 Example: 6 K + N2 2 K3N

State of Matter Solid = (s) Liquid = (l) Gas = (g) Solution = (aq)

Example: 6 K(s) + N2(g) 2 K3N(s)

Catalysts A catalyst is a substance which speeds up a

reaction, but is chemically unchanged at the end of the reaction .e.g conc. H2SO4 in many different reactions

Adding a catalyst has exactly this effect on activation energy. A catalyst provides an alternative route for the reaction. That alternative route has a lower activation energy

Draw a standard energy profile and then draw a new line to represent the inclusion of a catalyst

5 Types of Chemical Reactions1. Combustion2. Synthesis3. Decomposition4. Single Displacement5. Double Displacement

5 Types of Chemical ReactionsGeneralizations:Combustion: AB + oxygen oxides of A &

B + heatSynthesis: A + B CDecomposition: AB A + BSingle Displacement: A + BC AC + BDouble Displacement: AB + CD AD + CB

Combustion Reactions Combustion Reaction: the reaction of

a substance with oxygen, producing oxides and energy

Also know as burning

For a combustion reaction to occur 3 things must be present:1. Fuel2. Oxygen3. Heat

Combustion Reactions C

Type of Reaction: Synthesis

Example C + O2

OO C + O O C

OO COO C OO C OO C OO

C OO

C OO

CO

O

C OO

C O

O C O

O C OO C

OO C OO C

General: A + B AB

Synthesis Reaction Characteristics Two or more substances (elements or

compounds) react to form ONE product. Combination of smaller atoms/molecules into larger molecules.

Usually exothermic (energy is produced) Can occur naturally or by an initial application

of energy (heat, flame, UV light, use of catalyst)

Predicting Products of Synthesis Reactions Metal + oxygen → metal oxide (basic oxide)

EX. 2Mg(s) + O2(g) → 2MgO(s) Nonmetal + oxygen → nonmetallic oxide (acidic oxide)

EX. C(s) + O2(g) → CO2(g) Metal oxide + water → metallic hydroxide (base)

EX. MgO(s) + H2O(l) → Mg(OH)2(s) Nonmetallic oxide + water → acid

EX. CO2(g) + H2O(l) → ; H2CO3(aq) Metal + nonmetal → salt

EX. 2 Na(s) + Cl2(g) → 2NaCl(s) A few nonmetals combine with each other.

EX. 2P(s) + 3Cl2(g) → 2PCl3(g) These two reactions should be remembered:

N2(g) + 3H2(g) → 2NH3(g) NH3(g) + H2O(l) → NH4OH(aq)

Type of Reaction: Decomposition

Example: NaCl

General: AB A + B

Cl Na Cl + Na

Type of Reaction: Decomposition

Example 2HgO

O Hg

O Hg

Hg

O O Hg

+

General: AB A + B

Decomposition Reaction Characteristics ONE reactant produces two or more products.

Splitting of large molecules into elements or smaller molecules.

Usually endothermic (requires energy) Can require energy in the form of heat,

electricity, catalyst, UV light *some decomposition rxns occur at room

temperature

Predicting Products of Decomposition Reactions

Metallic carbonates, when heated, form metallic oxides and CO2(g). EX. CaCO3(s) → CaO(s) + CO2(g)

Most metallic hydroxides, when heated, decompose into metallic oxides and water.

EX. Ca(OH)2(s) → CaO(s) + H2O(g) Metallic chlorates, when heated, decompose into metallic chlorides and

oxygen. EX. 2KClO3(s) → 2KCl(s) + 3O2(g)

Some acids, when heated, decompose into nonmetallic oxides and water. EX. H2SO4 → H2O(l) + SO3(g)

Some oxides, when heated, decompose. EX. 2HgO(s) → 2Hg(l) + O2(g)

Some decomposition reactions are produced by electricity. EX. 2H2O(l) → 2H2(g) + O2(g) EX. 2NaCl(l) → 2Na(s) + Cl2(g)

Type of Reaction: Single Displacement

Example: Zn + CuCl2

ZnClCl Cu +

General: AB + C AC + B

ClCl Zn Cu+

Type of Reaction: Double Displacement

Example: MgO + CaS

General: AB + CD AD + CB

SOMg Ca+

O S

Mg Ca+

Chemical Reactions

combustion: AB + oxygen oxides of A & Bsynthesis: A + B Cdecomposition: AB A + Bsingle displacement: A + BC AC + Bdouble displacement: AB + CD AD + CB