Chemical Reactions

The 5 Basic Classifications

SYNTHESISSYNTHESIS

A reaction in which two or more reactants yield a single product.

Also called composition or combination

General EquationA + B AB

EX: 2 Li + Se ---> Li2Se

Decomposition ReactionDecomposition Reaction

One Reactant Breaking Down into two or more products

General Equation AB ---> A + B

Example: 2 HgO 2 Hg + O2

One or moreOne or more reactants combine with reactants combine with oxygen oxygen releasing releasing heatheat or or lightlight

Any combustion reaction must include Any combustion reaction must include the reactant oxygen, Othe reactant oxygen, O22

General Equation:General Equation:

A + OA + O22 AOAO

Example:Example:

2Mg(s) + O2Mg(s) + O22(g) 2MgO(s)(g) 2MgO(s)

What IsWhat Is Combustion?Combustion?

Single ReplacementSingle Replacement Sometimes called Sometimes called DisplacementDisplacement

One element replaces a One element replaces a similar similar elementelement in a in a compoundcompound

A + BX A + BX AX + BAX + B

2 Na + 2 HOH ----> 2 NaOH + H2

Double ReplacementDouble Replacement

Reaction that has the interchanging of Reaction that has the interchanging of twotwo ionsions from from two different compoundstwo different compounds..

general form: general form:

AB + CD----> AB + CD----> AD + CBAD + CB

Example:Example:Pb(NOPb(NO33))2 2 + 2 KI ----> PbI+ 2 KI ----> PbI2 2 + 2 + 2

KNOKNO33

Double Double ReplacementReplacement

Equation consists of two reactants that have both a cation and anion.

During a reaction the cations (or anions) switch places.

The products usually consist of a precipitate.

Reaction Checklist

1) Is O2 a reactant? (combustion)

2) One product? (synthesis)

3) One reactant? (decomposition)

4) Is an element being replaced? (single)

5) 2 switches? (double)

EXOTHERMIC & ENDOTHERMIC REACTIONSEXOTHERMIC & ENDOTHERMIC REACTIONS

Exothermic process: a change (e.g. a chemical reaction) that releases heat.

A release of heat corresponds to a decrease in enthalpy

Exothermic process: H < 0 (at constant pressure)

Burning fossil fuels is an exothermic

reaction

Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat.

An input of heat corresponds to an increase in enthalpy

Endothermic process: H > 0 (at constant pressure)

Photosynthesis is an endothermic reaction

(requires energy input from sun)

Forming Na+ and Cl- ions from NaCl is

an endothermic

process

Measuring HeatMeasuring Heat

reaction

reaction

Exothermic reaction, Exothermic reaction, heat heat given offgiven off & temperature of & temperature of

water water risesrises

Endothermic reaction, Endothermic reaction, heat taken inheat taken in & &

temperature of water temperature of water dropsdrops

How do we relate change in temp. to the energy transferred?

Heat capacity (J/oC) = heat supplied (J)

temperature (oC)

Heat Capacity = heat required to raise temp. of an object by 1oC

• more heat is required to raise the temp. of a large sample of a substance by 1oC than is needed for a

smaller sample

Specific heat capacity is the quantity of energy required to change the temperature of a 1g sample

of something by 1oC

Specific Heat Capacity (Cs)

Heat capacity

Mass

=

J / oC / g J / oC

g

=

VaporisationVaporisation

Energy has to be supplied to a liquid to enable it to overcome forces that hold molecules together

• endothermic process (H positive)

Melting

Energy is supplied to a solid to enable it to vibrate more vigorously until molecules can move past each other and

flow as a liquid

• endothermic process (H positive)

Freezing

Liquid releases energy and allows molecules to settle into a lower energy state and form a solid

• exothermic process (H negative)

(we remove heat from water when making ice in freezer)

Reaction EnthalpiesReaction Enthalpies

All chemical reactions either release or absorb heat

Exothermic reactions:

Reactants products + energy as heat (H -ve)

Endothermic reactions:

Reactants + energy as heat products (H +ve)

e.g. burning fossil fuels

e.g. photosynthesis

Bond StrengthsBond Strengths

Bond strengths measured by bond enthalpy HB (+ve values)

• bond breaking requires energy (+ve H)

• bond making releases energy (-ve H)

Lattice EnthalpyLattice Enthalpy

A measure of the attraction between ions (the enthalpy change when a solid is broken up into a gas of its ions)

• all lattice enthalpies are positive

• I.e. energy is required o break up solids

Enthalpy of hydration Enthalpy of hydration HHhydhyd

• the enthalpy change accompanying the hydration of gas-phase ions

•Na+ (g) + Cl- (g) Na+ (aq) + Cl- (aq)

• -ve H values (favourable interaction)

WHY DO THINGS DISSOLVE?

• If dissolves and solution heats up : exothermic

•If dissolves and solution cools down: endothermic

Breaking solid into ions

Ions associating with water

Dissolving+ =

Lattice Enthalpy

+ Enthalpy of Hydration

= Enthalpy of Solution

Substances dissolve because energy and matter tend to disperse (spread out in disorder)

2nd law of Thermodynamics