Chemical Rxn Types

Chemical Reactions The 5 Basic Classifications

SYNTHESIS 

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

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Also called composition or combination

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General Equation A + B  AB

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EX: 2 Li + Se ---> Li Se

Decomposition Reaction One Reactant Breaking Down into two or more products General Equation AB ---> A + B Example: 2 HgO  2 Hg + O2

What Is Combustion? •

One or more reactants combine with oxygen releasing heat or light –

Any combustion reaction must include the reactant oxygen, O2

–

General Equation: A + O2  AO

Example:

2Mg(s) + O2(g)

2MgO(s)

Single Replacement  

Sometimes called Displacement One element replaces a similar element in a compound

A + BX  AX + B

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

Double Replacement 

Reaction that has the interchanging of two ions from two different compounds.

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general form: AB + CD----> AD + CB

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Example: Pb(NO3)2 + 2 KI ----> PbI2 KNO3

+2

Double Replacement 

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

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

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The products usually consist of a precipitate.

Reaction Checklist 

1) Is O2 a reactant? (combustion)

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2) One product? (synthesis)

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3) One reactant? (decomposition)

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4) Is an element being replaced? (single)

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5) 2 switches? (double)

EXOTHERMIC & 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 Heat

reaction

reaction

Exothermic reaction, heat given off & temperature of water rises

Endothermic reaction, heat taken in & temperature of water drops

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

Vaporisation 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 Enthalpies All chemical reactions either release or absorb heat Exothermic reactions: Reactants

products + energy as heat (∆H -ve) e.g. burning fossil fuels

Endothermic reactions: Reactants + energy as heat e.g. photosynthesis

products (∆H +ve)

Bond Strengths Bond strengths measured by bond enthalpy ∆HB (+ve values) • bond breaking requires energy (+ve ∆H) • bond making releases energy (-ve ∆H) Lattice 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 ∆ Hhyd • the enthalpy change accompanying the hydration of gasphase 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

Lattice Enthalpy

+

+

Ions associating with water

Enthalpy of Hydration

=

Dissolving

=

Enthalpy of Solution

Substances dissolve because energy and matter tend to disperse (spread out in disorder) 2nd law of Thermodynamics