Thermochemistry FlashCards

8/13/2019 Thermochemistry FlashCards

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THERMOCHEMISTRY


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Thermochemistry:

The Law of Conservation of Energyenergycan neitherbecreatednor destroyed but

it can be changedfrom one form to the other form

Exothermic reactiona chemical reaction thatgives out heatto the surrounding

Endothermic reactiona chemical reaction that absorbs heatfrom thesurrounding.

Surroundings do not involve in thereactions. Example: water, container, the air,

solvent and thermometer


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Heat of reactionthe heat changewhen thenumber of moles of reactantsin thechemical

equationreacts toform productsin standard conditions.

Standard conditions: temperature (25C / 298 K), pressure (1 atm), concentration of solution(1.0 mol dm-3), reactants and products are at their normal physical states.

Heat of precipitationthe heat changewhen one mole of a precipitate is formed from their

ionsin aqueous solution.

Heat of displacement

theheat changewhen one mole of a metal isdisplaced from its saltsolution by a more electropositive metal

Heat of neutralisationthe heat changewhen one mole of wateris formed from the

reaction betweenan acid and an alkali

Heat of combustionthe heat changewhen one mole of a substanceiscompletely burnt inoxygenunder standard conditions.


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Exothermic reaction

Chemical energy> Heat energy

The heat energy is transferred to the surrounding.

Temperature of the surrounding increases


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Example of Chemical Reactions:

Respiration

Burning of metal

Reactionof an alkaline metals (Group 1) with water

Reactionof a reactive metal with acid

Neutralisation reaction between acid and alkali

Reaction of a carbonate with acid

Combustion of carbon compound

Displacement reaction of metals

Rusting of iron


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Example of Physical Processes:

Freezing process

Condensationprocess

Dissolving an alkali in water

Dissolving an concentrated acid in water


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ENDOTHERMIC REACTION


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Endothermic reaction

Heat energy> Chemical energy

The heat is absorbed from the surrounding.

Temperature of the surrounding decreases

Example of Chemical Reactions:

Photosynthesis

Decomposition of nitrate salts

Decompositon of carbonates salts

Reaction between acid with hydrogencarbonates


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Example of Physical Processes:

Melting process

Boiling process

Sublimation process

Dissolving of ammonium salts in water

Dissolving of potassium salts in water

Dissolving of thiosulphate in water


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Heat of Reaction

Enthalpy (H)absolute energy content of a substance.

Change in energy content (H) absolute energy content cannot be

determined, but H can be determined when the reactants are

converted to the products.

1 kJ (kilojoule) = 1000 J

The unit H is kJ


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ENERGY DIAGRAM


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Energy Level Diagram

H = H productH reactant = negative value.

Example: CH4(g) + 2O2(g)> CO2(g) + 2H2O(l) H = -890 Kj

The value of H is negative = exothermic reaction

H = H productH reactant = positive value.

Example: N2(g) + 3H2(g)> 2NH3(g) H = +91.8kJ

The value of H is positive = endothermic reaction


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Energy Change during Formation and Breaking of Bonds

Bond breaking

Usually chemical bonds ofthereactant.

Heatenergy isabsorbed

Bond forming

Usually new chemical bondsof the product.

Heatenergy is given out.

Relationship between energy change and the formation and breaking of bonds

In a chemical reaction, if theheat energy absorbed in bond breaking islower than the

heat energy given out in bond forming, the reaction is anexothermic reaction

Example: H (bond breaking) = +600 kJ, H (bond forming) = -800 kJ, H (heat of

reaction) = [(+600) + (-800)] kJ= -200 kJ


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Applications of Exothermic and Endothermic Reaction in Everyday Life

Hot pack

Contains of anhydrous calcium chloride / anhydrous magnesium sulphate / wet iron powder

and sodium chloride / calcium oxide.

Uses: reduce swelling and muscles or joint sprain.

Cold pack or Ice pack

Contains of ammonium nitrate / potassium nitrate / sodium thiosulphate.

Uses: reduce swelling, muscles or joint sprain and reduce fever.


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You must be able to

1. Calculate the number of moles of salt precipitated / metal displaced / water

produced / fuel used;

2. Calculate the heat energy released (H); and

3. Calculate the heat of precipitation / heat of displacement / heat of neutralization

/ heat of combustion.


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Heat of Precipitation (Form 4, Chapter 8 Salts)

Heat of precipitationthe heat change when one mole of a precipitate is formed

fromtheir ions in aqueous solution.

Precipitation reaction = double decomposition which is used to prepare insoluble salts

Heat change of a solution = mcJoule [m = mass of the solution (g), c = specific heat

capacity of the solution (J g-1C-1), = temperature change in the solution (C)]

Heat change in a reaction, mc= n x H

Heat of reaction / Heat of precipitation, H= mc/ n


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Example 1:

Chemical reaction: Pb(NO3)2(aq) + 2KI(aq)> PbI2(s) + 2KNO3(aq)

Ionic reaction: Pb2+(aq) + 2I-(aq)> PbI2(s)

Heat of precipitation of PbI2 =Heat change / Number of moles of PbI2

Example 2:

Chemical reaction: BaCl2(aq) + Na2SO4(aq)> BaSO4(s) + 2NaCl(aq)

Ionic reaction: Ba2+(aq) + SO42-(aq)> BaSO4(s)

Heat of precipitation of BaSO4 =Heat change / Number of moles of BaSO4


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Heat of Displacement (Form 4, Chapter 6 Electrochemistry & Form 5, Chapter 3

Oxidation and Reduction)

Heat of displacementthe heat change when one mole of a metal is

displaced from its salt solution by a more electropositive metal.

Heat change of the reaction mixture / Heat energy released / Heat given out in the

reaction = mcJoule

Heat change in a reaction, mc= n x H

Heat of reaction / Heat of displacement, H = mc/ n


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Example 1:

Chemical equation: Mg(s) + FeCl2(aq)> MgCl2(aq) + Fe(s)

Ionic equation: Mg(s) + Fe2+(aq)> Mg2+(aq) + Fe(s)

Example 2:

Chemical equation: Zn(s) + CuSO4(aq)> ZnSO4(aq) + Cu(s)

Ionic equation: Zn(s) + Cu2+(aq)> Zn2+ (aq) + Cu(s)


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Heat of Neutralisation (Form 4, Chapter 7 Acids and Bases)

Heat of neutralisationthe heat change when one mole of water is formed from the

reaction between an acid and an alkali.

Neutralisationa reaction between an acid reacts with a base (alkali) to form

a salt andwater.

Hydrogen ion from acid reacts with hydroxide ions from alkali to form water. H+(aq) + OH-

(aq)> H2O(l)

Neutralisation reaction gives out heat and always an exothermic reaction.


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Example 1: (Strong acidsmonoprotic acid and strong alkalis)

Chemical equation: HCl(aq) + NaOH(aq)> NaCl(aq) + H2O(l)

Ionic equation: H+(aq) + OH-(aq)> H2O(l)

Heat of neutralisation of strong acids and strong alkalis are the

same (H = -57.3 kJ mol-1)

Example 2: (Strong acidsdiprotic acid and strong alkalis)

Chemical equation: H2SO4(aq) + NaOH(aq)> Na2SO4(aq) +

2H2O(l)

Ionic equation: 2H+(aq) + 2OH-(aq)> 2H2O(l)

Heat of neutralisation of strong acids and strong alkalis are thesame (H = -57.3 kJ mol-1)


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Example 3: (Weak acids and strong alkalis)

Chemical equation: CH3COOH(aq) + NaOH(aq)> CH3COONa(aq) + H2O(l)


Heat of neutralisation of weak acids and strong alkalis are lower (H = -55.0 kJ mol-1)

than heat of neutralisation of strong acids and strong alkalis (H = -57.3 kJ mol-1).

Example 4: (Strong acids and weak alkalis)

Chemical equation: HCl(aq) + NH4OH(aq)> NH4Cl(aq) + H2O(l)


Heat of neutralisation of strong acids and weak alkalis are lower (H = -51.5 kJ mol-1)

than heat of neutralisation of strong acids and strong alkalis (H = -57.3 kJ mol-1).


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Example 5: (Weak acids and weak alkalis)

Chemical equation: CH3COOH(aq) + NH4OH(aq)> NH4Cl(aq) + H2O(l)


Heat of neutralisation of strong acids and weak alkalis are lower (H = -50.4 kJ mol-1) than

heat of neutralisation of strong acids and strong alkalis (H = -57.3 kJ mol-1).


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Heat of Combustion (Form 5, Chapter 2 Carbon Compounds)

Heat of combustionthe heat change when one mole of a substance is completely burnt inoxygen under standard conditions.

Combustionredox reaction between substance (fuel) reacts rapidly with oxygen with the

production of heat energy.

Combustion reaction gives out heat and always an exothermic reaction.

Heat evolved in combustion of fuel = Heat absorbed by water.

Bomb calorimeter is used to determine the heat of combustion.

The more carbon and hydrogen atoms per molecules in a fuel, the more heat that isreleasedwhen 1 mol of fuel combusts.


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Chemical equation H (kJ mol-1)

H2(g) + O2(g)> H2O(l) -286

C(s) + O2(g)> CO2(g) -392

CH4(g) + 2O2(g)> CO2(g) + 2H2O(l) -890

CH3OH(l) + 3/2 O2(g)> CO2(g) +

2H2O(l)

-728

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

-1376

C3H7OH(l) + 9/2 O2(g)> 3CO2(g) +

4H2O(l)

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There are differences in heats of combustion:

The selection of suitable fuel:


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Fuel value (the amount of heat energy given out when one gram of the fuel is completely

burnt in excess of oxygen): The higher the fuel value, the more energy is released.

Effect on the environment: Production of soot which caused air pollution. Hydrogen fuel isknown as clean fuels (no soot or poisonous gases).


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Qualities of a fuel are based on the following:

Easily available

Cheap in cost

High fuel value

Do not pollute the environment

Less storage space

The Existence of Various Energy Sources

The Sun: solar cell (still expensive and inefficient)


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Fossil Fuels: relatively high fuel value and convenient to use (non-renewable and cause

greenhouse effect and acid rain)

Water: hydroelectric power. It is clean, renewable, convenient and economical to use (high

cost of construction and destruction of the surrounding environment)

Biomass: plants and droppings of animalsbiodiesel (large areas of land to grow plants)

Radioactive substances: uranium and plutonium (non-renewable and very destructive if an

accident occurs)

Thermochemistry FlashCards

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