PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy,...

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THERMOCHEMISTRY CP Unit 9 Chapter 17

Transcript of PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy,...

Page 1: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

THERMOCHEMISTRYCP Unit 9

Chapter 17

Page 2: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

Thermochemistry 17.1p Thermochemistry is the study of energy

changes (HEAT) that occur during chemical reactions and changes in state.

Page 3: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

ENTHALPY 17.2p Enthalpy = a type of chemical energy, sometimes

referred to as “heat content”, ΔH (the heat of reaction for a chemical reaction)

p endothermic reactions (feels cold):n q = ΔH > 0 (positive values)

p exothermic reactions (feels hot):n q (heat) = ΔH (enthaply, heat of rxn) < 0 (negative

values)

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Endo vs. Exo-Endothermic reactions:

absorbs heat from surroundings (+). n If you touch an

endothermic reaction it feels COLD

Exothermic reactions: release heat to the surroundings (-)n If you touch an

exothermic reaction it feels HOT

Page 5: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

Magnitude of Heat Flow• Units of heat energy:

• 1 kcal = 1,000 cal = 1 Cal (nutritional)• 1 kJ = 1,000 J• 1 calorie = 4.184 J• 1 kcal = 4.184 kJ

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Thermochemical Equations• A chemical equation that shows the

enthalpy (DH) is a thermochemical equation.

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Rule #1The magnitude (value) of DH is directly proportional to the amount of reactant or product.

H2 + Cl2 ® 2HCl DH = - 185 kJ

* meaning there are 185 kJ of energy RELEASED for every:

1 mol H21 mol Cl22 moles HCl

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Rules of ThermochemistryExample 1:H2 + Cl2 ® 2HCl DH = - 185 kJ

Calculate DH when 2.00 moles of Cl2reacts.

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Rules of ThermochemistryExample 2: Methanol burns to produce carbon dioxide and water:

2CH3OH + 3O2 ® 2CO2 + 4H2O DH = - 1454 kJ

What mass of methanol is needed to produce 1820 kJ?

Page 10: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

Rule #2DH for a reaction is equal in the magnitude but opposite in sign to DH for the reverse reaction.

(If 6.00 kJ of heat absorbed when a mole of ice melts, then 6.00 kJ of heat is given offwhen 1.00 mol of liquid water freezes)

Page 11: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

Rules of ThermochemistryExample 1:Given: H2 + ½O2 ® H2O DH = -285.8 kJ

Reverse: H2O è H2 + ½O2 DH = +285.8 kJ

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Example 2CaCO3 (s) è CaO (s) + CO2 (g) DH = 178 kJ

What is the DH for the REVERSE RXN?

CaO (s) + CO2 (g) è CaCO3 (s) DH = ?

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Alternate form of thermochem. eq.p Putting the heat content of a reaction

INTO the actual thermochemical eq.p H2 + ½O2 ® H2O DH = -285.8 kJ

Exothermic

(DH is negative)

Heat is RELEASED as a PRODUCT

The alternate form is this:

H2 + ½O2 ® H2O +285.8 kJ

Page 14: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

EX:2 NaHCO3 + 129 kJ è Na2CO3 + H2O + CO2

Put in the alternate form

The alternate form is this:2 NaHCO3 è Na2CO3 + H2O + CO2 DH = + 129 kJ

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Put the following in alternate form1. H2 + Cl2 è 2 HCl DH = -185 kJ

2. 2 Mg + O2 è 2 MgO + 72.3 kJ

3. 2 HgO è 2 Hg + O2 DH = 181.66 kJ

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Enthalpies of Formationenthalpy

change (delta)

standard conditions

formation

°D fH

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Enthalpies of Formationpusually exothermicp see table for DHf

° value (Table A3)penthalpy of formation of an element

in its stable state = 0p these can be used to calculate DH°

for a reaction

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Standard Enthalpy ChangeStandard enthalpy change, DH°, for a given thermochemical equation is = to the sum of the standard enthalpies of formation of the product – the standard enthalpies of formation of the reactants.

) H(-) H(H reactantsfproductsfrxn°°° DD=D SS

sum of (sigma)

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Standard Enthalpy Changep elements in their standard states can be omitted:

2 Al(s) + Fe2O3(s) ® 2 Fe(s) + Al2O3(s)

ΔHrxn = S(ΔHf°products) - S(ΔHf

°reactants)

ΔHrxn = ΔHf°Al2O3 - ΔHf

°Fe2O3

ΔHrxn = (-1676.0 kJ) – (-822.1 kJ)

ΔHrxn = -853.9 kJ

Page 20: PPT - 1 - Thermochem eqns - ATF€¦ · ENTHALPY 17.2 pEnthalpy = a type of chemical energy, sometimes referred to as “heat content”, ΔH (the heat of reaction for a chemical

Standard Enthalpy Changep the coefficient of the products and reactants in

the thermochemical equation must be taken into account:

O2(g) + 2SO2 (g) ® 2SO3 (g)

ΔHrxn = S(ΔHf°products) - S(ΔHf

°reactants)

ΔHrxn = 2ΔHf°SO3 - 2ΔHf

°SO2

ΔHrxn = 2(-395.7 kJ) – 2(-296.8 kJ)

ΔHrxn = -197.8 kJ

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Standard Enthalpy Change - Calculate the standard heat for formation of benzene, C6H6, given the following thermochemical equation:

C6H6(l) + 15/2 O2(g) ® 6CO2(g) + 3H2O(l) DH° = -3267.4 kJ

-3267.4kJ = [6(-393.5kJ)+3(-285.8kJ)]–X

X = +49.0 kJ

-3267.4kJ = -3218.4–X

-49.0kJ = – X

X Total-393.5kJ -285.8kJ

Don’t forget the coefficents!