New Way Chemistry for Hong Kong A-Level Book 11 Chapter 6 Energetics 6.1 What is Energetics? 6.2 Ideal Enthalpy Changes Related to Breaking and Formation

New Way Chemistry for Hong Kong A-Level Book 11

Chapter 6Chapter 6EnergeticsEnergetics

6.16.1 What is Energetics? What is Energetics?

6.2 6.2 Ideal Enthalpy Changes Related to Ideal Enthalpy Changes Related to Breaking and Formation of BondsBreaking and Formation of Bonds

6.3 6.3 Standard Enthalpy ChangesStandard Enthalpy Changes


6.4 6.4 Experimental Determination of Experimental Determination of EnthEnthalpy Changes by Calorimetryalpy Changes by Calorimetry

6.5 6.5 Hess’s LawHess’s Law

6.6 6.6 Calculations involving Enthalpy Calculations involving Enthalpy Changes of ReactionsChanges of Reactions


What is Energetics?Energetics is the study of energy changes associated with chemical reactions.Energetics is the study of energy changes associated with chemical reactions.

Thermochemistry is the study of heat changes associated with chemical reactions.Thermochemistry is the study of heat changes associated with chemical reactions.

Some terms

Enthalpy(H) = heat content in a substance

Enthalpy change(H) = heat content of products - heat content of reactants= Hp - Hr

6.1 What is Energetics? (SB p.140)


Internal Energy and Enthalpy

e.g. Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Heat change atconstant pressure

= Change in internal energy +

Work done on the surroundings

(Heat change at constant volume)Enthalpy change



Exothermic and Endothermic Reactions

An exothermic reaction is a reaction that releases heat energy to the surroundings. (H = -ve)An exothermic reaction is a reaction that releases heat energy to the surroundings. (H = -ve)



Exothermic and Endothermic Reactions

An endothermic reaction is a reaction that absorbs heat energy from the surroundings. (H = +ve)An endothermic reaction is a reaction that absorbs heat energy from the surroundings. (H = +ve)



Enthalpy Changes Related to Breaking and Forming of Bonds

e.g. CH4 + 2O2 CO2 + 2H2O

6.2 Enthalpy Changes Related to Breaking and Formation of Bonds? (SB p.143)

In an exothermic reaction, the energy required in breaking the bonds in the reactants is less than the energy released in forming the bonds in the products (products contain stronger bonds).

In an exothermic reaction, the energy required in breaking the bonds in the reactants is less than the energy released in forming the bonds in the products (products contain stronger bonds).


Enthalpy Changes Related to Breaking and Forming of Bonds

In an endothermic reaction, the energy required in breaking the bonds in the reactants is more than the energy released in forming the bonds in the products (reactants contain stronger bonds).

In an endothermic reaction, the energy required in breaking the bonds in the reactants is more than the energy released in forming the bonds in the products (reactants contain stronger bonds).



Bond Enthalpies

To be discussed in later chapters.


Bond Mean Bond Enthalpy (kJ mol-1)

H – H

C – C

C≡C

C≡ C

N – N

N ═ N

N≡ N

436

348

612

837

163

409

944


Standard Enthalpy Changes

CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ mol-1

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890 kJ mol-1

As enthalpy changes depend on temperature and pressure, chemists find it convenient to report enthalpy changes based on an internationally agreed set standard conditions:

1. elements or compounds in their normal physical states;2. a pressure of 1 atm (101325 Nm-2); and3. a temperature of 250C (298 K)

1. elements or compounds in their normal physical states;2. a pressure of 1 atm (101325 Nm-2); and3. a temperature of 250C (298 K)

6.3 Standard Enthalpy Changes (SB p.144)

Enthalpy change under standard conditions denoted by symbol: H

ø


Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of Neutralization

e.g. The standard enthalpy change of neutralization between HNO3 and NaOH is -57.3 kJ mol-1

e.g. The standard enthalpy change of neutralization between HCl and NaOH is -57.1 kJ mol-1


The standard enthalpy change of neutralization (Hneut) is the enthalpy change when one mole of water is formed from the neutralization of an acid by an alkali under standard conditions.

The standard enthalpy change of neutralization (Hneut) is the enthalpy change when one mole of water is formed from the neutralization of an acid by an alkali under standard conditions.

ø

e.g. H+(aq) + OH-(aq) H2O(l) Hneut = -57.3 kJ mol-1ø


Standard Enthalpy Changes of neutralization

H+(aq) + OH-(aq) H2O(l)


-57.1

-57.2

-52.2

-68.6

NaOH

KOH

NH3

NaOH

HCl

HCl

HCl

HF

Hneu AlkaliAcid ø


Standard Enthalpy Changes of Reactions

Standard Enthalpy Change of Solution

Note that enthalpy changes of solution associate with physical changes.


The standard enthalpy change of solution (Hsoln) is the enthalpy change when one mole of a solute is dissolved to form an infinitely dilute solution under standard conditions.

The standard enthalpy change of solution (Hsoln) is the enthalpy change when one mole of a solute is dissolved to form an infinitely dilute solution under standard conditions.

ø

e.g. NaCl(s) + water Na+(aq)+Cl-(aq) Hsoln=+3.9 kJ mol-1ø

e.g. LiCl(s) + water Li+(aq) + Cl-(aq) Hsoln=-37.2 kJ mol-1ø



Standard Enthalpy Changes of solution

-44.7

+3.9

-57.8

+20.0

NaOH

NaCl

KOH

KBr

Hsoln(kJ mol-1)Salt ø


Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of Formation

Standard enthalpy change of formation of NaCl is -411 kJ mol-1.


The standard enthalpy change of formation (Hf) is the enthalpy change of the reaction when one mole of the compound in its standard state is formed from its constituent elements under standard conditions.

The standard enthalpy change of formation (Hf) is the enthalpy change of the reaction when one mole of the compound in its standard state is formed from its constituent elements under standard conditions.

ø

e.g. 2Na(s) + Cl2(g) 2NaCl(s) H = -822 kJ mol-1

ø

Na(s) + ½Cl2(g) NaCl(s) Hf = -411 kJ mol-1

1 mole

ø



The enthalpy change of formation of an element is always zero.The enthalpy change of formation of an element is always zero.

N2(g) N2(g)


What is Hf [N2(g)] ?

ø

Hf [N2(g)] = 0

ø


Standard Enthalpy Changes of ReactionsStandard Enthalpy Change of Combustion

e.g. C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) H1 = -2220 kJ

2C3H8(g) + 10O2(g) 6CO2(g) + 8H2O(l) H2 = ?

H2 = -4440 kJ

It is more convenient to report enthalpy changes per mole of the main reactant reacted/product formed.




Standard Enthalpy Change of Combustion

The standard enthalpy change of combustion of propane is -2220 kJ mol-1


The standard enthalpy change of combustion (Hc) of a substance is the enthalpy change when one mole of the substance burns completely under standard conditions.

The standard enthalpy change of combustion (Hc) of a substance is the enthalpy change when one mole of the substance burns completely under standard conditions.

ø

e.g. C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l)

Hc = -2220 kJ mol-1 1 mole

ø


Standard Enthalpy Changes of Reactions6.3 Standard Enthalpy Changes (SB p.150)

-285.8

-395.4

-393.5

-283.0

-890.4

H2(g)

C (diamond)

C (graphite)

CO(g)

CH4(g)

Hc (kJ mol-1)Substance

ø


Experimental Determination of Enthalpy Changes by CalorimetryCalorimeter = a container used for measuring the temperature change of solution

Determination of Enthalpy Change of Neutralization

6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 151)



Heat evolved = (m1c1 + m2c2) ΔT

Where m1 is the mass of the solution,

m2 is the mass of calorimeter,

c1 is the specific heat capacity of the solution,

c2 is the specific heat capacity of calorimeter,

And Δ T is the temperature change of the reaction.


Determination of Enthalpy Change of Combustion6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 153)


Heat evolved = (m1c1 + m2c2) ΔT

Where m1 is the mass of water in the calorimeter,

m2 is the mass of the calorimeter,

c1 is the specific heat capacity of the water,

c2 is the specific heat capacity of calorimeter,

And Δ T is the temperature change of the reaction.



Hess’s Law

A + B C + DRoute 1

H1

E

H2 H3

Route 2 H1 = H2 + H3 H1 = H2 + H3

Hess’s Law states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place.

Hess’s Law states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place.

Why?

6.5 Hess’s Law (p. 157)


Importance of Hess’s Law

• the reactions cannot be performed in the laboratory

• the reaction rates are too slow• the reactions may involve the formation of side

products

The enthalpy change of some chemical reactions cannot be determined directly because:

But the enthalpy change of such reactions can be determined indirectly by applying Hess’s Law.

6.5 Hess’s Law (p. 158)


Enthalpy Change of Formation of CO(g)

H2

+ ½O2(g)

CO2(g)H1

+ ½O2(g)

= -393.5 - (-283.0 ) = -110.5 kJ mol-1

6.5 Hess’s Law (p. 158)

Given: Hf [CO2(g)] = -393.5 kJ mol-1; Hc [CO(g)] = -283.0 kJ mol-1ø ø

C(graphite) + ½O2(g) CO(g)Hf [CO(g)]ø

Hf [CO(g)] + H2 = H1Hf [CO(g)] + H2 = H1

ø

Hf [CO(g)] = H1 - H2

ø


Enthalpy Change of Formation of CO(g)6.5 Hess’s Law (p. 158)

Hf CO(g)] + H2 = H1Hf CO(g)] + H2 = H1

ø


Enthalpy Change of Hydration of MgSO4(s)6.5 Hess’s Law (p. 160)

aq

MgSO4(s) + 7H2O(l) MgSO4·7H2O(s)

Mg2+(aq) + SO42-(aq) + 7H2O(l)

ΔH

ΔH2 aq

ΔH1

ø

ΔH = enthalpy of hydration of MgSO4(s)

ΔH1 = molar enthalpy change of solution of anhydrous magnesium

sulphate(VI)

ΔH2 = molar enthalpy change of solution of magnesium

sulphate(VI)-7-water

ΔH = ΔH1 - ΔH2

ø

ø


Calculations involving standard enthalpy changes of reactions

reactants productsHreaction

6.6 Calculations involving Enthalphy Changes of Reactions (p. 161)

elements

- Hf [reactants]

ø Hf [products]ø

Hreaction = Hf [products] - Hf [reactants]Hreaction = Hf [products] - Hf [reactants]ø ø


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New Way Chemistry for Hong Kong A-Level Book 11 Chapter 6 Energetics 6.1 What is Energetics? 6.2 Ideal Enthalpy Changes Related to Breaking and Formation