ENERGETICS

Hess's Law

Example of question

Use the following enthalpy change data:


Reaction 1 2H2(g)+O2(g) → 2H2O(l) ΔH1=−572kJmol−1
Reaction 2 2H2(g)+O2(g) → 2H2O(g) ΔH2=−484kJmol−1


to work out the enthalpy change for the process:
H2O(l) → H2O(g)

Method 1 (Enthalpy Cycle)

  1. Draw the diagram of the reaction (the enthalpy cycle)
  2. Look at the direction of the formation of the main equation as shown below:







  1. Then, solve the question by using the cycle as your guide
  2. The same direction, add,different direction, subtract

Method 2 (Manupilating Equation)

  1. Look at the equation given:
    Reaction 1 2H2(g)+O2(g) → 2H2O(l) ΔH1=−572kJmol−1
    Reaction 2 2H2(g)+O2(g) → 2H2O(g) ΔH2=−484kJmol−1

.

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Measuring Energy Changes

Terms

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the energy needed to break one mole of bonds in gaseous molecules under standard conditions

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Average bond enthalpy: the energy needed to break one mole of bonds in gaseous molecules under standard conditions averaged over similar compound.

More atoms in a molecule, more energy requires to break the bond

Break the bond (energy absorbed)

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Form the bond (energy released)

∆H (reaction = ∑∆H(product) – ∑∆H(reactant)

Standard enthalpy change of Formation

Definition
The enthalpy change when one mole of a substance is formed from its elements in their standard stats under standard condition

Definition
The enthalpy change accompanying a chemical reaction is independent of the pathway between initial and final states

ΔHr=ΣΔHf(products) − ΣΔHf(reactants)

Example of Question
Given these standard enthalpy change values (kJmol−1):
∆Hf [NH3(g)] = −46; ∆Hf [NO(g)] = 90; ∆Hf [H2O(g)] = −242
calculate the enthalpy change for the reaction:
4NH3(g)+5O2(g) → 4NO(g)+6H2O(g)

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Bond in O2 and ozone break when absorbing sufficient UV radiation

Planck's equation :

E (photon) = hν = (h x c)/λ

Using ∆Hr=Σ∆Hf (products) − Σ∆Hf (reactants)


Σ∆Hf (products)=(4×90)+(6×−242)= −1092 kJmol−1
Σ∆Hf (reactants)=4×−46=−184 kJmol−1


Note: ∆Hf for O2(g) is zero because it is an element in its standard state.

Ozone Depletion

Exothermic

Endothermic

Bond Enthalpies

Heat: form of energy that flows from a higher temp. to a lower. temp

Temp: measure of the average kinetic energy of particles

Internal energy: total amount of energy (kinetic + potential

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Ex(it)othermic

En(ter)dothermic

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Neutralisation

Measuring enthalpy, H=mc

Combustion

Solution

Evaporation of alcohol and water

Heat loss to the surrounding

Incomplete combustion of alcohol

Assuming C of reactant alike with water

Heat loss to the surrounding

C of the cup not taking into account

Absorption of heat from surrounding

Assuming C of mixture with water

Mass of salt does not taken into account in mass of solution

(Draw structural formula for see the bond clearly)

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∆H (absorbed)= ∆H (released)

(Formation) O2 absorbed high-energy of UV radiation --> 2 O radical

1 O radical + O2 --> O3

(Depletion) O3 absorbed low-energy UV radiation --> O radical + O2

O3 + O radical --> 2 O2

Formation and depletion of ozone

(usually used to find wavelength)

Bond formed (Exothermic Reaction) - Helps in maintaining relatively high temperature of stratosphere by producing heat

Enthalpy of combustion
Q=mcْθ