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)
- Draw the diagram of the reaction (the enthalpy cycle)
- Look at the direction of the formation of the main equation as shown below:
- Then, solve the question by using the cycle as your guide
- The same direction, add,different direction, subtract
Method 2 (Manupilating Equation)
- 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