CHEM C5: Enthalpy Changes

Here’s today’s topic:

  • What is Enthalpy?
  • Energy Changes: Exothermic & Endothermic
  • What is a standard enthalpy change?
  • Types of standard enthalpy changes
    • formation
    • combustion
    • hydration
    • solution
    • neutralization
    • atomization
  • What causes enthalpy?
    • Bond energy
    • Lattice energy

Let’s go!


What is Enthalpy?
Total value of energy of a system.
Since energy exists as heat, it is the measure of heat content of a substance at a constant pressure.

It is denoted by the letter H.

It is impossible to determine the exact value of H for a system at a single moment, but the DIFFERENCE between the H before & after a reaction CAN.

The DIFFERENCE is ΔH.


Enthalpy changes can be EXOTHERMIC or ENDOTHERMIC:

Exothermic Energy is RELEASED by the system

Enthalpy change is NEGATIVE (ΔH = -n)

Surrounding temperature will RISE

Endothermic Energy is ABSORBED by the system

Enthalpy change is POSITIVE (ΔH = +n)

Surrounding temperature will FALL

 


What is Standard Enthalpy Change?
ΔH of a reaction measured under STANDARD CONDITIONS:

  • Pressure: 101.3 kPa (1 atm)
  • Temperature: 298 K (25 C)
  • Reactants & products in standard physical states
  • Solutions must be in 1 moldm-3

 


What is a Thermochemical Equation?
Balanced chemical equation that shows enthalpy relation between the product & the reactant.

Must include:

  • ΔH
  • physical state of ALL species (s, l, g, aq)
  • Number of moles
  • if the equation is balanced by multiplying both sides, REMEMBER to multiply the ΔH as well

 


There are a few TYPES of STANDARD ENTHALPY CHANGE:

Std. Enthalpy Change of… Symbol Definition:

Heat change when…

Enthalpy Change is…
REACTION ΔHr Amounts of reactants react to form products under standard conditions Exothermic or Endothermic
FORMATION ΔHf 1 mole of a compound is formed from its elements at standard condition Usually Exothermic
COMBUSTION ΔHc 1 mole of substance is burnt completely in excess oxygen at standard conditions ALWAYS Exothermic
SOLUTION ΔHsol 1 mole of substance is dissolved in a solvent to form an infinitely dilute solution at standard conditions Exothermic for soluble substances

 

Endothermic for insoluble substances

HYDRATION ΔHhyd 1 mole of gaseous ions is dissolved in a large amount of water to form an infinitely dilute solution at standard conditions ALWAYS Exothermic
HYDRATION of an Anhydrous Salt ΔH 1 mole of hydrated salt is formed from 1 mole of anhydrated salt at standard conditions ALWAYS Exothermic
NEUTRALISATION ΔHn 1 mole of water is formed by reaction of an acid with an alkali at standard conditions ALWAYS Exothermic
ATOMISATION ΔHat 1 mole of gaseous atoms form from an element at standard conditions Usually Endothermic

 


Why do reactions have an Enthalpy Change?
This is all due to BOND ENERGY.
Bonds store energy.

In a reaction, old bonds are broken, & new bonds are formed.
When bonds are broken, they ABSORB energy (+ΔH).
When bonds are formed, they RELEASE energy (-ΔH).

Besides this, there is also LATTICE ENERGY.
This is the energy needed to release lattice particles from their lattices (such as ions from an ionic lattice).

The net enthalpy change of a reaction is the final value of ΔH!


How do you calculate Enthalpy Change of a reaction?
There are multiple ways to calculate either the THEORETICAL value or EXPERIMENTAL value of enthalpy change.

To calculate theoretical values:

  • Applying Hess’ Law (see here)
    • For covalent compounds: Hess Cycles
    • For ionic compounds: Born-Haber Cycles
  • Finding the sum of bond enthalpies (see here)

To calculate experimental values, we use CALORIMETRY (see here).

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