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What is enthalpy change?

The 1st Law of Thermodynamics states that energy can neither be created nor destroyed, but it can certainly be transferred from one area to another or converted from one form to another.

  • Enthalpy change is a preferred expression given to heat transfer in a system under constant pressure.
  •  Eg.  Boiling water in an open container is a constant pressure process
  • Open systems can be considered as constant pressure
  • In a sealed system, pressure can vary.
  • Eg. air pressure in a ball will vary according to gas law
  • Most chemical reactions occur at constant pressure, so enthalpy change is more often used to measure heats of reaction than internal energy.
  • Enthalpy change tells us how much heat is added or removed from a substance in a chemical process under constant pressure.
  • The unit for enthalpy is joule/mole.
  • Enthalpy  change is governed by the equation

ΔH = ΔE  +  (p•ΔV)

where ΔH = change in enthalpy
ΔE = change in internal energy
p = atmospheric pressure
ΔV = change in volume due to system expansion(eg. gas expanding in a chemical reaction)

In a reaction where pressure remains constant, there is no work done.

Enthalpy change is given by the equation ΔH = ΔU

 

Change in internal energy of a system analogy

 Example showing work done by the system

Below is an example of work done by a cistern. The gas in the cistern is heated up and it expands. The expansion causes work to be done against the surrounding. Heat is lost to the surrounding so work done becomes negative.

change in internal energy of a chemical syste,

 Example showing work done on the system

Energy is transferred from the surrounding into the system. Work done is positive because the system gains energy. There is a decrease in the volume of the piston. If this is done very quickly, the temperature in the gas will rise.
Example work done on a system - piston pushing down

Predicting whether a reaction is exothermic or endothermic

When ΔH < 0, the reaction is exothermic. It gives off heat.

When ΔH > 0, the reaction is endothermic. It absorbs off heat.

When ΔH = 0, the system is in equilibrium.

Propane combustion

In this example, propane is burned and you are required to work out the enthalpy change. The experiment shows how you are measuring the heat change indirectly. You do this by measuring the rise in temperature of the water. It is assumed all the heat from burning the propane will be transferred to the water. You'd use the change in water temperature to calculate the change in enthalpy of the gas. This method isn’t very accurate because some heat is lost to the environment (not all goes to the water) and the loss isn’t recorded. But for a small experiment held in the laboratory, this is an effective method to learn the concept.

enthalpy change - propane combustion example

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