Hess' Law, Flammability Limits and Terminology

Question 1

What are the fundamentals of combustion?

Answer 1

• Combustion is an oxidation process.
• Oxidation is an exothermic process - heat energy is liberated.
• The liberated energy is called the heat of combustion
  
  • Enthalpy is the correct term at constant pressure
• The Standard enthalpy of combustion refers to standard conditions (298K, 1 atmosphere). It assumes complete combustion and the formation of water, as a gas.

Question 2

What does Hess’ Law state?

Answer 2

The enthalpy change for a reaction is independent of the route by which the reaction is achieved, but depends only on the initial and final stages.

This is because the enthalpy content of a substance is a property of the substance; it does not depend on how the substance is made.

Question 3

What is the lower flammability limit?

Answer 3

• The lowest concentration of flammable gas in air that can sustain combustion is called the “lower flammability limit”. Below this is fuel lean.
• It exists because fuel vapours are capable of burning in 21% oxygen (air) only in a certain range of concentrations.
• If the concentration is too low, it cannot burn. The enthalpy of combustion will not generate enough heat to sustain the flame.

Question 4

What is the upper flammability limit?

Answer 4

If the concentration of flammable gas is too high, it will not burn. This is because there is not enough oxygen to sustain the reaction.

The highest concentration of flammable gas in air that can sustain combustion is the “upper flammability limit”. Above this is fuel rich.

Question 5

What do flammability limits depend on?

Answer 5

• Flammability limits usually apply to fuels in 21% oxygen at 1 atmosphere. Changing the pressure will change combustion properties.
• White’s rule of thumb: The lower flammability limit for a gaseous hydrocarbon is half the stoichiometric concentration.

Question 6

What is the flash point of a substance?

Answer 6

Flashpoint: “The minimum temperature at which the vapour produced by a liquid can be ignited momentarily in air”

• Vapour pressure of a liquid increases with temperature. The FP is the temp at which the vapour conc. reaches the lower flammability limit.
• The ignition source is external
• Resultant flame does not sustain at this temperature, after ignition source is removed. Influenced by enthalpy of combustion, heat capacity of products etc.

Question 7

What is the fire (flame) point of a substance?

Answer 7

“The minimum temperature at which sufficient vapour is produced by a liquid to sustain combustion after ignition in air”.

• Few degrees higher than flash point
• After ignition source is removed, heat of combustion must balance heat loss from flame, so temperature doesn’t drop
• Vapour must also be produced at a rate sufficient to maintain concentration above the lower flammability limit

Question 8

What is the (auto) ignition temperature of a substance?

Answer 8

• “The minimum temperature at which a fuel will ignite on its own without external source of ignition
  
  • Activation energy required to initiate combustion with oxygen is supplied by ambient thermal energy.
  • For gas/liquid, measured by injecting fuel into container at measured temperatures.
• Concept can be extended to solids.
  
  • Measured by heating in oven and monitoring temperature.