The tutor introduces a topic that academic chemists love.  Enthalpy appears in high school chemistry.

The definition of enthalpy, H, is

H=E_int+PV

where E_int is the internal energy content of the product, while PV is work done by volume increasing against pressure (relating to gases) as the reaction proceeds. Note that enthalpy is called by the variable H rather than E.

What does enthalpy mean, conceptually? For the concept to be useful, I think the internal energy content is the important part to focus on. After the reaction that forms a given substance is over, the PV part vanishes. The substance’s internal energy content is then what might distinguish it from another substance in terms of enthalpy.

Ethane (C₂H₆), which is found in natural gas, has a standard enthalpy of -84.68 kJ/mol. Oxygen gas, O₂, has a standard enthalpy of 0. CO₂ has a standard enthalpy of -393.5 kJ/mol, while H₂O_gas has -241.8 kJ/mol. The precise numbers aren’t important. The point to realize is that when ethane burns thus:

2C₂H₆+70₂→4CO₂+6H₂0

you can perceive the reaction as a change in enthalpy like so:

Notice that, from left to right, the enthalpy values decrease.

When you’re near a natural gas heater that’s burning, the enthalpy being lost is released to the environment: it’s the heat you feel. What’s actually being lost is internal energy from the fuel’s bonds as they are broken and replaced with lower energy ones. A reaction that has negative enthalpy change, aka, -ΔH, releases heat to the environment. A reaction with +ΔH draws in heat from the environment. In that case, the heat is used to form higher energy bonds than the original molecules had; the products’ internal energy goes up.

HTH:)

Sources:

www.energy.alberta.ca/naturalgas

Mortimer, Charles E. Chemistry, sixth Ed. Belmont: Wadsworth, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.