Question

enthalpy change

Answer 1

refers to the energy change for chemical system, when reactants change to products

does that mean the products are also part of the system? not the surrounding?

Answer 2

Yes!

The system refers to whatever is being studied (in this case, the chemical reaction, which includes the reactants, products and enthalpy). The surroundings are everything else (their change in enthalpy is opposite that of the reaction).

Answer 3

ok thank you!!

just one more:

whats the difference between molar enthalpy and enthalpy change?

is enthalpy change the amt of heat energy lost or gained of h whole system, products and reactants

and molar enthalpy is the amt of heat energy lost or gained of one specific reactant undergoing change? so no products and just one reactant?

this is what i think right now, not sure if its right

Answer 4

The enthalpy change is kind of more general. It's referring to how much heat was gained/lost by the specific reaction you're performing (i.e. it will depend on how much reactant you use).

The molar enthalpy is more specific and refers to how much heat is gained/lost PER mole of something. The something can be either a product or a reactant, but usually it refers to how much heat is gained/lost by the reaction as written. For instance:

A + 2B --> C + 4D ∆H = -100 kJ/mol

This means that 100 kJ are released from this reaction for every mole of A used, or every 2 moles of B used, or every mole of C produced, or every 4 moles of D produced.

Make sense?

Answer 5

oh i see, so enthalpy change is the overall heat energy transferred from system to surrounding vice versa

and molar enthalpy is energy transfer from reactant or product to surrounding but every whatever kj per mole of a specific reactant or product

Answer 6

so for example given a balanced eqn like you had, it asked for molar enthalpy of product D

you find it, lets say 550 kj/mole, since its four moles of D wouldnt you have to divide the molar enthalpy 555kj/mol by 4 to get PER mole?

or is 4D per mole... which is kind confusing as its a moles of D

Answer 7

Yeah depends on the question. Sometimes the question might say per mole of ___ which makes it easy.

If it just says kJ/mol without specifying anything else than you assume it's as written like in my example above, in which case you would need to divide the enthalpy again by the coefficient of whichever reactant/product you're concerned about.

So in your example, you can divide by 4 if you're interested in finding how much heat was released when x moles of D were produced.

Answer 8

Thanks! :)