what is enthalpy in chemistry?

Question

i have an idea, but i cannot picture what it actually is. please describe so it is easily understandable.

Answer 1

Enthalpy is the heat change that evolves as a chemical reaction takes place. It is specifically a measure of this heat.

Answer 2

Enthalpy is the heat content or the energy stored within a compound.

Enthalpy of fusion(heat of fusion) is the amount of energy needed to melt a substance.

Enthalpy of vaporization(heat of vaporization) is the amount of energy needed to melt a substance.

So if you measure how much energy is needed to melt ice at 32F to water at 32F you are measuring the enthalpy of fusion. Likewise, you can measure it in the reverse direction to get the enthalpy of freezing which will be the same value but the opposite sign.

Another way of thinking about it is if you were a molecule and you climbed on a roof of a building, your body is storing energy as you get higher. So if you measured your body at this point is has one value of enthalpy(stored energy). Now if you go back down, your body has lower energy or a lower enthalpy.

In actual molecules, the energy is stored within the bonds and depend on which atoms are bonded to each other, the stored energy can be higher or lower.

Answer 3

Review

You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin.

Problem

Hydrogen peroxide decomposes according to the following thermochemical reaction:

H2O2(l) --> H2O(l) + 1/2 O2(g); DH = -98.2 kJ

Calculate the change in enthalpy, DH, when 1.00 g of hydrogen peroxide decomposes.

Solution

The thermochemical equation tells us that DH for the decomposition of 1 mole of H2O2 is -98.2 kJ, so this relationship can be used as a conversion factor. Using the Periodic Table, the molecular mass of H2O2 is 34.0, which means that 1 mol H2O2 = 34.0 g H2O2. Using these values:

DH = 1.00 g H2O2 x 1 mol H2O2 / 34.0 g H2O2 x -98.2 kJ / 1 mol H2O2

DH = -2.89 kJ

Answer

-2.89 kJ

Answer 4

The R is right but the heat must be measured at constant pressure.

Answer 5

It's a thermodynamic property equal to the sum of the internal energy of the system i guess. it's also the product of pressure and volume of the system. that's what i know it as.