What is enthalpy and how can it be calculated?

Enthalpy is a thermodynamic property that chemists can use to describe the heat energy transferred throughout a chemical reaction. It is especially useful given that it is defined as "a measure of heat energy present in a chemical system at constant pressure". Interestingly, it is the constant pressure aspect of the definition that makes it so useful as most reactions - at least certainly the more straightforward reactions - can be conducted under such constant pressure conditions.
As an example, we can consider the bond strengths of individual molecules. Comparing the reactants to the products bond strengths give a numerical value of enthalpy that allows us to compare between different reactions.
CH4 + 2O2 --> CO2 + 2H2O [complete combustion of methane]
C-H bond enthalpy (here as a measure of bond strength) = 411 kJ mol-1O=O bond enthalpy = 494 kJ mol-1C=O bond enthalpy = 799 kJ mol-1O-H bond enthalpy = 459 kJ mol-1
Therefore, calculation to conduct (Reactants - Products) for enthalpy change of combustion = [4(411) + 2(494)] - [2(799) + 4(459)] = - 802 kJ mol-1 [exothermic]

Answered by Chemistry tutor

1794 Views

See similar Chemistry A Level tutors

Related Chemistry A Level answers

All answers ▸

Why are zinc and Scandium not transition metals?


How do acid buffer solutions work?


Explain how the electron pair repulsion theory can be used to deduce the shape of, and the bond angle in, NH3.


What is the difference between structural isomers and stereoisomers?


We're here to help

contact us iconContact ustelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

© MyTutorWeb Ltd 2013–2025

Terms & Conditions|Privacy Policy
Cookie Preferences