Enthalpy Changes Flashcards
memorisation
Enthalpy change
change in thermal energy
Enthalpy Changes Equation
delta H= H products-H reactants
What is an enthalpy change?
any substance that undergoes any chemical of physical change, requires heat, and ONLY happens under constant pressure.
When is delta H positive?
Delta H is positive when enthalpy in produts is greater than the reactants. Signifying the system absorbs energy.
When is delta H negative?
Delta H is negative when the enthalpy of the products is less than the reactants. Signifying that the system releases energy.
Molar Enthalpy
Change in Delta Hr
units are J/mol
r= the type of reaction occurring
How to calculate moles?
moles=mass / molar mass
4 Ways to show an enthalpy change…
- As apart of a balanced chemical equation
- exothermic on products
- endothermic on reactants - Writing balanced chemical equation and stating enthalpy change. as delta H=x
- Stating Molar enthalpy of specific reaction
-endothermic = -
-exothermic = + - Drawing energy level diagrams
Bond energies
- Covalent bonds need to be broken so that new bonds can form.
Bond dissociation energy
Average quantity of energy required to break a chemical bond.
Bond energies formula
delta H - sigma moles times bonds broken value - sigma times moles times bonds formed
3 Factors that affect the bond energy of a chemical bond include…
- Bond length
- Bond type
- Electronegativity difference between 2 atoms
Steps
- Chemical equation
- List out all chemical bonds
- Bond energies
- Formula for enthalpy change
Hess’s Law
The enthalpy change for a reaction is the sum of the enthalpy change for the individual steps of a reaction.
Rules
If you reverse the sign of a reaction that means you have flipped it
if the coefficients in a balanced chemical equation are multiplies by a factor, the value delta H must also be multiplied by the same factor.
