R1.2 - enthaly changes in reactions Flashcards
What type of reaction is bond breaking?
Endothermic reaction which requires energy.
What type of reaction is bond making?
Exothermic which releases energy.
What is bond enthalpy?
Bond enthalpy - energy needed to break 1 mole of bonds of gaseous molecules under standard conditions.
What is average bond enthalpy?
Average bond enthalpy - energy needed to break 1 mole of bonds of gaseous molecules under standard conditions averaged over similar compounds.
What is a covalent bond?
Electrostatic attraction between a shared pair of negative electrons and the positive nuclei of a bonded atom.
What is the relationship between bond enthalpy and bond length, amount of bonds and polarity?
Shorter bond length = Higher bond enthalpy
Longer bond length = Lower bond enthalpy
More bonds = Higher bond enthalpy
Less bonds = Lower bond enthalpy
Polar bonds = Higher bond enthalpy
Non-Polar bonds = Lower bond enthalpy
How can you calculate bond enthalpy?
Bond enthalpy = bond energies broken - bond energies made
Bond enthalpy = products - reactants
What is Hess’s Law?
Hess’s Law - states that the enthalpy change for a reaction is independent of it’s pathway between initial and final states, with reactants and products being the same.
What is standard enthalpy change of formation?
Standard enthalpy change of formation - enthalpy change that occurs when 1 mole of a substance is formed from it’s elements in their standard states when they are stable (283K and 100kPa).
What is standard enthalpy change of combustion?
Standard enthalpy change of combustion - enthalpy change that occurs when 1 mole of a substance burns completely under standard conditions.
- Must have a negative value.
In what direction does Hess’s triangle face for enthalpies of combustion and formation?
Formation - Formed in Heaven - upwards
Combustion - Burned in Hell - downwards
When giving a question asking to calculate enthalpy of formation/combustion, but you are given the opposite values, which values should you use?
Use the values you are told and given
Why are standard enthalpy changes of formation calculations useful?
- Gives a measure of the stability of a substance relative to its elements
- Can be used to calculate the enthalpy changes of all reactions; either hypothetical and real.
What is the standard enthalpy of formation for an element?
0
What are standard states?
The form of an element at standard conditions - usually the most stable.
What is a Born-Haber cycle?
Born-Haber cycle - an application of Hess’s law, used to show the energy changes in the formation of an ionic compound.
What is first ionization energy?
First ionization energy - the minimum energy required to remove 1 mole of electrons from 1 mole of gaseous atoms.
What is electron affinity?
First electron affinity - enthalpy change when one mole of gaseous electrons is added to 1 mole of gaseous atoms.
What is lattice enthalpy?
Lattice enthalpy - enthalpy change that occurs when 1 mole of a solid ionic compound is separated into gaseous ions under standard conditions.
What is enthalpy change of atomization?
Enthalpy change of atomization - enthalpy change that occurs when 2 moles of gaseous atoms is formed from the element in its standard state.
Why are experimental lattice enthalpies larger than theoretical lattice enthalpies?
- Theoretical values are based on an ionic model and doesn’t take into account additional covalent contributions to the bonding.
What type of reaction is lattice enthalpy?
Endothermic reactions - all values are positive.
What type of reaction is enthalpy change of atomization?
Exothermic
What type of reaction is enthalpy change of formation?
Exothermic
What type of reaction is ionization enthalpy?
Endothermic
What type of reaction is bond enthalpy?
Endothermic
What type of reaction is first electron affinity?
Exothermic
What type of reaction is second electron affinity?
Endothermic
What is the relationship between bond enthalpy and enthalpy change of atomization?
Bond enthalpy = x2 Enthalpy of atomization
Bond enthalpy - starts with 1 mole
Enthalpy of atomization - makes 1 mole
What is the trend between lattice enthalpy and ionic charge?
Lattice enthalpy is proportional to the product of the charges on the positive and negative ions.
- The larger the ionic charge, the larger the lattice enthalpy.
What is the trend between lattice enthalpy and ionic radius?
Lattice enthalpy is inversely proportional to the sum of the ionic radii of the positive and negative ions.
- The larger the ionic radius, the smaller the lattice enthalpy.
