Energetics + Thermodynamics Flashcards
Enthalpy Change
? The Energy transferred to or from the system when a chemical change happens. In constant pressure
Standard Enthalpy of Combustion
? The enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions, all substances in the standard state.
Standard Enthalpy of Formation
? The enthalpy change when one mole of a substance is formed from its elements under standard conditions, all substances in their standard state.
Standard state
? Whether it is a solid, liquid or gas @ 298 K(25oC) 1cm3 = 1g
Calorimetry
? Process of measuring the heat change for a given chemical reaction
Bond dissociation enthalpy
? The enthalpy change when one mole of COVALENT bonds are broken with all the species in the gaseous state. All enthalpies are endothermic.
Mean bond enthalpy
? The enthalpy change when one mole of covalent bond is broken averaged over a range of compounds with everything in the gas phase. If this is a diatomic molecule, you need to define bond enthalpy instead.
Hess Law
? The enthalpy change in a reaction is independent of the route taken
Lattice enthalpies
? THe measure of the strength of the forces between ions in an ionic solid.
Factors that increase attraction
- High charge
- Small size
Enthalpy of atomisation
? Enthalpy changes when one mole of gaseous atom is formed from its elements in its standard states. (may not happen in standard conditions)
Electron affinity (opposite of ionisation)
? Enthalpy changes when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous ions
Lattice formation enthalpy
? The enthalpy of lattice formation is the standard enthalpy change when 1 mole of an solid ionic lattice is formed its constituents ions in gaseous form
Born Haber Cycle
? Used to calculate some to the types of enthalpy changes from experimental data.
? Special type of Hess Cycle that represents the formation of an ionic compound from its elements in a series of steps.
Enthalpy of Hydration
? The enthalpy change when one mole of gaseous ions dissolves to form one mole of aqueous ions.
Enthalpy of Solution
? The enthalpy changes when one mole of a solid crystal is dissolved in a large excess of solvent
Entropy
? Measure of disorder
? Number of ways molecules can be arranged and the number of ways their energy can be arranged between them.
Free Energy
? A simple way to determine if the reaction is feasible (includes both enthalpy and entropy)
Gibbs free energy
? A measure of whether the reaction is spontaneous (depends on the experimental conditions)
What do the letters in this equation represent? q=mcΔT
q - heat transferred
m - mass of the substance
c - specific heat capacity
ΔT - change in temperature
How does Calorimetry work?
Dry the cup after washing
Put the polystyrene in a beaker for insulation and support
Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
Clamp thermometer into place making sure the thermometer bulb is immersed in the solution
Measure the initial temperatures of the solution or both solutions (if 2 are used). Do this at regular intervals of 2-3 min (or as instructed by the method)
At minute 3/6 transfer the second reagent to the cup. Solid reagent is used, then weigh the solid then add it into the cup.
Stir the mixture (ensures that all of the solution is at the same temperature)
What are the errors in calorimetry
Energy transfer to the surroundings
Reaction or dissolving may incomplete
Density of solution is taken to be same as water
Assumes all solution to have a SHC of water
How do you calculate ΔH
Energy (BB) - Energy (FB)
Why do the mean bond values from the book differ from the ones calculated in the Hess cycle.
Remember that calculated values of enthalpy of combustions will
be more accurate if calculated from enthalpy of formation data than
if calculated from average bond enthalpies. This is because
average bond enthalpy values are averaged values of the bond
enthalpies from various compounds.
Define Bond dissociation enthalpy
The bond dissociation enthalpy is the standard molar enthalpy change
when one mole of a covalent bond is broken into two gaseous
atoms (or free radicals)
Define first ionisation energy
The first ionisation enthalpy is the enthalpy change required to
remove 1 mole of electrons from 1 mole of gaseous atoms to form
1 mole of gaseous ions with a +1 charge
Equation for first ionisation energy
X (g) –> X+ (g) + e-
Enthalpy of lattice dissociation
? The Enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form.
Second electron affinity
? The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.
Trends in Lattice enthalpies
Size -
Charge
Describe a perfect ionic model
Theoretical lattice enthalpies assumes a perfect ionic model where the ions
are 100% ionic and spherical and the attractions are purely electrostatic.
Why does calcium chloride have a formula of CaCl2, not CaCl or CaCl3.
Calculate the enthalpy of formation for each case. The one with the most exothermic enthalpy of formation will be the one that will be formed as it is the most thermodynamically stable.
Factors affecting entropy
- Temperature
- Physical state
- number of moles
Equation for entropy
Total ΔS = ΣSsystem + ΣSsurroundings
Unit for entropy
J K^-1 mol^-1
Equation for Gibbs free energy
ΔG = ΔH - TΔS
the reaction will be spontaneous if G<0
Define spontaneous
A process that will proceed on its
own without any external influence.
(e.g. diffusion)
what happens to ∆G during phase changes?
As physical phase changes like melting and boiling are
equilibria, the ∆G for such changes is zero
An equation to calculate enthalpy change of solution
ΔH solution = Δ HL dissociation + ΣΔ hydH
