Thermodynamics Flashcards
What is the enthalpy change of atomisation. Write the general equation and the equation for chlorine?
the enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
1/2Cl2 (g) →Cl (g)
X(s) → X(g)
What is the enthalpy change of hydration enthalpy? Write the equation for magnesium.
the enthalpy change to form one mole of aqueous ions from one mole of gaseous ions
Mg2+ (g) → Mg 2+ (aq)
What is the enthalpy change of solution? Write the equation for magnesium chloride.
The enthalpy change when one mole of a sold ionic compound is dissolved in water to produce aqueous ions
MgCl2 (s) → Mg2+ (aq) + 2Cl - (aq)
What is the lattice formation enthalpy? Write the equation for magnesium.
the enthalpy change to form one mole of a solid ionic lattice from its ions in the gaseous state
exothermic
Mg2+ (g) + 2Cl- (g) → MgCl2
What is the lattice dissociation enthalpy? Write the general equation
the enthalpy change to break one mole of a solid ionic lattice into its gaseous ions
endothermic
MgCl2(s) → Mg2+ (g) + 2Cl- (g)
What is the enthalpy of vaporisation? Write the equation for sodium.
enthalpy change when one mole of liquid is turned into gas
Na (l) → Na (g)
What is the enthalpy of fusion? Write the equation for sodium
The enthalpy change when one mole of a solid is turned into a liquid
Na (s) → Na (l)
What is the standard enthalpy change of formation? Write the equation for magnesium chloride .
The enthalpy change to form one mole of a compound from its element, with all substances in their standard states, at 100kPA and the stated temperature
Mg (s) + Cl2 (g) → MgCl2 (s)
What is the standard enthalpy change of combustion? Write the for ethane
the enthalpy change when ONE mole of a substance reacts completely with oxygen under standard conditions and all of the substances are in their standard state
C2H6 (g) + 3 1/2 O2 (g) → 2 CO2 (g) + 3H2O (l)
What is the definition for ionisation enthalpy? Write the equation for magensium
The enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ion
Mg+ (g) → Mg2+ (g) + e-
What is the definition for the enthalpy for electron affinity? Write the equation for chlorine
the enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.
Cl (g) + e → Cl- (g)
What is the definition for the mean bond dissociation enthalpy? Write the equation for chlorine
the enthalpy change to break 1 mole of bonds in 1 mole of gaseous molecules (average over a number of compounds).
Cl-Cl (g) → 2Cl (g)
What are the two main factors that affect enthalpy change?
size: smaller ions have a larger attraction due to higher density charge
charge on the ion: larger charge have a larger attraction for the water molecules due to larger charge density
What does it mean when an ion is hydrated and what happens to this ion?
The ion is surrounded by water molecules.
If the ion is positive an attraction is formed with the delta negative oxygen atoms in H2O
If the ion is negative an attraction is formed with the delta positive hydrogen atoms in H2O
Why is hydration exothermic?
energy is released when attractions are formed
What is the difference in exothermic hydration enthalpies with smaller and larger ions and why is this?
Smaller ions have more exothermic hydration enthalpies as they have a stronger attraction to water molecules due to their higher charge density (the charge is more concentrated over the volume of the ion)
larger ions have less exothermic hydration for the opposite reasons
What must happen for a substance to dissolve?
the attractions formed with water molecules must be the same strength if not greater than those broken
Why do small ions generally have stronger attractions?
because they have a higher charge density
What is theoretical lattice enthalpy and what is it based on? What does it assume?
lattice enthalpy changes made by calculations based on the pure ionic model of the lattice
based on radius and charges of ions
assumes that all ions are spherical, ions are a known distance apart and that the charge is evenly distributed around the ion
What is experimental lattice enthalpy and what is it based on?
hess cycle/ Born harber cycle
takes into account extra attractions unlike theoretical that holds ions together that makes the lattice have high magnitude
(usually higher than theoretical lattice enthalpy)
How do you know when the theoretical value is not using a perfect ionic structure?
if there are much more discrepancies between the theoretical and experimental value
What does covalent character mean?
When a positive ion attracts electron density from the negative ion. The positive ion “polarises”/ distorts the electron density on the negative ion towards the positive ion.
This means that the negative ion has become non-spherical resulting in a degree of covalency due to some electron density being shared between the two ions
When does covalent character occur?
it only happens if the positive ion has a high charge density to distort the negative ion and if the negative ion is not good at holding on to outer electrons
What are the properties of a positive and negative ion with high convalency?
Postive:
- good at attracting electron density
- small and highly charged (as they will have a high charge density
Negative:
- large = outer electrons further away from the nucleus (positive ion can distort easily)
- high negative charge = higher build up of electrons on the outside = positive ion more able to attract them
How does high covalency affect theoretical and experimental values
leads to a bigger difference between them, as there are more additional attractions
What is entropy
a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles
the measure of disorder in particles
What are the factors that contribute to a high entropy change?
- lots of particles (as there are more ways the particles and their energy can be arranged) aka the complexity of a molecule
- the particles containing a high amount of energy
*
What is a spontaneous change?
one that tends to go naturally (without assistance)
What does it mean if a reaction is spontaneous/feasible?
it can go of its own accord thermodynamically
Describe the spontaneous change in an exothermic reaction
From a higher energy state to a lower energy state = release energy and the enthalpy change is negative
Describe the spontaneous change in an endothermic reaction
from a lower state to a higher state = takes in energy and the enthalpy change is postive
Why are reactions feasible/spontaneous?
Because substances always tend towards disorder = they are more energetically stable when there’s more disorder → particles will move to increase their entropy
What is the formula for the entropy change?
entropy change of products - entropy change of reactants
What is the definition of the standard entropy of a substance?
The entropy of one mole of that substance under standard conditions (at a pressure of 100 kPa and a temperature of 298 K)
What are the factors that govern the feasibility of a chemical reaction?
- the enthalpy change
- the entropy change
- sometimes temperature
What is the equation for Gibbs free energy and how do we use this to see whether a reaction is feasible or not?
When a reaction is 0 or negative
Explain why the bond enthalpy of a cl-cl bond is greater than that of a Br-Br bond?
- The bonding pair is closer to the nucleus
- larger attractions with each other
Explain the meaning of the term perfect ionic model (4)
- only ionic bonding/ electrostatic attractions
- assumes that all ions are perfectly spherical
- no covalent character
- ions are point charges
By describing the nature of the attractive forces involved, explain why the value of the enthalpy of hydration for the chloride ion is more negative than that of the bromide ion
- chlorine is a smaller ion so it has a higher charge density
- So the force of attraction between chloride ions and water is stronger
- chlorine ions attract the s+ H ions of water
Explain why the free-energy change for the dissolving of potassium chloride in water is negative, even though the enthalpy change is positive
entropy increases
Because 1 mol (solid) → 2 mol (aqueous ions) / no of particles increases
Therefore T∆S > ∆H
In terms of electrostatic forces, suggest why the electron affinity of fluorine has a negative value.
There is an attraction between the nucleus / protons and (the added) electron(s) Energy is released (when the electron is gained)
Suggest why hydration of the chloride ion is an exothermic process.
Water is polar / water has Hδ+ (Chloride ion) attracts (the H in) water molecules
Why is the second electron affinity endothermic?
If we want to add one more electron after the addition of one electron then the incoming electron will face repulsion from the electrons in valence shells. So we need to provide some energy if we want to add one more electron to negative − ion
