Thermodynamics Flashcards
Define enthalpy change of combustion
Enthalpy change when 1 mole of a substance undergoes complete combustion in oxygen under standard conditions
Define enthalpy change of formation
Enthalpy change when 1 mole of a substance is formed from its elements in their standard States under standard conditions
Define enthalpy change of ionisation
The first ionisation energy is the enthalpy change when 1 mole of gaseous atoms loses 1 electron per atom to become 1 mole of gaseous 1+ ions
The second ionisation energy is the enthalpy change when 1 mole of 1+ gaseous ions loses another electron to become 1 mole of gaseous 2+ ions
Define enthalpy change of solution
Enthalpy change when 1 mole of a solute is dissolved in enough solvent that the ions are separated and no further enthalpy change occurs on further dilution
Define enthalpy change of atomisation
Enthalpy change when 1 mole of gaseous atoms are formed from an element or compound in its standard state under standard conditions
Define lattice enthalpy of formation
Enthalpy change when 1 mole of a solid ionic compound is formed from its constituent gaseous ions
Define electron affinity
The first electron affinity is the enthalpy change when 1 mole of gaseous atoms gains 1 electron per atom to become 1 mole of gaseous 1- ions
The second electron affinity is the enthalpy change when 1 mole of gaseous 1- ions gain another electron to become 1 mole of gaseous 2- ions
Define enthalpy change of hydration
Enthalpy change when 1 mole of gaseous ions are fully dissolved in water to form 1 mole of aqueous ions. This always gives out energy (EXO (negative))
Define enthalpy change
the heat energy transferred in a reaction at constant pressure
Define bond dissociation enthalpy
Enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken
Theoretical lattice enthalpies are often different from experimental values. Explain
You can work out a theoretical lattice enthalpy by doing some calculations based on the purely ionic model of a lattice. The purely ionic model of a lattice assumes that all the ions are spherical and have their charge evenly distributed, there is no covalent character. But the experimental lattice enthalpy from born Haber cycle is usually different. This is evidence that ionic compounds usually have some covalent character. The positive and negative ions in a lattice aren’t usually exactly spherical. Positive ions polarise neighbouring negative ions to different extents and the more polarisation the more covalent the bonding will be
What happens when a solid ionic lattice dissolves in water
The bonds between the ions break (ENDO). This enthalpy change is the lattice enthalpy of dissociation. Bonds between the ions and water are made (EXO). This enthalpy change is enthalpy change of hydration. The enthalpy change of solution is the overall effect on the enthalpy of these 2 things
What do you need to know to be able to complete an enthalpy cycle
Lattice dissociation enthalpy of the compound and enthalpies of hydration of the ions
How does physical state affect entropy
Solid particles wobble around a fixed point, hardly any randomness so lowest entropy
Liquids have some disorder
Gas particles have most disorder, most random arrangements of particles so highest entropy
How does dissolving affect entropy
Dissolving a solid increases entropy as dissolved particles can move freely as they’re no longer held in one place. Increases vibrations
How do more particles affect entropy
The more particles, the more entropy as the more ways they and their energy can be arranged. Entropy increases as number of moles increases
Entropy increase may explain spontaneous endothermic reactions. Explain
A spontaneous (or feasible) reaction is one that’ll happen by itself (don’t have to give it energy). Normally have to supply energy to make an endothermic reaction happen, but in some reactions the entropy increases such a lot that the reaction will happen by itself, without a supply of energy
Reactions won’t happen unless the total entropy change is ……
Positive
During a reaction, there’s an entropy change between reactant and products (entropy change of the system). The entropy change of the surroundings alters too ( as energy is transferred to or from the system). The total entropy change is the sum of the entropy changes of the system and the surroundings.
Define lattice enthalpy of dissociation
Enthalpy change to separate 1 mole of a substance into its gaseous ions
Mean bond enthalpies are only approximations. Explain
Type of bond will vary in strength in different compounds. Only bond enthalpies in diatomic molecules will always be same. So calculations done using mean bind enthalpies will never be perfectly accurate. Will get much more exact results from experimental data obtained from specific compounds
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
What things affect entropy
Physical state
Dissolving
More particles
In terms of the forces acting on the particles, suggest 1 reason why the first electron affinity of oxygen is an exothermic process
There is a strong net attraction between the nucleus and outer electrons, takes a lot of energy to break attraction
Explain the interactions between water molecules and fluoride ions when the fluoride ions become hydrated
Water is polar
The F- ions attracts water (attracted to the positive charge on the hydrogen)
Explain why the electronegativity of fluorine is greater than that of chlorine
The fluorine atom is smaller, there is less shielding
The bond pair of electrons are attracted more strongly to the nucleus
Explain why the hydration enthalpy of the fluoride ion is more negative than that of the chloride ion
Fluoride ions are smaller, they have a larger charge density
The positive charge attracts the positive charge on the hydrogen on water more strongly
