energetics 1 and 2 Flashcards
Enthalpy
measure of the energy content of the system
Enthalpy change
difference between the quantity of heat absorbed to break the bonds in the reactants and that released during the formation of new bonds in the products at constant pressure
units of enthalpy change
kj mol-1
activation energy
minimum amount of energy that the reactant particles must possess before they can collide successfully to form products
enthalpy change of reaction
energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions (ie. 1 bar and 298K)
enthalpy change of formation
energy change when 1 mole of the pure substance in a specified state is formed from its constituent elements in their standard states under standard conditions (ie. 1 bar and 298K)
enthalpy change of combustion
energy released (cause always exo) when 1 mole of the substance is completely burnt in excess oxygen under standard conditions (ie. 1 bar and 298K)
enthalpy change of neutralisation
between an acid and a base is the energy change when the acid and the base react to form 1 mole of water under standard conditions (ie. 1 bar and 298K)
value for enthalpy change of neutralisation for strong acid and base
-57.3 kj mol-1
explanation for less exo neutralisation for weak acid strong base
- weak acid exists mainly as molecules hence further dissociation is required to produce more H+ ions
- dissociation involves the breaking of the O-H bond (for the case of CH3COOH) which is an endothermic process and the hydration of the CH3COO- and H+ ions which is an exothermic process
- overall, the dissociation process is endothermic
- thus as energy is consumed for further dissociation, the energy released by the reaction will be reduced and it will be less exothermic
enthalpy change of atomisation
energy absorbed when 1 mole of gaseous atoms is formed from the element under standard conditions (ie. 1 bar and 298K)
enthalpy change of atomisation of a compound
energy absorbed (always endo cos breaking bonds) when 1 mole of the compound is converted to gaseous atoms under standard conditions (ie. 1 bar and 298K)
enthalpy change of hydration
of an ion is the energy released (always exo) when 1 mole of gaseous ions is dissolved in water to form 1 mole of aqueous ions under standard conditions (ie. 1 bar and 298K)
- proportional to charge over radius (charge density)
enthalpy change of solution
of a substance is the energy change when 1 mole of the substance is completely dissolved in a solvent to form an infinitely dilute solution (does not produce any further enthalpy change when more solvent is added) under standard conditions (ie. 1 bar and 298K)
bond dissociation energy
BDE of a X-Y bond is the energy required to break 1 mole of that particular X-Y bond in a particular compound in gaseous state
always positive
bond energy
of a X-Y bond is the average energy required to break 1 mole of the X-Y bond in the gaseous state
ionisation energy 1st and 2nd
The first ionisation energy (1st IE) of an element is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly positively charge gaseous ions
The second ionisation energy (2nd IE) of an element is the energy required to remove 1 mole of electrons from 1 mole of singly positively charged gaseous ions to form 1 mole of doubly positively charge gaseous ions
electron affinity
→ The first electron affinity (1st EA) of an element is the energy CHANGE when 1 mole of gaseous atoms acquires 1 mole of electrons to form 1 mole of singly negatively charged gaseous ions
→ The second electron affinity (2nd EA) of an element is the energy ABSORBED when 1 mole of singly negatively charged gaseous ions acquires 1 mole of electrons to form 1 mole of doubly negatively charged gaseous ions
lattice energy
of an ionic compound is the energy released when 1 mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions
specific heat capacity/heat capacity
quantity of heat required to raise the temperature of 1g of the substance by 1˚C
quantity of heat required to raise the temperature of the substance by 1˚C
assumptions for the calculations
- negligible heat loss to the surroundings due to insulation
- specific heat capacity of solution is approx that of water (4.18J g-1 K-1)
- density of solution is approx that of water (1.00g cm-3)
Hess Law
enthalpy change of reaction is determined by the initial and final states of the system and is independent of the pathways taken
explain theoretical and experimental LE deviation
in agreement
- predominantly ionic
- fits in the purely ionic model which consists of spherical ions with evenly distributed charge
differ by quite a bit
- not purely ionic
- has covalent character due to the substantial polarisation of the anion by the cation
- if the value is more negative, suggests that bonding is stronger than that suggested by the purely ionic model
dissolving steps
- Separation of the ionic crystal lattice into its gaseous ions
- breaking down of the solid ionic crystal lattice
- endothermic - Hydration of the gaseous ions
- formation of ion dipole moments between the gaseous ions and water molecules to form hydrated ions in aqueous solution
- exothermic
for dissolution of smth in water
= -LE + enthalpy change of hydration for each ion
Solubility
- more soluble when enthalpy change of solution is more negative (less when positive)
- when it is negative, the hydration energy released is sufficient to compensate for the energy required to break the solid ionic crystal lattice
and vice versa
Spontaneous and non spontaneous
Spontaneous
- once started will continue without external assistance
(not part of definition but always trying to achieve a lower energy state and reach a state of greater entropy)
Non spontaneous
- will not occur unless external assistance is continuously applied
Entropy
of a system is a measure of the disorder of matter and energy in the system. The more ways matter in the system can be arranged, the more ways energy in the system can be dispersed, the more disordered the system is and the larger is its entropy
entropy change
measure of change of disorder of a system
