Chemical energetics

what is enthalpy

the total chemical energy of a substance

describe endothermic

delta h more than 0, product hv higher energy content, rxn absorbs heat from the surroundings, temperature of surroundings decrease and reaction is endothermic

describe exothermic

delta H less than 0, product have lesser energy content than the reactants, rxn releases heat to the surroundings, temp of surroundings increases,

what is activation energy

minimum energy which reacting particles must possess in order to overcome the energy barrier before becoming products

what is standard condition and standard state

298k (25c) 1 bar and 1 mol;/dm³

define standard enthalpy change of reaction

the enthalpy change when molar quantities of reactants as specified by the chemical equation to form products at 1 bar and 298k1

standard enthalpy change of formation

the enthalpy change when one mole of substance is formed from its constituent elements in their standard states at 298k and 1 bar

standard enthalpy change of combustion

the heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298k and 1 bar

standard enthalpy change of neutralisation

the heat evolved when one mole of water is formed int he neutralization rxn between and acid and a base at 298k and 1 bar

equation to determine heat evolved

q (heat change/J) = m(mass/g) . c (specific heat capacity/J g-1 K-1) . T (change in temp/K/C)

endothermic reaction equation

delta H = + q(heat energy)/ n (number of moles)

exo rxn eqn

delta H = - q(heat energy)/ n (number of moles)

define Hess’ law of constant heat summation

the enthalpy change of a reaction is determined only by the initial and final states and is independent of the rxn pathway taken

eqn w standard enthalpy change of formation

enthalphy change = sum of coeff x enthaly Hf(products) - sum of coeff Hf (reactants).

define bond energy

bond energy is the energy required to break 1 mole of covalent bond in the gaseous state (alw postive since enrgy is needed to pull the atoms apart)

equation with bond energy

enthalpy change of reaction = sum of bonds broken - sum of bonds formed

define standard enthalpy change of atomisation (for both element and compound)

enthalpy change of atomisation for an element is the energy required when 1 mole of gaseous atoms is formed from the elements at 298K and 1 bar

enthalpy change of atomisation for a compound is the energy required to convert 1 mole of the compound into gaseous atoms at 298K and 1 bar

define lattice energy

the heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions

equation to calculate lattice energy

le = cation charge x anion charge/ cation radii + anion radii

factors affecting lattice energy

charge of the ions and sizes of the ions

Ionisation energy define

the first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly chagred gaseous cations

electron affinity define

the first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous anions

define standard enthalpy changes of hydration

the heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar

alw negative! heat evolved in forming ion-dipole interactions between the ions and the polar water molecules. also depends on charge density

define standard enthalpy change of solution

the enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at 298K and 1 bar

eqn between enthalpy change of solution, lattice energy and enthalpy change of hydration

enthalpy change of solution = sum of enthalpy change of hydration - lattice energy

Entropy define

entropy is a measure of the randomness or disorder in the system. reflected in the number of ways that the energy of a system can be distributed through the motion of its particles

factors that affect entropy

temp (inc = inc ke = more ways to disperse energy)

phase (sol to liq : particles move more freely, become disordered, more ways to distribute the particles and their energy in the liquid state

number of particles (inc = inc disorder = inc ways particles and energy can be distributed)

mixing of particles (inc volume = inc ways to distribute the particles and hence their energy)

disssolution of an ionic solid (inc S since ions free to move in water, dec S since water molecules now bond to atoms and not free) depending on which is greater, causes the sign of entropy to change

equation for gibbs free energy change of reaction

gibbs free energy change of reaction = enthalpy - temperature x change in entropy

describe the different scenarios

G<0: rxn takes place spontaneously

G=0: the system is at equilibrium, there is no net reaction in the forward or backward direction

G>0: the rxn cannot take place spontaneously (spontaneous in the reverse direction)