Bond Enthalpies and Entropies Flashcards
What is an enthalpy?
Unit of heat gained or lost. = -Q/n KJ/mol
What is an enthalpy of combustion?
Heat produced when one mole of a substance is burned completely in standard conditions
What is an enthalpy of formation?
Heat produced when one mole of a substance is formed from elements in standard states.
What is an enthalpy of reaction?
the total enthalpy of a reaction. Total = sum of reactions - sum of the products. Difficult to measure due to side reactions and slow rates
What is an enthalpy of ionization?
The energy needed to remove the first valence electron from one mole of metal in a gas state to form its ions.
= E/mol
What is an enthalpy of electron affinity?
Energy that is transferred by the addition of one mole of electrons to one mole of gaseous atoms.
What is an enthalpy of atomization?
Heat exchanged when one mole of gaseous atoms of an element is formed from the elements in its standard state.
What is a lattice enthalpy?
Energy absorbed when one mole of a solid ionic compound is decomposed to form its gaseous ions.
H(latt) = sum of all processes (IE, EA, F)
What is an enthalpy change of solution?
Change in energy when one mole of a substance is dissolved in a large excess of a pure solvent.
What is an enthalpy change in hydration?
Change when one mole of gaseous ions is added to water to form a dilute solution.
Definition of energy
a measure of an ability to do work in joules.
Thermal energy definition
energy available to a substance as a result of the motion of its molecules.
Definition of chemical potential energy
Energy store in chemical bonds (enthalpy)
What is the difference between an open, closed and isolated system?
Open: Can exchange matter and energy with the surroundings
Closed: Can only exchange energy with the surroundings
Isolated: Neither energy nor matter can be exchanged with the surroundings.
What is an exothermic reaction?
Transfers energy to the surroundings, occurs in most chemical reactions, all combustion and neutralization reactions. Has a negative enthalpy as there is a loss of heat in the system.
What is an endothermic reaction?
Thermal energy is absorbed from the surroundings, occurs in a few chemical reactions. Has a positive enthalpy as the system gains heat.
What are the conditions for standard enthalpy changes?
- T = 298K or 25 degrees
- 100 kPa
- [ ] = 1 mol/dm^3
- all substances are in their standard states (most thermodynamically stable state)
What is the heat change equation?
Q = mc(delta)t
Q = heat change
m = m (g)
c = specific heat capacity
t = temp
Enthalpy relationship of products and reactants
It is expected that the H of products is less than the H of the reactants because compounds are more stable at a lower E.
What are some sources of energy for combustion calorimetry?
- Heat lost to surroudings
- On complete combustion
- Heat gained by the metal calorimeter cup
What is polystyrene calorimetry?
Reactions in solution. Single or double displacement reactions are common. Heat is still lost to surroundings.
What is Hess’s law?
Application of the law of conservation of energy. Implies that no process is 100% efficient and a measured difference may occur. States: The value of any change of H of reaction can be written in steps that equals the sum of the change of enthalpy (direction changes mean -ve or +ve).
What is a bond enthalpy?
The energy required to break one mole of a covalent bond in gaseous state under standard conditions
Why do we use average bond enthalpies?
Because occasionally when the same bond type is broken it can have different enthalpies as it is affected by the bonds and atoms around it.
Trends in bonds
Shorter bonds require more energy to break, related to electronegativity difference
Making bonds
An exothermic reaction, energy is given off. We can track breaking and formation of bonds through:
(delta)H = sum of bonds broken (reactants) - sum of bonds formed (products)
What is the Born-Haber cycle?
The energy cycle based on Hess’s law, represents the formation of an ionic compound from its elements in its standard states.
Process of a Born-Haber cycle
Starts at enthalpy of formation - enthalpy of atomization of each reactant - first and second enthalpy of ionization - first enthalpy of electronic affinity which goes down - second enthalpy of electron affinity which goes back up - lattice enthalpy is the sum of all these steps
What are the factors affecting the lattice enthalpy?
- Charges: larger charges implies greater attraction which implies higher lattice enthalpy
- Size of ions: Bigger ions imply a larger separation between the nucleus of the cation and the e cloud of the anion. size and lattice enthalpy is inversely proportional.
What is an entropy?
A measure of disorder or the dispersal of E in a system. Measured in J/K/mol
Laws of Thermodynamics
- Law of the conservation of energy
- The entropy of the universe must always increase
Predicting changes in entropy
Effect of changing state:
Solid to gas means an increase in entropy. Gas to solid means a decrease in entropy.
Effect of temperature change:
higher temp - higher movement - higher disorder
Effect of change in # of particles:
mole coefficients, more particles = more disorder, doubling particles doubles entropy
Effect of mixing particle:
breaking a substance into its ions
Quantifying the change in entropy of the surroundings
2nd law of thermodynamics states that S of the system and surroundings must add to greater than 0.
S of surroundings = -Hsys/T
S total = S sys - Hsys/t
Explain spontaneity
Disorder in the system is sufficient to compensate for the order created in the surroundings by an endothermic reaction, therefore, the reaction will occur spontaneously for most temperatures
Gibbs free energy formula
Change in G of system = Change in H of sys - T (change in S of system)
- Change in G is free E available to do work
- T(change of S) is unfree E
- Change in H is total energy change at a constant temp
How to determine spontaneity?
If G = positive the reaction is endo and not spontaneous
If G = negative the reaction is exo and spontaneous
What is Gibbs free energy of formation?
Free E changes when 1 mole of a compound is formed from its element in a standard state under SATP
How to determined the percentage error for a combustion rxn?
% = |theo-exper/theo| ×100
