Thermochemistry Flashcards
Thermochemistry
heat changes of chemical reactions
helps to determine if a particular reaction will occur and if it will release or absorb energy as it occurs
two principles of thermodynamics
Laplace law
Hess’ Law
The Laplace Law
the heat change (or enthalpy change) of a chemical reaction is exactly equal but opposite in sign for the reverse reaction
∆Hforward reaction = ∆Hbackword reaction
(a) CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) [here, ∆H0 = -890.3 kJ mol-1]
(b) CO2 (g) + 2H2O (l) → CH4 (g) + 2O2 (g) [here, ∆H0 = +890.3 kJ mol-1]
Enthalpy Notations
enthalpy change 𝚫H(delta H) is the heat energy transferred in a reaction at constant pressure. The units are Kj mol⁻1
Standard conditions (𝚫Hᶿ)
100 kPa pressure and a stated temperature
Energy
stored in the bonds of chemical compounds.
Chemical energy may be released during a chemical reaction, often in the form of heat; such reactions are called exothermic.
Work
transfer of mechanical energy between two systems
Heat
transfer of thermal energy between systems
Exothermic reactions
have a negative enthalpy change value, because heat is given out (the chemicals lose energy)
Endothermic reactions
have a positive enthalpy change value, because heat energy is absorbed (the chemicals lose energy)
Lattice formation enthalpy
enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions
Lattice dissociation enthalpy
enthalpy change when 1 mole of a compound is formed from its elements in their standard states
Bond dissociation enthalpy
enthalpy change when all the bonds of the same type in 1 mole of gaseous atoms are broken
First ionisation energy
enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms
First electron affinity
enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous ions
three types of thermodynamics systems
open
closed
isolated
isolated system
thermodynamic system that cannot exchange either energy or matter outside the boundaries of the system
1. The system may be so distant from another system that it cannot interact with them.
2. The system may be enclosed such that neither energy nor mass may enter or exit.
Closed systems
only closed to matter, energy can be exchanged across the system’s boundaries
Open system
exchange both energy and matter with its surroundings
Hess’ Law
The enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction occurs.
1st law of thermodynamics
a systems energy content remains constant as well as not being able to be created or destroyed
Activation energy
minimum energy required
Increase temp effect on Maxwell-Boltzmann diagram
move right
Reaction Rate for a given chemical reaction
measure of the change in concentration of the reactants or the change in concentration of the products per unit time