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What is the enthalpy change when a reaction is endothermic
positive
What is the enthalpy change when a reaction is exothermic
negative
What are the conditions for standard enthalpy change
100 kPa
All solutions have a concentration of 1 mols/dm3
All substances in their standard states
How can you calculate the heat loss/gain of a sample
Q = heat change
m = mass
c = specific heat capacity
ΔT = change in temperature
Q = mcΔT
What is Hess’s Law
The overall enthalpy change of a chemical reaction is independent of the pathway taken. It allows us to calculate the enthalpy change of a reaction by combining the enthalpy changes of other reactions. Provided that the starting and finishing conditions are the same in reactants and products
What is the standard enthalpy change of formation
The standard enthalpy change of formation is the heat energy released or absorbed when one mole of a compound is formed from its elements in their standard states in standard conditions. It is a measure of the stability and energy content of a compound.
What is the lattice enthalpy of a substance
The lattice enthalpy of a substance is the energy released when one mole of an ionic compound is formed from its constituent ions in the gaseous state.
What are the steps of a born haber cycle
Enthalpy of formation
Enthalpy of atomisation (of all reactants)
(potentially bond enthalpy)
Ionization
Affinity
Lattice enthalpy
What affects lattice enthalpy
Increasing ionic radius, decreases attraction, therefore decreases lattice enthalpy
Increasing ionic charge, increases attraction, therefore increases lattice enthalpy
What is entropy
Measure of the disorder or randomness in a system. It increases with the number of possible arrangements of particles in a system.
How do you calculate the entropy of a reaction
Entropy of products - entropy of reactants
What is the enthalpy change of breaking bonds
positive (endothermic)
What is the enthalpy change of making bonds
negative (exothermic)
What does the hydration enthalpy give
The strength of interaction between the polar water molecules and the separated ions
How do exothermic reactions change the entropy
increase in the surroundings entropy
What is the equation to calculate entropy of surroundings
Entropy of surroundings (ΔS) is calculated using the equation ΔS = -ΔH/T, where ΔH is the change in enthalpy of the system and T is the temperature in Kelvin.
What are the units of entropy
J K-1 mol-1
What is the second law of thermodynamics
For a spontaneous change
ΔS (total) = ΔS system + ΔS surroundings
Is greater than 0
How can endothermic reactions occur
If the change in entropy is great enough to compensate for the negative entropy change of the surroundings as heat flows surroundings to system
What is the Gibbs free energy equation
ΔG = ΔH - TΔS, where ΔG represents the change in Gibbs free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
What does Gibbs free energy show us
ΔG must be negative for a reaction to be spontaneous.
How does temperature affect the feasibility of a reaction according to Gibbs free energy
At a low temperature all exothermic reactions can occur
All reactions are feasible at high temperature due to it being high enough to make the effect of ΔH negligible
Note
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