Gen 8 - Enthalpy & Thermochemistry Flashcards
What is thermochemistry ?
The study of energy absorbed or released in chemical reactions or any physical transformations (ie. phase change)
What equation represents the enthalpy change of a reaction ?
What does a positive enthalpy change (delta H+) represent ?
The flow of heat into a system as a reaction occurs - E.g. An Endothermic reaction
What does a negative enthalpy change represent ?
Heat energy is given off to its surroundings - an exothermic reaction.
What is the law of conservation of energy ?
The total energy of an isolated system remains constant over time (if there is no associated external force)
What is the second law of thermodynamics ?
The entropy or randomness of the universe increases in any real system.
For a reaction to proceed, the entropy of the system must increase.
What is the difference between enthalpy and entropy ?
Enthalpy is the heat transfer that takes place at constant pressure.
Entropy gives an idea of the randomness of a system.
What is the first law of thermodynamics ?
Energy can neither be created nor destroyed.
What is delta Hf ?
The standard enthalpy (or heat) of formation is the change of enthalpy that would occur when 1 mole of a substance is formed from its elemental constituents, in a standard state reaction.
What is the standard enthalpy of formation of an element in its standard state ?
Zero, as no enthalpy change is involved in their formation.
What are the SI units for enthalpy ?
KJ/mole
What is delta Hrxn ?
The standard enthalpy change for a reaction - the change of enthalpy that would occur if one mole of matter is transformed by a chemical reaction with all reactants and products under standard state.
What is Hess’s law ?
Why is it important to define standard conditions in terms of Thermochemistry ?
Because enthalpy changes depend on the conditions under which the reactions are carried out.
What is standard pressure ?
1 atmosphere, approx 1 barr
What is standard temperature with regards to thermochemistry ?
Generally 25 ºC if unspecified
What is the standard physical state of an element ?
The “natural” physical state of an element under standard temperature and pressure
What is the standard temperature for an ideal gas ?
0ºC
What is the bond dissociation energy ?
A measure of the bond strength within a particular molecule - the standard enthalpy change in the homolytic cleavage of a chemical bond.
Why are bond energies always positive values ?
Because it always takes energy to break bonds apart.
Describe bond dissociation energy and bond energy in relation to CH4 - why are they not the same ?
The bond dissociation energies for each of the homolytic C-H bonds cleavage are all slightly different. The average of the four bond dissociation energies is known as the bond energy.
When is bond dissociation energy and bond energy the same ?
In all diatomic molecules because there is cleavage of only one bond.
Define bond energy
A measure of bond strength. The energy required to sever 1 mole of a chemical bond in a gas.
How would you calculate Energy transferred (Q) in a thermochemical reaction ?
When is heat absorbed or released but without a change in temperature of the surroundings ?
During a phase change
What is a calorie ?
The amount of thermal energy required to raise the temperature of 1g of water by 1ºC
What is a calorie in joules ?
1 cal = 4.184 Joules
Describe the 3 enthalpy phase changes ?
- Heat of fusion Solid –> Liquid
- Heat of vaporization Liquid –> Gas
- Heat of sublimation Solid –> Gas
How do we calculate the heat required for a phase change ?
Q = ML
Where:
Q = heat transferred to system
M = mass of sample
L = Heat of transformation
What is Calorimetry ?
The science of measuring the heat evolved or exchanged due to a chemical reaction.
What are the most spontaneous transformation with regards to the 2nd law of thermodynamics ?
Transformations which require the smallest amount of energy and lead to the largest disorder of the system are the most spontaneous.
What is entropy (S) ?
A state function which measures the degree of disorder in a system.
Generally speaking, it increases from a solid to a liquid to a gas.
Describe entropy in terms of a rubber band
Entropy decreases when the elastic band is stretched. In the relaxed elastic band, the molecules of the rubber polymer are coiled up and form a disorganised structure.
As the rubber is stretched, the molecules tend to line up with each other and adopt a more organised structure.
What are the units of entropy ?
J/K or Cal/K
What is the law of disorder ?
The natural tendancy is for systems to move to the direction of maximum disorder, not vice versa.
E.g. Your room never cleans itself !
What is the equation for entropy change ?
What is Gibbs free energy ?
A state function of entropy that can be used as a criterion for spontaneity.
How do you calculate Gibbs free energy ?
What are the unit of Gibb’s free energy ?
KJMol-1
How does the value of delta G carried out at constant pressure, determine spontaneity of a reaction ?
A reaction carried out at constant pressure is spontaneous is delta G <0. It is not spontaneous if delta G >0.
How is a reaction proceeding if delta G = 0 ?
The reaction is at equilibrium.
Describe the best time to add milk to a hot drink in order to maximally cool it ?
Heat moves to an environment fastest when there is the greatest temperature difference, therefore for maximal heat loss, milk should be added just before drinking.
What is the difference between a burn from steam at 100ºC, and a burn from water at 100ºC ?
The steam burn would be more severe as it has more heat energy to be released onto the skin. Energy is lost to the skin by condensation.
