Section C- Thermodynamics Flashcards
What 3 types of chemical systems are there and what exchanges with the surroundings do each allow?
Isolated system- no exchange of matter or energy. Open system- exchange of both matter and energy. Closed system- exchange of energy but a fixed amount of matter.
What is heat?
Energy transferred as a result of temperature difference. The transfer of heat energy is a random process.
What is temperature?
The distribution of thermal energy and a measure of the average energy in a system.
Define specific heat capacity.
The enthalpy change in Joules when 1kg of substance is heated by 1 degree Celsius. (or 1g by 1K)
What is work (w)?
Energy exchange as a result of motion against an opposing force. It is a coordinated motion of molecules.
What is a state function?
A function whose value only depends on the beginning and end points of a process and not on the path taken e.g. altitude but not distance.
What is the molar heat capacity?
The energy required to raise the temperature of 1 mol of substance by 1K.
What are extensive and intensive properties?
Extensive- depends on the quantity of the substance that is present. Intensive- do not depend on the quantity.
Define enthalpy change (delta H). Is it a state function?
The heat transferred at constant pressure by a chemical reaction or process. Yes it is a state function as it only takes into account the start and end values and not the path taken (e.g. what method is used to transfer heat).
Define standard enthalpy change of vapourization.
The enthalpy change when 1 mol of a liquid vapourizes at its boiling point (Tb), and 1 bar pressure.
Define standard enthalpy change of fusion.
The enthalpy change when 1 mol of a substance melts at its melting point (Tm), and 1 bar pressure.
What is the equation used to calculate the enthalpy change of sublimation?
Enthalpy change of fusion + enthalpy change of vapourization
Define the standard enthalpy change of reaction.
The enthalpy change at 1 bar for a reaction with all the components in their standard states. The value obtained refers to the molar amounts given in the equation and are reported at 298K.
Define the standard enthalpy change of formation.
The enthalpy change when 1 mol of substance is formed under standard conditions from its constituent elements in their standard states.
What is the value of the standard enthalpy change of formation for elements in their standard states?
0
What does Hess’s law state?
That the total enthalpy change for a chemical reaction is independent of the path by which the reaction occurs as long as the starting and finishing states are the same for each reaction path.
How can the enthalpy change of the reaction be calculated?
(sum of enthalpies of formation of the products) - (sum of the enthalpies of formation of the reactants)
Define the standard enthalpy change of combustion.
The enthalpy change when 1 mol of a substance reacts completely with excess oxygen gas at 1 bar pressure.
What is the bond dissociation enthalpy? And the mean bond dissociation enthalpy?
The enthalpy change per mole when a particular chemical bond is broken under standard conditions in the gas phase. The mean is the average across a range of related compounds.
What information does the sign of the energy change provide?
Positive- energy gained by a system (heat absorbed or work done on the system)
Negative- energy lost by a system (heat released or work done by the system)
What is the zeroth law of thermodynamics?
Energy flows between objects such as to equalise their temperatures.
What is the first law of thermodynamics?
Delta U= q + w
Where q is the energy transferred as heat, amd w is the energy transferred as work.
Also called the law of conservation of energy :
- Energy can be neither created nor destroyed, only interconverted between forms.
- The total quantity of energy in the universe is constant.
- The internal energy of an isolated system is constant (DeltaU=0)
What is the internal energy (U)? Can it be measured?
The sum of the kinetic and potential energies of molecules in a sample. It cannot be measured, only the change in internal energy can.
What is the equation used to calculate DeltaU for a process at a constant volume?
DeltaU = qv
(only dependant on the heat transferred)
What equations show the relationship between the enthalpy change and change of internal energy?
DeltaH = DeltaU + pDeltaV
DeltaH = DeltaU + DeltangasRT
What is the value of the gas constant (R)?
8.314 JK-1
How can DeltaU be measured?
Using a bomb calorimeter which is an insulated device and material is combusted within it, causing a temperature rise associated with a particular physical or chemical change.
What is entropy?
Entropy is a measure of the randomness or disorder in a system.
It is related to the number of arrangements that a system can adopt by the Boltzmann formula:
S = kBlnW
Where kB is the Boltzmann constant (1.381x10-23JK-1), W is the number of ways of arranging the molecules and S is the entropy.
What is the second law of thermodynamics?
Spontaneous processes increase the total entropy of the universe.
The total entropy is the entropy of the system plus the entropy of the surroundings.
What is the third law of thermodynamics?
The entropy of a perfect crystal at zero kelvin is zero.
What is the standard entropy of a substance?
The entropy of 1 mol at 298 K and 1 bar pressure. Determined by adding the entropy changes in heating 1 mol of the substance from 0K to 298K taking into account any phase changes that occur.
For a chemical change to be spontanous, what must the entropy change of the Universe be?
Positive.
What is an adiabatic system?
One in which energy is only transferred in the form of work.
What does the Maxwell-Boltzmann distribution describe?
The speeds of molecules or the occupation of energy levels.
What is entropy change?
The ratio of heat transferred to temperature. (q/T)
What is the equation to calculate entropy (S)?
S = klnW
Where W is the number of microstates (degrees of freedom)
k is the Boltzmann constant (1.381x10-23 JK-1)
Therefore entropy is a measure of how dispersed the energy is.
It is a measure of the quality of the energy.
What is G?
What is the equation used to calculate it under constant pressure?
Gibbs free energy- the energy available for useful work.
G = H -TS
For a spotaneous process, what will the value of DeltaG be?
Negative.
Name some entropy-driven processes.
Mixing and unmixing
Protein folding
Formation of membranes
