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Thermodynamics

A-Level AQA Chemistry

This flashcard pack covers key concepts related to thermochemistry and reaction energetics, including various types of enthalpy changes (formation, atomisation, bond dissociation, ionisation, electron affinity, lattice formation), the Born-Haber cycle, Gibbs free energy, entropy, and factors affecting reaction feasibility. It also addresses the calculation of standard entropies, the relationship between entropy, enthalpy, and temperature in chemical reactions, and the thermodynamics of phase changes and solubility.

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"How can the temperature at which a reaction becomes feasible be calculated?"	"T = H / S"
"How can balanced chemical equations be used to predict the sign of entropy change?"	"Balanced chemical equations can often be used to predict if the entropy change is positive or negative."
"What are standard entropies?"	"Standard entropies (S°) are the entropies per mole for substances in their standard states."
"Can a substance have a standard entropy less than zero?"	"No, it is not possible for a substance to have a standard entropy less than zero."
"Describe the relationship between entropy and enthalpy in determining the feasibility of a reaction."	"The balance between entropy and enthalpy determines the feasibility of a reaction."
"Define Gibbs free energy."	"Gibbs free energy is a term that combines the effect of enthalpy and entropy into one number."
"What is the equation for Gibbs free energy change?"	"G = H - T S"
"How does increasing entropy and exothermicity affect the feasibility of a reaction?"	"A reaction that has increasing entropy (+ve S) and is exothermic (-ve H) will make G negative and always be feasible."
"What is the condition for a reaction to be feasible?"	"If G is negative, the reaction is feasible, but there is still a possibility that it may not occur or occur very slowly."
"What can cause a positive entropy change?"	"A change of state from solid or liquid to gas, or a significant increase in the number of molecules between products and reactants."
"Describe the relationship between Gibbs free energy and phase changes."	"During phase changes, the Gibbs free energy for such changes is zero."
"Describe the effect of temperature on the feasibility of a reaction involving an increase in entropy."	"Increasing temperature makes it more likely for the reaction to occur, as it makes the value of -TS in the equation G = H - TS more negative."
"Describe the effect of temperature on the feasibility of a reaction involving a decrease in entropy."	"Increasing temperature makes it less likely for the reaction to occur, as -TS will be small and G will not change much."
"Define the enthalpy of lattice dissociation."	"The enthalpy of lattice dissociation is the energy needed to break up the lattice of an ionic substance and convert it into gaseous ions."
"Describe the relationship between the size and charge of an ion and the strength of the lattice enthalpy."	"The smaller the ion and the higher its charge, the stronger the lattice enthalpy."
"Describe the enthalpy of solution of NaCI."	"The enthalpy of solution of NaCI is the energy change that occurs when NaCI dissolves in water."
"What is the relationship between hydration enthalpies and energy?"	"Hydration enthalpies are exothermic as energy is given out when water molecules bond to the metal ions."
"Define lattice enthalpy of formation."	"The lattice enthalpy of formation is the energy change that occurs when one mole of a compound is formed from its constituent ions in the gas phase."
"How does the charge density of ions affect their hydration enthalpy?"	"The higher the charge density, the greater the hydration enthalpy, as the ions attract the water molecules more strongly."
"Describe the solubility of a substance based on its H solution."	"In general, a substance is more likely to be soluble if the H solution is exothermic. If H solution is endothermic, the substance may be insoluble if the lattice enthalpy is much larger than the hydration enthalpy."
"Describe the enthalpy of atomisation."	"The enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state."
"What is bond dissociation enthalpy?"	"The standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms or free radicals."
"Explain first ionisation enthalpy."	"The enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge."
"What is the second ionisation enthalpy?"	"The enthalpy change required to remove 1 mole of electrons from one mole of gaseous 1+ ions to produce one mole of gaseous 2+ ions."
"Describe the first electron affinity."	"The enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge."
"Explain the second electron affinity."	"The enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions."
"What is the enthalpy of lattice formation?"	"The standard enthalpy change when a lattice is formed from its constituent ions in their gaseous states."
"Describe the process of calculating the lattice enthalpy using a Born-Haber cycle."	"The lattice enthalpy is calculated by linking together changes for which data are available and applying Hess's law."
"What is the purpose of a Born-Haber cycle?"	"The purpose of a Born-Haber cycle is to calculate the lattice enthalpy of a compound indirectly."
"Define lattice enthalpy."	"Lattice enthalpy is the energy released when one mole of an ionic compound is formed from its gaseous ions."
"How can the lattice enthalpy be determined indirectly?"	"The lattice enthalpy can be determined indirectly by using a Born-Haber cycle and linking together changes for which data are available."
"Define enthalpy change of formation."	"The energy transferred when 1 mole of a compound is formed from its elements under standard conditions (298K and 100kPa), with all reactants and products in their standard states."
"How does the ionization energy affect the enthalpy of formation in the Born Haber cycle?"	"The ionization energy contributes to the enthalpy of formation in the Born Haber cycle by balancing the lattice enthalpy."
"Describe the stability of CaCl2 compared to CaCI and CaCI3."	"CaCl2 is the most stable form as it has the most exothermic enthalpy of formation compared to CaCI and CaCI3."
"What is a spontaneous process?"	"A process that proceeds on its own without any external influence."
"What is activation energy?"	"The energy required to start a reaction."
"Describe the relationship between exothermic reactions and thermodynamic stability."	"Exothermic reactions result in products that are more thermodynamically stable than the reactants."
"What is entropy?"	"Entropy is a description of the number of ways atoms can share quanta of energy."
"How does the entropy of substances with more ways of arranging their atoms and energy compare to substances with fewer ways?"	"Substances with more ways of arranging their atoms and energy have higher entropy."
"What is the relationship between the entropy of solids, liquids, and gases?"	"Solids have lower entropies than liquids, which are lower than gases."
"What happens to the entropy of a solid as its temperature increases?"	"The entropy of a solid increases as its temperature increases."
"What is the relationship between boiling and entropy?"	"There is a bigger jump in entropy with boiling than with melting."

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How can the temperature at which a reaction becomes feasible be calculated?

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T = H / S

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