Thermodynamics Flashcards
Define enthalpy change
the heat energy transferred in a reaction at constant pressure.
What are the standard conditions?
100kPa and 298K
Do exothermic reactions have a negative or positive enthalpy change?
negative
What is lattice formation enthalpy?
the enthalpy change when one mole of a solid compound is formed from its gaseous ions
What is lattice dissociation enthalpy?
the enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous ions
What is enthalpy change of formation?
the enthalpy change when one mole of a compound is formed from its elements in their standard states
Ca(s) + Cl2(g) –> CaCl2(s)
What is bond dissociation enthalpy?
the enthalpy change when all the bonds of the same type in one mole of gaseous molecules are broken
Cl2(g) –> 2Cl(g)
What is the enthalpy change of atomisation of an element?
the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state
1/2Cl2(g) –> Cl(g)
What is the enthalpy change of atomisation of a compound?
the enthalpy change when one mole of a compound in its standard state is converted to gaseous ions
NaCl(s) –> Na(g) + Cl(g)
What is the first ionisation energy?
the enthalpy change when one mole of gaseous 1+ ions is formed from one mole of gaseous ions
Mg(g) –> Mg+(g) + e-
What is the second ionisation energy?
the enthalpy change when one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions
Mg+(g) –> Mg2+(g) + e-
What is the first electron affinity?
the enthalpy change when one mole of gaseous 1- ions is made from one mole of gaseous atoms
O(g) + e- –> O-(g)
What is the second electron affinity?
the enthalpy change when one mole of gaseous 2- ions is made from one mole of gaseous 1- ions
O-(g) + e- –> O2-(g)
What is the enthalpy of hydration?
the enthalpy change when one mole of aqueous ions is formed from gaseous ions
Na+(g) –> Na+(aq)
What is enthalpy of solution?
t he enthalpy change when one mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution
What does the purely ionic model of lattice assume?
all the ions are spherical and have their charge evenly distributed around them
What is covalent character?
the negative and positive ions aren’t exactly spherical, positive ions polarise neighbouring negative ions to different extents. the more polarisation, he more covalent the bonding is
What does it show if the experimental and theoretical lattice enthalpies for a compound are very different?
the compound has a lot of covalent character
What happens when a solid ionic lattice dissolves in water?
the bonds between the ions break to give gaseous ions (lattice enthalpy of dissociation) and bonds between the ions and the water are made (enthalpy change of hydration)
When calculating enthalpy change of solution, what is enthalpy change 1?
Lattice dissociation enthalpy
When calculating enthalpy change of solution, what is enthalpy change 2?
enthalpy of hydration of both the positive and negative ions
When calculating enthalpy change of solution, what is enthalpy change 3?
enthalpy change of solution
What is entropy and what symbol represents it?
entropy tells you how much disorder there is, it is represented by the letter S
What three things affect entropy?
physical state, dissolution and number of particles
What state of matter has the highest entropy?
gas
How can you increase the entropy of a solid?
dissolve a solid in solution
How does number of particles affect entropy?
more particles=more entropy
What is the calculation for working out entropy change?
entropy change=entropy of products-entropy of reactants
What is free-energy change?
a measure used to predict whether a reaction is feasible
What is a feasible reaction?
once started, it will carry on to completion without energy being supplied to it
What is the formula for free-energy change?
free energy change(J/mol)=enthalpy change(J/mol)- temperature(K) x entropy change(J/K/mol)
If the reaction is exothermic, how does this affect the entropy?
the entropy change will be positive
If the reaction is endothermic, how does this affect the entropy?
the entropy change will be negative
When will a reaction not be feasible?
when free-energy change is positive, when enthalpy and entropy are positive it won’t be feasible below certain temperatures
When free-energy change is zero, is the reaction feasible?
yes, it is just feasible
