Thermodynamics Flashcards
Give the definition for Standard conditions
Standard conditions – 298K, 100kPa, solutions of 1 mol dm-3
Give the definition for Standard enthalpy of reaction ΔrHθ
Standard enthalpy of reaction ΔrHθ – The enthalpy change when substances react under standard conditions given by the equation for the reaction.
Give the definition for Standard enthalpy of formation ΔfHθ
Standard enthalpy of formation ΔfHθ – The enthalpy change when 1 mole of a compound is formed from its constituent elements with all reactants and products in standard states under standard conditions.
Give the definition for Standard enthalpy of combustion ΔcHθ
Standard enthalpy of combustion ΔcHθ – The enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions.
Give the definition for Mean bond energy
Mean bond energy – the energy required to break one mole of a covalent bond averaged across compounds containing that bond.
Give the definition for Enthalpy of lattice dissociation ΔLHθ
Enthalpy of lattice dissociation ΔLHθ– The enthalpy change when one mole of an ionic compound is separated into its component gaseous ions.
Give the definition for Enthalpy of lattice formation ΔfHθ
Enthalpy of lattice formation ΔfHθ – The enthalpy change when one mole of an ionic compound is formed from its constituent ions in the gaseous state.
Give the definition for Enthalpy of atomisation ΔatHθ
Enthalpy of atomisation ΔatHθ – The enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state.
Give the definition for Bond dissociation energy
Bond dissociation energy– The enthalpy change when one mole of a covalent bond is broken under standard conditions in gaseous state.
Give the definition for First ionisation energy ΔIE1Hθ.
First ionisation energy ΔIE1Hθ - The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.
Give the definition for Second ionisation energy ΔIE2Hθ
Second ionisation energy ΔIE2Hθ– The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a single positive charge to form one mole of gaseous ions with a 2+ charge.
Give the definition for Third ionisation energy ΔIE3Hθ
Third ionisation energy ΔIE3Hθ - The enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a 2+ charge to form one mole of gaseous ions with a 3+ charge.
Give the definition for First Electron affinity ΔEA1Hθ
First Electron affinity ΔEA1Hθ– The enthalpy change when one mole of gaseous atoms forms one mole of negative ions with a single negative charge.
Give the definition for Second electron affinity ΔEA2Hθ
Second electron affinity ΔEA2Hθ– The enthalpy change when one mole of gaseous atoms with a single negative charge forms one mole of negative ions with a double negative charge.
Give the definition for Enthalpy of solution ΔsolHθ
Enthalpy of solution ΔsolHθ - The enthalpy change when one mole of a solute dissolves in water.
Give the definition for Enthalpy of hydration ΔhydHθ –.
Enthalpy of hydration ΔhydHθ – The enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.
Give the definition for Entropy change Sθ
Entropy change Sθ– the difference between the number of reacting particles in solution and the number of product particles in solution.
Give the definition for Enthalpy
Enthalpy – A thermodynamic property of a system linked to the internal energy. Represented by H.
Give the definition for ΔH
ΔH – change in enthalpy. Negative for an exothermic reaction, positive for an endothermic reaction.
What happens to the enthalpy of atomisation for diatomic molecules?
For gaseous diatomic elements (e.g. Cl2, O2 etc) the bond dissociation energy is twice the enthalpy of atomisation.
What does the value of the lattice enthalpy tell you?
The value of the lattice enthalpy tells you how strong the ionic bond is.
What 2 factors affect the value of the lattice enthalpy?
This depends on the charge on the ion and the size.
What combination of charge and size gives stronger ionic bonds?
Smaller ions and higher charges give stronger ionic bonds.
Why do smaller ions give a higher bond energy?
Smaller ions are closer so are more attracted.
Why do higher charges give a higher bond energy?
Higher charges are more attracted. They have stronger the electrostatic attraction.
Are all compounds either ionic or covalent?
Not all compounds are 100% ionic or covalent.
When are covalent compounds 100% covalent?
If covalent compounds are non polar they are 100% covalent.
What happens to polar covalent compounds?
Polar covalent compounds have some ionic character.
When are ionic compounds covalent?
A small highly charged cation can distort the anion giving some covalent character (Al2O3). The ions are distorted and not spherical.
What does the perfect ionic model assume?
The perfect ionic model assumes:
If the bond energy values are different from the calculated values what does it show?
If the values are different then the compound has covalent character. The greater the difference the more covalent it is.
Are values for hydration exothermic or endothermic and why?
Values for hydration are exothermic as energy is released when the ions are attracted to the polar water molecules.
Do smaller ions attract water molecules more or less strongly?
Smaller anions ions attract the δ+ of the water molecule more strongly.
What effect does the charge have on the attraction to water molecules?
The more charge it has the more it attracts the water molecule.
What part of the water molecule does the cation attract? What can make this attraction greater?
Cations attract the δ- of the oxygen on the water molecule. The smallerand more charged it is the more it attracts.
How does the group number affect the enthalpy of hydration?
Group 2 (2+ oxidation state) need 2x the enthalpy of hydration for the anion. e.g. 2 x Cl in CaCl2.
What happens to ionic compounds when they dissolve in water?
Ionic compounds dissolve when the ionic lattice breaks up and the polar water molecules form bonds with the ions.
How do you calculate the enthalpy of solution?
Enthalpy of solution = Enthalpy of lattice dissociation + Enthalpy of hydration
Which process in dissolving are exothermic and which are endothermic?
Breaking up the ionic lattice is endothermic. Forming bonds with the water is exothermic. The balance of these gives the enthalpy of solution.
What does the feasibility of a reaction depend on?
The feasibility of a reaction depends upon the enthalpy change and the entropy change and the temperature.
What does Gibbs free energy tell you and what value does it need to have?
Gibbs free energy ΔGθ is used to predict the feasibility of a reaction. For it to be feasible it must be equal or less than 0.
Give the equation for Gibbs free energy.
ΔGθ = ΔHθ - TΔSθ
How do you calculate the temperature that something becomes feasible
To calculate the temperature that something becomes feasible use:
What are the units for ΔGθ and ΔHθ
ΔGθ and ΔHθ have units of KJ mol-1
What are the units for ΔSθ
ΔSθ has units of JK-1mol-1 remember to divide by 1000 before using!
What does Entropy measure?
Entropy is the measure of disorder.
What are the units of entropy?
It is measured in JK-1mol-1
What is the entropy value at 0K?
Absolute values are for one mole based on a scale where the substance has an entropy value of zero at 0K.
What does a higher entropy value tell you?
The higher the absolute entropy the higher the degree of disorder.
Which are more order, solids, liquids or gases?
Solids are more ordered than liquids which are more ordered than gases.
What happens to entropy as you turn solids to liquids and liquids to gases?
There is an increase in entropy from solid to liquid and an even greater increase from liquid to gas.
What is the link between Mr and entropy?
The higher the Mr value of a substance in the same state the higher the entropy value.
How do you calculate entropy?
ΔSθ= Σsθproducts – Σsθreactants
From the equation for a reaction how can you tell if ΔSθ is negative or positive?
From the equation for a reaction you can tell if ΔSθ is negative or positive by looking at the states and number of moles of the reactants and products.
Why may a feasible reaction not happen at a particular temperature?
A feasible reaction may not occur at a particular temperature as the activation energy may be too high for it to proceed.
What is a spontaneous reaction?
A spontaneous reaction is one that is feasible and occurs under standard conditions.
What must happen for a reaction to be spontaneous?
To be spontaneous ΔGθ must be less than 0 and the activation energy must be low enough for it to proceed.
How do you determine the temperature at which a reaction changes from not feasible to feasible
To determine the temperature at which a reaction changes from not feasible to feasible divide ΔHθ by ΔSθ (remember to change the units).
During a change of state what does ΔGθ =?
During a change of state ΔGθ = 0 kJ mol -1.
ΔGθ is positive for temperatures below the melting points?
ΔGθ is positive for temperatures below the melting points as it is not feasible.
ΔGθ is negative for temperatures above the melting points
ΔGθ is negative for temperatures above the melting points as it is feasible.
Is the same true for boiling points?
The same is true for the boiling points.
What value does ΔGθ when a system is at equilibrium?
When ΔGθ =0, the system is at equilibrium.
How can you plot the equation ΔGθ = ΔHθ - T ΔSθ
The equation ΔGθ = ΔHθ - T ΔSθ can be plotted as a straight line graph.
How does the equation ΔGθ = ΔHθ - T ΔSθ link to a line graph?
ΔGθ = ΔHθ - TΔSθ
How do you plot the line graph?
ΔGθ is plotted against temperature in K (add 273 to oC).
What are the units for ΔSθ when you are plotting the graph?
ΔSθ is in kJ K-1 mol-1. Multiply by 1000 to get ΔSθ.
