Thermodynamics Flashcards by Lucy Williams

What is thermodynamics?

The branch of physical chemistry that includes heat, energy, temperature and the physical properties of matter

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What are energy cycles?

Representations of enthalpy changes for ionic cimpounds using the principles of Hess’s law

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What is meant by enthalpy change?

The amount of heat energy transferred during a chemical reaction under standard conditions

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What symbol is used to represent enthalpy change?

ΔH

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What is meant by enthalpy of formation?

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions

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What symbol is used to represent enthalpy of formation?

ΔHfꝋ

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What are standard conditions?

298K
100 kPa

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Is the enthalpy of formation endothermic or exothermic?

Can be either as the energy change is the sum of bonds broken and formed

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How is the equation for enthalpy of formation written, using sodium chloride as an example?

Na (s)+ ½Cl₂ (g) → NaCl (s) ΔHfꝋ = -

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What is the enthalpy of formation of elements?

Zero

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What is meant by ionisation enthalpy?

The amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions

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What symbol is used to represent ionisation enthalpy?

ΔHᵢₑꝋ

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Is ionisation enthalpy endothermic or exothermic?

Endothermic as energy is needed to overcome the attraction between electron and nucleus

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What is meant by first ionisation energy?

The energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions

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How is the equation for first ionisation energy written, using sodium as an example?

Na (g) → Na+ (g) + e– ΔHieꝋ = +

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What is meant by the standard enthalpy change of atomisation?

The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions

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What symbol is used to represent the standard enthalpy change of atomisation?

ΔHatꝋ

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Is the standard enthalpy change of atomisation endothermic or exothermic?

Endothermic as energy is always required to break bonds between atoms of element

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How is the equation for standard enthalpy change of atomisation written, using sodium as an example?

Na (s) → Na (g) ΔHatꝋ = +

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What is meant by the bond dissocation energy?

The amount of energy required to break one mole of a specific coavlent bond in the gas phase

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What symbol is used to represent bond dissociation energy?

E(bond broken)

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Is bond enthalpy endothermic or exothermic?

Endothermic as it is usually treated as a bond breaking process

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How is the equation for bond dissociation energy written, using chlorine as an example?

Cl₂ (g) → 2Cl (g) E(Cl-Cl) = +

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What is the difference in the expression of atomisation enthalpy and bond enthalpy?

Atomisation produces 1 mole of atoms
Bond per mole of bonds

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What is the value of atomisation enthalpy in relation to bond enthalpy?

Half the value

What is meant by lattice enthalpy?

The enthalpy chanfe when 1 mole of ionic crystal is formed from the gaseous ions at standard conditions

What symbol is used to represent lattice enthalpy?

ΔHlattꝋ

Is lattice enthalpy endothermic or exothermic?

Exothermic as when ions combine a large amount of energy is released

What does the large negative value of lattice enthalpy suggest about ionic compounds?

More stable than gaseous ions

Why do some ionic compounds have a more exothermic lattice enthalpy?

Stronger ionic binds within lattice

What is used to find the lattice enthalpy of an ionic compound?

Multiple experimental values and energy cycle

How is the equation for lattice enthalpy written, using sodium chloride as an example?

Na+(g) + Cl-(g) →NaCl (s) ΔHlattꝋ = -

What is meant by electron affinity?

The energy change when one mole of electrons is gained by one mole of gaseous atoms of an element to form one mole of gaseous ions under standard conditions

Is first electron affinity endothermic or exothermic?

Exothermic as energy is released when electrons are attracted to the atoms

Is second electron affinity endothermic or exothermic?

Can be endothermic as large amount of energy required to overcome large force of repulsion between negatively charged ion and second electron

How is the equation for first electron affinity written, using chlorine as an example?

Cl (g)+ e– → Cl- (g) ΔHeaꝋ = -

How is the equation for first electron affinity written, using oxygen as an example?

O– (g) + e– → O₂- (g) ΔHeaꝋ = +

What is a Born-Haber cycle?

Specific application of Hess's law for ionic compounds

What can the Born-Haber cycle be used to calculate?

Lattice enthalpy

Why is the Born-Haber cycle used to calculate lattice enthalpy?

Cannot be found by experiment

What is the basic principle of a Born-Haber cycle?

Diagram showing energy increases going up the diagram

What does the Born-Haber cycle show?

Steps needed to turn atoms into gaseous ions and from gaseous ions into ionic lattice

Why are arrows in the Born-Haber cycle drawn upwards?

Endothermic energy change

Is the enthalpy of formation an upwards or downwards arrow on a Born-Haber cycle?

Either Can be exothermic or endothermic

How is enthalpy of formation calculated using a Born-Haber cycle?

ΔHfꝋ = ΔHatꝋ + ΔHatꝋ + IE + EA + ΔHlattꝋ

What information is needed to calculate a theoretical value for the lattice enthalpy of an ionic solid?

Geometry of ionic solid Charge on ions Distance between ions

What assumptions is made when calculating the theoretical value for the lattice enthalpy of an ionic solid?

Substance is a highly ionic compound with only electrostatic attraction between cations and anions

What is implied when there is a difference between theoretical and experimental lattice enthalpies?

Bonding is not purely ionic and some covalent character is present

What trend can be seen in lattices across a period?

Become less ionic and more covalent Bigger differences in theoretical and experimental lattice enthalpy values

What are the two key factors that affect lattice enthalpy?

Charge of ions Radius of ions

How does the radius of ions affect lattice enthalpy?

Becomes less exothermic as radius increases

Why is lattice enthalpy less exothermic with larger ions?

Charge is more spread out Ions are further apart So electrostatic forces of attraction between oppositely charged ions weaker

How does the charge of ions affect lattice enthalpy?

More exothermic as charge increases

Why is lattice enthalpy more exothermic with more positive ions?

Higher charge density Stronger electrostatic attraction between oppositely charged ions

What is meant by the standard enthalpy change of solution?

The enthalpy change when 1mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution

What symbol is used to represent the standard enthalpy change of solution?

ΔHsolꝋ

Why is the symbol (aq) used to show?

Solid is dissolved in sufficient water

Is standard enthalpy of solution exothermic or endothermic?

Can be either

What two ways can the equation for enthalpy of solution be written, using potassium chloride as an example?

KCl(s) + aq → KCl(aq) KCl(s) + aq → K+(aq) + Cl-(aq)

What is meant by the standard enthalpy change of hydration?

The enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution

What symbol is used to represent the standard enthalpy of hydration?

ΔHhydꝋ

How is the equation for enthalpy of hydration written, using hydration of magnesium as an example?

Mg²⁺(g) + aq → Mg²⁺(aq)

Is standard enthalpy of hydration exothermic or endothermic?

Exothermic

What is formed when an ionic solid dissolves in water?

Positive and negative ions formed

Is water polar or non-polar?

Polar

How do water molecules act in solution?

δ- oxygen atom form ion-dipole attractions with positive ions present δ+ hydrogen atoms form ion-dipole attractions with negative ions present

How are enthalpy of solution and hydration enthalpies related?

Enthalpy of solution = lattice formation enthalpy + hydration enthalpy

What is hydration enthalpy in enthalpy of solution = lattice formation enthalpy + hydration enthalpy?

Sum of hydration enthalpies of each ion

What is the indirect route from gaseous ions to ions in aqueous solution?

Gaseous ions → Ionic solid → Ions in aqueous solution

How can hydration enthalpy be calculated from lattice enthalpy and enthalpy of solution?

ΔHhydꝋ = ΔHlattꝋ + ΔHsolꝋ

What is meant by entropy?

The number of possible arrangements of particles and their energy in a given system

How else can entropy be described?

A measure of how disordered or chaotic a system is

What happens to entropy when a system becomes more disordered?

Increases

What does an increase in entropy mean for a system?

Becomes more energetically stable

Why are gas molecules more disordered than solid?

Constantly moving around

What happens in terms of disorder and entropy when a solid melts into liquid?

System becomes more disordered as particles are more randomly arranged in liquid so can move around each other Entropy increases

What kind of entropy is energetically favourable?

System with higher entropy

What does the second law of thermodynamics broadly state?

The entropy of the universe is always increasing

What is meant by feasilibity?

Whether a reaction can occur spontaneously or not

What is included in the term system?

The molecules that are reacting in a chemical reaction

What is included in the term surroundings?

Everything not in system Including solvent, air, test tube

What unit is used for entropy?

J K⁻¹ mol⁻¹

How do entropy changes compare to enthalpy changes?

Smaller order of magnitude

What symbol is used to represent standard entropy change?

ΔSsystemꝋ

How can standard enthalpy change be calculated for a reaction?

ΔSsystemꝋ = ΣΔSproductsꝋ - ΣΔSreactantsꝋ

What entropy value is given to elements?

Not zero

What information is used to calculate entropy change of reaction?

Stoichiometry State of compounds

What two factors determine the feasibility of a reaction?

Enthalpy change Entropy change

What is the Gibbs free energy equation?

ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ

What units are given to ΔGꝋ in Gibbs free energy equation?

kJ mol⁻¹

What units are given to ΔHreactionꝋ in Gibbs free energy equation?

kJ mol⁻¹

What units are given to T in Gibbs free energy equation?

What units are given to ΔSsystemꝋ in Gibbs free energy equation?

J K⁻¹ mol⁻¹

What must units of ΔSsystemꝋ be converted to in Gibbs free energy equation?

kJ K-1 mol-1 Divided by 1000

What two ways can value of ΔGꝋ be calculated?

From ΔHꝋ and ΔSꝋ values From ΔGꝋ values of all substances present

How can the value of ΔGꝋ be calculated from ΔHꝋ and ΔSꝋ values?

Using Gibbs free energy equation

How can the value of ΔGꝋ be calculated from other ΔGꝋ values?

ΔGꝋ = ΣΔGproductsꝋ - ΣΔGreactantsꝋ

Describe simply the difference between enthalpy change, emtropy change and free energy.

Enthalpy change is difference between energy in to break bonds and out to make new Entropy change is cost of carrying out reaction Free energy is whats left

What is the Gibbs free energy equation used to calculate?

Whether reaction is feasible or not

What does a negative ΔGꝋ mean for feasibility?

Reaction is feasible and likely to occur

What does a positive ΔGꝋ mean for feasibility?

Reaction is not feasible and unlikely to occur

What is the difference between feasible and spontaneous?

Feasible describes reaction energetically favourable that should go Spontaneous describes reactions that go on own accord

If ΔH is negative and ΔS is positive, what is ΔG?

Negative

If ΔH is negative and ΔS is positive, is the reaction spontaneous?

Always

If ΔH is positive and ΔS is negative, what is ΔG?

Positive

If ΔH is positive and ΔS is negative, is the reaction spontaneous?

Never

If ΔH is negative and ΔS is negative, what is ΔG?

Negative at low T Positive at high T

If ΔH is negative and ΔS is negative, is the reaction spontaneous?

Spontaneous at low T

If ΔH is positive and ΔS is positive, what is ΔG?

Negative at high T Positive at low T

If ΔH is positive and ΔS is positive, is the reaction spontaneous?

Spontaneous at high T

What factor can affect the feasibility of a reaction?

Temperature

Is ΔH positive or negative in exothermic reactions?

Negative

What happens to feasibility if ΔS is positive in exothermic reaction?

Both ΔH and TΔS are negative Negative ΔGꝋ so reactiona feasible at all temperatures

What happens to feasibility if ΔS is negative in exothermic reaction?

ΔH positive but TΔS negative At high temp, TΔS will be large and positive to overcome ΔH ΔGꝋ is positive at high temperatures and not feasible

Is ΔH positive or negative in endothermic reactions?

Positive

What happens to feasibility if ΔS is negative in endothermic reaction?

Both ΔH and TΔS positive Positive ΔG so reaction not feasible at all temperatures

What happens to feasibility if ΔS is positive in endothermic reaction?

ΔH positive and TΔS negative At low temp, TΔS will be small and negative so not overcome ΔH ΔGꝋ is positive at low temperatures and not feasible

What value of ΔGꝋ is needed for a reaction to be feasible?

Zero or negative