module 5: physical chemistry and transition elements Flashcards
what is lattice enthalpy?
the enthalpy change when one mole of an ionic compound is formed from its gaseous ions, under standard conditions
is the value for lattice enthalpy change always positive or negative, and why?
negative, lattice enthalpy involves ionic bond formation from separate gaseous ions which is an exothermic change
what are the key features about lattice enthalpy?
- lattice enthalpy is an exothermic change, lattice enthalpies have a negative sign because energy is given out when ionic bonds are being formed from gaseous ions
- lattice enthalpy is a measure of the strength of ionic bonding in a giant ionic lattice, a large negative value shows that there are strong electrostatic forces of attraction between the oppositely charged ions in the lattice
- a covalent substance does not have a lattice enthalpy because there are no ions in its structure
- lattice enthalpies cannot be measure directly because it is impossible to form a mole of an ionic lattice from gaseous ions
how are lattice enthalpies calculated indirectly?
Born-Haber cycle
what is the standard enthalpy change of atomisation?
the enthalpy change when one mole of gaseous atoms are formed from the element in its standard state under standard conditions
why is the standard enthalpy change of atomisation always an endothermic process?
energy is taken in for bond breaking to form gaseous atoms
what is the difference between electron affinity and ionisation energy?
electron affinity is the opposite of ionisation energy
- electron affinity measures the energy to gain electrons
- ionisation energy measures the energy to lose electrons
what is the first electron affinity?
the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
why can lattice enthalpies not be determined directly?
you cannot form one mole of an ionic lattice from gaseous ions
what are the equations for making the solid sodium chloride?
- atomisation of sodium
- atomisation of chlorine
- ionisation of sodium
- electron affinity of chlorine
what is the standard enthalpy change of formation?
the enthalpy change when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
what is the standard enthalpy change of atomisation?
the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
what is the first ionisation energy?
the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
what is the first electron affinity?
the enthalpy change when one electron is added to one mole of gaseous atoms to form one mole of gaseous 1- ions
are second electron affinities exothermic or endothermic, and why?
- second electron affinities are endothermic
- a second electron is being gained by a negative ion, which repels the electron away
- so energy must be put in to force the negatively charged electron onto the negative ion
what is the enthalpy change of hydration?
the enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions, under standard conditions
what are the properties of ionic compounds?
- high melting and boiling points
- soluble in polar solvents
- conduct electricity when molten or in aqueous solution
what are the two factors that affect lattice enthalpy?
ionic size and ionic charge
how does ionic size affect lattice enthalpy?
- ionic radius increases
- attraction between ions decreases
- lattice energy becomes less negative (less exothermic)
- melting point decreases
how does ionic charge affect lattice enthalpy?
- ionic charge increases
- attraction between ions increases
- lattice enthalpy becomes more negative (more exothermic)
- melting point increases
what are the two factors affecting hydration enthalpy?
ionic size and ionic charge
how does ionic size affect the hydration enthalpy?
- ionic radius increases
- attraction between ion and water molecules decreases
- hydration energy becomes less negative
how does ionic charge affect hydration enthalpy?
- ionic charge increases
- attraction with water molecules increases
- hydration enthalpy becomes more negative
what is entropy?
a measure of the dispersal of energy and disorder within the chemicals making up the chemical system
how does entropy vary across the different states of matter?
- solids have the smallest entropies
- liquids have greater entropies
- gases have the greatest entropies
what factors control whether a reaction can occur?
a reaction can happen if the products have a lower overall energy than the reactants
what is the free energy change?
the overall change in energy during a chemical reaction, denoted as ΔG
what are the two types of energy that make up the free energy change?
the enthalpy change and the entropy change
what is the condition for feasibility according to the Gibb’s equation?
for a reaction to be feasible, there must be a decrease in free energy: ΔG < 0
what are the limitations of predictions made by ΔG about feasibility?
- many reactions have a negative ΔG value but do not seem to take place
- a large activation energy can result in a very slow rate of reaction
- although the sign indicates thermodynamic feasibility, it takes no account of the kinetics or rate of a reaction
what is reduction?
the gaining of electrons, decrease in oxidation number
what is oxidation?
the loss of electrons, increase in oxidation number
what is an oxidising agent?
takes electrons from the species being oxidised, contains the species being reduced
what is a reducing agent?
adds electrons to the species being reduced, contains the species being oxidised
what are the standard conditions?
- a temperature of 298K
- a pressure of 100kPa
- concentration of 1.00 mol dm-3
which electrode does oxidation and reduction occur at?
oxidation happens at the anode, reduction happens are the cathode
which electrode is the positive terminal?
the cathode
which electrode is the negative terminal?
the anode
what is the function of the salt bridge?
in electrochemical cells, the salt bridge allows charge balance to be maintained in the cell
what is a transition element?
a d-block element that can form at least one ion with a partially filled d-orbital
where are the transition elements located in the periodic table?
the d-block
what are the two elements in the d-block that are not considered as transition metals?
scandium and zinc
why are scandium and zinc not considered as transition metals?
they do not form ions with partially filled d-orbitals
- scandium only forms a 3+ ion, where the d-orbitals are empty
- zinc only forms a 2+ ion, where the d-orbitals are full
which electrons do transition metals lose first when forming ions?
4s electrons
what are some characteristic physical properties of transition metals?
- metallic
- high density
- high melting and boiling point
- shiny
- good conductors of heat and electricity
what are the characteristic chemical properties of transition metals?
- variable oxidation states
- coloured compounds/ions in solution
- good catalysts
- form complex ions
what are the two main ways in which transition metals act as an effective catalyst?
- they provide a surface on which reaction can take place
- they change oxidation states to form intermediates required for pathways with lower activation energy
what is a complex ion?
a transition metal bonded to ligands by coordinate bonds
what is a ligand?
a molecule or ion that can donate an electron pair to the central metal ion to form a coordinate bond
what is a coordinate bond?
a shared pair of electrons in which the bonded pair has been provided by only one of the bonding atoms
what does the coordination number indicate?
the number of coordinate bonds between the metal ion and the ligands
what is a monodentate ligand?
a ligand that is able to donate one pair of electrons to the central metal ion to form one coordinate bond
what is a bidentate ligand?
a ligand that is able to donate two pairs of electrons to the central metal ion to form two coordinate bonds
what is a multidentate ligand?
a ligand that forms three or more coordinate bonds with the central metal ion
what are the most common bidentate ligands?
1,2-diaminoethane and the ethanedioate ion
EDTA is a chelating agent, what does that mean?
EDTA decreases the concentration of metal ions in the solution by binding to them and forming complex ions
why is carbon monoxide toxic?
- CO also coordinately bonds with the central Fe 2+ ion
- CO bonds more strongly than oxygen and this binding is irreversible
- stops oxygen gas from binding to the haemoglobin
- so oxygen cannot be transported around the body
