Chemistry: physical chl

Metal + steam →

Metal oxide + hydrogen

Give a commercial use of a group 2 compound (2 examples)

• Mg(OH)₂ is used as an antacid to neutralise excess stomach acid

• Ca(OH)₂ is used in agriculture to neutralise excess acid in soil

Other than Mg(OH)₂,what compound is used as an antacid?

CaCO₃

Explain why it is difficult to predict whether MgF₂ or MgI₂ will have more or less exothermic enthalpy change of solution

• Ionic radius increases down the group so F- is smaller than I-

• Lattice enthalpy for MgI₂ is less exothermic than for MgF₂

• I- has less attraction for H₂O than F- so Hydration enthalpy is less exothermic for MgI₂

• Difficult to predict whether lattice enthalpy or hydration enthalpy has a bigger effect

Entropy definition

A measure of the dispersal of energy in a system which is greater when the system is more disordered

Why is a buffer solution formed from acid and base?

The acid is partially neutralised and forms conjugate base (salt of weak acid - give example!)

When a solid ionic lattice is dissolved in solution, what happens to entropy?

Entropy increases as ions are more disordered

Equation used to calculate entropy change

What are the limitations of feasibility predictions made using gibbs’ free energy equation?

• Activation energy may be too high

• Rate of reaction may be too slow

Oxidising agent

A species that is reduced in a reaction and causes another species to be oxidised

Standard conditions for gas pressure

1 atm (100 kPa)

Standard hydrogen half cell

How to make a simple salt bridge?

Filter paper dissolved in aqueous solution of potassium or ammonium nitrate

Which states can be used in heterogeneous Kc?

aq only

Precipitation reaction adding NH₃ dropwise (reaction with water)

NH₃ + H₂O → NH4⁺+ OH^-

Blood buffer (carbonic acid)

H⁺+ CO₃^- → CO₂ + H₂O

Equivalence point

The point in the titration when the volume of one solution has reacted exactly with the volume of the 2nd solution ([OH^-]=[H⁺])

How to select an indicator for titration

The pH range of the indicator coincides with the vertical section

End point

pH at which there are equal concentrations of the weak acid and its conjugate base ([HA]=[A^-])

Conjugate acid-base pair

Two species that transform into each other by gain or loss of a proton

Limitations of K_a

• Stronger weak acids have more dissociation of HA and [H⁺] becomes significant and sig diff. between HA and H⁺ (if K_a > 10^-²)

• Weaker weak acids are not valid for approximation [H⁺] ~ [A^-], if pH > 6, the dissociation of water becomes significant

Lattice enthalpy

Enthalpy change the accompanies the formation of 1 mole of an ionic compound from its gaseous ions in their standard states

Lattice enthalpy of formation

Enthalpy change when 1 mole of a compound is formed from it elements in their standard states under standard conditions

Lattice enthalpy of solution

Enthalpy change when one mole of a compound is dissolved in water under standard conditions

First ionisation energy

Energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions

Enthalpy change of hydration

Enthalpy change when one mole of isolated gaseous ions is dissolves in water forming one mole of aqueous ions under standard conditions

Comparing lattice enthalpies of MgS and Na₂O (ICSAE)

• Mg²⁺ ion has a greater charge than Na⁺ ion and Mg²⁺ is also smaller

• The O^2- ion is smaller than the S^- ion

• Mg²⁺ attracts negative ions more strongly than Na⁺ however O^2- attracts positive ions more strongly than S^2-

• It is hard to predict which lattice enthalpy is the most exothermic

Compare hydration enthalpy of Cl^- and O^2- (ICSAE)

• O^2- has a greater charge than Cl^- and it is smaller

• There is a greater attraction between O^2- ions and H₂O molecules than between Cl^- ions

• O^2- has a more exothermic hydration enthalpy

What units is temperature in for Q=mcΔT?

Degrees C

How does dative covalent bonds differ in bond enthalpy from covalent bonds?

Not at all - they have the same bond enthalpies!