Physical Thermodynamics Exam Reported Questions Well Answered.

Endothermic reaction - correct answer To break bonds, the surroundings need to give energy to the molecule. The bonds gains energy, as they take in energy so have a positive sign. The system loses energy so gets colder Exothermic reaction - correct answer To make bonds, the molecule give energy to the surroundings. The bonds lose energy, as they give out energy so have a negative sign. The system gains energy so gets hotter What are the standard conditions - correct answer 298K, 1 moldm3 and 100kPa pressure Enthalpy of atomisation - correct answer The enthalpy when one mole of gaseous atoms is formed from the element in its standard state Na(s) --> Na(g) - for a metal 1/2Cl2(g) --> Cl(g) - for a non metal First ionisation enthalpy - correct answer The energy required to form one mole of gaseous unipositive ions from one mole of gaseous atoms Na(g) --> Na+(g) + e- First electron affinity - correct answer The enthalpy change when one mole of gaseous atoms each gain an electron to form one mole of gaseous negative ions Cl(g) + e- --> Cl-(g) Second electron affinity - correct answer The enthalpy change when one mole of gaseous ions each gain an electron to form one mole of gaseous negative ions O-(g) + e- --> O2-(g) Lattice enthalpy of formation - correct answer The enthalpy change when one mole of an ionic solid is formed from gaseous ions Na+(g) + Cl-(g) --> NaCl(s) Lattice enthalpy of dissociation - correct answer The enthalpy change when one mole of an ionic solid is separated into its gaseous ions NaCl(s) --> Na+(g) + Cl-(g) Features of born haber cycle - correct answer Arrows going up = endothermic = will result in changes above Arrows going down = exothermic = will result in changes below Symbols and state symbols need to be written on each level Arrow are not to scale Constructing a born haber cycle - correct answer Start in the standard states Atomise metal Ionise metal (if ion has more than one charge, second ionisation has to be shown) Atomise non metal First electron affinity non metal to become an ion (if ion has more than one charge, second electron affinity has to be shown) Lattice enthalpy of formation of two ionised species, CAN CHECK THE CALCULATED NUMBER IS RIGHT BY STARTING ANYWHERE ON THE CYCLE, ANSWER SHOULD BE ZERO Born haber cycle - correct answer Na2O born haber cycle - what additional steps are needed - correct answer Second electron affinity Oxygen forms O2- ion 2 sodium atoms are needed in the ionic lattice so need to double the atomisation enthalpy and ionisation enthalpies 2 sodiums will each need to lose an electron in the ionisation phase Covalent character of ionic bonds - correct answer Smaller ions, larger charge = higher lattice enthalpy = stronger ionic bond Smaller ions are more closely packed in the lattice and are more attracted to each other Ions with a high charge are more attracted to each other as well Comparing formation enthalpies of ionic compounds - correct answer THE MORE NEGATIVE THE VALUE, THE MORE FAVOURABLE THE REACTION The lattice enthalpy determines whether or not an ionic compound can be formed and how much a substance is purely ionic Why are theoretical values and calculated values different - correct answer The difference is caused by the bonding in the real ionic solid being less than 100% ionic In the calculations, it is assumed to be completely perfectly ionic (spherical ions and equal charge distribution) whereas most of the time the positive ion polarises the negative ion meaning unequal charge Which will have the lattice enthalpy of greatest magnitude and why? LiF or NaF - correct answer LiF as lithium is smaller than sodium Which will have the lattice enthalpy of greatest magnitude and why? CaO or MgO - correct answer MgO as magnesium is smaller than calcium (Mg2+ is smaller than Ca2+) Which will have the lattice enthalpy of greatest magnitude and why? NaCl or MgCl2 - correct answer MgCl2 as magnesium is smaller (due to increasing nuclear charge so electrons pulled in closer) and higher charge than sodium Explain why the magnitude of the lattice enthalpy of KCl (-701) is less than CaCl2 (-2237) - correct answer Calcium is smaller and has a higher charge than potassium so a stronger attraction