Thermochemistry

Question 1

what is an intensive property?

Answer 1

a property that does not depend on the amount of substance

eg. temperature

Question 2

Kelvin temperature

Answer 2

K = ˚C + 273

Question 3

what is temperature?

Answer 3

a measure of the average kinetic energy of particles in a sample
same Ek = same temp

Question 4

change in the energy of a system results in

Answer 4

equal and opposite change in its surroundings

Question 5

heat define

Answer 5

q
transfer of energy due to temperature difference

an EXTENSIVE property - depends on the amount

Question 6

heat transfer between objects of different temperatures

Answer 6

thermal equilibrium

Question 7

heat capacity define

Answer 7

C
quantity of heat required to raise temperature by 1˚C
depends on composition AND mass

Question 8

define specific heat capacity

Answer 8

quantity of heat required to raise temp of 1g of pure substance by 1˚C
Jg-1˚C-1

for the same heat transfer, the object w/ LOWER heat capacity gets hotter

Question 9

heat capacity formula

Answer 9

C = mc

heat capacity = mass x specific heat capacity

Question 10

q = mcΔT

Answer 10

heat = mass x specific heat cap x change in temp

remember it by the units of specific heat cap

as C=mc, q=CΔT
q= heat capacity x change in temp

Question 11

find specific heat cap from heating graph

Answer 11

sloped section
c = heat added over range / temp range
J ˚C-1

Question 12

what three changes requires energy?

Answer 12

disrupting inter particle forces (solid–liquid–gas)

breaking chemical bonds

separation of charge

Question 13

what two changes releases energy?

Answer 13

neutralising charge = lattice energy
Na+ + Cl- –> NaCl

dispersing charge over a bigger volume
hydrated cation Na(OH2)6 + is LOWER ENERGY than Na+

Question 14

separating and bringing together particles that are attracted to one another energy

Answer 14

separating = ends

bringing together = exo

Question 15

what does a Maxwell-Boltzmann distribution show?

Answer 15

the distribution of energies for particles in a sample at a given temperature

fraction probability per Jmol-1 vs energy

looks like a bell curve skewed to the right
0,0 but the tail never reaches 0 at the right

Question 16

total energy of a sample of gas vs individual molecules

Answer 16

total energy of sample constant at a given temp

energy of individual molecules is constantly changing due to 10^9 collisions per sec

Question 17

what does the peak of the Maxwell-Boltzmann curve show?

Answer 17

the fraction of particles having the most probable energy

on the left side - the line of average energy is higher than most probable energy

Question 18

what does the area under a Maxwell-Boltzmann curve show?

Answer 18

related to the fraction of atoms/molecules having energies in the specified range

Question 19

how does the Maxwell-Boltzmann curve look different for reactions of various temperatures?

Answer 19

higher temp, flatter curve (broader distribution)
same starting point (0,0)

same area under curve = same number of particles

Question 20

what does a calorimeter measure?

Answer 20

the heat transferred by a reaction

Question 21

q(reaction) =

Answer 21

= q(system)
= - q(surroundings)
= -m(soln) x c(sln) x T(sln)

so if ΔT is -ve, q is +ve, endothermic

Question 22

define enthalpy

Answer 22

a characteristic of the reaction at standard temp/pressure

calculate by measuring heat transfer at constant pressure for a specific amount of reaction

∆rH = q / n
unit kJmol-1

does NOT depend on pathway!

Question 23

define ∆H, and formula associated with it

Answer 23

observed enthalpy change

∆H / ∆rH = n(reaction)

Question 24

Calculate ΔrH˚ from standard enthalpy of formation

Answer 24

= Σ(n x ΔfH˚(products)) - Σ(n x ΔfH˚(reactants))

Question 25

Define bond enthalpy

Answer 25

Energy required for dissociation into the component atoms in the GAS phase

A-B(g) —> A(g) + B(g)

Bond breaking is always ENDO, +ve

Question 26

standard state C

Answer 26

solid

Question 27

standard state N

Answer 27

N2 gas

Question 28

standard state calcium carbonate

Answer 28

CaCO3 (s)

Question 29

define standard enthalpy change

Answer 29

enthalpy change when all reactants/products are in STANDARD STATE

delta H˚

fractions may be used for combustion

Question 30

define standard enthalpy of formation

Answer 30

delta fH˚
enthalpy change for the reaction in which ONE MOL of substance in the standard state is formed from elements in their standard state

coefficient of product is 1, fractions used for others

Question 31

calculate standard enthalpy from standard enthalpy of formation

Answer 31

sum of fH˚(products) - sum of fH˚(reactants)

Question 32

define bond enthalpy

Answer 32

(D)
the energy required for dissociation into the component atoms in the GAS phase

A-B(g) –> A(g) + B(g)

requires energy to break bonds, ENDO, +Ve

Question 33

find enthalpy of reaction from bond enthalpies

Answer 33

rH(reaction) = sum of D(broken) - sum of D(formed)

Question 34

define energy of atomisation for metals

Answer 34

also energy of sublimation
= energy change on conversion of one mol of the metal in its standard state to its atoms in the gas phase

delta subH = delta aH
M(s) –> M(g)

endo

Question 35

enthalpies of atomisation for non-metals molecular

Answer 35

conversion of the elemental form in its standard state to atoms.

eg. delta aH(X2): X2 –> 2X(g)

Question 36

define ionisation enthalpy

Answer 36

GAS PHASE CATION formation

E(g) –> E+(g) + e-(g)

delta iH

Question 37

electron affinity enthalpy

Answer 37

delta EAH
gas phase ANION formation for any atom
enthalpy change for ATTACHING AN e-

Question 38

lattice formation enthalpy and trends in enthalpy

Answer 38

GAS phase ions to lattice, solid
M+(g) + X-(g) –> MX(s)

strongly exothermic
generally less exothermic for ionic solids having larger ions

Question 39

e- transfer between atoms overall energy

Answer 39

ENDOthermic

Question 40

Born-Haber cycle

Answer 40

METAL     +     NONMETAL 
metallic/covalent bond breaking (AiH, AeaH) 
GAS PHASE ATOM 
e- loss or gain (AaH) 
GAS PHASE IONS 
form lattice (AfH) 
IONIC SOLID 

Metal+nonmetal straight to ionic = direct path, lattice enthalpy

Question 41

ΔsolnH

Answer 41

enthalpy change on dissolving 1 mol of substance in sufficient solvent so that particles are well separated

Question 42

two steps of dissolving ionic solid and the enthalpies involved

Answer 42

1. separate ions in lattice
   ALH, lattice dissociation enthalpy, endo
2. forming complexes
   eg. M+(g) –(H2O)–> M+(aq)
   AhydH = hydration enthalpy, exothermic (dispersing charge over larger vol)