Chemical Energetics Flashcards
What is ΔH
Enthalpy change
overall energy absorbed or released for one mole of a substance/ molar quantities specified in a chemical reaction
sum of energy absorbed in bond breaking (reactants) + sum of energy released in bond forming (products)
expressed in kJ/mol
What is q
Heat change
q= ΔHr x moles of limiting reagent
What is H
Enthalpy
Energy content in a substance
Bond breaking and Bond forming
BBAE
BFRE
Exothermic
sum of energy absorbed in bond breaking lesser than sum of energy released in bond forming
ΔH -ve
Products are more energetically stable than reactants
Examples: Neutralisation, Condensation, Combustion
Endothermic
sum of energy absorbed in bond breaking greater than sum of energy released in bond forming
ΔH +ve
Reactants are more energetically stable than products
Examples: Photosynthesis, dissolving NH4NH3 (aq) , melting
Standard state
298K
1 bar
Thermochemical equation
Balanced chemical equation
Shows state symbols of substances
Is associated with enthalpy change
Factors affecting the magnitude and sign of ΔH
- Amount of substances
when multiplying n to both sides of the equation , ΔH must be multiplied by n as well - Physical states of reactants and products
3.ΔH of forward and backward reactions
the magnitude of ΔH is the same for forward and backward reactions however
if forward reaction is ΔH -ve, backward reaction will be ΔH +ve
if forward reaction is ΔH +ve backward reaction will be ΔH -ve
Standard enthalpy change of formation ΔHf
The heat change when one mole of a substance is formed from its constituent elements in their standard state 298K, 1 bar
Standard enthalpy change of reaction ΔHr
The heat change when molar quantities of substances react together based on their stochiometric ratio at the standard state 298K and 1 bar
Standard enthalpy change of combustion ΔHc
The heat released when one mole of a substance is completely burnt in excess oxygen at 298K and 1 bar
Standard enthalpy change of neutralisation ΔHn
The heat energy released when an acid reacts with an alkali to give one mole of water at 298K and 1 bar
Standard enthalpy change of hydration ΔHhyd
The energy released when one mole of gaseous ions are hydrated to form an indefinitely dilute solution at 298K and 1 bar
Standard enthalpy change of solution ΔHsol
The energy change when one mole of substance is dissolved in a solvent to form an indefinitely dilute solution at 298K and 1 bar
Standard enthalpy change of atomisation ΔHat
The energy absorbed when one mole of gaseous atoms is formed from its constituent elements at 298K and 1 bar
Bond Energy
The energy absorbed when one mole of covalent bonds between two atoms in gaseous molecules is broken to form gaseous atoms
The accuracy of BE data for diatomic molecules vs polyatomic molecules
BE data for diatomic molecules is accurately presented in data booklet however BE data for polyatomic molecules is inaccurate as they are an average. Same type of covalent bonds in different polyatomic molecules may have different bond energy values due to different types of chemical environments.
Chemical environment affected by
- Presence of electron withdrawing or donating group
- Type of hybridisation
- Ring strain in cyclic compounds
- Intermediate bond order due to resonance
Ionisation energy
1st: The energy absorbed in removing one mole of most loosely held electron from one mole gaseous cation to form one mole of singly positively charged gaseous cation
2nd: The energy absorbed in removing one mole of most loosely held electron from one mole of singly charged gaseous cation to form one mole of doubly charged positively cation
Electron Affinity
1st: The energy change when one mole of gaseous atoms gain one mole of electrons to form one mole singly negatively charged anion
2nd: The energy change when one mole of singly charged negatively gaseous anions gain one mole of electrons to form one mole of doubly charged negatively anions
Lattice energy
The energy released when one mole of ionic solid is formed from its gaseous constituent ions
Factors affecting magnitude of lattice energy
The greater the charge on the cation and the anion the greater the magnitude. More exothermic
The greater the ionic radii of cation and anion the lesser the magnitude. Less exothermic.
Why weak acids have less exothermic neutralisation reactions compared to strong acids?
Part of the energy released from the neutralisation is used to provide energy for complete disassociation of the acid
Direct method of measuring ΔH
Calorimetry
measurement of ΔT to obtain Heat change (q)
ΔT for exothermic: Tmax-Ti
ΔT for endothermic: Tmin- Ti
ΔHr= -mcΔT/n of limiting reagent x coefficient of limiting reagent
final energy of surrounding - initial energy of surrounding = -energy of products + energy of reactants
Assumptions made during calorimetric experiments
- There is negligible heat loss to surroundings
- The density of solution is the same as that of water 1.00g/cm3
- The heat capacity of the solution is the same as that of water 4.18J/gK
Combustion of fuel using calorimetry
1.Spirit burner containing fuel
2. Known volume of water in calorimeter
Mass of fuel used in combustion measured by weighing mass of spirit burner before and after experiment Wi-We
Limitations of using calorimetry and why it results in less exothermic reaction when used in combustion. |ΔHc|∝|mcΔT/n of substance burnt |
- There is heat loss to the surroundings, to the calorimeter which are not accounted for. Hence not all heat is not absorbed by the water in the calorimeter
- There might have been incomplete combustion
- Calculating the difference in the mass of spirit burner and fuel initially and after is not accurate to measure the amount of fuel that has combusted as highly volatile fuels might have evaporated
HENCE, for 1 and 2 the ΔT will be lesser than actual and so will ΔHc
For 3, greater fuel will be recorded as burnt hence amount of fuel is greater, ΔHc recorded is smaller
Bomb calorimeter
Finding ΔHc using heat capacity of the calorimeter
ΔHc= CΔT/n (C J/K- heat capacity of calorimeter amount of energy required raise the calorimeter by 1K)
Features of bomb calorimeter
- Insulated calorimeter
Reduce heat loss to the surroundings - Temperature regulated water jacket
Reduce heat loss to the surroundings by regulating temperature of the water in the calorimeter by ensuring there is no temperature gradient between water in calorimeter and jacket so no heat transfer. - Oxygen supplied at high pressure in the steel bomb calorimeter
Ensures complete combustion - The formula takes into account the heat lost to /absorbed by the calorimeter by measuring the heat capacity of the calorimeter CΔT
Hess’ Law
Energy cycle/ Energy level
ΔH1= ΔH2+ ΔH3 at standard state
FLIPPER
Formation
ΔHf = ΣnΔHf of products - ΣmΔHf of reactants
n and m are stochiometric ratios
CRAP
Combustion
ΔHr = ΣnΔHc of reactants - ΣmΔHc of products
BURP
Bond energy
ΔHr= ΣBE of bonds broken in reactants - ΣBE of bonds formed in products
Born- Haber Cycle
FAIL
for construction of energy level
- Formation
- Atomisation of cation b4 anion
- Ionisation of cation b4 anion
- L.E.
Comparison between theoretical and experimental LE values
The theoretical value of LE deviates from the experimental value of LE as it is calculated using a model where ions are spherical and their electron clouds evenly distributed. However that is not true in a real ionic compound
The percentage difference between theoretical LE and experimental LE can be used to determine how much theoretical deviates from experimental
Extent of deviation of theoretical from experimental
- cations with greater polarising power as they distort the electron cloud of the anion greater
eg Sodium halides vs Silver halides
Silver has a greater polarising power as it is a d block element , its nuclei is poorly shielded by d electron. Hence for same halide, Ag distorts the electron cloud of the halides greater than Na - Polarisability of anion (greater, more the deviation)
Dissolution of ionic compound
S-LOTH
ΔHsol= -L.E. + ΣΔHhyd ions
When ΔHsol of an ionic compound is negative the compound is soluble in water.
When ΔHsol of an ionic compound is positive it may or may not be soluble. Its solubility will be determined from the compounds’ ΔG (Gibbs free energy)
Greater the ΔHsol the more exothermic the reaction and hence more soluble the compound
q=mc Δt is +ve when rxn is exothermic
q=mcΔt is - ve when rxn is endothermic
Factors affecting magnitude of ΔHhyd
1.strength of ion dipole interactions
Stronger- greater the magnitude- more exothermic the ΔHhyd
Strength of ion dipole affected by charge to size ratio (charge of ion and ionic radius)
