Thermodynamics- 3 Flashcards
what is heat capapcity
It is the amount of heat required to raise the temperature of a system/substance through 1 degree celsius
Denoted by “C”
what is specific heat capacity
It is the amount of heat required to raise the temperature of 1g of a substance through 1 degree celsius
denoted by “c”
what is molar heat capacity
It is the amount of heat required to raise the temperature of 1mole of a substance through 1 degree celsiuis.
denoted by “cₘ”
what are thermochemical reactions
a balanced chemical reaction involving the physical states, the amounts of different reactant and products and the heat absorbed or evolved is called a thermochemiclal reactions.
what are the properties of thermocehmical eqns
- for exothermic reactions, the value of dH is -ve. for endothermic reactions, the valeu of dH is +ve.
-The stoichiometric coefficients denote the number of moles of products, reactants involved
-The physical states of ractnats and products must be mentioned because the heat absorbed or evolved depends on the phsyical states
- If the stoichiometric coefficients are multiplied or divided by a number, then the valeu dH should also be multiplied or divided by the same number.
- If a thermochemical reaction is reversed, then the value of dH remains same in magnitude but reversed in sign.
what is heat of reaction/ enthalpy of reaction/ reaction enthalpy dᵣH
It is the amount of heat absorbed or evolved in a chemical reaction when the no of moles,specified by the balanced chemical eqn, have completely reacted.
dᵣH= (sum of enthalpied of products)-(sum of enthalpies of reactants)
= ∑aᵢH products - ∑bᵢHreactants
(Here symbol ∑ (sigma) is used for
summation and ai
and bi
are the stoichiometric
coefficients of the products and reactants
respectively in the balanced chemical
equation. For example, for the reaction
how is knowing reaction enthalpy used ?
Enthalpy change is a very useful quantity.
Knowledge of this quantity is required when
one needs to plan the heating or cooling
required to maintain an industrial chemical
reaction at constant temperature. It is also
required to calculate temperature dependence
of equilibrium constant.
what is standard enthalpy of reactions
The
standard enthalpy of reaction is the enthalpy change for a reaction when all the participating substances are in their standard states.
The standard state of a substance at a specified temperature is its pure form at 1 bar pressure, 298K temperature..
Standard conditions are denoted by adding
the superscriptV to the symbol ∆H, e.g., ∆H
V
what is enthalpy of fusion
The enthalpy change that accompanies
melting of one mole of a solid substance
in standard state is called standard
enthalpy of fusion or molar enthalpy of
fusion, ∆fusH
H2O(s) —-> H2O( l); 6.00 kJ mol
what is enthalpy of vaporisation
Amount of heat required to vaporize
one mole of a liquid at constant
temperature and under standard pressure
(1bar) is called its standard enthalpy of
vaporization or molar enthalpy of
vaporization, ∆vapH
H2O(l)—-> H2O(g);
what is enthalpy of sublimation
Standard enthalpy of sublimation,
∆subH
0
is the change in enthalpy when one
mole of a solid substance sublimes at a
constant temperature and under standard
pressure (1bar).
Sublimation is direct conversion of a solid
into its vapour. Solid CO2
or ‘dry ice’ sublimes
at 195K with ∆subH
0
=25.2 kJ mol –1 ;
naphthalene sublimes slowly and for this
what does the enthalpy of transformation between phases depend upon
he magnitude of the enthalpy change
depends on the strength of the intermolecular
interactions in the substance undergoing the
phase transfomations. For example, the strong
hydrogen bonds between water molecules hold
them tightly in liquid phase. For an organic
liquid, such as acetone, the intermolecular
dipole-dipole interactions are significantly
weaker. Thus, it requires less heat to vaporise
1 mol of acetone than it does to vaporize 1 mol
of water
