Internal Energy
The total of all the possible kind of energies of a system is called the internal energy i.e.
E = E rotational + E translational + E vibrational
It is expressed in Joules or Calories.
Enthalpy
The heat content of the system called enthalpy. It is
the sum of internal energy E and PV work.
H = E + PV
It is Expressed in KJ molar K-cal mol-
02. Differentiate between internal energy change and enthalpy change.
Internal Energy Change
Heat provided to the system increases its internal
energy from E1 to E2. The difference of the two
energies is the change in energy.
∆E = E2- E1
If the volume of the system is not allowed to
change i.e. ∆V= 0 than the q heat is used only to
increase the internal energy of the system, hence,
∆E = qv
Enthalpy Change
If the pressure of the system is kept constant the
q heat is used to increase its enthalpy. So
∆H = qp
It is the difference of enthalpy of the system in
initial and final state:
∆H = H2- H1
03. Differentiate between Exothermic and Endothermic Reactions.
Exothermic Reaction
i. Heat is evolved from the system.
ii. The temperature of the system rises.
iii. Enthalpy of products is less than the enthalpy of
reactants.
iv. The sign of enthalpy change is negative.
v. They are normally spontaneous.
vi. 2 H2 + O2 2H2O
∆H = -285.58 kJ mol-1
Endothermic Reaction
i. Heat is absorbed by the system.
ii. Temperature of the system decreases.
iii. Enthalpy of products is more than enthalpy of
reactants.
iv. Sign of enthalpy change is positive.
v. They are normally non-spontaneous.
vi. 2H2O2 H2 + O2
∆H = +285.58 kJ mol-1
04. Differentiate between Heat and Temperature.
Heat
i. The transfer of energy caused by the difference in
temperature between the two objects is called
heat.
ii. It measures the total K.E.
iii. It is denoted by “q”.
iv. It is measured by the calorimeter.
v. It is expressed in KJ molar K-cal mol-
Temperature
i. The degree of hotness and coldness in a body is called temperature.
ii. It measures the average K.E.
iii. It is denoted by “T”.
iv. It is measured by Thermometer.
v. It is expressed in °C, °F or K.
05. Explain that the burning of the candle is a spontaneous process.
All combustion reactions are exothermic and spontaneous. However, some amount of energy is needed to initiate the chemical reaction.
06. Is it true that non-spontaneous are never happens in the universe?
Non-spontaneous processes need energy. Naturally, this energy is not available so non-spontaneous never happens in the universe. For example pumping of water uphill is a non-spontaneous process that never happens naturally.
07. Why is it necessary to mention the physical states of reactants and products in thermodynamic equations?
It is necessary to mention the physical states because different physical states in reaction have different values of heat of reaction. For example, when H2 and O2 react to for liquid water the heat of reaction is -285.58 kJ mol-1
. But if steam is formed the heat of reaction is 241.8 kJ mol-1. 08.For what system ∆E ≈ ∆H and why?
For the liquid and the solids systems, there is no significant change in volume with the change in the state.
So, ∆E ≈ ∆H for these systems.
∆H = ∆E + P∆V
∆H = ∆E + P(0)
∆H = ∆E
09. Why the enthalpy of neutralization is same for strong acids and strong bases?
The main chemical change when strong acids are reacted with strong bases is the formation of water i.e.
H + + OH− H2O. Thus the enthalpy change in the formation of water is the same for the strong acids and the strong bases.
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