Hey there! We receieved your request
Stay Tuned as we are going to contact you within 1 Hour
One of our academic counsellors will contact you within 1 working day.
Click to Chat
1800-5470-145
+91 7353221155
Use Coupon: CART20 and get 20% off on all online Study Material
Complete Your Registration (Step 2 of 2 )
Sit and relax as our customer representative will contact you within 1 business day
OTP to be sent to Change
This law states that the amount of heat evolved or absorbed in a process, including a chemical change is the same whether the process takes place in one or several steps.
Suppose in a process the system changes from state A to state B in one step and the heat exchanged in this change is q. Now suppose the system changes from state A to state B in three steps involving a change from A to C, C to D and finally from D to B. If q1, q2 and q3 are the heats exchanged in the first, second and third step, respectively then according to Hess’s law
q1 + q2 + q3 = q
Hess’s law is simply a corollary of the first law of thermodynamics. It implies that enthalpy change of a reaction depends on the initial and final state and is independent of the manner by which the change is brought about.
Sample problem.
In this case express ΔH in terms of ΔH1, ΔH2, ΔH3 .
Solution: ΔH = ΔH1 + ΔH2 + ΔH3
Sample problem. H2O (l) ® H2(g) + O2(g) DH = + 890.36 kJ / mole
What is DH for H2O (l) from its constituent elements
Solution: H2O(l) —> H2(g) + 1/2 O2 (g) ΔH = + 890.36 kJ / mole
H2(s) + 1/2 O2 (g) —> H2O(l) ΔH = – 890.36 kJ / mole
∴ ((ΔHf))H2O = –890.36 kJ / mole
Exercise:
(i) C + 1/2 O2 —> CO2(g) ΔH = –94 Kcals
C + 1/2 O2 —> CO(s) ΔH = –26.4 Kcals
CO + 1/2 O2 —> CO2(g) ΔH =?
(ii) What is heat evolved using neutralisation of HCN by a strong base? Heat of ionization of HCN is 10.8 Kcal.
To read more, Buy study materials of Thermodynamics comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.
Get your questions answered by the expert for free
You will get reply from our expert in sometime.
We will notify you when Our expert answers your question. To View your Question
THIRD LAW OF THERMODYNAMICS:- In all heat engines,...
Enthalpy of Reaction It is the enthalpy change...
Enthalpy of a System The quantity U + PV is known...
Specific Heat Capacity or Specific Heat [c] Table...
Level 2 Objective Problems of Thermodynamics Level...
Work Done During Isothermal Expansion Table of...
Introduction to Thermodynamics Table of Content...
Application of Hess's Law 1. Calculation of...
The First Law of Thermodynamics:- The first law of...
Objective Questions of Thermodynamics and Answers...
Work Done During Adiabatic Expansion Table of...
Solved Problems on Specific Heat, Latent Heat and...
Solved Problems on Thermodynamics:- Problem 1:- A...
Second Law of Thermodynamics Table of Content...
Specific Heat Capacity and Its Relation with...
Miscellaneous Exercises Thermal Physics:- Problem:...
Gibbs Free Energy This is another thermodynamic...
Reversible and Irreversible Process:- Reversible...
BOMB CALORIMETER The bomb calorimeter used for...
Application of bond energies (i) Determination of...
Relationship between free Energy and Equilibrium...
Solved Examples on Thermodynamics:- Problem 1 :- A...
Thermodynamic Change or Thermodynamic Process...
Macroscopic Properties He properties associated...
Thermodynamic State of a System and Macroscopic...