All Students Notes

Hesse's law Related Problems   Question By applying, Hess’s law calculates the enthalpy change for the formation of an aqueous solution of NH4Cl from NH3 gas and HCl gas. The results for the various reactions and pressures are as follows.  Solution Question Calculate the heat of formation of ethyl alcohol from the following information.  (i) Heat of combustion of ethyl alcohol is -1367 kJmol-1.  (ii) Heat of formation of carbon dioxide is -393.7 kJmol-1. (iii) Heat of formation of water is -285.8k Jmol-1.  Solution Question If the heat of combustion of C2H4, H2 and C2H6 are -337.2, -68.3 and -372.8 k calories respectively, then calculate the heat of following reaction. C2H4(g) + H2(g) C2H6(g)  Solution . According to Hess’s law above reaction can be broken into three steps Question Graphite and diamond are two forms of Carbon. The enthalpy of combustion of graphite at ...

Question. Hydrazine (N2H4) is a rocket fuel. It burns in O2 to give N2 and H2O.   N2H4(ℓ) + O2(g) --------------->N2(g) + 2H2O(g)  1. 00 g of N2H4 is burned in a bomb calorimeter. An increase of temperature 3.51 OC is recorded. The heat capacity of calorimeter is 5.5 k JK-1. Calculate the quantity of heat evolved. Also, calculate the heat of combustion of 1 mole of N2H4.  Solution:  Question: Octane (C8H16) is a motor fuel. 1.80 g of a sample of octane is burned in a bomb calorimeter having heat capacity 11.66 kJk-1. The temperature of the calorimeter increases from 21.36OC to 28.78OC. Calculate the heat of combustion for 1. 8 g of octane. Also, calculate the heat for 1 mole of octane. Solution: Next

Question. What is lattice energy? How does Born- Haber cycle help to calculate the lattice energy of NaCl?  Answer.   Lattice energy: Amount of energy released when one mole of the ionic compound is formed from gaseous ions.  Now we extend the above triangle to show the various stages involved in finding the ∆ . The complete cycle is called a Born Hyber cycle. This cycle shows all arrows of endothermic processes pointing upward and those for exothermic processes pointing downward. It is clear from Born-Hyber cycle that  Question. 50 cm3 of 1.0 M HCl is mixed with 50 cm3 of 1.0 M NaOH in a glass calorimeter. The temperature of the resultant mixture increases from 21.0OC to 27.5OC. Assume that calorimeter losses of heat are negligible. Calculate the enthalpy change mol-1 for the reaction. The density of solution to be considered is 1 gm cm-3 and specific heat is 4.18Jg-1k-1.  Answer. Specific heat of water s =4.18 J...

Question. Define and explain Hess’s law of constant heat summation. Explain it with examples and give its application.  Answer.  Hess’s Law: If a reaction takes place in one step or in several steps, the overall enthalpy change is the same for each route. Or total heat change for a reaction is same and does not depend upon the route.  Explanation:  Let us assume that reactant A can be converted to D directly in a single step and heat evolved is . If reaction have indirect root from A B C as shown below.  Question: Hess’s law helps us, to calculate the heat of those reactions, which can’t be carried out in laboratory. Explain it.  Answer.  Application of Hess’s Law: In many cases, direct determination of enthalpies of formation and enthalpies of reactions are not possible. This may be due to incompletion of reaction or side reaction or slow speed of reaction. For example, heat of formation of CO ca...

Question. How will you differentiate between ∆E and ∆H ? Is it true that ∆H and ∆E have the same values for the reaction taking place in the solution state?  Answer. H is amount of heat evolved or absorbed in a reaction at constant pressure whereas ∆E is amount of heat evolved or absorbed in a reaction at constant volume. Yes it is true that both ∆E and ∆H have the same values for the reaction taking place in solution phase. This is due to fact that in solution state volume of the system remains constant.  Question. What is the difference between heat and temperature? Write a mathematical relationship between these two parameter.  Answer. Heat: Transfer of energy between two bodies due to temperature difference is called heat. It is represented by letter q.  Temperature: It is measure of average kinetic energy of all particles of a system. Temperature can be used to determine the flow of heat.  Mathematica...

Question . What is first law of thermodynamics? How does it explain that (i) qv=∆E (ii) qp=∆H  Answer.   First law of thermodynamics: This law states that energy is neither created nor destroyed but can be changed from one form to another form. First law of thermodynamics is also called law of conservation of energy. OR Total energy of a system and its surroundings remains constant.  Explanation. Consider a gas enclosed in a cylinder fitted with a frictionless piston. Let us assume that E1 be initial energy of the system. Now, there are two ways of transferring energy to system: by heating the system or by doing work on the system. We transfer the energy by both ways. If q is quantity of heat supplied to system and w is work done on the system by compressing piston from V1 to V2 and internal energy of system changes to E2, then           ...

Question. Define the following enthalpies and give two examples each.   Answer.  Standard enthalpy of reaction: Standard enthalpy of reaction is enthalpy change, measured, at 25O C and 1 atm pressure, when molar quantities of reactant as given in balance chemical equation react completely. It is symbolized by ∆H. Its units are kJ. 2H2(g) + O2(g) -----> 2H2O(ℓ) ∆H0 = − 571.6kJ   C(g) + O2(g) ----------> CO2(ℓ) ∆H0 = − 393.7 kJ  Standard enthalpy of formation: Standard enthalpy of formation is enthalpy change, measured, at 25O C and 1 atm pressure, when one mole of a product is formed from reactants. It is symbolized by ∆Hoc­­­ . Its units are kJmol-1.   C2H5OH(g) + O2(g) -----> 2H2O(ℓ) ∆H0 = − 1368 kJmol-1   CH4(g) + O2(g) ----------> CO2(ℓ) ∆H0 = − 890 kJmol-1   Standard enthalpy of combustion: Standard enthalpy of combustion ...

Thermochemistry Question 7.1: Define the following terms and give three examples of each.  (i) system (ii) surrounding (iii) state function  Answer.  System: Anything (material) under test in the laboratory. or Any portion of universe which is under study is called system. Examples: Reaction between Zn andCuSO4 solution. (ii) One mole of oxygen enclosed in a cylinder fitted with piston (iii) Neutralization reaction of HCl and NaOH.   Surrounding: All the material of universe which surrounds the system but do not form a part of system. Examples: Consider the reaction between Zn and CuSO4 which is taking place in a cylinder lying on the table and air is surrounding it. If the reaction is our system then cylinder, table and air are surroundings.  State function: The macroscopic property of system whose value depends only on the current state of the system is called state function. The amount of change in a state function depends only on the initial and final st...