Loading [MathJax]/extensions/TeX/AMSsymbols.js
Skip to main content

Thermochemistry

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 can not be measured directly due to the formation of CO2 with in it. If the enthalpy of combustion of graphite to form CO2 and the enthalpy of combustion of CO to form CO2 are known, we determine the enthalpy of formation for CO. The oxidation of carbon can be written as follows. So enthalpy change for the formation of CO is 110.0 kJmol-1.


                                       Next

Labels

Labels:

Comments

Post a Comment

Popular posts from this blog

Class 9 Chemistry Notes for Federal Board

Class 9 Chemistry Notes for Federal Board Introduction Class 9 Chemistry notes for the Federal Board have been written according to the National Curriculum 2006. It develops various skills such as questioning, observations, predicting, measuring, classifying, designing experiments, analyzing and interpreting the data. among students.  We have made these notes very simple.  After reading these notes, students can easily understand all topics. Students can  answer all types of questions including  essay type, short and multiple choice, self assessments and think tank. We have also solved exercises according to the Federal board examination pattern.  Students, after going through 9th class chemistry notes can get maximum marks in their exams. They can lay down a strong foundation of scientific knowledge among students. In future, notes for class 9 will be available in pdf format. Students will be able to note pdf free download  very soon. Students will ha...
1 comment
Read more

Class 10 Chemistry Notes for Federal Board

Class 10 Chemistry Notes for Federal Board Introduction Class 10 Chemistry Notes for Federal Board have been written according to  the National Curriculum 2006.  These notes will make students able to learn topics by their own. 10th class notes for all chapters are very and easy to understand. After studying these notes, students will become confident to answer all type of questions. Chemistry notes for class 10  provide solution to all kinds of  questions including essay type, short and multiple choice, self assessments and think tank. These notes also contain all past papers of reduced syllabus  as well as complete syllabus. Soon, students will have access to 10th class chemistry notes in Urdu pdf download form. We are also working hard to provide all chemistry notes in pdf format and student will be able to download these notes. We also trying to upload video lectures about class 10 chemistry. Don't forget, you can test your chemistry by using online test. Fu...
3 comments
Read more

Theories of Covalent Bonding and Shapes of Molecules: MCQs

  chemicalAlgebra 1. The bond energies of F 2 , Cl 2 , Br 2 and I 2 are 37, 58, 46 and 36 Kcal, respectively. The strongest bond is formed in A) F 2 B) Cl 2 C) Br 2 D) I 2 ... Answer is B) Description: The bond energy is proportional to bond length. So, the bond strength of Cl 2 is higher. show Answer 2. The percentage of ionic character of bond between two atoms is calculated from the difference between their A) Ionic B) Electronegativities C) Electron affinities D) Ionization energies ... Answer is B) Description: The percentage of ionic character depends upon the difference of E.N. difference between the bonded atom. Greater the E. N. difference, greater the ionic character and hence greater the bond energy show Answer 3. The geometry of PF 5 molecule is A) Planer B) Square planer C) Trigonal pyramidal D) Tetrahedral ... Answer is C) Description: ...
3 comments
Read more

Chemistry

T he Federal Board of Intermediate & Secondary Education (FBISE) is an autonomous body that conducts the examinations in the institutions affiliated with it in Pakistan as well as abroad.  Punjab government has ten boards of Intermediate & Secondary Education including Bahawalpur Board, D.G. Khan Board, Faisalabad Board, Gujranwala Board, Lahore Board, Multan Board, Rawalpindi Board and Sargodha Board and each board conducts exams independently.   Federal Board generally follows the international norms and  standards. Federal Board paper pattern is quite tricky and similar to O-Level and A Level formats. These questions need conceptual approach to be solved. Punjab board’s Paper pattern is comparatively easy up to some extent.  We are currently focusing on the subject of Chemistry for all classes affiliated with Federal Board ( IX ,  X ,  XI ,  XII )  and Punjab Board (  XI   XII ). We intend to extend it to all subjects i...
Post a Comment
Read more

Basic Definitions| Matter, Substance, Element, Compound. Mixture, Homogeneous mixture and Heterogeneous mixture Examples (for Grade - 9 students)

Basic Definitions| Matter, Substance, Element, Compound. Mixture, Homogeneous mixture and Heterogeneous mixture Examples (for Grade - 9 students) Question: Define following terms:  Matter Substance Element Compound Mixture Homogeneous mixture Heterogeneous mixture Answer Matter:   Matter can be anything that occupies space (volume) and has mass. Example, book, table, walls, chairs, plants, animal, etc.  Substance:   Substance is a matter that has a particular set of characteristics that differ from the characteristics of another kind of matter. For example, oxygen, water, carbon monoxide, carbon dioxide, urea, glucose, common salt etc are different substances.  Element:   A substance that cannot be converted to other simpler substances is known as an element. or A substance which has all atoms of same atomic number is called element. For example substances like oxygen, hydrogen, iron, copper, aluminum etc. are elements.  Compound:  ...
1 comment
Read more

Experimental Techniques in Chemistry

Analytical Chemistry: The branch of chemistry that deals with the quantitative and qualitative analysis of substances is called Analytical Chemistry. Question 2.1: What is difference between quantitative and quantitative analysis? What are various steps involved in quantitative analysis? Answer:   Qualitative analysis: The analysis in which elements are identified is called qualitative analysis.  Quantitative analysis: The analysis in the form of which relative amounts of elements are determined is called quantitative analysis.  There are four main steps involved in quantitative analysis. 1. Obtaining a sample for analysis 2. Separation of desired constituent 3. Measurement and calculations of results 4. Drawing conclusions from analysis Question 2.2: Define filtration. How do we select the filter media? Answer:   The process of separating solids or suspended particles from liquids by using a filter medium is called filtration. Question...
11 comments
Read more

Alkyl halides

The compounds in which one hydrogen atom of alkane is replaced by a halogen atom is called alkyl halide. Alternatively, The halogen derivatives of alkanes with general formula R-X are Halide called alkyl halide. For example CH4  ( Alkane )            CH3-X  ( Alkyl halide )             X =   F, Cl, Br and I  Table Of Contents Types Mono, di, tri or poly haloalkane depending upon the number of halogen atom. For example C lassification of Alkyl Halides Alkyl halides can be classified as primary,  secondary and Tertiary alkyl halides. 1.  Primary Alkyl Halides The alkyl halides in which a halogen atom is bonded with primary carbon is called primary alkyl halide.  Primary carbon is the carbon that is attached  one or no carbon atom.    The Secondary Alkyl Halides The alkyl halides in which a halogen atom is bonded with secondary carbon ...
Post a Comment
Read more

CHAPTER # 15 ORGANIC COMPOUNDS

  ORGANIC COMPOUNDS Modern definition. C ompounds of carbon, hydrogen and their derivatives, are called organic compounds. For example, CH 4  , CH 3 Cl etc. Contents Sources of organic Compounds:  Following are the main sources of organic compounds: i. Fossil fuels or Fossil remains.  ii. Plants and Natural product  iii. Partial &Total Synthesis iv.  Fermentation and Biotechnology Fossil fuels or Fossil remains:   The main sources of fossil fuels or fossil remains are:  a. coal b. petroleum c. gas a. Coal:  Coal is one of the organic compounds. Coal major source of into coke, and coal law" on pyrolysis Com can be converted destructive distillation. More than 200 organic compounds" have been directly isolated from coal tar, which are used. in ...drug. dyes, other. preparation of perfumes.. photographic developers and. b. Petroleum: Petroleum Sticky Liquid ground in that is block Thick seeps out of the s...
Post a Comment
Read more

Shielding Effect (Grade 9 Students) and its trend in Periodic Table.

Question: What is shielding  effect? Give its trends in the periodic table. Answer . Shielding Effect  The decrease in force of attraction between nucleus  and valence electrons due to presence of inner shell electrons is called shielding effect.  It is also known as screening effect. It increases with increase in inner shell electrons. For example, Mg has greater shielding effect than Be. This is because Mg has eight more inner-shell electrons than Be. Trend in Periodic Table In period As we move from left to right in a period, shielding effect remains constant. Reason is that the number of inner shell electrons remains constant along the period. In group As we move from top to bottom in a group, shielding effect increases. This is due to fact that the number of inner shell increases and hence, inner shell electrons also increase. For example, Na has greater shielding effect than Li because of eight more inner shell electrons than Li...
Post a Comment
Read more

Class 9 Electrochemistry MCQs and Answers with explanation

1. In which of the following changes, the nitrogen atom is reduced? a) N 2 to NO b) N 2 to NO 2 c) N 2 to NH 3 d) N 2 to HNO 3 ... Answer is b) Explanation: Oxidation number of an elemnet in free state is zero. So Oxidation No. of N 2 = 0 Calculate oxidation No. of nitrogen in each compound. Option a) Let, oxidation No. of N in NO is X. Sum of oxidation numbers of all atoms present in a neutral compound is zero. Oxidation No. of N + (Oxidation No. of O ) = 0 X-2 = 0 X = +2 X = +2 Oxidation No. of N in NO 2 is +2 Option b) Let, oxidation state of N in NO 2 is X. Sum of oxidation numbers of all atoms present in a neutral compound is zero. Oxidation No. of N + 2(Oxidation No. of O ) = 0 X + 2(-2) = 0 X -4 = 0 X = +4 Oxidation No. of N in NO 2 is +4 Option c) Let, oxidation state of N in NH 3 is X. Sum of oxidation numbers of all atoms present in a neutral compound is zero. Oxidation No. of N + 3(Oxidation No. of H )...
Post a Comment
Read more

For All Topics Click here

Show more

Contact Form

Name

Email *

Message *