Rate of Reaction or Reaction Rate in chemistry is defined as the speed or the rate at which a chemical reaction takes place. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time. This can vary drastically. Chemical reactions proceed at extensively different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. 

Rate of Reaction: Instantaneous & Average Rate of Reaction, Definition, Formula, Factors Affecting, Units, Graph, Difference between Rate of Reaction and Rate Constant

by

Rate of Reaction or Reaction Rate in chemistry is defined as the speed or the rate at which a chemical reaction takes place. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time. This can vary drastically. Chemical reactions proceed at extensively different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. 

In this article, you will understand the meaning of rate of a chemical reaction, rate of reaction order, the unit of the rate of reaction, and formulas.

What is the Rate of Reaction

Rate of Reaction is the speed at which a chemical reaction occurs. A low-rate reaction means that the rearrangement of molecules by breaking old bonds and creating new bonds is slow. Some reactions can take hundreds of years to occur, whereas others can occur in less than a second. Consider how long it takes plants and ancient fish to become fossils if you want to conceive of an extremely slow reaction (carbonization). The Speed of Reaction is also affected by the type of molecules combined. The reaction will be slower if an essential element or compound is present in low concentrations.

Rate of Reaction Meaning

The Rate of Reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.

Rate of Reaction or Reaction Rate in chemistry is defined as the speed or the rate at which a chemical reaction takes place. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time. This can vary drastically. Chemical reactions proceed at extensively different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. 

Rate of Reaction Formula

The Rate of Reaction is proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit time. And can be defined as,

  • Rate of Reaction = Decrease in the concentration of a reactant R / Time Interval
  • Rate of Reaction = Increase in the concentration of a product P / Time interval

If we consider, [R1] and [P1] are the molar concentrations of the reactant and the product respectively at any time t1 and [R2] and [P2] are the concentrations of reactant and product at time t2, then changes in concentrations of the reactant and product will be d [R] = R2 – R1 and d [P] = P2 – P1 and time interval is dt = t2 – t1  and the rate of reaction in terms of reactant or product is given by

  • Rate of reaction = -(R2 – R1)/ (t2 – t1) = + (P2 – P1)/(t2 – t1)
  • Rate of reaction = – Δ[R]/Δt =+ Δ[P]/Δt.

Significance of Negative and Positive Signs

The sign in the rate of reaction tells about the increase and decrease in the concentration. 

  • Negative sign indicates that the concentration of the reactant is decreasing.
  • Positive sign indicates that the concentration of the product is increasing.

Expressing the Rate of Reaction in terms of different Reactants and Products 

 Let us consider a reaction ⇒ a A + b B ⇢  x X + y Y

Then the Rate of Reaction is given by

Rate = – 1/a d[A]/dt = – 1/b d[B]/dt = + 1/x d[X]/dt = + 1/y d[Y]/dt

where

  • d[A], d[B] represent small decrease in the concentrations of A and B respectively
  • d[X] and d[Y] represent small increase in the concentrations of X and Y respectively in the small interval of time dt

Average Rate of Reaction

From beginning to end the rate of reaction does not remain the same, it can vary from time to time. Therefore, the rate of reaction is defined as the ‘Average Rate of Reaction’.

Mathematically, the Average rate of reaction is given by, 

rav = -Δ[R]/Δt = +Δ[P]/Δt

Instantaneous Rate of Reaction

The Rate of Reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time. From the Average Rate of Reaction, we can understand it as when the change in the time interval is very less i.e. Δt→0 then the Rate of Reaction is termed as Instantaneous Rate of Reaction. Let us suppose that the small change in concentration is given dx in the small interval of time dt. Then the rate of reaction at that instant is given by dx/dt. It is given by the tangent to the curve of Concentration of Reactant/Product vs Time.

Mathematically, the Instantaneous Rate of Reaction is given by,

rinst = dx/dt

rinst  = -Δ[R]/Δt as Δt ⇢ 0 = -d[R]/dt = -slope

rinst  = +Δ[P]/Δt as Δt ⇢ 0 = +d[P]/dt = +slope     

Rate of Reaction or Reaction Rate in chemistry is defined as the speed or the rate at which a chemical reaction takes place. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time. This can vary drastically. Chemical reactions proceed at extensively different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. 

Unit of Rate of Reaction

The Rate of Reaction, in general, can be measured as a change in concentration of reactant or product where concentration is denoted in moles/liter and time in seconds or minutes. 

So the Unit of Rate of Reaction = mol L-1 s-1 or mol L-1 min-1.

Factors Affecting Rate of Reaction

There are various factors that affect the reaction rate which are listed below:

  • Reactant Concentration
  • Order of Reaction
  • Nature of Reactant
  • Pressure
  • Temperature
  • Solvent
  • Electromagnetic Radiation
  • Presence of Light
  • Presence of Catalyst
  • Surface Area
  • Activation Energy

Reactants Concentration 

From the Collision Theory, we know that the more the reactant more will be the collision and hence more will be the reaction, hence Rate of Reaction is directly proportional to the concentration of Reactants. Also as the concentrations of the reactants decrease, the rate of reaction decreases. Hence, the Rate of Reaction for a given reaction in terms of concentration of Reactant can be given as

aA + bB ⇢ cC + dD

Rate ∝ [A]x [B]y

Rate = k[A]x [B]y

where 

  • k is the Rate Constant 
  • x and y may or may not be equal to the coefficient of the reactant.

The above representation of the Rate of Reaction in terms of concentration of Reactant is called Rate Law.

Order of Reaction

In the above reaction, the sum of x and y is called as Order of Reaction. Say for example if (x + y = 0) then the Order of Reaction is Zero and the Rate of Reaction is independent of the concentration of the reactants.

Nature of Reactant

The Rate of Reaction is dependent on the Nature of the Reactant in the manner that the reaction happens fastest if the Reactants are in Gaseous Phase, slower in Liquid Phase, and slowest in the Solid phase.

Pressure

The effect of Pressure is applicable to the reactants in the gaseous phase. The effect is pressure is the same as the effect of concentration in a reaction. High Pressure of gas means higher concentration and hence the rate of reaction increases.

Temperature

The rate of reaction increases with an increase in the temperature. In many cases, the Rate Constant of reaction becomes nearly double for a 10° rise in temperature. However, an accurate explanation of the dependence on temperature was given by Arrhenius. He gave the below expression for the dependence of the rate constant on temperature

k = A -Ea/RT

where,

  • k is Rate Constant
  • A is the Pre-Exponential Factor or the Frequency Factor
  • Ea is Activation Energy
  • R is Gas Constant 
  • T is Temperature

Solvent

Solvent provides the medium for the solute to dissolve. A higher concentration of solute in the solvent will increase the rate of reaction. 

Electromagnetic Radiation

Electromagnetic Radiation provides external energy which increases the rate of reaction.

Presence of light

Some reactions do not take place in the dark but can proceed in the presence of light. For Example: H2 + Cl2 ⇢ 2 HCl. This reaction is known as a “photochemical reaction.”

Presence of Catalyst

The main purpose of the catalyst is to increase the rate of reaction without itself involving in the reactions. So the catalyst increases the reaction rate. A catalyst reduces the activation energy barrier and hence provides an alternate path for the reaction to happen.

Surface area

Greater surface area means more collision to take place and hence, greater is the rate of reaction.

Activation Energy

Activation Energy refers to the minimum amount of energy possessed by the reactant to proceed with the reaction. When the molecules of reactant collide they form an intermediate, to form this intermediate minimum amount of energy is required. This minimum energy is called Activation Energy and the intermediate so formed is called Activated Complex. The Rate of Reaction depends on the Activation Energy in the manner that if the Activation Energy is high then the Rate of Reaction will be low and vice versa. Hence, Activation Energy and Rate of Reaction are inversely related to each other.

Learn more about, Factors Affecting Rate of a Chemical Reaction

Difference between Rate of Reaction and Rate Constant

The difference between the Rate of Reaction and Rate Constant is tabulated below:

Rate of ReactionRate Constant
It is the change in the concentration of reactants or the change in the concentration of products per unit of time.The rate constant is the proportionality constant related to the rate of a particular reaction. 
It depends on the molar concentration of Reactants and Products.It doesn’t depend on the molar concentration of Reactants and Products.
It indirectly depends on the temperature. This directly depends on the temperature 
It is time-dependent.It is time-independent. 

FAQs on Rate of Reaction

What is Rate of Reaction Meaning?

The rate of reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.

What is Rate Law?

Rate law refers to the representation of the Rate of Reaction in terms of the concentration of Reactants where the Rate of Reaction is directly proportional to the concentration of the Reactant species each raised to some power which may or may not be equal to the coefficient of the reactants.

What is the Unit of Rate of Reaction?

The Unit of Rate of Reaction is molL-1s-1

What is the Difference between Average and Instantaneous Rate of Reaction?

Instantaneous Rate of Reaction: The rate of reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time.

Average Rate of Reaction: It is the change is concentration of reactant or product over a time period.

How is Instantaneous Rate of Reaction calculated?

The Instantaneous Rate of Reaction can be calculated by taking tangent to the curve at that particular instant of time.

What is Order of Reaction?

In Rate law, the sum of powers of the concentration of the reactants is called as Order of Reaction. It basically tells the Rate of Reactants is dependent on concentration of how many reactants. For Example, in Zero Order Reaction rate of reaction is independent of the concentration of reactant, in case of First Order reaction, rate is determined by concentration of one of the reactant.

How is Rate of Reaction dependent upon Temperature?

The Rate of Reaction get doubles if temperature is increased by 10°, however accurate dependence is given by Arrhenius Equation given by k = A -Ea/RT

How is the Rate of Reaction dependent upon Activation Energy?

Rate of Reaction and Activation Energy are inversely related to each other. Higher Activation Energy will lead to a lower Rate of Reaction and Vice Versa.

What is the Rate of Reaction Formula?

  • Rate of Reaction = Decrease in the concentration of a reactant R / Time Interval
  • Rate of Reaction = Increase in the concentration of a product P / Time interval

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.

...

CBSE Class 12 Chemistry Syllabus Download PDF

Below is the CBSE Class 12 Syllabus along with the marking scheme and time duration of the Chemistry exam.

S.NoTitleNo. of PeriodsMarks
1Solutions107
2Electrochemistry129
3Chemical Kinetics107
4d -and f -Block Elements127
5Coordination Compounds127
6Haloalkanes and Haloarenes106
7Alcohols, Phenols and Ethers106
8Aldehydes, Ketones and Carboxylic Acids108
9Amines106
10Biomolecules127
Total70

CBSE Class 12 Chemistry Practical Syllabus along with Marking Scheme

The following is a breakdown of the marks for practical, project work, class records, and viva. The total number of marks for all parts is 15. The marks for both terms are provided in the table below.

Evaluation Scheme for ExaminationMarks
Volumetric Analysis08
Salt Analysis08
Content-Based Experiment06
Project Work and Viva04
Class record and Viva04
Total30

CBSE Class 12 Chemistry Syllabus (Chapter-wise)

Unit -1: Solutions

  • Raoult's law.
  • Colligative properties - relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, osmotic pressure, determination of molecular masses using colligative properties, abnormal molecular mass.
  • Solutions, Types of solutions, expression of concentration of solutions of solids in liquids, solubility of gases in liquids, solid solutions.
  • Van't Hoff factor.

Unit -2: Electrochemistry

  • Redox reactions, EMF of a cell, standard electrode potential
  • Nernst equation and its application to chemical cells
  • Relation between Gibbs energy change and EMF of a cell
  • Kohlrausch's Law
  • Electrolysis and law of electrolysis (elementary idea)
  • Dry cell-electrolytic cells and Galvanic cells
  • Conductance in electrolytic solutions, specific and molar conductivity, variations of conductivity with concentration.
  • Lead accumulator
  • Fuel cells

Unit -3: Chemical Kinetics

  • Rate of a reaction (Average and instantaneous)
  • Rate law and specific rate constant
  • Integrated rate equations and half-life (only for zerfirst-order order reactions)
  • Concept of collision theory (elementary idea, no mathematical treatment)
  • Factors affecting rate of reaction: concentration, temperature, catalyst;
  • Order and molecularity of a reaction
  • Activation energy
  • Arrhenius equation

Unit -4: d and f Block Elements  

  • Lanthanoids- Electronic configuration, oxidation states, chemical reactivity and lanthanoid contraction and its consequences.
  • Actinoids- Electronic configuration, oxidation states and comparison with lanthanoids.
  • General introduction, electronic configuration, occurrence and characteristics of transition metals, general trends in properties of the first-row transition metals – metallic character, ionization enthalpy, oxidation states, ionic radii, color, catalytic property, magnetic properties, interstitial compounds, alloy formation, preparation and properties of K2Cr2O7 and KMnO4.

Unit -5: Coordination Compounds  

  • Coordination compounds - Introduction, ligands, coordination number, color, magnetic properties and shapes
  • The importance of coordination compounds (in qualitative analysis, extraction of metals and biological system).
  • IUPAC nomenclature of mononuclear coordination compounds.
  • Bonding
  • Werner's theory, VBT, and CFT; structure and stereoisomerism

Unit -6: Haloalkanes and Haloarenes  

  • Haloarenes: Nature of C–X bond, substitution reactions (Directive influence of halogen in monosubstituted compounds only). Uses and environmental effects of - dichloromethane, trichloro methane, tetrachloromethane, iodoform, freons, DDT.
  • Haloalkanes: Nomenclature, nature of C–X bond, physical and chemical properties, optical rotation mechanism of substitution reactions.

Unit -7: Alcohols, Phenols and Ethers   

  • Phenols: Nomenclature, methods of preparation, physical and chemical properties, acidic nature of phenol, electrophilic substitution reactions, uses of phenols.
  • Ethers: Nomenclature, methods of preparation, physical and chemical properties, uses.
  • Alcohols: Nomenclature, methods of preparation, physical and chemical properties (of primary alcohols only), identification of primary, secondary and tertiary alcohols, mechanism of dehydration, and uses with special reference to methanol and ethanol.

Unit -8: Aldehydes, Ketones and Carboxylic Acids   

  • Carboxylic Acids: Nomenclature, acidic nature, methods of preparation, physical and chemical properties; uses.
  • Aldehydes and Ketones: Nomenclature, nature of carbonyl group, methods of preparation, physical and chemical properties, mechanism of nucleophilic addition, the reactivity of alpha hydrogen in aldehydes, uses.

Unit -9: Amines    

  • Diazonium salts: Preparation, chemical reactions and importance in synthetic organic chemistry.
  • Amines: Nomenclature, classification, structure, methods of preparation, physical and chemical properties, uses, and identification of primary, secondary and tertiary amines.

Unit -10: Biomolecules     

  • Proteins -Elementary idea of - amino acids, peptide bond, polypeptides, proteins, structure of proteins - primary, secondary, tertiary structure and quaternary structures (qualitative idea only), denaturation of proteins; enzymes. Hormones - Elementary idea excluding structure.
  • Vitamins - Classification and functions.
  • Carbohydrates - Classification (aldoses and ketoses), monosaccharides (glucose and fructose), D-L configuration oligosaccharides (sucrose, lactose, maltose), polysaccharides (starch, cellulose, glycogen); Importance of carbohydrates.
  • Nucleic Acids: DNA and RNA.

The syllabus is divided into three parts: Part A, Part B, and Part C. Part A consist of Basic Concepts of Chemistry, which covers topics such as atomic structure, chemical bonding, states of matter, and thermochemistry. Part B consists of Topics in Physical Chemistry, which includes topics such as chemical kinetics, equilibrium, and electrochemistry. Part C consists of Topics in Organic Chemistry, which covers topics such as alkanes, alkenes, alkynes, and aromatic compounds.

Basic Concepts of Chemistry:

  • Atomic structure: This section covers the fundamental concepts of atomic structure, including the electronic configuration of atoms, the Bohr model of the atom, and the wave nature of matter.
  • Chemical bonding: This section covers the different types of chemical bonds, including ionic, covalent, and metallic bonds, as well as the concept of hybridization.
  • States of the matter: This section covers the three states of matter - solid, liquid, and gas - and the factors that influence their properties.
  • Thermochemistry: This section covers the principles of thermochemistry, including the laws of thermodynamics and the concept of enthalpy.

Chapters in Physical Chemistry:

  • Chemical kinetics: This section covers the study of the rate of chemical reactions and the factors that influence it, including the concentration of reactants, temperature, and the presence of catalysts.
  • Equilibrium: This section covers the principles of chemical equilibrium, including the concept of Le Chatelier's principle and the equilibrium constant.
  • Electrochemistry: This section covers the principles of electrochemistry, including the concept of half-cell reactions, galvanic cells, and electrolysis.

Chapters in Organic Chemistry:

  • Alkanes: This section covers the properties and reactions of alkanes, including their structure, isomerism, and combustion.
  • Alkenes: This section covers the properties and reactions of alkenes, including their structure, isomerism, and addition reactions.
  • Alkynes: This section covers the properties and reactions of alkynes, including their structure, isomerism, and addition reactions.
  • Aromatic compounds: This section covers the properties and reactions of aromatic compounds, including their structure, isomerism, and electrophilic substitution reactions.

In addition to the topics covered in the syllabus, the CBSE Class 12 Chemistry exam also tests students on their analytical and problem-solving skills, as well as their ability to apply the concepts learned in the classroom to real-world situations.

Students can also check out the Tips for the Class 12 Chemistry Exam. They can easily access the Class 12 study material in one place by visiting the CBSE Class 12 page at ANAND CLASSES (A School Of Competitions). Moreover, to get interactive lessons and study videos, download the ANAND CLASSES (A School Of Competitions) App.

Frequently Asked Questions on CBSE Class 12 Chemistry Syllabus

Q1

How many chapters are there in the CBSE Class 12 Chemistry as per the syllabus?

There are 10 chapters in the CBSE Class 12 Chemistry as per Syllabus. Students can learn all these chapters efficiently using the study materials provided at ANAND CLASSES (A School Of Competitions).

Q2

What is the marking scheme for CBSE Class 12 Chemistry practical exam according to the syllabus?

The marking scheme for CBSE Class 12 Chemistry practical exam, according to the syllabus, is 8 marks for volumetric analysis, 8 marks for salt analysis, 6 marks for the content-based experiment, 4 marks for the project and viva and 4 marks for class record and viva.

Q3

Which is the scoring chapter in Chemistry as per CBSE Class 12 syllabus?

The chapter Electrochemistry in Chemistry is the scoring chapter as per CBSE Class 12 syllabus.

© 2025 Anand Classes • Published by Anand Technical Publishers
Privacy Policy Terms Contact

Sitemap