Difference WikiChemistry

Difference Between Order and Molecularity

Main Difference

The difference between Order and Molecularity is that the Order of a reaction is the algebraic sum of the powers to which the concentration of atoms is raised in a reaction, whereas Molecularity is the number of reactants taking part in an elementary reaction that is indicted by its chemical equation.

Order vs. Molecularity

Order of a reaction is the algebraic sum of the powers to which the concentration of atoms is raised in a reaction, while on the other hand, molecularity is the number of reactants taking part in a chemical reaction that is indicted by its chemical equation. Order of a reaction reduces a single reactant is in excess in a reaction, while on the other hand, in case of molecularity, there is no such dependency on reactants.

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The order of a reaction is generally 1, 2, or 3 or maybe zero or in a fraction or negative, but on the flip side, the molecularity of a reaction can always be in a natural number. Order of a reaction can only be determined by experimenting and cannot be predicted based on the balanced chemical equation, whereas the molecularity of a reaction can easily be predicted based on its balancing equation, and no complicated experimentation is required.

The order of a reaction may vary whenever a reaction is subjected under changing pressure, temperature, and concentration, etc., contrary to this, as molecularity, is just several reactants that are taking part in a chemical reaction which makes it irrelevant to these changes in the atmosphere and makes it invariant. For calculating the order of a reaction, there is no rate-determining step, and the overall reaction is used to calculate the order of a reaction; however, the rate-determining step is used to obtain the molecularity, and the overall reaction is not required.

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There are Zero-order, first-order, and second-order reactions that are classified based on order; however, based on molecularity, there are unimolecular reactions, bimolecular reactions, and trimolecular reactions. Order of a reaction applies to simple and elementary reactions both, whereas, molecularity is calculated for simple reactions only, there is no sign of calculating the molecularity of a complex reaction.

Comparison Chart

OrderMolecularity
The algebraic sum of the powers to which the concentration of atoms is raised in a reactionThe number of reactants taking part in a chemical reaction
Determination
Can only be determined by experimentingNo complicated experimentation is required
Dependence
Order of a reaction reduces if a single reactant is in excessMolecularity does not depend on the excessiveness of a reactant.
Value
Order of a reaction is generally 1, 2 or 3 or maybe zero or in fraction or negativeMolecularity of a reaction can always be in a natural number
Rate Determining Step
No rate-determining step and the overall reaction is used to calculate the order of a reactionThe rate-determining step is used to obtain the molecularity
Effect of the External Environment
The order of a reaction may vary whenever a reaction is subjected to changing pressure, temperature, and concentration, etc.Molecularity of a reaction is invariant to these changes in the atmosphere.
Classification of Reactions
Reactions are classified based on their order as

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· Zero-Order Reaction

· 1st Order Reaction

· 2nd Order Reaction

Reactions are classified based on their molecularity as

· Unimolecular Reaction

· Bimolecular Reaction

· Trimolecular Reaction

Application
Order of a reaction applies to a simple and elementary reactionMolecularity is calculated for simple reactions only there is no sign of calculating molecularity of a complex reaction

What is Order?

The rate order of a reaction is defined as the algebraic sum of the powers to which the concentration of atoms is raised in a reaction. It is also referred to as the power dependence of the rate of each reactant during the reaction. It is the sum of exponents of the rate law derived, and it may not depend on the stoichiometric coefficients of each reactant. Due to this reason, the rate of law is determined experimentally.

It is a quantitative measure regarding the rate of a reaction. The rate law is the equation raised. The order of a reaction is generally 1, 2, or 3 or maybe zero or in fraction or negative. The rate order of a reaction can only be resolute by experimenting and usually cannot be expected based on the balanced chemical equation. In an example like X + Y + Z → A + B + C, the rate law will be concluded as R = k [X] p [Y] q [Z] r .

Where R is the short form for the rate of reaction, X, Y, and Z are reactants, p, q, and r are orders of the reaction for X, Y, and Z. K here is considered as proportionality constant, and it mirrors the character of the reaction. Chemists discuss the sum of p, q, and r as the kinetic direction of the reaction. The order of these values is calculated experimentally.

The order of this reaction will be calculated by considering the overall reaction, unlike in molecularity, where rate-determining steps determine the molecularity. There are several types of reactions based on their order. There are Zero-order reactions in which the rate of a reaction does not rest on the concentration of reactants. First-order reactions are those who are reliant on the deliberation of reactants and correspond to a uni-molecular reaction. A one-second order reactant or two first order reactants are the second-order reactions that may hang on the deliberation of these two orders.

The order of a reaction may vary whenever a reaction is subjected to changing pressure, temperature, and concentration, etc. For calculating the order of a reaction, there is no rate-determining step, and the overall reaction is used to calculate the order of a reaction, and the order of a reaction applies to simple and elementary reactions both.

What is Molecularity?

Molecularity is the number of reactants participating in a chemical reaction that is indicted by its chemical equation. In the case of Molecularity, there is no dependency on the excessiveness of reactants as it was in the order of reactions. Molecularity of a reaction can always be in a natural number as it is the number of reactants in a particular reaction.

Molecularity of a reaction can easily be predicted based on its balancing equation, and no complicated experimentation is required. It does not depend upon the external environment as it is just a quantity of reactants that are taking part in a chemical reaction, which makes it irrelevant to these changes in the atmosphere and makes it invariant. The rate-determining step is used to obtain the molecularity, and the overall reaction is not required.

Based on molecularity, there are unimolecular reactions, bimolecular reactions, and trimolecular reactions. In unimolecular reactions, a single molecule undergoes amendments while reaction proceeds, and it has only one reactant and a single rate-determining step. In bimolecular reactions, two reactants are involved and are completing the reaction. However, in trimolecular reactions, three reactants are involved in the rate-determining step. Molecularity is just calculated for simple reactions; only there is no sign of calculating the molecularity of a complex or multi-step reaction.

Key Differences

  1. Order of a reaction is the algebraic sum of the powers for the concentration of atoms in a reaction, whereas molecularity is just the number of reactants taking part in a chemical reaction.
  2. Zero-order, first-order, and second-order reactions are classified based on order; however, based on molecularity, there are unimolecular reactions, bimolecular reactions, and trimolecular reactions.
  3. Order of a reaction reduces as if a single reactant is found in excess in a reaction, while on the other hand, in case of molecularity, there is no such dependency on reactants.
  4. The order of a reaction is generally 1, 2, or 3 or can be zero or infraction or negative, but on the other hand, the Molecularity of a reaction can always be in a natural number.
  5. The rate order of a reaction can only be determined by experimenting, whereas, the molecularity of a reaction can be predicted based on its balancing equation.
  6. The order of a reaction may vary whenever a reaction is subjected under changing pressure, temperature and concentration, etc., contrary to this, as molecularity is just several reactants that are taking part in a chemical reaction and are not affected by the external environment conditions.
  7. There is no rate-determining step, and the overall reaction is used to calculate the order of a reaction; however, the rate-determining step is used to obtain the molecularity.
  8. Order of a reaction applies to simple and complex reactions or reactions that are completed in multi-steps both, whereas, molecularity is calculated for simple reactions only.

Conclusion

Order of a reaction is the algebraic sum of the powers to which the concentration of atoms is raised in a reaction that can be in fraction or negative number, whereas, molecularity is the number of reactants taking part in an elementary reaction and are in neutral number.

Harlon Moss

Harlon currently works as a quality moderator and content writer for Difference Wiki. He graduated from the University of California in 2010 with a degree in Computer Science. Follow him on Twitter @HarlonMoss

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