10.1
Rate Laws and Equilibrium Constants for Elementary Reactions
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Q1: What is molecularity and how does it affect reaction likelihood?
Molecularity is the number of molecules reacting in a single elementary step. Unimolecular reactions involve one molecule, bimolecular reactions involve two, and termolecular reactions involve three. Higher molecularity reactions are rarer because simultaneous collisions of multiple molecules are statistically unlikely.
Q2: How does the rate constant relate to temperature and concentration?
The rate constant, denoted by 'k', is a proportionality constant that is concentration-independent but temperature-dependent. It determines how fast a reaction proceeds at a given temperature. As temperature increases, the rate constant increases, causing the overall reaction rate to accelerate.
Q3: What is the relationship between rate constants and equilibrium constants?
For reversible elementary reactions, the equilibrium constant Kc equals the ratio of the forward reaction rate constant to the reverse reaction rate constant. When the forward rate constant far exceeds the reverse rate constant, Kc is much greater than one, indicating strong product favorability at equilibrium.
Q4: What defines the rate law for a chemical reaction?
The rate law expresses how the reaction rate depends on reactant concentrations. The reaction rate is proportional to reactant concentrations raised to their stoichiometric coefficients. This mathematical relationship allows prediction of how changes in concentration affect the speed at which reactants convert to products.
Q5: What are elementary steps and intermediates in reaction mechanisms?
Reactions proceed through multi-step mechanisms where each elementary step is a single process. Intermediates are species that appear only between successive elementary steps—they are produced in one step and consumed in a later step. Understanding elementary steps is essential for analyzing reaction mechanisms using the rate limiting step approximation.
Q6: Why are termolecular reactions less common than unimolecular or bimolecular reactions?
Termolecular reactions require three molecules to collide simultaneously, which is statistically improbable. Unimolecular and bimolecular reactions are far more common because they require fewer molecules to interact at once. The rarity of higher molecularity reactions reflects the low probability of multi-molecule collisions occurring together.
Q7: How do forward and reverse reaction rates relate at chemical equilibrium?
At equilibrium, the rates of forward and reverse reactions are equal, meaning reactants convert to products at the same rate as products convert back to reactants. This dynamic balance determines the equilibrium constant Kc, which reflects the ratio of forward to reverse rate constants at that temperature.