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Understanding Chemical Reaction Rates
Sep 24, 2024
Notes on Rate of Chemical Reactions
Definition of Reaction Rate
The rate of a chemical reaction is defined as the change in the concentration of a reactant or product over change in time.
Units: Concentration in moles per liter (molar) and time in seconds, leading to rate expressed in molar per second (mol/s).
Relationship to Physics
The concept of rate in chemistry is analogous to average velocity in physics.
Average velocity = Change in position (x) / Change in time.
Simple Reaction Example
Reaction Overview
Example: Reactant A turns into Product B.
Initial concentration of A at time = 0: 1 M (molar).
Initial concentration of B at time = 0: 0 M.
Time Interval
After 2 seconds:
Concentration of A: 0.98 M.
Concentration of B: 0.02 M (since 0.02 M of A has converted into B).
Calculation of Average Rate
For Product B:
Rate = (Final concentration of B - Initial concentration of B) / (Final time - Initial time)
Rate = (0.02 - 0) / (2 - 0) = 0.01 mol/s.
For Reactant A:
Rate = (Final concentration of A - Initial concentration of A) / (Final time - Initial time)
Rate = (0.98 - 1) / (2 - 0) = -0.02 / 2 = -0.01 mol/s.
In chemistry, rates are reported as positive, so:
Rate = -(-0.02) / 2 = 0.01 mol/s.
General Rate Expression
Rate can be expressed as:
Rate = - (Change in concentration of A / Change in time) = (Change in concentration of B / Change in time).
More Complex Reaction Example
Reaction Equation
Decomposition of dinitrogen pentoxide (N₂O₅) into nitrogen dioxide (NO₂) and oxygen (O₂).
Given Rate
Rate of formation of O₂ = 9 × 10⁻⁶ mol/s.
Calculating Rates for Other Products
For NO₂:
Mole Ratio: 4 moles NO₂ for every 1 mole O₂.
Rate of formation of NO₂ = 4 * (9 × 10⁻⁶) = 3.6 × 10⁻⁵ mol/s.
For N₂O₅:
Mole Ratio: 2 moles N₂O₅ lost for every 1 mole O₂ formed.
Rate of disappearance of N₂O₅ = -2 * (9 × 10⁻⁶) = -1.8 × 10⁻⁵ mol/s.
Expressing Overall Rate
The overall rate of reaction can be expressed in terms of:
Rate = Change in concentration of O₂ / Change in time = 9 × 10⁻⁶ mol/s.
Rate = Change in concentration of NO₂ / Change in time = 3.6 × 10⁻⁵ mol/s.
Rate = Change in concentration of N₂O₅ / Change in time = -1.8 × 10⁻⁵ mol/s.
Coefficient Patterns
Coefficients from the balanced equation dictate the relationship between the rates:
N₂O₅: Coefficient = 2 → Negative rate expression.
NO₂: Coefficient = 4 → Multiplier for rate expression.
O₂: Coefficient = 1 → Rate is directly expressed.
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