Rate of a Chemical Reaction — Revision Notes

⚡ 30-Second Revision

Definition: — Change in concentration of reactant/product per unit time.
Units: — $M s^{-1}$ or $mol,L^{-1}s^{-1}$.
Average Rate: — $\text{Rate}_{avg} = -\frac{\Delta[R]}{\Delta t} = +\frac{\Delta[P]}{\Delta t}$
Instantaneous Rate: — $\text{Rate}_{inst} = -\frac{d[R]}{dt} = +\frac{d[P]}{dt}$ (slope of tangent).
Stoichiometry: — For $aA + bB \rightarrow cC + dD$, $\text{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = +\frac{1}{c}\frac{d[C]}{dt} = +\frac{1}{d}\frac{d[D]}{dt}$.
Negative Sign: — For reactants, ensures rate is positive as $[R]$ decreases.

2-Minute Revision

The rate of a chemical reaction measures how quickly reactants are consumed and products are formed. It's expressed as the change in concentration per unit time, typically in $M s^{-1}$. We differentiate between average rate, which is the overall speed over a time interval ($\text{Rate}_{avg} = \frac{\Delta[C]}{\Delta t}$), and instantaneous rate, which is the speed at a specific moment ($\text{Rate}_{inst} = \frac{d[C]}{dt}$).

The instantaneous rate is graphically represented by the slope of the tangent to the concentration-time curve. When expressing rates, a negative sign is used for reactants ($-\frac{\Delta[R]}{\Delta t}$) because their concentration decreases, ensuring the rate value is positive.

For a balanced reaction $aA + bB \rightarrow cC + dD$, the overall reaction rate is normalized by stoichiometric coefficients: $\text{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = +\frac{1}{c}\frac{d[C]}{dt}$.

This ensures a consistent rate value regardless of the species monitored. Key for NEET: master calculations involving average rate, instantaneous rate from graphs, and stoichiometric relationships between rates of different species.

5-Minute Revision

The rate of a chemical reaction is a fundamental concept in chemical kinetics, quantifying the speed of a chemical change. It's defined as the change in concentration of a reactant or product per unit time, with standard units being $M s^{-1}$ (moles per liter per second).

There are two main types of rates: average rate and instantaneous rate. The average rate is calculated over a finite time interval, providing an overall speed: $\text{Rate}_{avg} = -\frac{\Delta[R]}{\Delta t}$ for reactants, and $+\frac{\Delta[P]}{\Delta t}$ for products.

For example, if $[A]$ changes from $0.6,M$ to $0.3,M$ in $60,s$, $\text{Rate}_{avg} = -\frac{(0.3-0.6),M}{60,s} = 0.005,M s^{-1}$. The instantaneous rate is the rate at a specific moment in time, determined by the slope of the tangent to the concentration-time curve at that point: $\text{Rate}_{inst} = -\frac{d[R]}{dt}$ or $+\frac{d[P]}{dt}$.

This gives a more accurate picture as reaction rates often change over time. A crucial aspect is relating the rates of different species in a balanced chemical equation. For $aA + bB \rightarrow cC + dD$, the overall reaction rate is expressed as $\text{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = +\frac{1}{c}\frac{d[C]}{dt} = +\frac{1}{d}\frac{d[D]}{dt}$.

This normalization ensures a single, consistent rate value. For instance, if $2X \rightarrow Y$, and $\frac{d[Y]}{dt} = 0.02,M s^{-1}$, then $-\frac{1}{2}\frac{d[X]}{dt} = 0.02,M s^{-1}$, so $-\frac{d[X]}{dt} = 0.

04,M s^{-1}$. Remember the negative sign for reactants is a convention to keep the rate positive. NEET questions frequently involve these calculations and conceptual distinctions.

Prelims Revision Notes

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Definition: — Reaction rate is the change in concentration of a reactant or product per unit time. It's a measure of reaction speed.
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Units: — Standard units are $M s^{-1}$ or $mol,L^{-1}s^{-1}$. For gaseous reactions, $atm,s^{-1}$ can be used.
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Average Rate:

* Calculated over a finite time interval ($\Delta t$). * For reactant R: $\text{Rate}_{avg} = -\frac{\Delta[R]}{\Delta t} = -\frac{([R]_{\text{final}} - [R]_{\text{initial}})}{\Delta t}$. * For product P: $\text{Rate}_{avg} = +\frac{\Delta[P]}{\Delta t} = +\frac{([P]_{\text{final}} - [P]_{\text{initial}})}{\Delta t}$. * The negative sign for reactants ensures the rate is a positive value.

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Instantaneous Rate:

* Rate at a specific moment in time ($t$). * Determined by the slope of the tangent to the concentration vs. time curve at that instant. * For reactant R: $\text{Rate}_{inst} = -\frac{d[R]}{dt}$. * For product P: $\text{Rate}_{inst} = +\frac{d[P]}{dt}$.

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Stoichiometric Relationship: — For a general balanced reaction $aA + bB \rightarrow cC + dD$, the overall reaction rate is:

$$\text{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = +\frac{1}{c}\frac{d[C]}{dt} = +\frac{1}{d}\frac{d[D]}{dt}$$

This equation is crucial for relating the rates of different species.

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Graphical Interpretation:

* Concentration vs. Time graph: Reactant concentration decreases, product concentration increases. * Slope of tangent at any point gives instantaneous rate. * Slope of secant line between two points gives average rate.

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Key Points for NEET:

* Always ensure rates are positive values. * Pay close attention to stoichiometric coefficients in calculations. * Understand the difference between average and instantaneous rates. * Be proficient in unit conversions if required. * Do not confuse reaction rate (kinetic property) with thermodynamic properties like $\Delta H$ or $\Delta G$.

Vyyuha Quick Recall

To remember the rate expression for $aA \rightarrow bB$: Reactants Reduce, Products Progress. Rate = -1/a $\Delta$[A]/$\Delta$t = +1/b $\Delta$[B]/$\Delta$t (Think 'R' for Reactant, 'R' for Reduce, 'R' for Ratio (1/a); 'P' for Product, 'P' for Progress, 'P' for Positive, 'P' for Ratio (1/b))