Chemical Kinetics PDF

### Chemical Kinetics **(3)** **Wurtz Fitting Reaction (Wurtz Fitting reaction):** - This is a reaction between an alkyl halide and aryl halide with sodium metal in dry ether to form a higher alkane and an arylalkane. **Chemical Kinetics:** - It is that branch of chemistry that deals with the study of rates and mechanisms of chemical reactions. **Based on the rate of reaction, Chemical reactions are generally divided into the following categories:** * **Instantaneous Reaction (Fast Rxm):** - These reactions occur at a very fast rate. - Therefore, their rate cannot be measured by normal methods. - These are usually ionic reactions. - **Examples:** - **Acid + Base -> Salt + H2O** (1012– 106 sec) - **NaCl(aq) + AgNO3(aq) -> AgCl(s) + NaNO3(aq)** (White ppt) * **Very Slow Reaction (Very slow Rxm):** - These reactions take a long time to complete. - Their rate is immeasurable, or it takes years to complete. - **Examples:** - Rusting of Iron, conversion of graphite to diamond, formation of a planet by the fusion of dust particles. * **Moderately Slow Reaction (Slow Rxm):** - These reactions take a definite time to complete. - Their rate can be measured. - These reactions occur between molecules. ### **Example** **CH3- S - CH3(aq) + H-O-O2S(aq) -> CH3 - S - O2HS (aq) + H2O** **(i) CH3Cl2(g)+ O2(g) -> COCl2(g) + 2HCl(g)** **(ii) C12H22O11(aq) + H2O(g) -> C6H12O6(aq) + C6H12O6(aq)** - (Glucose) (Fructose) **Rate of a chemical reaction:**- - It is the change in concentration of a reactant or product per unit time. Let us consider a reaction of the type, **R -> P** **Rate of Reaction (r) = Change in concentration of reactant or product/time taken for the change** **Rate of reaction (r) = - (ΔR/Δt) = + (ΔP/Δt)** - The negative sign is used for reactants because their concentration decreases with time, while the positive is used for products because their concentration increases. Let's consider that at time t1, the concentration is R1 and P1 and, at time t2, the concentration is R2 and P2. **Δt = t2 - t1, ΔR = R2 – R1, ΔP = P2 – P1 ** **Average Rate (average velocity):** - It is the change in molar concentration of a reactant or product per unit time in a given time interval. **Average Rate = [Change in concentration of reactant or product]/[Time interval]** **[Reactant]t2 – [Reactant]t1 / t2 - t1 = Average Rate** **[Product]t2 - [Product]t1 / t2 - t1 = Average Rate** #### **Note:** - The negative sign in the expression of the rate is just to indicate that the concentration of reactants is decreasing. - **The rate should always be positive** because rate indicates **how fast a reaction is occurring**. **Instantaneous Rate:** - The rate of a reaction at a particular instant of time, or, rate of reaction at a particular point on the concentration vs time curve. - The instantaneous rate of a reaction is represented as the slope of the tangent drawn at a particular point on the concentration vs time curve for the reactant. **Average rate of reaction:** - The average rate of reaction is represented as the slope of the secant drawn between the two points on the concentration vs time curve for the reactant. ### **Rate of disappearance of a reactant:** - It is the ratio of change in concentration with respect to change in time. - It can be calculated using the following formula: **V = - (ΔC/Δt)** #### **Note:** - In general **(in average rate)**: - Change in concentration of a substance is measured by **(C2 - C1)** , as given below: **Vav. = (ΔC/Δt)= (C2 - C1)/(t2 - t1)** - In the second case: [t1 =0 & t2 = t] **Vav. = (ΔC/Δt) = (C2 - C1)/ (t)** ### **Rate of formation of a product:** - It is the ratio of the change in concentration of the product with respect to the change in time. It can be calculated using the following formula: **V = (ΔC/Δt)** **Vav. = (ΔC/Δt) = (C2 - C1)/(t2 - t1)** - In the second case: [t1 =0 & t2 = t] **Vav. = (ΔC/Δt) = ** **C2 – C1)** / **t)** **Note:** - The negative sign is used for the rate of disappearance of a reactant - The positive sign is used for the rate of formation of a product because the concentration of reactants decreases with time, while the concentration of products increases with time. **Rate of disappearance of a Reactant** **-d(R)/ dt** **Rate of formation of a Product** **+d(P)/dt** **Average rate of reaction (Vav):** **-(ΔR/Δt) = + (ΔP/Δt)** **NOTE:** - The rate is always positive. - The rate of disappearance of reactants is always negative. - The rate of formation of products is always positive. - The rate of change of concentration is proportional to the rate of reaction. - The rate of reaction is directly proportional to the **slope of the tangent drawn at a particular point**. #### **Factors affecting the rate of reaction:** **(1)** **Concentration of reactants:** - In general, the rate of reaction increases with an increase in the concentration of reactants. - The rate is directly proportional to the product of the concentrations of reactants raised to some power, called **order of reaction**. **Note:** - For zero-order reactions the concentration of the reactants has no effect on the rate of a reaction. **(2)** **Physical state of reactants:** - The rate of reaction is influenced by the physical state of the reactants. - **A gas -> A liquid -> A solid** - The reactants are most reactive in gaseous phase, then in liquid phase, and least reactive in solid phase. **Reasons:** - **Larger surface area:** As the surface area of the reactant increases, the rate of reaction increases. **(3) Collision of reactants:** - For a reaction to proceed, the reactant particles need to collide. - This collision must take place with sufficient energy, referred to as activation energy. **(4)** **Effect of Temperature** - The rate of almost all reactions increases with an increase in temperature. - **Activation Energy (Ea):** - The minimum amount of energy that must be supplied to the reactants to start a chemical reaction **(5)** **Effect of Catalyst:** - Catalyst is a substance that alters the rate of a reaction but remains chemically unchanged at the end of the reaction. A catalyst **lowers the activation energy**. - A catalyst **increases the rate of a reaction** by providing an alternative pathway with a **lower activation energy** for the reaction. - **Positive Catalyst:** - A positive catalyst **increases the rate of a reaction**. - **Example:** - Iron catalyst **increases the rate of the Haber process** - **Negative Catalyst:** - A negative catalyst **decreases the rate of a reaction**. - It increases the activation energy thus reducing the rate of reaction. - Example: - **Antioxidants** used in food preservation are negative catalysts. **Rate Law:** - It states that the rate of a reaction is proportional to the product of the concentrations of the reactants, each raised to a power which is experimentally determined. **Rate = k [A]m [B]n** Where: - k is the rate constant. - [A] and [B] are the molar concentrations of reactants. - m and n are the orders of the reaction with respect to A and B respectively. - m + n is the overall order of the reaction. **Rate Constant (k):** - It is the proportionality constant in the rate law. - It represents the rate of the reaction when the concentration of each reactant is unity. - It **depends on temperature** and the order of reaction. **Difference between Rate Constant and Rate of Reaction:** | Feature | Rate Constant | Rate of Reaction | |---|---|---| | Meaning | Represents the **rate of the reaction when the concentration of each reactant is unity.** | Represents the **change in concentration of a reactant or product per unit time.** | | Dependence on Concentration | **Independent** of the concentration of reactants | **Dependent** on the concentration of reactants | | Dependence on Temperature | **Increases** exponentially with increasing temperature. | **Increases** with increasing temperature. | | Units | Varies depending on the order of the reaction | mol L-1 s-1 or mol L-1 min-1 | **Determination of Rate Constant from Experimentally Determined Rate Law and Time/Concentration Data** - **Step 1: Determine the order of the reaction.** - **Step 2: Determine the rate constant (k) from the experimental data.** - **Step 3: Determine the units of k.** **Units of Rate Constant:** - The units of the rate constant (k) depend on the order of the reaction. The order of the reaction is the sum of the exponents in the rate law for the reaction. - The units of k are (mol L-1)1-n s-1 , where n is the overall order of the reaction. **Units of Rate Constant for Different Orders:** - **Zero order** ⇒ mol L-1 s-1 - **First order** ⇒ s-1 - **Second order** ⇒ mol-1 L s-1 - **Third order** ⇒ mol-2 L2 s-1 **Note:** - The rate constant depends on the **temperature** and the **order of the reaction**. - It is **independent of the concentration of the reactants**. **The Relationship between Rate of Reaction and Rate Constant:** - The **rate of reaction** depends on **both the rate constant (k) and the concentration of reactants**, whereas the **rate constant k** is independent of the concentration of reactants. - The rate constant(k) is **temperature dependent**, while the rate of reaction is both **concentration and temperature dependent.** - The rate of reaction **doesn't depend on the order of the reaction** but **k depends on the order of the reaction.** **Units of rate of reaction:** - The units of the rate of reaction are typically **mol L-1 s-1** (moles per liter per second), **mol L-1 min-1 ** (moles per liter per minute), or **mol L-1 h-1** (moles per liter per hour). **Units of Rate Constant:** - The units of the rate constant (k) depend on the order of the reaction. The order of the reaction is the sum of the exponents in the rate law for the reaction. - The units of k are (mol L-1)1-n s-1 , where n is the overall order of the reaction. **Difference between Rate of Reaction and Rate Constant** | Feature | Rate of Reaction | Rate Constant | |---|---|---| | Definition | The change in concentration of a reactant or product per unit time. | The proportionality constant in the rate law. | | Dependence on Concentration | Dependent | Independent | | Dependence on Temperature | Dependent | Dependent | | Units | mol L-1 s-1 or mol L-1 min-1 | Varies depending on the order of the reaction | ### Summary of Key Concepts - The rate of a chemical reaction depends on the concentration of reactants, the temperature, and the presence of a catalyst. - The rate law expresses the relationship between the rate of a reaction and the concentration of reactants. - The rate constant (k) is a proportionality constant in the rate law. - The units of the rate constant depend on the order of the reaction.

Chemical Kinetics PDF

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