A2 Kinetics II.docx

**GYANI EDUCATION** **THE GOAT EDUCATION PLATFORM PRESENTS** **IAL CHEMISTRY Kinetics II** **KNOWLEDGE ORGANIZERS** ***Introduction:** Chemical kinetics is the measurement of how quickly reactions occur. If changes in conditions affect the speed of reaction, we can determine how the reaction hap...

**GYANI EDUCATION** **THE GOAT EDUCATION PLATFORM PRESENTS** **IAL CHEMISTRY Kinetics II** **KNOWLEDGE ORGANIZERS** ***Introduction:** Chemical kinetics is the measurement of how quickly reactions occur. If changes in conditions affect the speed of reaction, we can determine how the reaction happens. Kinetic studies are important in understanding reactions, and they also have practical implications. By understanding how a reaction takes place, many processes can be improved. It is important to know how long to hold the reaction at one stage before moving on, to make sure that reaction has finished before starting the next one. In addition, reagents might be added that would make certain steps in the reaction happen more easily.* **[Kinetic Rates:]** **Rate of reaction:** change in concentration of a **chemical species** (atom, molecule or and ion that is taking part in the chemical reaction) over a period of time taken for the change to occur in a chemical reaction. Simply **rate of change of product/reactant concentration**.  **Determine rate of reaction using experimental techniques:** rate of reaction of particular experiment can only be determined by conducting experiments and measuring: - Volume of gas given off - Change in mass - Change in concentration - Change in color - Change in electrical conductivity - Change in pH of a solution - Measuring any other physical properties that shows a significant and measurable change **Relationship of concentration and rate of reaction:** rate of reaction is directly proportional to concentration of chemical species in a reaction. **Example:**  **Observe:** - A directly proportional relationship between the rate of reaction and concentration of D - is observed when the results are plotted on a graph: - The graph represents Rate of reaction over various concentrations of D (**it is a rate-conc graph**) - This leads to an expression: **Rate∝ \[D\] or Rate=k\[D\]** - This rate expression means that if the concentration of **D is doubled, then the rate doubles**. Equally, if the concentration of **D halves, then the rate halves** **Calculating rate from gradient:** - From linear graph just calculate the gradient - For nonlinear graph by drawing tangent then calculating the gradient - Bigger the section of the triangle taken from the graph more accurate the calculated gradient will be. C6 D) Calculating Rate of Reaction from Graphs -- Edexcel Combined Science - Elevise **[Rate equation and orders:]** Rate equation: Rate equation **links with rate and concentration** of substances in a chemical reaction. It expresses the mathematical relationship between the rate of reaction and concentration of the reactants. *where **k** is a **rate constant** that is a number that allows us to equate rate and concentration. The **larger the value of k faster the rate** of reaction.* Rate constant for every type of reaction is **unique** and rate constant is not affected by any other factors **except temperature.** **Explaining temperature effect on k:** Because it is only fixed at a particular temperature. So, k increases and temperature increases and vice versa. Because as the temperature increases, the particles have more kinetic energy and they collide more often. This increases the rate but the concentration of the substances remains the same they don't change. **Order of reaction:** an order is the power to which a concentration is raised to in the rate equation. Shows how the concentration of the substance affects the rate. Orders can only be determined by experiments. You cannot work them out by looking into the equation. - **Zero order:** changes in concentration has no effect on rate. - **1st Order:** change in concentration has a proportional change on rate. - **2nd Order:** changes in concentration has a squared proportional change on rate. - **Overall order:** the sum of all the individual order. Overall order= m+n **Rate Law:** rate constants and the arrhenius equation - These **exponents are not related to the stoichiometric coefficient** from the chemical equation and must be **determined experimentally**. - **Reactants, products or catalyst can appear** in the rate equation but **NOT intermediates** formed in the multistep reaction. **Example:**    **[Determining rate equation from experimental data of initial rate:]**   **Calculating rate: rate can be calculated using rate equation.**  **Calculating rate constant: rate constant can be calculated using rate equation.**  **Complex example:** Some rate tables do not have a substance where the concentration remains constant. For example: A reaction occurs between A and B and the experiment was repeated 3 times varying the concentration of A and B. Give the rate equation for this reaction using the data below.  First work out the order with respect to A Ideally, we need to compare experiments where \[A\] is changing and \[B\] is constant. Experiment 1 and 2 \[A\] has trebled x3. The initial rate has increased x9. The order with respect to A is 2^nd^ order. Second work out order with respect to B. This is tricky as \[A\] changes in each experiment. **Add another column showing the change in initial rate only for A. Looking at Experiment 2 and 3, we see \[A\] doubles so rate must quadruple as we know it is 2^nd^ order with respect to A. This gives us a rate of 15.1x**[**10**^ **−** **3**^]{.math.inline} ***Finally, there is a difference in the rate so B must have an effect on the rate. You can see it is about half. This caused by halving \[B\] so 1^st^ order with respect to B.* Rate=k**[**\[A\]**^**2**^ **\[B\]**]{.math.inline} **[Reaction order graphs:]** - **Rate Graphs:** Rate-concentration graphs can be used to identify the order. - **Concentration-time graph:** graph that is created by conducting experiment and collecting aliquots at regular intervals and calculate their concentration by titration. - **Rate-concentration graph:** graph that is created using the value of rate. The value of rate can be found by taking the gradient at various points on a concentration-time graph. **ZERO ORDER:** Concentration is inversely proportional to time as a result over time the concentration decreases. The rate on a straight-line graph is constant in the rate-concentration graph. The rate-concentration graph shows a horizontal line. Changing concentration doesn't change the rate. **This is ZERO ORDER. Rate=k** this means the gradient will give you the value of rate as well as the value of rate constant in conc-time graph. **FIRST ORDER:** The rate on a shallow curve in conc-time graph changes in equal amounts because the concentration of reactant decreases over time. The rate-concentration graph shows a straight diagonal line passing through the origin. Changing concentration changes the rate equally. **This is FIRST ORDER. Rate=k\[X\] the rate increases proportionally with concentration and vice versa.** **SECOND ORDER:** The rate on a steep curve in conc-time graph changes in unequal amount because the concentration of reactants decreases more steeply over time. The rate-concentration graph shows a curved line. Changing concentration changes the rate squared. **This is SECOND ORDER. Rate=k**[**\[X\]**^**2**^]{.math.inline} **the rate increases more as the concentration increases and vice versa.** **Reaction order and half-life:** **Half-Life: Half-life (**[**t**~**1/2**~]{.math.inline}**)** is the time taken for **half of the reactants to be used up.** Half-life can be **calculated from concentration-time graph**.  **Example: Calculating half-life for the decomposition of Hydrogen peroxide.** **[Obtaining rate data using experimental methods:]** **Volume of gas given off:** - **Process:** two methods available: **the gas syringe method and the inverted measuring cylinder in water**. The gas syringe method involves measuring the amount of gas collected over a period of time. - **Suitability:** The **gas syringe method has a higher precision and lower degree of uncertainty because gas syringe has smaller resolution than a measuring cylinder.** Also, gases like carbon dioxide that are **soluble in water should be collected using a gas syringe**; otherwise, some gas will dissolve in water in the other technique. Volume of gas evolved-time graph can be plotted to determine the change in concentration over time that is the rate.   **Change in mass:** - **Process:** Applicable for a reaction that gives off a gas. Initially, the reaction mixture and conical flask are weighed on a digital balance and set to zero. As the reaction proceeds, the **value starts to decrease from zero and thus becomes negative**. The magnitude of the value shown after the reaction is complete gives the value of change in mass. - Suitability: This **method is suitable for a type of reaction where a gas of high Mr is given off**. If Mr gas, like hydrogen gas (Mr=2), is given off, then the **measured value is very small using a 2 or 3 dp balance**, and for that, the **percentage uncertainty will increase (absolute uncertainty remains the same). Mass loss-time graph can be plotted to determine the rate. Also, some liquid may be lost from the flask as spray. This can be overcome by putting a cotton wool on the flask mouth.**.  Chemistry Topic 6: Rate of reaction **Change in concentration:** - **Process:** This is done **by titrating aliquots collected at regular intervals**. Aliquots are small samples of the reaction mixture taken out of the reaction and placed in a separate labelled test tube, which undergoes titration. But before that, the **aliquots must be stopped to stop the reaction** or otherwise, the **aliquot sample will continue its reaction and the concentration will change**. In order to slow down a reaction, some methods include dilution with deionized water, cooling the sample, and adding chemicals **known as quenching agents** to stop the reaction. This process is known as **quenching**. Concentration-time graph is then plotted to determine the rate. - **Suitability:** This method is suitable when an acid, alkali or iodine is a reactant or products. Acid can be titrated with a standard alkali, alkalis with a standard acid and iodine with a standard solution of sodium thiosulfate. - **Safety:** Care must be taken that the quenching agent does not react with one of the reagents to form same products that is being measured. Frozen Aliquotters for Extracting Aliquots - NBS Scientific **Change in color:** If a reaction involves a change in color, this can be measured using  **colorimeter(required to know it's set up and function for Core practical 9)**. It measures the **absorbance of light by a colored sample**. The more concentrated a sample is, **the darker its color**, and hence the **lighter absorbed**. Light intensity is related to the concentration so we can plot light intensity -- time graph to determine the concentration. Also, a simpler method can be used that is the **clock experiment technique**, where the reaction vessel is place on a white paper with a cross and the time taken for the cross to disappear or appear depending on the color of the solution initially is measured. Note this method is not suitable for monitoring the formation of colored precipitates as the light will be scattered or blocked by the precipitate.  **Using Infrared spectroscopy:** Infrared spectroscopy can be used in a similar way to colorimetry. The spectrometer is set at a particular frequency and the amount of infrared radiation absorbed at that frequency is measured at regular time intervals. The oxidation of propan-2-ol to propanone by acidified potassium dichromate (VI) can be followed by setting the spectrometer at 1700 [cm^ − 1^]{.math.inline} (the absorption frequency die tp the stretching of the C=O bond) and the measuring the increase in absorption as the CHOH group is oxidized to the C=O group. **Change in Electrical conductivity:** **Process:** During a chemical reaction, there may be a change in the total number of ions present in the system. It can **increase or decrease the number of ions that will increase or decrease the overall electrical conductivity** because **the total charge changes Since Q=ne**. We can experimentally determine the change in number of ions present in the reaction over intervals and calculate the rate. **Suitability:** Reaction include ions in the reactant side or product side and overall number changes due to production or reduction of ions in the reaction. **Change in pH level:** The pH of a reaction mixture can change over time if **H+ ions get used up or produced**. A pH meter is used to measure the pH of the reaction mixture at regular intervals and measure the concentration of H+ ions. Measuring any other physical properties that shows a significant and measurable change: there are other physical properties like dilatometry, refractive index. Etc.  **[Initial Rate method:]** **Determines the rate at time t=0**. It is one of the procedure to calculate rate equation for a reaction by determining the order of reaction. We can run several trials of the experiment and vary the initial rate by changing the concentration of one reactant at a time.   **[The Clock Experiment:]** A clock reaction is measuring the time taken for a reaction to occur. The clock reaction method is used to simplify the initial rate method and more convenient approach to determine the initial rate using a single measurement. Essentially, the quicker the clock reaction, the faster the initial rate of reaction is. The time taken for a specific visual (observable) change in the reaction is measured. This method is ideal for color reaction. **As a result, the rate of the clock reaction is a good estimate of the initial rate of the reaction.** **3 assumptions made:** - The temperature of the reaction remains constant - The concentration of reactants doesn't change significantly during the time period of the reaction. - Reaction has not proceeded too far when the end point is seen. - Initial rate is proportional to [\$\\frac{1}{t}\$]{.math.inline} **Example: Iodine clock experiment.** Most reaction like iodine clock experiment is monitored by placing the reaction vessel on some paper with a cross mark. Time taken for the cross to be invisible is measured with a stopwatch. Therefore, the time taken for the color change to be monitored is measured. This is also called the end point. For more precision colorimetry device can be used to determine the color change.   **[Continuous Monitoring Method:]** **Continuous Monitoring method:** it involves **collecting experimental data throughout the course of reaction** to plot a concentration-time graph. Common methods include: measuring the volume of gas evolved, measuring the reactant mass loss over time, measuring the absorbance of color change using colorimetry. **When carrying out rate experiment:** - describe all the steps in the procedure - name all the apparatus - draw date table including headings or quantity and units - draw a graph showing labels, unit and best fit lines - determine an initial gradient or at any point in the curve **Core practical 9: Experiment of iodination of propanone**. It is a suitable experiment to demonstrate continuous monitoring method because rate of reaction can be measured using a colorimeter throughout the reaction. Reaction between propanone and iodine in **present of acid catalyst**. In this reaction there is a color change from **orange solution to colorless**. The orange color is due to presence of **aqueous iodine**. **Iodine** **decolorizes** as the reaction proceeds as it turn into **iodopropanone** and **hydrogen iodide.** Note: **absorbance of light is directly proportional to the concentration of colored solution**(Beer-lamberts law). Since iodine is orange in color. The device will be absorbing the complimentary color to orange and give us an arbitrary value. As the reaction proceeds the concentration of iodine decreases so the absorbance of light. **Procedure:** **Setting up the colorimetry device**: Turn it on and set the **dialer to absorbance** and then choose the suitable filter. Add plain solvent in the test tube and put it inside the colorimetry and **calibrate the absorbance to zero**. **Setting up the burettes:** We have **4 burettes**: water, followed by acid solution, iodine solution and lastly propanone solution. **Taking samples in beaker:** We have two separated beaker, one filled with propanone and the other beaker is field with water, acid solution and iodine solution. For accuracy, close the burette when the bottom of the meniscus sits on the line of the final volume. Add the iodine, acid plus water solution into the propanone beaker and **immediately start the stopwatch**. Then pour an amount of solution in an empty and clean test tube and put it inside the colorimetry device. Then measure the first absorbance when the time is 30 seconds. Followed by 60 seconds and over time by 30 seconds intervals.  **Tabulate the results:** this is one table; you repeat the whole process by varying the initial concentration of iodine to determine the rate of reaction. That will give us 4 set of results at same set of axis. With equal gradient. ***Meaning, the rate is unaffected by the change in concentration of iodine. So, the order of reaction with respect to iodine is zero.***        **[Rate determining steps from equations:]** **Rate determining Step (RDS): the rate determining step is the slowest step in a multi-step reaction**. Most reaction occurs via a few different steps and intermediates (**species that is found in elementary steps but not in the overall equation**) are formed. Out of these steps one is rate determining step or RDS in short. The whole reaction rate depends on how quick the rate determining step is. ***Rate law can be determined directly from the RDS and by this method the exponent (reaction order of each reactant concentration) will be equal to the stoichiometric coefficients of the reactants from the overall equation.*** **Example to understand RDS :** Baking a Victoria sponge cake can be seen as a multi-step process: - **Gather the ingredients (10 minutes)** - **Mix the ingredients (5 minutes)** - **Bake in the oven and cool afterwards (1 hour 30 minutes)** - **Dust with icing, add fillings (5 minutes)** Total time taken for this cooking is 1 hour 50 min. **The RDS is step 3** since it takes the maximum time.\ In order to increase the rate and decrease the overall time, we need to **speed up step 3** by let's say cooling in fridge, or fan to cool. ***Similarly in a chemical reaction, we have the same slow step that is our RDS. Except to speed up a step we could use a catalyst or change the temperature.*** In the rate equation, the reactants that appear affect the rate of reaction. These reactants (or substances derived from them) must appear in the rate determining step. **Substances not in the rate equation won't be in the Rate Determining Step.** It is not just the reactants that can appear in the rate equation but also the catalyst can as well. **Substance presents in the rate equation but not in the chemical equation is a catalyst.** **RDS and rate equation:** rate equation can be found from a **multi-step reaction**.  The RDS is step 2 because it is the **slowest step**. One of the reactant is C so this must appear in the rate equation. **Notice the 2 next to it this tells us the power is "2". In other words, it is in SECOND order with respect to C**. C is an **intermediate** which is **formed from (derived) A+B**. for this reason both A and B must be in the rate equation too. There are only 1 of A and B so the **order is FIRST order with respect to A and B.** **T**he rate equation can be deduced: [**Rate=**\[**A**\]\[**B**\]**\[C\]**^**2**^]{.math.inline} ***Note: C is not a reactant as it doesn't appear in the overall equation. C is a catalyst. This reaction is an example of autocatalysis (further in topic 15)*** **Rate determining step can be found from the rate equation:** Determine the ratio of molecules: 2 NO and 1 [*O*~2~]{.math.inline}. Looking at the reaction step: there isn't one step with this ratio. So, we need to start from step 1 and score off the molecules/ atoms needed to match the ratio in the rate equation. **We stop when we have accounted for them all**. **This step will be the rate determining step that is the step we stop at and we have accounted all of the molecules/atoms.** Step-1: We have 2NO, we score them off but we still have [*O*~2~]{.math.inline} to get. We continue to step 2...  Step-2: We have the [*O*~2~]{.math.inline} molecule in this step. We have now accounted for all molecules in the rate equation in the step 2 elementary reaction so we stop. **The RDS is step 2.**  **Rate determining step and reaction mechanism:** Reaction mechanism can be found from rate determining step. It is the slowest step in the mechanism. **Single step or Elementary Reaction:** a reaction taking place **in a single collision** between the **two reactant particles**. From an elementary reaction, rate law can be **directly deduced from the stoichiometric equation**. An elementary reaction is a type of chemical reaction in which the reactants directly form the products. **Multi-step reaction:** a non-elementary or complex reaction is one in which **intermediates form**, which go on to form the final products. Intermediates are species that appears in mechanism step but doesn't appear in the overall reaction since it is formed and used up in the reaction. **Mechanism**: A reaction that is not elementary takes place via **a series of interconnected elementary reactions** that are collectively called the mechanism for the reaction. Mechanism 1 is a singles step mechanism. Mechanism 2 is a multi-step mechanism. **RDS of Propanone and Iodine:**  **Key points from the given information:** - Iodine does not appear in the rate equation so it will not get involved in the reaction unitl AFTER the rate determining step. - Propanone and H+ do appear in the rate equation so they (or something made from it) will appear in the rate determining step. - H+ acts as a catalyst in this reaction so must be regenerated in a different step. H+ and propanone are both 1^st^ order. - The rate determining step must use 1 molecule each. Since the rate equation contains both of these reactants in step 1, they both must react first to produce an intermediate.  **Mechanism of Halogenoalkanes:** **Basics from AS:** Halogenoalkanes can be **hydrolyzed** (split) by hydroxide ions. They follow different mechanism depending if they are primary, secondary or tertiary.  **Bond Polarity and Nucleophiles**: **Halogenoalkanes have a polar bond and are attacked by nucleophiles.** Halogens are more electronegative than carbon so they pull electrons towards themselves in a covalent bond. This leads to a polar pond. This polar bond means halogenoalkane can be attacked by a nucleophiles. A nucleophile is a substance that is an electron pair donor. And example of nucleophile is a hydroxide ion.  **Reaction with Hydroxide ions:** Halogenoalkanes react with hydroxide ions via nucleophillic substitution. The conditions: Warm aqueous sodium hydroxide (source of [OH^−^]{.math.inline} ions) and carried out under reflux. Nucleophile will attack the ժ+ carbon and willl replace the halogen on the haloalkane. Hence substitution occurs. The C-X breaks, both electrons move from the bond to the halogen (**Heterolytic fission occurs**). A new bond is formed between the [OH^−^]{.math.inline} ions and carbon. Overall reaction: R-X + NaOH ROH + NaX (R=alkyl group and X= Halogen) **Types of mechanism for Halogenoalkane:** - [**S**~**N**~**2** (Substitution Nucleophillic Bimolecular)**:** **reaction** **mechanism** **with** **single** **step** **and** **goes** ]{.math.inline}**\ through transition state.**[ ]{.math.inline} **It includes two reactant species colliding together in the reaction which are in included in the rate determining step. Rate equation include two species:\ Rate= k\[A\]\[B\] and the overall order is second order. Therefore, the rate of reaction depends on the halogenoalkane (\[A\]) and on the nucleophile (\[B\]) this is why it is called S= Substitution, N= Nucleophilic, 2=2^nd^ order.** - [**S**~**N**~**1(Substitution** **Nucleophillic** **Unimolecular)**]{.math.inline}**: reaction mechanism is in 2 step and reactions only have 1 reactant species in the rate determining step**. It **includes intermediate** compounds, ions or radicals in one of the steps. Rate equation include one species:\ **Rate=k\[A\] and overall order is 1^st^ order.** Therefore, the rate of reaction only depends on the halogenoalkane (\[A\]) not on the nucleophile (\[B\]) this is why it is called S= Substitution, N= Nucleophilic, 1=1^st^ order. As there are two reactants but order with respect of the nucleophile is in zero order. **Degree of halogenoalkane and mechanism.** - **Primary halogenoalkane reacts via an** [**S**~**N**~**2**]{.math.inline} **mechanism.** - **Secondary halogenoalkane reacts via an** [**S**~**N**~**1** **and** **S**~**N**~**2**]{.math.inline} **mechanism.** - **Tertiary halogenoalkane reacts via** [an **S**~**N**~**1**]{.math.inline} **mechanism.** - **Rate of reaction with** [**S**~**N**~**2**]{.math.inline} **mechanism decreases in the order: Primary\>Secondary\>Tertiary because of the increasing steric hindrance by the alkyl groups on the attacking nucleophile.** - **Rate of reaction with** [**S**~**N**~**1**]{.math.inline} **mechanism increases in the order: Primary\