Name: measuring reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 53 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Reaction Rate

Een chemische reactie omvat de chemische omzetting van reactanten in producten. Naarmate de reactie vordert, neemt de concentratie van reactanten af terwijl de concentratie van producten toeneemt. Dit verschil in concentraties van reactanten en producten kan als functie van de tijd in een grafiek worden uitgezet.De snelheid waarmee een reactie verloopt, wordt de reactiesnelheid genoemd. Het meet de snelheid waarmee reactanten verdwijnen of de snelheid van het verschijnen van producten en wordt uitgedrukt in eenheden van molariteit per seconde. De gemiddelde reactiesnelheid kan worden berekend uit de verandering in concentraties van reactanten of producten gedurende een bepaalde tijdsperiode.De molaire concentratiewaarde wordt tussen haakjes aangegeven, t staat voor de tijdsperiode en het deltasymbool vertegenwoordigt veranderen in'Omdat reactanten uitgeput raken tijdens een chemische reactie, is de waarde van de concentratieverandering van reactanten altijd negatief. Daarom wordt een reactiesnelheid berekend in termen van reactanten aangevuld met een negatief teken, om de totale waarde positief te maken. De reactiesnelheid is niet uniform tijdens een reactie.De snelheid aan het begin van de reactie, op tijd nul'wordt de initiële reactiesnelheid genoemd. Als de concentraties van reactanten afnemen, neemt de reactiesnelheid af of vertraagt de reactie. De reactiesnelheid op een bepaald tijdstip, of de momentane reactiesnelheid, wordt gemeten door de helling te berekenen van een raaklijn aan de reactiekromme, voor reactanten of producten, op het moment van interesse.De hellingswaarde of de momentane snelheid op een specifiek tijdstip is gelijk voor alle reactanten en producten. De reactiesnelheid van een chemische reactie weerspiegelt ook de werkelijke stoichiometrische coëfficiënten van reactanten en producten. Dus voor elke gebalanceerde reactie, waarbij een mol A reageert met b mol B en c mol C en d mol D produceert, kan de reactiesnelheid worden uitgedrukt en berekend met behulp van de algemene formule:minus 1/a maal verandering in concentratie van reactant A met de tijd, gelijk aan minus 1/b maal verandering in concentratie van reactant B met de tijd.Deze waarden zijn hetzelfde als plus 1/c maal, verandering in concentratie van product C met de tijd, wat hetzelfde is als plus 1/d maal verandering in concentratie van product D met de tijd. Het bepalen van de reactiesnelheden is fundamenteel voor het bestuderen van chemische kinetiek, wat helpt te begrijpen hoe snel een medicijn, katalysator of synthetische reactie werkt, zodat deze beter kan worden gecontroleerd en worden geoptimaliseerd in zijn functie.

reactiesnelheid

De reactiesnelheid is de verandering in de hoeveelheid van een reactant of product per tijdseenheid. Reactiesnelheden worden daarom bepaald door het meten van de tijdsafhankelijkheid van een eigenschap die verband kan houden met de hoeveelheden reactant of product. De reactiesnelheden waarbij gasvormige stoffen worden verbruikt of geproduceerd, kunnen bijvoorbeeld gemakkelijk worden bepaald door veranderingen in volume of druk te meten. De wiskundige weergave van de verandering in de concentratie van reactanten en producten in de loop van de tijd is de snelheidsuitdrukking voor de reactie. De haakjes geven de molaire concentraties aan, en het symbool delta (Δ) geeft ‘verandering in’ aan. De concentratie waterstofperoxide, H2O2, in een waterige oplossing verandert bijvoorbeeld langzaam in de loop van de tijd terwijl deze ontleedt volgens de vergelijking:  <img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;"> De snelheid waarmee waterstofperoxide ontleedt, kan worden uitgedrukt in termen van de snelheid waarmee de concentratie verandert:  <img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;"> [H2O2]t1 vertegenwoordigt dus de molaire concentratie van waterstofperoxide op tijdstip t1; op dezelfde manier vertegenwoordigt [H2O2]t2 de molaire concentratie van waterstofperoxide op een later tijdstip t2; en Δ[H2O2] vertegenwoordigt de verandering in de molaire concentratie van waterstofperoxide gedurende het tijdsinterval Δt (dat wil zeggen t2 − t1). Omdat de concentratie van de reactanten afneemt naarmate de reactie vordert, is Δ[H2O2] een negatieve grootheid. Reactiesnelheden zijn, volgens afspraak, positieve grootheden, en dus wordt deze negatieve verandering in concentratie vermenigvuldigd met −1. Gemiddelde reactiesnelheid en momentane reactiesnelheid Reactiesnelheden variëren met de tijd en nemen af naarmate de reactie vordert. Aan de hand van de concentraties aan het begin en het einde van deze periode, waarin de reactiesnelheid verandert, kan een gemiddelde reactiesnelheid over een tijdsinterval worden berekend. Op elk specifiek tijdstip staat de snelheid waarmee een reactie verloopt bekend als de momentane snelheid. De momentane snelheid van een reactie op ‘tijdstip nul’, wanneer de reactie begint, is de initiële snelheid. De momentane snelheid van een reactie kan op een van de twee manieren worden bepaald. Als experimentele omstandigheden het meten van concentratieveranderingen over korte tijdsintervallen mogelijk maken, dan levert de berekening van gemiddelde snelheden redelijk goede benaderingen op van momentane snelheden. Als alternatief kan een grafische procedure worden gebruikt. In het voorbeeld van de ontleding van waterstofperoxide kan bijvoorbeeld, door de concentratie waterstofperoxide tegen de tijd uit te zetten, de momentane ontledingssnelheid van H2O2 op elk moment ‘t’ worden berekend op basis van de helling van een raaklijn die op dat moment aan de curve wordt getrokken.  <img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;"> Deze grafiek toont een grafiek van de concentratie versus de tijd voor een 1.000 M oplossing van H2O2. De snelheid is op elk moment gelijk aan het negatief van de helling van een lijn die op dat moment de curve raakt. Raaklijnen worden weergegeven op t = 0 uur (“initiële snelheid”) en op t = 12 uur (“onmiddellijke snelheid” op 12 uur). Relatieve reactiesnelheden De snelheid van een reactie kan worden uitgedrukt als de verandering in de concentratie van een reactant of product. Voor elke gegeven reactie zijn deze snelheidsuitdrukkingen allemaal aan elkaar gerelateerd volgens de reactie-stoichiometrie. De snelheid van de algemene reactie aA ⟶ bB kan worden uitgedrukt in termen van de afname van de concentratie van A of de toename van de concentratie van B. Deze twee snelheidsuitdrukkingen houden verband met de stoichiometrie van de reactie, waarbij:  <img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;"> Merk op dat er een negatief teken is opgenomen als factor om rekening te houden met de tegengestelde tekenen van de twee hoeveelheidsveranderingen (de hoeveelheid reactant neemt af terwijl de hoeveelheid product toeneemt).

Reactiesnelheid

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time ...

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Chemical Kinetics

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KEY TERMS AND CONCEPTS

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measuring reaction rates

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts.

For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes over time. In this expression, brackets mean molar concentration, and the delta symbol means change in amount.

Hydrogen peroxide provides a clear example. As H2O2 decomposes in water, its concentration slowly decreases with time. The reaction rate can be written as the rate of change of the H2O2 concentration. If the concentration at time t1 is compared with the concentration at a later time t2, the change in concentration over the time interval Δt is found.

Because the reactant concentration goes down, Δ[H2O2] is negative. Reaction rates are reported as positive values, so the negative sign is reversed by multiplying by -1. This keeps the reaction rate positive even when the reactant amount is falling.

Reaction rates are not constant. They usually change as the reaction continues, so an average reaction rate is calculated over a time interval using the starting and ending concentrations. The rate at one exact moment is the instantaneous rate, and the instantaneous rate at time zero is the initial rate.

Instantaneous rate can be estimated in two ways. If concentration changes can be measured over very short time intervals, the average rate gives a good approximation. A graph can also be used. When concentration is plotted against time, the instantaneous rate at a given time equals the negative slope of a tangent line to the curve. In the hydrogen peroxide example, tangents can be drawn at t = 0 h for the initial rate and at t = 12 h for the instantaneous rate at 12 h.

The rate of a reaction can be written using either a reactant or a product. For a general reaction aA → bB, the decrease in A and the increase in B are linked by the reaction stoichiometry. The two rate expressions are related by that stoichiometry, and a negative sign is included to account for the reactant decreasing while the product increases.

Measuring Reaction Rates

reaction rate

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

Tracking Reaction Kinetics with Light and Pressure

Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time. Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes...

concentration and rate law

Reaction Order from Concentration

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

determining order of reaction

Finding Rate Law from Initial Rates

Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

the-integrated-rate-law-the-dependence-of-concentration-on-time

Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

Reaction Half-Life in Kinetics

Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

temperature dependence on reaction rate

Collision Theory and Reaction Speed

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

arrhenius plots

Determining Activation Energy from Temperature

The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

Elementary Steps in Reaction Mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

Enzyme Action and Inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

Learn how reaction rates are measured using concentration, volume, pressure, average rate, and instantaneous rate with the hydrogen peroxide example.

Measuring Reaction Rates | JoVE K12

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North Carolina K-12 Science Standard Course of Study

9-12

Matter and its Interactions

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Pennsylvania State Academic Standards

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Chemical Reactions

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Matter And Its Interactions

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

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New Jersey Student Learning Standards for Science

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Matter and Its Interactions: Energy

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New York State Science Learning Standards

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Measuring Reaction Rates

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