Name: reaction order from concentration
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Concentration and Rate Law

קצב התגובה תלוי לעיתים קרובות בריכוזי המגיב. בכל תגובה, הקשר בין קצב התגובה לריכוזי המגיב יכול לבוא לידי ביטוי מתמטי תוך שימוש בחוק הקצב או במשוואת קצב. במשוואת הקצב, k הוא ה-קבוע הפרופורציה, או קבוע הקצב, ו n הוא סדר התגובה ביחס למגיב יחיד, שערכו הוא לעתים קרובות מספר שלם.במשוואת קצב עבור תגובות מרובות מגיבים, סדר התגובה הכללי הוא סכום כל סדרי המגיבים. עבור כל מגיב, קצב התגובה, קבוע הקצב, ריכוז וסדר התגובה, נקבעים נסיונים. משוואת הקצב מבטאת את הקשר בין כל הפרמטרים הללו.הסדר של מגיבים אינדיבידואלים הוא בדרך כלל בערך של 0, 1 או 2, ועל בסיס סדר התגובה הכולל, ניתן לקטלג תגובות כימיות כתגובות בעלות סדר אפס, סדר ראשון או שני. תגובה של מגיב יחיד, או תגובה כימית אונימולקולרית, שבה קצב התגובה נשאר קבוע לכל אורכה היא תגובת מסדר אפס. סדר המגיב בתגובה סדר אפס הוא אפס, ובהתאם למשוואת הקצב, ריכוז המגיב עולה בחזקת אפס.כיוון שהערך של כל מספר בחזקת אפס הוא 1, קצב התגובה של תגובה מסדר אפס שווה לקצב הקבוע ולכן הוא אינו תלוי בריכוז המגיב. לכן בתגובה מסדר אפס, גם כשריכוז המגיב יורד, קצב התגובה לא מאט. תגובה כימית אונימולקולרית שבה קצב התגובה פרופורציונלי באופן ישיר לריכוז המגיב היא תגובה מסדר ראשון.סדר המגיב בתגובה מסדר ראשון הוא 1, ולפי משוואת הקצב, ריכוז המגיב עולה בחזקת 1. כיוון שהערך של כל מספר שמועלה בחזקת אחד לא משתנה, קצב התגובה בתגובה מסדר ראשון תלוי באופן ישיר בריכוז המגיב. ככל שריכוז המגיב יורד, קצב התגובה יורד באופן פרופורציונלי באופן ליניארי.תגובה כימית אונימולקולרית שבה קצב התגובה תלוי בריבוע הריכוז של המגיב היא תגובה מסדר שני. סדר המגיב הוא 2 וריכוז המגיב עולה בחזקה שנייה. בהתאם לכך, קצב התגובה בתגובה מסדר שני תלוי באופן ישיר בריבוע של ריכוז המגיב.ככל שריכוז המגיב יורד, קצב התגובה יורד באופן מעריכי ריבועי.

חוק ריכוז וקצב

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קצב התגובה מושפע מריכוזי המגיבים. חוקי קצב (חוק קצב דיפרנציאלי) או משוואות קצב הם ביטויים מתמטיים המתארים את הקשר בין קצב תגובה כימית לריכוז המגיבים שלה. 

לדוגמה, בתגובה גנרית aA + bB⟶ תוצרים, כאשר a ו-b הם מקדמים סטוכיומטריים, ניתן לכתוב את חוק הקצב כך:
<img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;"> 

[A] ו-[B] מייצגים את הריכוזים המולריים של מגיבים, ו-k הוא קבוע הקצב, שהוא ספציפי לתגובה מסוימת בטמפרטורה מסוימת.

המעריכים m ו-n הם סדרי התגובה והם בדרך כלל מספרים שלמים חיוביים, אם כי הם יכולים להיות שברים, שליליים או אפס.

קבוע הקצב k וסדרי התגובה m ו-n נקבעים בניסוי על ידי התבוננות כיצד קצב התגובה משתנה עם שינוי ריכוזי המגיבים. קבוע הקצב k אינו תלוי בריכוזי המגיבים, אך משתנה בהתאם לטמפרטורה.

סדרי התגובה בחוק קצב מתארים את התלות המתמטית של הקצב בריכוזי המגיבים. בהתייחס לחוק הקצב הגנרי (קצב = k[A]m[B]n), התגובה היא מסדר m ביחס ל-A וסדר n ביחס ל-B. לדוגמה, אם m = 1 ו-n = 2, התגובה היא מסדר ראשון ב-A וסדר שני ב-B. סדר התגובה הכולל הוא פשוט סכום הסדרים עבור כל מגיב. עבור חוק הקצב לדוגמה , התגובה היא מסדר שלישי בסך הכל (1 + 2 = 3). 

גישה ניסיונית נפוצה לקביעת חוקי הקצב היא שיטת הקצב הראשון. שיטה זו כוללת מדידת קצב התגובה עבור ניסויים מרובים המבוצעים באמצעות ריכוזי מגיבים ראשוניים שונים. השוואת הקצבים הנמדדים עבור ניסויים אלה מאפשרת קביעת סדרי התגובה ולאחר מכן, קבוע הקצב, המשמשים יחד לגיבוש חוק הקצב. 

חוקי קצב עשויים להציג סדרים חלקיים עבור מגיבים מסוימים, ולעתים נצפים סדרי תגובה שליליים כאשר עלייה בריכוז של מגיב אחד גורמת לירידה בקצב התגובה. חשוב לציין שחוקי הקצב נקבעים על ידי ניסוי בלבד ואינם צפויים בצורה מהימנה על ידי סטוכיומטריה של התגובה. 

סדר התגובה קובע את הקשר בין קצב התגובה לריכוז המגיבים או התוצרים. 

<ul style="padding-right: 35px; list-style-type: disc;">
<li style="text-align: right;">בתגובה מסדר אפס, לריכוז המגיבים אין כל השפעה על קצב התגובה, שנשאר קבוע לאורך כל הדרך.</li> 
<li style="text-align: right;">בתגובה מסדר ראשון, קצב התגובה הוא פרופורציונלי ישיר ולינארי לשינוי בריכוז המגיבים. ככל שריכוז המגיבים יורד, גם קצב התגובה יורד באופן פרופורציונלי.</li>
<li style="text-align: right;">בתגובות מסדר שני או מסדר גבוה יותר, קצב התגובה פרופורציונלי לערך האקספוננציאלי של המגיבים. לכן, ככל שהתגובה מתקדמת וריכוז המגיבים יורד, קצב התגובה יורד באופן אקספוננציאלי.</li> </ul>

טקסט זה מותאם מ<a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033">Openstax, Chemistry 2e, Section 12.3: Rate Laws</a>.</div>

קצב התגובה מושפע מריכוזי המגיבים. חוקי קצב (חוקי קצב דיפרנציאליים) או משוואות קצב הם ביטויים מתמטיים ...

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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 are the reaction orders. They are usually positive integers, but they can also be fractions, negative numbers, or zero. The values of k, m, and n are found experimentally by measuring how the rate changes when reactant concentrations change.

The rate constant does not depend on the reactant concentrations, but it can change with temperature. The reaction orders show the mathematical link between concentration and rate. In the expression rate = k [A]m [B]n, the reaction is m order with respect to A and n order with respect to B. For example, if m = 1 and n = 2, the reaction is first order in A and second order in B.

The overall reaction order is the sum of the orders for each reactant. In the example above, the reaction is third order overall because 1 + 2 = 3. A common way to find a rate law is the method of initial rates. In this method, reaction rates are measured in several trials that start with different initial reactant concentrations. Comparing those rates makes it possible to find the reaction orders and then the rate constant.

Rate laws can also include fractional orders. In some cases, a reactant may have a negative order. That means increasing its concentration can lower the reaction rate. Rate laws are determined by experiment only, and they cannot be reliably predicted from reaction stoichiometry. The reaction order also describes how the rate changes as reactant or product concentration changes.

A zero-order reaction has a rate that does not change with reactant concentration. The rate stays constant throughout the reaction. In a first-order reaction, the rate is directly proportional to the change in reactant concentration. As the reactant concentration drops, the rate drops by the same proportion. In second-order or higher-order reactions, the rate depends on the reactants raised to an exponent. As the reaction continues and reactant concentration decreases, the rate falls exponentially.

Reaction Order from Concentration

reaction rate

Measuring Reaction Rates

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concentration and rate law

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

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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 concentration affects reaction rate, how rate laws are written, and how initial rates reveal reaction order, the rate constant, and overall order.

Reaction Order from Concentration | JoVE K12

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Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of ...

Quantum Numbers

It is said that the energy of an electron in an atom is quantized; that is, it can be equal only to certain specific values and can jump from one energy ...

Balmer Series

The Bohr Model
Niels Bohr proposed a model for the hydrogen atom in 1913 that described discrete energy states are associated with a fixed electron orbit ...

Explain how atomic energy levels and emission spectra present evidence for the wave particle duality.

IPC.7.E

Science concepts. The student knows that relationships exist between the structure and properties of matter. The student is expected to:

Reaction Order from Concentration

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