Name: how reactant concentration sets reaction rate
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 33 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.12

SC.912.P.12.12

Rate Law and Reaction Order

עבור כל תגובה, ניתן לבטא את הקשר בין קצב התגובה לבין ריכוזי המגיבים באופן מתמטי באמצעות חוק קצב או משוואת הקצב. 
בחוק שיעור, 'k' הוא קבוע המידתיות, או קבוע הקצב, ו- 'n' הוא סדר התגובה ביחס למגיב יחיד, שערכו הוא לעתים קרובות מספר שלם. בחוקי הקצב של תגובות מרובות מגיבים, סדר התגובה הכולל הוא סכום כל סדרי המגיבים.
עבור כל מגיב, קצב התגובה, קבוע הקצב, הריכוז וסדר התגובה נקבעים כולם באופן ניסיוני.
פקודות מגיבים בודדות בדרך כלל מקבלות את הערכים 0, 1 או 2, ובהתבסס על סדר התגובה הכולל, תגובות כימיות יכולות להיות מסווגות כתגובות מסדר אפס, מסדר ראשון או מסדר שני.
תגובה כימית חד-תגובתית, או חד-גונית, שעבורה קצב התגובה נשאר קבוע לאורך כל משכה היא תגובה מסדר אפס. סדר המגיבים בתגובה מסדר אפס הוא אפס, וריכוז המגיבים מועלה לחזקת האפס.
מכיוון שהערך של כל מספר המועלה לחזקת האפס הוא אחד, קצב התגובה של תגובה מסדר אפס שווה לקבוע הקצב, ולכן אינו תלוי בריכוז המגיב.
תגובה כימית חד-מולקולרית שבה קצב התגובה עומד ביחס ישר לריכוז המגיב היא תגובה מסדר ראשון. סדר המגיבים לתגובה מסדר ראשון הוא אחד, ועל פי חוק הקצב, ריכוזו של המגיב מועלה לחזקה הראשונה.
מכיוון שהערך של כל מספר המועלה בחזקתו של אחד נשאר זהה, קצב התגובה של תגובה מסדר ראשון תלוי ישירות בריכוז המגיב. ככל שריכוז המגיבים יורד, קצב התגובה יורד באופן יחסי באופן ליניארי.
תגובה כימית חד-מולקולרית שבה קצב התגובה תלוי בריבוע ריכוז המגיב היא תגובה מסדר שני. ככל שריכוז המגיבים יורד, קצב התגובה יורד באופן מעריכי באופן ריבועי.

חוק הקצב וסדר תגובה

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 קצב התגובה מושפע מריכוזי המגיבים. חוקי קצב (חוקי קצב דיפרנציאלי) או משוואות קצב הם ביטויים מתמטיים המתארים את הקשר בין קצב תגובה כימית לריכוז המגיבים שלה.
 
 
 לדוגמה, בתגובה גנרית aA + bB ⟶ מוצרים, כאשר a ו-b הם מקדמים סטוכיומטריים, ניתן לכתוב את חוק הקצב כך:
 
 
 קצב = k[A]^m[B]^n
 
 
 [A] ו-[B] מייצגים את הריכוזים המולאריים 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, ערכים 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 חודש ביחס ל-A וסדר n'י ביחס ל-B. לדוגמה, אם m = 1 ו-n = 2, ה- התגובה היא מסדר ראשון ב-A וסדר שני ב-B. סדר התגובה הכולל הוא פשוט סכום הסדרים עבור כל מגיב. עבור חוק הקצב לדוגמה כאן, התגובה היא מסדר שלישי בסך הכל (1 + 2 = 3).
 
 
 גישה ניסיונית נפוצה לקביעת חוקי הקצב היא שיטת הקצבים התחלתיים. שיטה זו כוללת מדידת קצבי תגובה עבור מחקרים ניסויים מרובים המבוצעים באמצעות ריכוזי מגיבים התחלתיים שונים. השוואת הקצבים הנמדדים עבור ניסויים אלה מאפשרת קביעת סדרי התגובה, ולאחר מכן, קבוע הקצב, המשמשים יחד לגיבוש חוק קצב.
 
 
 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 סטוכיומטריית תגובה.
 
 
 סדר התגובה קובע את הקשר בין קצב התגובה לריכוז המגיבים או המוצרים.
 

 <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>

 הטקסט הזה 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>.

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קצב התגובה מושפע מריכוזי המגיבים. חוקי קצב (חוקי קצב דיפרנציאליים) או משוואות קצב הם ביטויים מתמטיים ...

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Thermodynamics and Chemical Kinetics

תרמודינמיקה וקינטיקה כימית

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

how reactant concentration sets reaction rate

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants.

For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at a specific temperature.

The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative values, or zero. 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 separate orders, so this example is third order overall.

Rate law values are found by experiment, not by stoichiometry. Scientists change the initial reactant concentrations in several trials and measure the reaction rate. This method, called the method of initial rates, helps determine the reaction orders and then the rate constant.

Some reactions show zero-order behavior. In a zero-order reaction, changes in reactant concentration do not change the rate, so the rate stays constant. In a first-order reaction, the rate changes in direct proportion to the reactant concentration. In second-order and higher-order reactions, the rate depends on the concentration raised to an exponent, so the rate drops more sharply as reactants are used up.

Rate laws can also show fractional orders, and some can even show negative orders. A negative order means that increasing one reactant can decrease the reaction rate. These patterns are useful for describing how reaction rate changes in real experiments. They are not reliably predicted from the reaction equation alone, so the rate law must be determined experimentally.

How Reactant Concentration Sets Reaction Rate

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rate law and reaction order

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants. For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at...

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Learn how rate laws connect reactant concentration to reaction speed, how reaction orders work, and how the method of initial rates is used to find them.

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Gravitation Between Spherically Symmetric Masses

The gravitational potential energy between two spherically symmetric bodies can be calculated from the masses and the distance between the bodies, ...

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Gravity between Spherical Bodies

Newton&#39;s law of gravitation describes the gravitational force between any two point masses. However, for extended spherical objects like the Earth, ...

Describe how the gravitational force between two objects depends on their masses and the distance between them.

SC.912.P.12.4

Linear Momentum

The term momentum is used in various ways in everyday language, most of which are consistent with the precise scientific definition. Generally, momentum ...

Apply the law of conservation of linear momentum to interactions, such as collisions between objects.

SC.912.P.12.5

A. Motion can be measured and described qualitatively and quantitatively. Net forces create a change in motion. When objects travel at speeds comparable to the speed of light, Einstein's special theory of relativity applies.

B. Momentum is conserved under well-defined conditions. A change in momentum occurs when a net force is applied to an object over a time interval.

C. The Law of Universal Gravitation states that gravitational forces act on all objects irrespective of their size and position.

D. Gases consist of great numbers of molecules moving in all directions. The behavior of gases can be modeled by the kinetic molecular theory.

E. Chemical reaction rates change with conditions under which they occur. Chemical equilibrium is a dynamic state in which forward and reverse processes occur at the same rates.

How Reactant Concentration Sets Reaction Rate

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