Name: chemical equilibrium and kc rules
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The Equilibrium Constant

חוק פעולת המסות מציג את קבוע שיווי המשקל המקשר ריכוזי שיווי משקל של המגיבים והתוצרים. עבור משוואה כימית מאוזנת שבה A ו-B הם מגיבים, וC ו-D הם תוצרים, dו a, b, c הם המקדמים הסטויכומטריים שלהם בהתאמה, הביטוי לקבוע שיווי המשקל מתקבל מחלוקת הריכוזים המולריים של התוצרים בריכוזים המולריים של המגיבים, כולם בחזקת המקדמים הסטויכומטריים שלהם. קבוע שיווי המשקל מבוטא בדרך כלל באותיות K או Kc, כאשר האות c מציינת כי קבוע שיווי המשקל בהתאמה לריכוז המולרי.בהיותו יחס בין ריכוזים מולריים, קבוע שיווי המשקל הוא מספר נטול יחידת מידה. משמעותו של קבוע שיווי משקל גדול משמעותית מ-1 היא שהמונה, ריכוז התוצרים, גדול יותר מהמכנה, ריכוז המגיבים. כך רואים ששיווי המשקל נוטה לעבר התוצרים ומעדיף את התגובה הישירה.בהתאמה, קבוע שיווי משקל קטן מ-1 מצביע על כך שהמכנה גדול מהמונה. כלומר ששיווי המשקל הכימי נוטה לעבר המגיבים ומעדיף את התגובה ההפוכה. קבוע שיווי משקל בעל ערך שווה או קרוב ל-1 מראה כי הריכוזים היחסיים של המגיבים והתוצרים כמעט זהים.זה מעיד על כך שלא התגבוה הישירה ולא התגובה ההפוכה עדיפות. שינויים במשוואה הכימית משנים גם את קבוע שיווי המשקל. אם תגובה כימית בעלת קבוע שיווי משקל של X מתהפכת, אזי נוסחת שיווי המשקל הכימי לתגובה ההפוכה תהיה הופכית לתגובה הישירה.מכאן שקבוע שיווי המשקל החדש יהיה 1 חלקי X.אם המקדמים מוכפלים בגורם נוסף, n, אזי קבוע שיווי המשקל יהיה בחזקת אותו n. קבוע שיווי המשקל החדש יהיה x בחזקת n. כששתי תגובות או יותר מחוברות זו לזו, קבוע שיווי המשקל של התגובה בכללותה הוא מכפלת קבועי שיווי המשקל.קבוע שיווי המשקל החדש יהיה y כפול z. בשיווי משקל הטרוגני, נוסחת קבוע שיווי המשקל לא תכלול חומרים מוצקים או נוזליים טהורים. לדוגמה, קבוע שיווי המשקל בפירוק של אמוניום חנקתי מוצק לחמצן דו חנקני במצב גז ולאדי מים, לא כולל אמוניום חנקתי מוצק טהור.זאת מפני שהריכוז המולרי היחסי של מוצק או נוזל טהור נותר קבוע במהלך תגובה.

קבוע שיווי המשקל

<div style="direction:rtl">

שקול את החמצון של דו תחמוצת הגופרית: 
<img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;"> 

עבור תגובה שמתחילה בתערובת מגיבים בלבד, ריכוז התוצר שווה בתחילה לאפס. ככל שהתגובה מתקדמת לקראת שיווי משקל בכיוון קדימה, ריכוזי המגיבים יורדים וריכוז התוצר עולה. כאשר מושג שיווי משקל, ריכוזי המגיבים והתוצרים נשארים קבועים. 

אם התגובה מתחילה רק כשהתוצרים קיימים, התגובה ממשיכה לכיוון שיווי משקל בכיוון ההפוך. ריכוז התוצר יורד עם הזמן וריכוזי המגיבים עולים עד שהריכוזים נעשים קבועים בשיווי משקל. 

חוק פעולת המסה קובע שהיחס בין ריכוז התוצרים לריכוז המגיבים בשיווי משקל, המועלה למקדמים הסטוכיומטריים שלהם, שווה לקבוע, הנקרא קבוע שיווי המשקל, K או Kc. 

לפיכך, ביטוי קבוע שיווי המשקל עבור התגובה לעיל נכתב כך: 
<img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;"> 

כאשר, הכיתוב 'c' מציין שקבוע שיווי המשקל מתייחס לריכוז המולרי של מגיבים ותוצרים. 

גודל קבוע שיווי המשקל משקף במפורש את ההרכב של תערובת תגובה בשיווי משקל. תגובה שמציגה K גדול תגיע לשיווי משקל כאשר רוב המגיב הומר לתוצר, ואילו K קטן מציין שהתגובה משיגה שיווי משקל לאחר שמעט מאוד מגיב הומר. חשוב לזכור שגודל K אינו מציין באיזו מהירות יגיע שיווי המשקל. שיווי משקל מסוימים נוצרים כל כך מהר עד שהם כמעט מיידיים, ואחרים כל כך באיטיות עד שלא נצפה שינוי מורגש במהלך ימים, שנים או יותר. ניתן להשתמש בקבוע שיווי המשקל לתגובה כדי לחזות את ההתנהגות של תערובות המכילות מגיבים ו/או תוצרים. כפי שהוכח על ידי תהליך חמצון הגופרית הדו-חמצנית שתואר לעיל, תגובה כימית תתקדם בכל כיוון הדרוש כדי להשיג שיווי משקל. 

שיווי משקל משולב

מערכות שיווי משקל רבות כוללות שתיים או יותר תגובות שיווי משקל משולבות, אלו המשותף לסוג מגיב או תוצר אחד או יותר. ערך K עבור מערכת הכוללת שיווי משקל מוצמד יכול להיות קשור לערכי K של התגובות הבודדות. שלוש מניפולציות בסיסיות מעורבות בגישה זו, כמתואר להלן: 
<ul style="padding-right: 35px; list-style-type: disc;">
	<li style="text-align: right;">שינוי כיוון של משוואה כימית בעצם מחליף את הזהויות של "מגיבים" ו"תוצרים", ולכן קבוע שיווי המשקל עבור המשוואה ההפוכה הוא פשוט ההדדיות של זה עבור המשוואה הקדמית.</li> 
	<li style="text-align: right;">שינוי המקדמים הסטוכיומטריים במשוואה בגורם x כלשהו מביא לשינוי אקספוננציאלי בקבוע שיווי המשקל באותו גורם.</li> 
	<li style="text-align: right;">הוספת שתי משוואות שיווי משקל או יותר יחדיו מניבה משוואה כוללת שקבוע שיווי המשקל שלה הוא מכפלה מתמטית של ערכי K של התגובה האינדיבידואלית.</li> 
</ul>

שיווי משקל ביטוי קבוע לשיווי משקל הטרוגני 

עבור שיוויי משקל הטרוגני, הכוללים מגיבים ותוצרים בשני שלבים שונים או יותר, הריכוזים של מוצקים טהורים או נוזלים טהורים אינם נכללים בביטוי קבוע שיווי המשקל, כפי שמומחש בדוגמה הבאה: 
<img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;"> 

הסיבה לכך היא שהריכוזים היחסיים של נוזלים טהורים ומוצקים טהורים נשארים קבועים במהלך התגובה. 

טקסט זה מותאם מ<a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a>.</div>

שקול את החמצון של דו תחמוצת הגופרית:

עבור תגובה שמתחילה בתערובת של מגיבים בלבד, ריכוז המוצר שווה בתחילה ל ...

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chemical equilibrium and kc rules

Chemical equilibrium and the equilibrium constant Kc describe how reactants and products are balanced in a reaction. In the oxidation of sulfur dioxide, a mixture that starts with reactants only has no product at first. As the reaction moves forward toward equilibrium, reactant concentrations decrease and product concentration increases. When equilibrium is reached, the concentrations of both stay constant.

A reaction can also begin with only products present. In that case, the reaction moves in reverse toward equilibrium. The product concentration falls with time, and the reactant concentrations rise until both become constant. This shows that a chemical reaction will move in whatever direction is needed to reach equilibrium.

The law of mass action links equilibrium to a constant value. It states that the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric coefficients, equals the equilibrium constant, K or Kc. The subscript c means the expression uses molar concentration. The size of K shows the mixture makeup at equilibrium. A large K means most reactant has changed to product before equilibrium is reached. A small K means only a little reactant changes to product. The value of K does not tell how fast equilibrium is reached.

Some equilibria form very quickly. Others take so long that no visible change may be seen for days, years, or longer. The equilibrium constant can still be used to predict how a mixture with reactants and/or products will behave. For coupled equilibria, two or more equilibrium reactions share one or more species. The K value for the overall system can be found from the K values of the separate reactions. Reversing a chemical equation changes K to its reciprocal. Multiplying the stoichiometric coefficients by a factor x changes K by that same factor as an exponent. Adding equilibrium equations gives an overall K equal to the product of the individual K values.

For heterogeneous equilibria, the reactants and products are in different phases. Pure solids and pure liquids are not included in the equilibrium constant expression. Their concentrations stay constant during the reaction, so they do not change the value of K. This rule is an important part of writing equilibrium expressions correctly.

Chemical Equilibrium and Kc Rules

the equilibrium constant

Chemical equilibrium and the equilibrium constant Kc describe how reactants and products are balanced in a reaction. In the oxidation of sulfur dioxide, a mixture that starts with reactants only has no product at first. As the reaction moves forward toward equilibrium, reactant concentrations decrease and product concentration increases. When equilibrium is reached, the concentrations of both stay constant. A reaction can also begin with only products present. In that case, the reaction moves...

homogeneous equilibria for gaseous reactions

homogeneous-equilibria-for-gaseous-reactions

Relating Kc and Kp in Gas Equilibria

Gas-phase equilibrium can be written using either concentration or pressure. The equilibrium constant may be shown as Kc, which uses molar concentration, or as Kp, which uses partial pressure of the reactants and products. The link between Kc and Kp comes from the ideal gas equation and the definition of molarity. Molarity is the number of moles divided by volume. In the ideal gas equation, P stands for partial pressure, V for volume, n for number of moles, R for the gas constant, and T for...

le chatelier's principle: changing concentration

le-chatelier-s-principle-changing-concentration

Shifting Equilibrium by Changing Concentration

Chemical equilibrium is a dynamic balance. At equilibrium, the forward and reverse reactions happen at equal rates. If the system is stressed in a way that changes one rate more than the other, it is no longer at equilibrium and will shift until balance is restored. This behavior is described by Le Châtelier’s principle. Changes in concentration are a major stress on an equilibrium system. Reaction rates depend mainly on concentration and temperature. For that reason, changing the amount of a...

calculating the equilibrium constant

Finding Kc from Equilibrium Data

The equilibrium constant, Kc, can be found from the equilibrium concentrations of reactants and products. When those concentrations are known, the calculation is straightforward. The values are placed into the Kc expression for the reaction. One example uses nitrogen dioxide and dinitrogen tetroxide. When 0.10 mol NO2 is added to a 1.0-L flask at 25 °C, the concentrations change until equilibrium is reached. At equilibrium, [NO2] is 0.016 M and [N2O4] is 0.042 M, and these values are used to...

reaction quotient

Using Q to Track Equilibrium Changes

Reaction quotient Q helps track how far a reversible reaction has moved toward equilibrium. For a balanced reaction such as m A + n B ⇌ x C + y D, Q comes directly from the stoichiometric coefficients in the equation. When concentrations are used, the expression is written as Qc. For gases, a similar form uses partial pressures. These expressions use relative concentration and pressure values, not absolute values. That means reaction quotients are dimensionless and have no units. As a reaction...

dynamic equilibrium

Chemical Equilibrium in Reversible Reactions

Chemical equilibrium in reversible reactions happens when a reaction moves in both directions at the same time. A reversible reaction uses a double arrow to show that products can turn back into reactants. At equilibrium, the concentrations of reactants and products stay constant over time because the forward and reverse reaction rates are equal. The video uses the reversible decomposition of colorless dinitrogen tetroxide to brown nitrogen dioxide as an example. At time zero, there is finite...

calculating equilibrium concentrations

Equilibrium Concentrations from an ICE Table

Equilibrium concentrations can be found from initial concentrations and an equilibrium constant. This skill matters in many areas of science and technology, including pharmaceutical product formulation and dosing. After a drug is ingested or injected, it may take part in several chemical equilibria that change its final concentration in the body system of interest. Scientists use these quantitative equilibrium relationships to help calculate a dosage that will produce the desired therapeutic...

le chatelier's principle: changing volume (pressure)

le-chatelier-s-principle-changing-volume-pressure

Gas Equilibrium Under Volume Changes

Gas-phase equilibrium can shift when the volume of the system changes. In these reactions, a change in pressure changes concentration because the partial pressure of an ideal gas is proportional to its molar concentration. That means pressure changes affect equilibrium in the same way as concentration changes. When the volume of a gas mixture changes, the concentrations of reactants and products change together. Since every gas in the equilibrium occupies the same space, the same volume change...

le chatelier's principle: changing temperature

le-chatelier-s-principle-changing-temperature

Le Chatelier's Principle in Heating and Cooling

Le Chatelier's principle helps explain how chemical equilibrium responds to heating and cooling. A system at equilibrium will shift to restore balance after a temperature change, but the equilibrium constant, K, will not stay the same. Instead, the mixture reaches a new equilibrium with a different composition. This temperature effect is different from a concentration change, which shifts equilibrium without changing K. To understand why temperature matters, the reaction can be written with...

the small x assumption

Estimating Equilibrium with Small Changes

A small equilibrium constant can make equilibrium calculations easier. In these reactions, the equilibrium position favors the reactants. When the initial reactant concentration is high and the Kc value is small, the change in concentration may be negligible. In that case, the equilibrium concentration is close to the initial concentration. This idea helps simplify weak acid and weak base problems. Chemists can assume that some equilibrium concentrations are equal to the initial concentrations.

key terms and concepts

Explore chemical equilibrium, Kc, and the law of mass action. Learn how concentration changes, reaction direction, and mixed-phase equilibria shape equilibrium expressions.

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PS.Chm.3.1

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

surface-tension-capillary-action-and-viscosity

Surface Tension, Capillary Action, and Viscosity

Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded ...

Hydrogen Bonds

Hydrogen BondsHydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Chemical Equilibrium and Kc Rules

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