Name: weak base ph and kb calculations
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 3 min 21 s
Channel: JoVE

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Physical Science

Matter and its Interactions

PS.PSc.2.5

Weak Base Solutions

בסיס חלש, כמו אמוניה, הוא בסיס ברונסטד שמקבל פרוטון ממים כדי לייצר יון הידרוקסיד. בסיס חלש מגיב באופן חלקי עם מים בהתאם לקבוע פירוק הבסיס, Kb, שהוא 10⁻⁵×1.76 עבור אמוניה. ניתן לבטא את ה-Kb באמוניה כמכפלה של ריכוז יוני אמוניום וריכוז יוני הידרוקסיד המחולקת בריכוז האמוניה בשיווי משקל.Kb יכול לסייע לקבוע את ריכוז יוני ההידרוקסיד בתמיסת בסיס חלש ובהתאם לכך לקבוע את ערכי pOH ו-pH. ריכוז יוני הידרוקסיד ו-pH של תמיסת אמוניה 0.23 מולר יכול להימצא על ידי שימוש בקבוע פירוק הבסיס ועל ידי הכנה של טבלת ICE הכוללת את הריכוזים ההתחלתיים וריכוזי שיווי המשקל של אמוניה, יוני אמוניום ויוני הידרוקסיד. הצבת ריכוזי שיווי המשקל בביטוי עבור Kb, Kb שווה x כפול x חלקי 0.23 פחות x.בעוד בסיס חלש מקיים פירוק חלקי, 0.23 פחות x יכול להיחשב לכ-0.23 בקירוב. כשפותרים את המשוואה, עולה כי x שווה 2 כפול 10 בחזקת מינוס 3 מולר. הקירוב ש0.23 פחות x שווה ל-0.23 תקף כאן, מכיוון שריכוז יוני ההידרוקסיד הוא רק 0.86%מ-0.23 מולר.על מנת לחשב את ה-pH של תמיסה זו, ראשית מצאו את ה-pOH על ידי הלוג השלילי של ריכוז יוני ההידרוקסיד, השווה 2.70. ה-pH יכול להיקבע על ידי שימוש בנוסחה:pH pOH 14 ומחושב כ-11.30. ניתן לחשב Kb של תמיסת בסיס חלש אם ה-pH של תמיסת הבסיס החלש ידוע.מתילאמין הוא בסיס חלש שמתפרק במים באופן חלקי ליוני מתילאמוניום וליוני הידרוקסיד. ניתן לבטא את הKb של מתילאמין כריכוז יוני מתילאמיניום כפול ריכוז יוני הידרוקסיד חלקי ריכוז המתילאמין בשיווי משקל. על מנת לחשב את ה-Kb של תמיסת מתילאמין 0.040 מולר עם pH של 11.6, ראשית יש לחשב את ה-pOH בתמיסה ולאחר מכן את ריכוז יוני ההידרוקסיד בה.כשה-pH הוא 11.60, ה-pOH הוא 2.40 וריכוז יוני ההידרוקסיד הוא 4.0 כפול 10 בחזקת מינוס 3. ניתן לבנות טבלת ICE באמצעות הריכוזים ההתחלתיים ובזמן שיווי משקל של המתילאמין, מתילאמוניום והידרוקסיד. הצבת ריכוזי שיווי המשקל בביטוי עבור Kb נותנת את ערך ה-Kb, שהוא 4.4 כפול 10 בחזקת מינוס 4.

תמיסות בסיסים חלשים

<div style="direction:rtl">תרכובות מסוימות מייצרות יוני הידרוקסיד כשהן מומסות על ידי תגובה כימית עם מולקולות מים. בכל המקרים, תרכובות אלו מגיבות רק באופן חלקי ולכן מסווגות כבסיסים חלשים. תרכובות מסוג זה מצויות בשפע בטבע וגם בסחורות חשובות בטכנולוגיות שונות. לדוגמה, הייצור העולמי של אמוניה בסיס חלש הוא בדרך כלל הרבה יותר מ-100 טון מדי שנה, בשימוש נרחב כדשן חקלאי, חומר גלם לסינתזה כימית של תרכובות אחרות ומרכיב פעיל בחומרי ניקוי ביתיים. כאשר היא מומסת במים, אמוניה מגיבה חלקית כדי להניב יוני הידרוקסיד, כפי שמוצג כאן: <img alt="Eq1" src="/files/ftp_upload/11401/11401_Equation_1.png" style="height: 50px;"> זוהי, בהגדרה, תגובת חומצה-בסיס, במקרה זה כוללת העברה של יוני H+ ממולקולות מים למולקולות אמוניה. בתנאים טיפוסיים, רק כ-1% מהאמוניה המומסת קיים כיוני NH4+. חישוב ריכוז יוני הידרוקסיד ו-pOH בתמיסת בסיס חלשה מצא את הריכוז של יון הידרוקסיד, ה-pOH וה-pH של תמיסה של 0.25 M של טרימתילאמין, בסיס חלש: <img alt="Eq2" src="/files/ftp_upload/11401/11401_Equation_2.png" style="height: 50px;"> טבלת ICE עבור מערכת זו היא <table style="margin-left: auto; margin-right: auto;"> <tbody> <tr> <td style="color: black; border: 1px solid black; text-align: center;"></td> <td style="color: black; border: 1px solid black; text-align: center;">(CH3)3N (aq)</td> <td style="color: black; border: 1px solid black; text-align: center;">(CH3)3NH+ (aq) </td> <td style="color: black; border: 1px solid black; text-align: center;">OH− (aq) </td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">ריכוז ראשוני (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">0.25</td> <td style="color: black; border: 1px solid black; text-align: center;">0</td> <td style="color: black; border: 1px solid black; text-align: center;">~0</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">שנה (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">−x</td> <td style="color: black; border: 1px solid black; text-align: center;">+x</td> <td style="color: black; border: 1px solid black; text-align: center;">+x</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">ריכוז שיווי משקל (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">0.25 − x</td> <td style="color: black; border: 1px solid black; text-align: center;">0 + x</td> <td style="color: black; border: 1px solid black; text-align: center;">~0 + x</td> </tr> </tbody> </table> החלפת מונחי ריכוז שיווי המשקל בביטוי Kb נותן <img alt="Eq3" src="/files/ftp_upload/11401/11401_Equation_3.png" style="height: 90px;"> בהנחה x << 0.25 ופתרון עבור x נותן את התוצאה: <img alt="Eq4" src="/files/ftp_upload/11401/11401_Equation_4.png" style="height: 50px;"> ערך זה נמוך מ-5% מהריכוז ההתחלתי (0.25), כך שההנחה מוצדקת. כפי שהוגדר בטבלת ICE, x שווה לריכוז שיווי המשקל של יון הידרוקסיד: <img alt="Eq5" src="/files/ftp_upload/11401/11401_Equation_5.png" style="height: 50px;"> ה-pOH מחושב להיות <img alt="Eq6" src="/files/ftp_upload/11401/11401_Equation_6.png" style="height: 50px;"> שימוש ביחס; <img alt="Eq7" src="/files/ftp_upload/11401/11401_Equation_7.png" style="height: 50px;"> מאפשר חישוב של pH: <img alt="Eq8" src="/files/ftp_upload/11401/11401_Equation_8.png" style="height: 50px;"> קביעת Kb מ-pH אם ה-pH של תמיסה של 0.28 M של אתילאמין (C2H5NH2) הוא 12.10, מה הוא ה-Kb שלו? <img alt="Eq9" src="/files/ftp_upload/11401/11401_Equation_9.png" style="height: 50px;"> כדי לחשב את ה-Kb של אתילאמין, ראשית יש לקבוע את ה-pOH ואת ריכוז יוני הידרוקסיד. מכיוון שה-pH הוא 12.10, ניתן לחשב את ה-pOH באופן הבא: <img alt="Eq10" src="/files/ftp_upload/11401/11401_Equation_10.png" style="height: 50px;"> מכיוון שה-pOH הוא 1.90, ניתן לחשב את ריכוז יוני הידרוקסיד של התמיסה באמצעות הנוסחה הזו: <img alt="Eq11" src="/files/ftp_upload/11401/11401_Equation_11.png" style="height: 50px;"> ניתן לבנות את טבלת ICE עבור מערכת זו באופן הבא <table style="margin-left: auto; margin-right: auto;"> <tbody> <tr> <td style="color: black; border: 1px solid black; text-align: center;"></td> <td style="color: black; border: 1px solid black; text-align: center;">C2H5NH2 (aq)</td> <td style="color: black; border: 1px solid black; text-align: center;">C2H5NH3+ (aq)</td> <td style="color: black; border: 1px solid black; text-align: center;">OH− (aq)</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">ריכוז ראשוני (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">0.28</td> <td style="color: black; border: 1px solid black; text-align: center;">0</td> <td style="color: black; border: 1px solid black; text-align: center;">~0</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">שינוי (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">−0.0126</td> <td style="color: black; border: 1px solid black; text-align: center;">+0.0126</td> <td style="color: black; border: 1px solid black; text-align: center;">+0.0126</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">ריכוז שיווי משקל (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">0.28 − 0.0126</td> <td style="color: black; border: 1px solid black; text-align: center;">0.0126</td> <td style="color: black; border: 1px solid black; text-align: center;">0.0126</td> </tr> </tbody> </table> 

מכיוון שה-0.0126 M הוא 4.5% מ-0.28 M, 0.28 - 0.0126 יכולים להיחשב כשווים כמעט ל-0.28 M לפי כלל ה-5%.

 לאחר החלפת הערכים לעיל בביטוי עבור Kb של אתילאמין, 

<img alt="Eq12" src="/files/ftp_upload/11401/11401_Equation_12.png" style="height: 90px;"> 

טקסט זה מותאם מ Openstax, Chemistry 2e <a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">Section 4.2: </a><a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">Classifying Chemical Reactions</a> and <a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases">14.3 Relative Strengths of Acids and Bases.</a></div>

תרכובות מסוימות מייצרות יוני הידרוקסיד כאשר הן מומסות על ידי תגובה כימית עם מולקולות מים. בכל המקרים, תרכובות אלה מגיבות רק באופן חלקי ו ...

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Acids and Bases

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weak base ph and kb calculations

Weak base pH and Kb calculations show how partially reacting bases change water chemistry. Weak bases produce hydroxide ions when they dissolve by reacting with water molecules. Because the reaction only happens partly, these compounds are classified as weak bases.

Weak bases are common in nature and in technology. Ammonia is one major example. Global production of ammonia is typically well over 100 metric tons each year. It is used as an agricultural fertilizer, a starting material for chemical synthesis, and an ingredient in household cleaners.

When ammonia dissolves in water, it reacts only partially to form hydroxide ions. This is an acid-base reaction because H+ ions move from water molecules to ammonia molecules. Under typical conditions, only about 1% of the dissolved ammonia is present as NH4+ ions.

The video also shows how to calculate hydroxide ion concentration, pOH, and pH for a weak base solution. For a 0.25 M trimethylamine solution, an ICE table is used to track initial, change, and equilibrium concentrations. The Kb expression is then written with the equilibrium terms, and the x &lt;&lt; 0.25 assumption is checked with the 5% rule before solving for x as the hydroxide ion concentration. From there, pOH is found and pH is calculated.

A second example shows how to determine Kb from pH. For a 0.28 M ethylamine solution with a pH of 12.10, the pOH is first found, then the hydroxide ion concentration is calculated. An ICE table is built for ethylamine, ethylammonium ion, and hydroxide ion. After applying the 5% rule, the values are substituted into the Kb expression to find the base dissociation constant.

Weak Base pH and Kb Calculations

bronsted-lowry acids and bases

Proton Transfer in Acids and Bases

Acids and bases can be understood through proton transfer. Early scientists described acids by their sour taste, their effect on dyes, and their reaction with limestone, which releases carbon dioxide. Robert Boyle also noted that acid solutions can lose these properties after contact with alkali, or base, solutions. The modern view developed step by step. Humphry Davy showed that hydrogen is the essential part of acids. Joseph Louis Gay-Lussac then described acids as substances that neutralize...

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Water can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base. When hydrogen fluoride dissolves in water, it donates protons to water. This is acid ionization. The products are hydronium ions and fluoride ions. Water can also accept protons from another substance. When pyridine dissolves in water, it accepts a proton from water molecules. This is base ionization. The products are hydroxide ions and pyridinium ions. These reactions show that water can play either role. A substance that...

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pH and pOH in Water

pH and pOH show how hydronium and hydroxide ions behave in water and other aqueous solutions. These ion concentrations are linked by the ion product of water, K w , so when one goes up, the other goes down. Because ion concentrations can differ by huge amounts, a logarithmic scale is used to describe them. pH is defined from the hydronium ion concentration, [H3O+], and pOH is the matching p-function for hydroxide ion concentration. These two values are also connected through the K w...

relative strengths of conjugate acid-base pairs

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How Ka and Kb Show Acid-Base Strength

Brønsted-Lowry acid-base chemistry is based on proton transfer. That idea helps show how the strength of an acid is linked to the strength of its conjugate base, and vice versa. Acid and base strength is measured with ionization constants, Ka and Kb, which tell how far an ionization reaction goes. For the conjugate pair HA and A−, the acid ionizes in water and the base reaction can be written for the partner species. When the two reactions are added, they give the autoionization of water. The...

strong acid and base solutions

Strong Acids, Strong Bases, and pH

Strong acids and strong bases change the pH of a solution by dissociating completely in water. A strong acid produces hydronium ions, which are H3O+ ions, and a strong base produces hydroxide ions, OH-. Because the dissociation is complete, the ion concentration can be found from the starting concentration and the molar ratio. For example, 0.20 M hydrobromic acid dissociates fully in water. It produces 0.20 M hydronium ions and 0.20 M bromide ions. A 0.015 M KOH solution also dissociates...

weak acid solutions

Weak Acid Equilibrium in Water

Weak acid equilibrium in water explains why many acids only partially react when dissolved. Most weak acid molecules stay in their original form, and only a small amount of hydronium ions forms. Weak acids are common in nature. They help create the tangy taste of citrus fruits, the stinging sensation from insect bites, and the smells linked to body odor. Acetic acid is a familiar weak acid. It is the main ingredient in vinegar. Its equation uses a double arrow to show that the reaction with...

weak base solutions

Weak base pH and Kb calculations show how partially reacting bases change water chemistry. Weak bases produce hydroxide ions when they dissolve by reacting with water molecules. Because the reaction only happens partly, these compounds are classified as weak bases. Weak bases are common in nature and in technology. Ammonia is one major example. Global production of ammonia is typically well over 100 metric tons each year. It is used as an agricultural fertilizer, a starting material for...

mixtures of acids

pH of Acid Mixtures

The pH of acid mixtures depends on the acids present, their dissociation constants, and their starting concentrations. When a solution contains two different acids, the pH can be found by choosing the method that fits their relative strengths. This makes acid mixtures a useful case for applying acid-base rules in chemistry. In a mixture of a strong acid and a weak acid, the strong acid dissociates completely and supplies almost all of the hydronium ions in the solution. The weak acid...

ions as acids and bases

Salt Hydrolysis and pH of Solutions

Salt hydrolysis explains why some salt solutions are acidic, basic, or neutral. Salts are ionic compounds made of cations and anions. In water, either ion may react with water through an acid or base ionization reaction. The pH of the solution depends on the relative acid-base strength of the ions in the salt. Ammonium chloride is a good example of an acidic salt. When it dissolves, it separates into ammonium ions and chloride ions. The ammonium ion is the conjugate acid of ammonia, so it can...

determining the ph of salt solutions

Salt Hydrolysis and pH in Solutions

Salt hydrolysis and pH in solutions depend on the ions released when a salt dissolves. Some ions react with water, while others do not. The balance between the cation and anion helps show whether the solution becomes acidic, basic, or neutral. Salts with pH-neutral anions and cations that produce hydronium ions give acidic solutions with pH below 7. Ammonium nitrate is one example. In NH4NO3, NO3− does not react with water, but NH4+ forms hydronium ions, so the solution is acidic. Other salts...

acid/base strengths and dissociation constants

acidbase-strengths-and-dissociation-constants

Acid and Base Strength in Water

Acid and base strength in water depends on how much a substance ionizes after it dissolves. If ionization is nearly complete, the substance is called strong. If only a small amount ionizes, it is called weak. Weak acids and weak bases are much more common than strong ones. The most common strong acids are HClO4, HCl, HBr, HI, HNO3, and H2SO4. The most common strong bases are LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. These strong acids and bases ionize essentially completely in water. <br...

polyprotic acids

Stepwise Ionization of Polyprotic Acids

Polyprotic acids lose hydrogen ions in steps. The number of ionizable protons in a molecule determines whether an acid is monoprotic, diprotic, or triprotic. This video also shows that polyprotic bases can accept more than one proton. Monoprotic acids have one ionizable hydrogen atom per molecule. Examples include HCl, HNO3, and HCN. Their reactions with water form hydronium ions and the matching anion. Acetic acid, CH3CO2H, is also monoprotic because only the hydrogen in the carboxyl group...

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How Bonding Affects Acid Strength

Acid strength depends on bonding, electronegativity, and oxidation number. These factors help explain why some hydrogen compounds act as strong acids, while others behave as weak acids or bases. For binary acids, which are hydrogen compounds with one other nonmetal, acidity increases as the H-A bond gets weaker down a group. That is why the order for group 17 is HF &lt; HCl &lt; HBr &lt; HI, and for group 16 it is H2O &lt; H2S &lt; H2Se &lt; H2Te. Across a period, acid strength increases as...

lewis acids and bases

Electron-Pair Reactions in Acid-Base Chemistry

Electron-pair reactions explain Lewis acid-base chemistry. In 1923, G. N. Lewis described acids and bases by how they accept or donate a pair of electrons. When that electron pair forms a coordinate covalent bond, one atom supplies both bonding electrons. This bond is also called a dative bond. A simple example is the reaction of water with a hydrogen ion to make a hydronium ion. Ammonia can also combine with a hydrogen ion to form an ammonium ion. In both cases, the reaction creates a Lewis...

key terms and concepts

Learn how weak bases form hydroxide ions, then use ICE tables to find pOH, pH, and Kb for trimethylamine and ethylamine examples.

Weak Base pH and Kb Calculations | JoVE K12

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

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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, ...

Carry out investigations to compare physical and chemical properties of matter.

PS.PSc.1.3

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

The Bohr Model

Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

Use models to interpret the data presented in Bohr diagrams and electron dot diagrams for neutral atoms of elements 1 through 18.

PS.PSc.1.4

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

Use models to compare representations of atoms, ions, and isotopes.

PS.PSc.1.5

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use the Periodic Table as a model to predict the relative properties (metallic/nonmetallic character, ionic charge, and reactivity) and arrangement of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.PSc.1.6

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

PS.PSc.2.1

Nomenclature of Alkanes

In the late 19th-century, the number of new chemical compounds discovered increased tremendously. Hence, the necessity arose to develop a naming system ...

Nomenclature of Alkenes

The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix of ...

Nomenclature of Alkynes

Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of ...

structure-and-nomenclature-of-alcohols-and-phenols

Structure and Nomenclature of Alcohols and Phenols

Overview 
Alcohols are one of the most important functional groups in organic chemistry. The name of alcohol comes from the hydrocarbon from which it is ...

Structure and Nomenclature of Ethers

Structure and Bonding
Ethers are organic compounds with an ether functional group which is characterized by an oxygen atom connected to two &#8212; ...

IUPAC Nomenclature of Aldehydes

Aldehydes are named based on the systematic nomenclature rules set by the IUPAC. For acyclic aldehydes, the longest carbon chain containing the aldehydic ...

Use models to apply International Union of Pure and Applied Chemistry (IUPAC) conventions to name and write formulas for simple compounds.

PS.PSc.2.2

Understand types, properties, and structure of matter.

Weak Base pH and Kb Calculations

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