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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

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Physical Science

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PS.PSc.2.5

Weak Base Solutions

Une base faible, comme l'ammoniac, est une base Brønsted qui accepte un proton de l'eau pour produire l'ion hydroxyde. Les bases faibles réagissent partiellement avec l'eau selon leur constante de dissociation de base, Kb, qui est de 1, 76 10⁻⁵ pour l'ammoniac. Le Kb pour l'ammoniac peut être exprimé comme la concnentration d'ion ammonium multipliée par la concentration d'ion hydroxyde divisée par la concentration d'ammoniac à l'équilibre.Le Kb peut être utilisé pour déterminer la concentration en ions hydroxyde dans une solution de base faible et par conséquent, le pOH et le pH de la solution. La concentration en ions hydroxyde et le pH de 0, 23 M de solution d'ammoniac peut être déterminé en utilisant sa constante de dissociation de base et en préparant un tableau ICE contenant les valeurs initiales et d'équilibre de l'ammoniac, d'ions d'ammonium et ions hydroxyde. Substitution des concentrations d'équilibre dans l'expression Kb, Kb est égal à x fois x divisé par 0, 23 moins x.Comme les bases faibles montrent une dissociation partielle, 0, 23 moins x peut être considéré comme égal à environ 0, 23. Lorsque l'équation est résolue, x est égal à 2 10⁻³ molaire. Le rapprochement 0, 23 moins x est égale à 0, 23 est valable ici car la concentration en ions hydroxyde est seulement de 0, 86%de 0, 23 molaire.Pour calculer le pH de cette solution, déterminez d'abord le pOH en prenant le log négatif de la concentration en ions hydroxyde, ce qui est égale à 2, 70. Le pH peut être déterminé en utilisant la formule:pH plus pOH égal à 14 et calculé à 11, 30. Le Kb pour une solution de base faible peut être calculé si le pH de la solution de base faible est connu.La méthylamine est une base faible qui se dissocie dans l'eau en ions méthylammonium et les ions hydroxyde. Le Kb de la méthylamine peut être exprimé en concentration en méthylammonium ionique multipliée par la concentration d'ion hydroxyde divisée par la concentration de méthylamine à l'équilibre. Pour calculer le Kb d'une solution de méthylamine de 0, 040 molaire avec un pH de 11, 6, il faut d'abord calculer le pOH suivi de sa concentration en ions hydroxyde.Comme le pH étant de 11, 60, le pOH est de 2, 40, et sa concentration en ions hydroxyde est de 4, 0 10⁻³. La table ICE peut être construite à partir de l'initiale et les concentrations d'équilibre de méthylamine, méthylammonium et hydroxyde. La substitution des concentrations d'équilibre dans l'expression pour le Kb donne la valeur de Kb, qui est de 4, 4 10⁻⁴.

solutions de base faible

Certains composés produisent des ions hydroxyde lorsqu'ils sont dissous par réaction chimique avec des molécules d'eau. Dans tous les cas, ces composés ne réagissent que partiellement et sont donc classés comme des bases faibles. Ces types de composés sont également abondants dans la nature et ce sont d'importants produits de base dans diverses technologies. Par exemple, la production mondiale de la base faible ammoniac dépasse généralement 100 tonnes métriques par an, étant largement utilisée comme engrais agricole, matière première pour la synthèse chimique d'autres composés et ingrédient actif dans les nettoyants ménagers. Lorsqu'il est dissout dans l'eau, l'ammoniac réagit partiellement pour produire des ions d'hydroxyde, comme illustré ci-dessous : 
 <img alt="Eq1" src="/files/ftp_upload/11401/11401_Equation_1.png" style="height: 50px;" />
 Il s'agit, par définition, d'une réaction acido-basique, dans ce cas impliquant le transfert d'ions H+ des molécules d'eau vers les molécules d'ammoniac. Dans des conditions normales, seulement environ 1 % de l'ammoniac dissous est présent sous forme d'ions NH4+. 
 Calcul des concentrations en ions d'hydroxyde et du pOH dans une solution de base faible 
 Trouvez la concentration de l'ion hydroxyde, le pOH, et le pH d'une solution de 0,25 M de triméthylamine, une base faible : 
 <img alt="Eq2" src="/files/ftp_upload/11401/11401_Equation_2.png" style="height: 50px;" />
 Le tableau ICE de ce système est 
 <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;">Concentration initiale (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;">Changement (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;">Concentration à l'équilibre (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>
 La substitution des termes de la concentration à l'équilibre dans l'expression de Kb donne 
 <img alt="Eq3" src="/files/ftp_upload/11401/11401_Equation_3.png" style="height: 90px;" />
 En supposant que x << 0,25 et en résolvant x, cela donne 
 <img alt="Eq4" src="/files/ftp_upload/11401/11401_Equation_4.png" style="height: 50px;" />
 Cette valeur est inférieure à 5 % de la concentration initiale (0,25), de sorte que la supposition est justifiée. 
 Comme défini dans le tableau ICE, x est égal à la concentration à l'équilibre de l'ion hydroxyde : 
 <img alt="Eq5" src="/files/ftp_upload/11401/11401_Equation_5.png" style="height: 50px;" />
 Le pOH est calculé comme étant 
 <img alt="Eq6" src="/files/ftp_upload/11401/11401_Equation_6.png" style="height: 50px;" />
 En utilisant la relation ; 
 <img alt="Eq7" src="/files/ftp_upload/11401/11401_Equation_7.png" style="height: 50px;" />
 Permet le calcul du pH : 
 <img alt="Eq8" src="/files/ftp_upload/11401/11401_Equation_8.png" style="height: 50px;" />
 Détermination de Kb à partir du pH 
 Le pH d'une solution 0,28 M d'éthylamine (C2H5NH2) est de 12,10 ; quel est son Kb ? 
 <img alt="Eq9" src="/files/ftp_upload/11401/11401_Equation_9.png" style="height: 50px;" />
 Pour calculer le Kb de l'éthylamine, il faut d'abord déterminer le pOH et la concentration en ions hydroxyde. Le pH est de 12,10 et donc le pOH peut être calculé comme suit : 
 <img alt="Eq10" src="/files/ftp_upload/11401/11401_Equation_10.png" style="height: 50px;" />
 Le pOH est de 1,90 et donc la concentration en ions hydroxyde de la solution peut être calculée à l'aide de la formule 
 <img alt="Eq11" src="/files/ftp_upload/11401/11401_Equation_11.png" style="height: 50px;" />
 Le tableau ICE peut être construit pour ce système comme suit 
 <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;">Concentration initiale (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;">Changement (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;">Concentration à l'équilibre (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>
 0,0126 M représente 4,5 % de 0,28 M et on peut donc considérer 0,28 − 0,0126 comme presque égal à 0,28 M grâce à la règle des 5 %. 
 Après avoir remplacé les valeurs ci-dessus dans l'expression du Kb de l'éthylamine, 
 <img alt="Eq12" src="/files/ftp_upload/11401/11401_Equation_12.png" style="height: 90px;" />
 Ce texte est adapté de Openstax, Chimie 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"> Classifier les réactions chimiques</a> et <a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases"> 14.3 Forces relatives des acides et des bases. </a>

Solutions de base faible

Certains composés produisent des ions hydroxyde lorsqu’ils sont dissous en réagissant chimiquement avec des molécules d’eau. Dans tous les cas, ces composés ne réagissent que partiellement et ...

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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

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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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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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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

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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

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determining the ph of salt solutions

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acid/base strengths and dissociation constants

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lewis acids and bases

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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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Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
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Colloids

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Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

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Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

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

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The Periodic Table

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Periodic Classification of the Elements

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Atomic Radii and Effective Nuclear Charge

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Noble Gases


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Types of Chemical Bonds

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Ionic Bonding and Electron Transfer

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Covalent Bonding and Lewis Structures

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Bonding in Metals

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

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Nomenclature of Alkanes

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Nomenclature of Alkenes

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Nomenclature of Alkynes

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Structure and Nomenclature of Alcohols and Phenols

Overview 
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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; ...

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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