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

9-12

Physical Science

Matter and its Interactions

PS.PSc.2.5

Weak Base Solutions

Słaba zasada, taka jak amoniak, to zasada Brønsted, która przyjmuje proton z wody w celu wytworzenia jonu wodorotlenkowego. Słabe zasady reagują częściowo z wodą zgodnie ze stałą dysocjacji zasady, Kb, która wynosi 1,76 × 10−5 dla amoniaku.
Kb dla amoniaku można wyrazić jako stężenie jonów amonowych pomnożone przez stężenie jonów wodorotlenkowych podzielone przez stężenie amoniaku w stanie równowagi.
Kb można wykorzystać do określenia stężenia jonów wodorotlenkowych w słabym roztworze zasady, a w konsekwencji pOH i pH roztworu.
Stężenie jonów wodorotlenkowych i pH 0,23 M roztworu amoniaku można oznaczyć za pomocą jego podstawowej stałej dysocjacji oraz poprzez przygotowanie tabeli ICE zawierającej wartości początkowe i równowagowe amoniaku, jonów amonowych i jonów wodorotlenkowych.
Podstawiając stężenia równowagowe w wyrażeniu Kb, Kb jest równe x razy x podzielone przez 0,23 minus x. Ponieważ słabe zasady wykazują częściową dysocjację, 0,23 minus x można uznać za około 0,23.
Gdy równanie zostanie rozwiązane, x wynosi 2 × 10−3 M.
Przybliżenie 0,23 minus x jest równe 0,23 jest tutaj ważne, ponieważ stężenie jonów wodorotlenkowych wynosi tylko 0,86% z 0,23 mola.
Aby obliczyć pH tego roztworu, najpierw oznacz pOH, biorąc ujemny logarytm stężenia jonów wodorotlenkowych, który wynosi 2,70. pH można określić za pomocą wzoru: pH plus pOH jest równe 14 i oblicza się je na 11,30.
Kb dla słabego roztworu zasady można obliczyć, jeśli znane jest pH roztworu słabej zasady.
Metyloamina jest słabą zasadą, która częściowo dysocjuje w wodzie na jony metyloamoniowe i jony wodorotlenkowe.
Kb dla metyloaminy może być wyrażony jako stężenie jonów metyloamoniowych pomnożone przez stężenie jonów wodorotlenkowych podzielone przez stężenie metyloaminy w stanie równowagi.
Aby obliczyć Kb w 0,040 M roztworze metyloaminy o pH 11,6, najpierw należy obliczyć pOH, a następnie stężenie jonów wodorotlenkowych. Ponieważ pH wynosi 11,60, pOH wynosi 2,40, a stężenie jonów wodorotlenkowych wynosi 4,0 × 10−3. 
Tabela ICE może być skonstruowana na podstawie początkowych i równowagowych stężeń metyloaminy, metyloamonu i wodorotlenku.
Podstawienie stężeń równowagowych w wyrażeniu dla Kb daje wartość dla Kb, która wynosi 4,4 × 10−4.

słabe roztwory zasadowe

Niektóre związki po rozpuszczeniu w wyniku reakcji chemicznej z cząsteczkami wody wytwarzają jony wodorotlenkowe. We wszystkich przypadkach związki te reagują tylko częściowo i dlatego są klasyfikowane jako słabe zasady. Tego typu związki są również bogate w przyrodzie i stanowią ważne surowce w różnych technologiach. Na przykład globalna produkcja słabego amoniaku zasadowego wynosi zwykle znacznie ponad 100 ton rocznie i jest on powszechnie stosowany jako nawóz rolniczy, surowiec do syntezy chemicznej innych związków oraz aktywny składnik domowych środków czyszczących. Po rozpuszczeniu w wodzie amoniak częściowo reaguje, tworząc jony wodorotlenkowe, jak pokazano poniżej:

<img alt="Eq1" src="/files/ftp_upload/11401/11401_Equation_1.png" style="height: 50px;" />

Z definicji jest to reakcja kwasowo-zasadowa, w tym przypadku polegająca na przeniesieniu jonów H+ z cząsteczek wody do cząsteczek amoniaku. W typowych warunkach tylko około 1% rozpuszczonego amoniaku występuje w postaci jonów NH4+.

Obliczanie stężeń jonów wodorotlenkowych i pOH w roztworze słabej zasady

Znajdźmy stężenie jonu wodorotlenkowego, pOH i pH 0,25 M roztworu trimetyloaminy, słabej zasady:

<img alt="Eq2" src="/files/ftp_upload/11401/11401_Equation_2.png" style="height: 50px;" />

Tabela ICE dla tego systemu to

<div class="overflow">
<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;">Stężenie początkowe (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;">Zmień (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;">Stężenie równowagowe (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>
</div>

Podstawienie składników stężenia równowagowego do wyrażenia Kb daje

<img alt="Eq3" src="/files/ftp_upload/11401/11401_Equation_3.png" style="height: 90px;" />

Zakładając, że x << 0,25 i obliczając x, otrzymujemy

<img alt="Eq4" src="/files/ftp_upload/11401/11401_Equation_4.png" style="height: 50px;" />

Wartość ta jest mniejsza niż 5% stężenia początkowego (0,25), więc założenie jest uzasadnione.

Jak zdefiniowano w tabeli ICE, x jest równe równowagowemu stężeniu jonu wodorotlenkowego:

<img alt="Eq5" src="/files/ftp_upload/11401/11401_Equation_5.png" style="height: 50px;" />

Oblicza się, że pOH wynosi

<img alt="Eq6" src="/files/ftp_upload/11401/11401_Equation_6.png" style="height: 50px;" />

Korzystanie z relacji;

<img alt="Eq7" src="/files/ftp_upload/11401/11401_Equation_7.png" style="height: 50px;" />

pozwala na obliczenie pH:

<img alt="Eq8" src="/files/ftp_upload/11401/11401_Equation_8.png" style="height: 50px;" />

Oznaczanie Kb na podstawie pH

Jeśli pH 0,28 M roztworu etyloaminy (C2H5NH2) wynosi 12,10, jaka jest jego Kb?

<img alt="Eq9" src="/files/ftp_upload/11401/11401_Equation_9.png" style="height: 50px;" />

Aby obliczyć Kb etyloaminy, najpierw należy określić pOH i stężenie jonów wodorotlenkowych. Ponieważ pH wynosi 12,10, pOH można obliczyć w następujący sposób:

<img alt="Eq10" src="/files/ftp_upload/11401/11401_Equation_10.png" style="height: 50px;" />

Ponieważ pOH wynosi 1,90, stężenie jonów wodorotlenkowych w roztworze można obliczyć za pomocą wzoru

<img alt="Eq11" src="/files/ftp_upload/11401/11401_Equation_11.png" style="height: 50px;" />

Tablicę ICE można skonstruować dla tego systemu w następujący sposób

<div class="overflow">
<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 (wodny)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">C2H5NH3+ (wodny)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">OH− (wodny)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Stężenie początkowe (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;">Zmiana (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;">Stężenie równowagowe (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>
</div>

Ponieważ 0,0126 M wynosi 4,5% z 0,28 M, 0,28 - 0,0126 można uznać za prawie równe 0,28 M zgodnie z zasadą 5%.

Po podstawieniu powyższych wartości do wyrażenia Kb etyloaminy,

<img alt="Eq12" src="/files/ftp_upload/11401/11401_Equation_12.png" style="height: 90px;" />

Słabe roztwory zasadowe

Niektóre związki wytwarzają jony wodorotlenkowe po rozpuszczeniu w reakcji chemicznej z cząsteczkami wody. We wszystkich przypadkach związki te reagują tylko częściowo i ...

Education

JoVE Core

Chemistry

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Jeden

Acids and Bases

Kwasy i zasady

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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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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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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

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Construct an explanation to classify matter as a pure substance or mixture; homogeneous or heterogeneous; element or compound; solution, colloid or suspension.

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Phase Transitions: Vaporization and Condensation

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Vapor Pressure

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The equilibrium between a liquid and its vapor depends on the temperature of the system; a rise in temperature causes a corresponding rise in the vapor ...

Phase Transitions: Melting and Freezing

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Phase Transitions: Sublimation and Deposition

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Heating and Cooling Curves

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Phase Diagrams

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Use models to compare the phases of matter and the physical changes they undergo.

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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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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

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Intermolecular Forces

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

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

PS.PSc.1.3

The Atomic Theory of Matter

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Subatomic Particles

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The Bohr Model

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Emission Spectra

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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.

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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

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Periodic Classification of the Elements

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Atomic Radii and Effective Nuclear Charge

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Ionic Radii

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Ionization Energy

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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

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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; ...

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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