Name: weak acid equilibrium in water
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 4 min 2 s
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Weak Acid Solutions

Un acide faible, comme l'acide cyanhydrique, est un acide de Brønsted car il donne un proton à la molécule d'eau et produit l'ion hydronium. Un acide faible se dissocie partiellement dans l'eau selon sa constante de dissociation acide, Ka, qui est de 4, 9 10⁻¹⁰ pour l'acide cyanhydrique. Pour l'acide cyanhydrique, le Ka est égal à la concentration d'hydronium multiplié par la concentration d'ions cyanure divisés par la concentration d'acide cyanhydrique.La constante de dissociation acide, Ka, peut être utilisé pour déterminer la concentration en ions hydronium dans une solution d'acide faible et par conséquent, le pH de la solution. La concentration d'ions hydronium et le pH d'une solution de 0, 15 molaire d'acide cyanhydrique peuvent être calculés en utilisant son expression d'équilibre et une table ICE. Les concentrations d'acide cyanhydrique, d'hydronium, et cyanure initialement et à l'équilibre peuvent être exprimés dans un tableau representant les concentrations initiale, de changement et d'équilibre de chacune des molécules.Pour atteindre l'équilibre, la concentration initiale des réactifs diminue alors que la concentration initiale des produits augmente en fonction de leur ratio molaire. Cette modification de la concentration des réactifs et des produits est désignée par x. En substituant les concentrations à l'équilibre dans l'expression du Ka, on obtient x fois x divisé par 0, 15 moins x.Dans de nombreux acides faibles, x, la quantité de dissociation, est susceptible d'être très faible par rapport à la concentration initiale de 0, 15 molaire. 0, 15 moins x peut être considéré comme étant approximativement égal à 0, 15. Lorsque l'équation est résolue, x est égal à 8, 6 10⁻⁶ molaire.Le rapprochement, 0, 15 moins x égal à 0, 15, n'est valable que si x est inférieur à 5%de 0, 15 molaire. Ici, x est 0, 0057%de 0, 15 molaire et donc ce rapprochement est valide. Par conséquent, la concentration d'hydronium est de 8, 6 10⁻⁶ molaire.Pour déterminer le pH, prenez le log négatif de la concentration d'hydronium ionique. La solution de ce problème montre que le pH de la solution d'acide cyanhydrique 0, 15 M est de 5, 07. Le pH d'une solution peut être utilisé pour déterminer le Ka d'un acide faible.Par exemple, l'acide acétique dissocie partiellement en ions hydronium et en ions acétate lorsqu'il est dissous dans l'eau. Le Ka de l'acide acétique peut être exprimé comme la concentration d'ion hydronium multipliée par la concentration en ions acétate divisée par la concentration d'acide acétique. Si le pH d'une solution d'acide acétique de 0, 20 molaire est de 2, 72, sa concentration d'hydronium peut être calculée, qui est de 1, 9 10⁻³ molaire.La table ICE peut être construite à partir des concentrations initiales et d'équilibre de l'acide acétique, des ions hydronium et des ions acétate. En utilisant des chiffres significatifs, 0, 20 moins 1, 9 10⁻³ est essentiellement égal à 0, 20. En substituant les valeurs d'équilibre dans l'expression Ka, Ka est égal à 1, 8 10⁻⁵.

solutions d'acides faibles

Peu de composés agissent comme des acides forts. Un nombre beaucoup plus élevé de composés se comportent comme des acides faibles et ne réagissent que partiellement avec l'eau, laissant une grande majorité de molécules dissoutes dans leur forme d'origine et générant une quantité relativement faible d'ions hydronium. Les acides faibles se trouvant généralement dans la nature, ce sont les substances en partie responsables du goût acidulé des agrumes, de la sensation de picotement des piqûres d'insectes, et des odeurs désagréables associées à l'odeur corporelle. L'acide acétique, principal ingrédient du vinaigre, est un exemple familier d'acide faible : 
 <img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_1.png" style="height: 50px;" />
 L'utilisation d'une double flèche dans l'équation ci-dessus indique l'aspect réaction partielle de ce processus. Lorsqu'elles sont dissoutes dans l'eau dans des conditions normales, seulement 1 % des molécules d'acide acétique sont présentes sous la forme ionisée, CH3COO−. 
 Calcul des concentrations en ions hydronium et du pH d' une solution d'acide faible 
 L’acide formique, HCO2H, est un irritant qui provoque la réaction du corps à certaines morsures et piqûres de fourmis. Quelle est la concentration de l'ion hydronium et le pH d'une solution d'acide formique à 0,534 M ? 
 <img alt="Eq2" src="/files/ftp_upload/11400/11400_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;"> HCO2H (aq) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3O+ (aq) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCO2− (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;">0,534 </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;">0,534 − 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>
 </tbody>
 </table>
 La substitution des termes de la concentration à l'équilibre dans l'expression de Ka donne 
 <img alt="Eq3" src="/files/ftp_upload/11400/11400_Equation_3.png" style="height: 90px;" />
 La concentration initiale relativement importante et la petite constante d'équilibre permettent pour simplifier de supposer que x sera beaucoup moins élevé que 0,534 et, ainsi, l'équation devient 
 <img alt="Eq4" src="/files/ftp_upload/11400/11400_Equation_4.png" style="height: 90px;" />
 La résolution de l'équation pour déterminer x donne 
 <img alt="Eq5" src="/files/ftp_upload/11400/11400_Equation_5.png" style="height: 90px;" />
 Il est possible d'estimer sa valeur absolue relative afin de vérifier la supposition que x est petit comparé à 0,534 : 
 <img alt="Eq6" src="/files/ftp_upload/11400/11400_Equation_6.png" style="height: 90px;" />
 Étant donné que x est inférieur à 5 % de la concentration initiale, la supposition est valable. Comme défini dans le tableau ICE, x est égal à la concentration à l'équilibre de l'ion hydronium : 
 <img alt="Eq7" src="/files/ftp_upload/11400/11400_Equation_7.png" style="height: 50px;" />
 Enfin, le pH est calculé à 
 <img alt="Eq8" src="/files/ftp_upload/11400/11400_Equation_8.png" style="height: 50px;" />
 Détermination de Ka à partir du pH 
 Le pH d'une solution de 0,0516 M d'acide nitreux, HNO2, est de 2,34. Quel est son Ka ? 
 <img alt="Eq9" src="/files/ftp_upload/11400/11400_Equation_9.png" style="height: 50px;" />
 La concentration de l'acide nitreux fournie est une concentration formelle qui ne tient pas compte des équilibres chimiques qui peuvent être établis en solution. De telles concentrations sont traitées comme des valeurs “ initiales ” pour les calculs d'équilibre utilisant l'approche du tableau ICE. On remarque que la valeur initiale de l'ion hydronium est indiquée comme étant approximativement nulle, car une faible concentration de H3O+ est présente (1 × 10−7 M) en raison de l'auto-ionisation de l'eau. Dans de nombreux cas, cette concentration est beaucoup moins importante que celle générée par l'ionisation de l'acide (ou de la base) en question et peut être négligée. 
 Le pH fourni est une mesure logarithmique de la concentration en ions hydronium résultant de l'ionisation acide de l'acide nitreux et représente donc une valeur d'équilibre pour le tableau ICE : 
 <img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_10.png" style="height: 50px;" />
 Le tableau ICE de ce système est alors 
 <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;"> HNO2 (aq)  </td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3O+ (aq) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">NO2− (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;">0,0516 </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;">−0,0046 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">+0,0046 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">+0,0046 </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;">0,0470 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">0,0046 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">0,0046 </td>
 </tr>
 </tbody>
 </table>
 Enfin, calculez la valeur de la constante d'équilibre à l'aide des données du tableau : 
 <img alt="Eq11" src="/files/ftp_upload/11400/11400_Equation_11.png" style="height: 90px;" />
 Ce texte est adapté de<a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions"> Openstax, Chimie 2e, Section 4.2 : Classifier les réactions chimiques</a> et <a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases"> Openstax, Chimie 2e,14.3 Forces relatives des acides et des bases. </a>

Solutions d'acides faibles

Peu de composés agissent comme des acides forts. Un nombre beaucoup plus important de composés se comportent comme des acides faibles et ne réagissent que partiellement avec l’eau, laissant une grande majorité de ...

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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 water is only partial. Under typical conditions, only about 1% of acetic acid molecules are present as the ionized form, CH3COO−.

Weak acid problems often use an ICE table to find hydronium ion concentration and pH. Formic acid, HCO2H, is one example because it can cause the irritation from some ant bites and stings. For a 0.534-M formic acid solution, the ICE table starts with formic acid, H3O+, and HCO2−, then tracks the change as x forms and the equilibrium amounts are written in terms of x. The Ka expression is set up from those equilibrium concentrations.

Because the initial formic acid concentration is much larger than the amount that reacts, x is assumed to be much less than 0.534. That lets the expression simplify and makes the calculation easier. After solving for x, the result is checked against the 5% rule. Since x is less than 5% of the initial concentration, the assumption is valid. The value of x equals the equilibrium hydronium ion concentration, and the pH is then calculated from that concentration.

The same ICE table method can also be used in reverse to find Ka from pH. For nitrous acid, HNO2, the given pH is 2.34 for a 0.0516-M solution. That concentration is a formal concentration, which means it does not include any equilibrium changes. In these setups, the initial hydronium ion is shown as approximately zero because water already contains a very small amount, about 1 × 10−7 M, from autoionization. In many cases, that amount is much smaller than the acid-produced hydronium ion and can be ignored.

The pH gives the equilibrium hydronium ion concentration for the nitrous acid solution. The ICE table then shows HNO2, H3O+, and NO2− changing to equilibrium values. Using those values in the equilibrium expression gives the Ka of the acid. This approach shows how pH and Ka connect to the behavior of weak acids in water.

Weak Acid Equilibrium in Water

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...

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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...

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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 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...

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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...

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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...

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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...

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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...

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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...

acid strength and molecular structure

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...

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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...

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Explore how weak acids partially ionize in water, use ICE tables to find hydronium ion concentration and pH, and calculate Ka from pH.

Weak Acid Equilibrium in Water | JoVE K12

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Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

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 ...

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.

Weak Acid Equilibrium in Water

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