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

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Chemistry

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PS.Chm.6.3

Weak Acid Solutions

Un ácido débil, como el ácido cianhídrico, es un ácido de Brønsted, ya que dona un protón a la molécula de agua y produce el ion de hidronio. Un ácido débil se disocia parcialmente en agua de acuerdo con su constante de disociación ácida, Ka, que es 4, 9 10⁻¹⁰ para el ácido cianhídrico. Para el ácido cianhídrico, el Ka es igual a la concentración de hidronio multiplicada por la concentración de iones de cianuro dividida por la concentración de ácido cianhídrico.La constante de disociación ácida, Ka, se puede utilizar para determinar la concentración de iones de hidronio en una solución de ácido débil y, en consecuencia, el pH de la solución. La concentración de iones de hidronio y el pH de una solución 0, 15 molar de ácido cianhídrico se pueden calcular utilizando su expresión de equilibrio y una tabla ICE. Las concentraciones de ácido cianhídrico, hidronio, y cianuro inicialmente y en equilibrio se pueden expresar en una tabla que muestra las Concentraciones Iniciales, de Cambio y Equilibrio de cada una de las moléculas.Para alcanzar el equilibrio, la concentración inicial de los reactivos disminuye a medida que aumenta la concentración inicial de los productos de acuerdo con sus relaciones molares. Este cambio en la concentración de reactivos y productos se denota con una x. Si se substituyen las concentraciones de equilibrio en la expresión Ka da como resultado x multiplicado por x dividido por 0, 15 menos x.En muchos ácidos débiles, x, la cantidad de disociación, es probable que sea muy pequeña en comparación con la concentración inicial de 0, 15 molar Se puede suponer que 0, 15 menos x es aproximadamente 0, 15. Cuando se resuelve la ecuación, x es igual a 8, 6 10⁻⁶ molar. La aproximación, 0, 15 menos x igual a 0, 15, es válida sólo si x es menor que el 5 de 0, 15 molar.Aquí, x es 0, 0057%de 0, 15 molar y, por lo tanto, esta aproximación es válida. Por lo tanto, la concentración de hidronio es de 8, 6 x 10⁻⁶ molar. Para determinar el pH, considere el logaritmo negativo de la concentración de iones de hidronio.Resolver esto muestra que el pH de la solución de ácido cianhídrico 0, 15 molar es 5, 07. El pH de una solución se puede utilizar para determinar el Ka de un ácido débil. Por ejemplo, el ácido acético cuando se disuelve en agua, se disocia parcialmente en iones de hidronio e iones de acetato.El Ka del ácido acético se puede expresar como la concentración de iones de hidronio multiplicada por la concentración de iones de acetato dividida por la concentración de ácido acético. Si el pH de una solución de ácido acético 0, 20 molar es 2, 72, se puede calcular su concentración de hidronio, que es 1, 9 x 10⁻³ molar. La tabla ICE se puede construir a partir de las concentraciones iniciales y de equilibrio de los iones de ácido acético, hidronio e iones de acetato.Utilizando cifras significativas, 0, 20 menos 1, 9 10⁻³ es esencialmente igual a 0, 20. Si se substituyen los valores de equilibrio en la expresión de Ka, Ka es igual a 1, 8 10⁻⁵.

soluciones de Ácidos débiles

Pocos compuestos actúan como ácidos fuertes. Un número mucho mayor de compuestos se comporta como ácidos débiles y sólo reaccionan parcialmente con el agua, dejando una gran mayoría de moléculas disueltas en su forma original y generando una cantidad relativamente pequeña de iones hidronio. Los ácidos débiles se encuentran comúnmente en la naturaleza, siendo las sustancias en parte responsables del sabor picante de los cítricos, la sensación de picadura de las picaduras de los insectos, y los olores desagradables asociados con el olor corporal. Un ejemplo familiar de un ácido débil es el ácido acético, el principal ingrediente del vinagre:
 <img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_1.png" style="height: 50px;" />
 El uso de una flecha doble en la ecuación anterior indica el carácter de reacción parcial de este proceso. Cuando se disuelven en agua en condiciones típicas, sólo alrededor del 1% de las moléculas de ácido acético están presentes en forma ionizada, CH3COO−.
 Cálculo de las concentraciones de iones hidronio y el pH de una solución ácida débil
 El ácido fórmico, HCO2H, es un irritante que causa la reacción del cuerpo a algunas mordeduras y picaduras de hormigas. ¿Cuál es la concentración de iones hidronio y el pH de una solución 0,534-M de ácido fórmico?
 <img alt="Eq2" src="/files/ftp_upload/11400/11400_Equation_2.png" style="height: 50px;" />
 La tabla ICE para este sistema es
 <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(ac)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3O+(ac)</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;">Concentración inicial (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;">Cambio (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;">Concentración de equilibrio (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>
 Al sustituir los términos de concentración en equilibrio en la expresión Ka queda
 <img alt="Eq3" src="/files/ftp_upload/11400/11400_Equation_3.png" style="height: 90px;" />
 La concentración inicial relativamente grande y la constante de equilibrio pequeña permiten la suposición de simplificación de que x será mucho menor que 0,534, y así la ecuación se convierte en
 <img alt="Eq4" src="/files/ftp_upload/11400/11400_Equation_4.png" style="height: 90px;" />
 Al resolver la ecuación para x se obtiene
 <img alt="Eq5" src="/files/ftp_upload/11400/11400_Equation_5.png" style="height: 90px;" />
 Para comprobar la suposición de que x es pequeña en comparación con 0,534, se puede estimar su magnitud relativa:
 <img alt="Eq6" src="/files/ftp_upload/11400/11400_Equation_6.png" style="height: 90px;" />
 Debido a que x es menos del 5% de la concentración inicial, la suposición es válida. Como se define en la tabla ICE, x es igual a la concentración del ion hidronio en equilibrio:
 <img alt="Eq7" src="/files/ftp_upload/11400/11400_Equation_7.png" style="height: 50px;" />
 Finalmente, el pH se calcula que es
 <img alt="Eq8" src="/files/ftp_upload/11400/11400_Equation_8.png" style="height: 50px;" />
 Determinación de Ka a partir del pH
 El pH de una solución de ácido nitroso de 0,0516 M, HNO2, es 2,34. ¿Cuál es su Ka?
 <img alt="Eq9" src="/files/ftp_upload/11400/11400_Equation_9.png" style="height: 50px;" />
 La concentración de ácido nitroso proporcionada es una concentración formal, que no tiene en cuenta ningún equilibrio químico que pueda establecerse en solución. Dichas concentraciones se tratan como valores “iniciales” para los cálculos de equilibrio utilizando el método de la tabla ICE. Observe que el valor inicial del ion hidronio se indica como aproximadamente cero porque hay una pequeña concentración de H3O+ (1 × 10−7 M) debido a la autoionización del agua. En muchos casos, esta concentración es mucho menor que la generada por la ionización del ácido (o base) en cuestión y puede ser desestimada.
 El pH proporcionado es una medida logarítmica de la concentración de iones hidronio resultante de la ionización ácida del ácido nitroso, por lo que representa un valor de “equilibrio” para la tabla ICE:
 <img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_10.png" style="height: 50px;" />
 La tabla ICE para este sistema es entonces
 <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 (ac)  </td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3O+(ac)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">NO2−(ac)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Concentración inicial (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;">Cambio (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;">Concentración de equilibrio (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>
 Finalmente, calcule el valor de la constante de equilibrio utilizando los datos de la tabla:
 <img alt="Eq11" src="/files/ftp_upload/11400/11400_Equation_11.png" style="height: 90px;" />
 Este texto es adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions"> Openstax, Química 2e, Sección 4.2: Clasificación de las Reacciones Químicas</a> y <a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases"> Openstax, Química 2e,14.3 Fuerzas Relativas de Ácidos y Bases.</a>

Soluciones de Ácidos Débiles

Pocos compuestos actúan como ácidos fuertes. Un número mucho mayor de compuestos se comportan como ácidos débiles y solo reaccionan parcialmente con el agua, dejando una gran mayoría 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 as an Amphiprotic Substance

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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relative-strengths-of-conjugate-acid-base-pairs

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, 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 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

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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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...

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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...

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

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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...

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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

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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Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

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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