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

Słaby kwas, taki jak kwas cyjanowodorowy, jest kwasem Brønsteda, ponieważ przekazuje proton cząsteczce wody i wytwarza jon hydroniowy. Słaby kwas dysocjuje częściowo w wodzie zgodnie ze swoją stałą dysocjacji kwasu, Ka, która wynosi 4,9 × 10−10 dla kwasu cyjanowodorowego.
W przypadku kwasu cyjanowodorowego Ka jest równe stężeniu hydronu pomnożonemu stężeniu jonów cyjanku podzielonemu przez stężenie kwasu cyjanowodorowego.
Stała dysocjacji kwasu, Ka, może być wykorzystana do określenia stężenia jonów hydroniowych w roztworze słabego kwasu, a co za tym idzie, pH roztworu.
Stężenie jonów hydroniowych i pH 0,15-molowego roztworu kwasu cyjanowodorowego można obliczyć za pomocą jego wyrażenia równowagowego i tabeli ICE.
Stężenia kwasu cyjanowodorowego, hydroniku i cyjanku początkowo i w stanie równowagi można wyrazić w tabeli, która pokazuje początkowe, zmienne i równowagowe stężenia każdej z cząsteczek.
Aby osiągnąć równowagę, początkowe stężenie reagentów zmniejsza się wraz ze wzrostem początkowego stężenia produktów zgodnie z ich stosunkami molowymi. Ta zmiana stężenia reagentów i produktów jest oznaczona przez x.
Podstawienie stężeń równowagowych w wyrażeniu dla Ka daje x razy x podzielone przez 0,15 minus x. 
W wielu słabych kwasach x, stopień dysocjacji, może być bardzo mały w porównaniu z początkowym stężeniem 0,15 mola. Można założyć, że 0,15 minus x wynosi około 0,15.
Gdy równanie zostanie rozwiązane, x jest równe 8,6 × 10−6 molowym.
Przybliżenie 0,15 minus x równe 0,15 jest ważne tylko wtedy, gdy x jest mniejsze niż 5% z 0,15 mola. Tutaj x wynosi 0,0057% z 0,15 mola, a zatem to przybliżenie jest prawidłowe.
Dlatego stężenie hydroniu wynosi 8,6 × 10−6 molowych. Aby określić pH, weź ujemny log stężenia jonów hydroniowych. Rozwiązanie tego pokazuje, że pH 0,15 M roztworu kwasu cyjanowodorowego wynosi 5,07.
pH roztworu można wykorzystać do oznaczenia Ka słabego kwasu.
Na przykład kwas octowy po rozpuszczeniu w wodzie dysocjuje częściowo na jony hydroniowe i jony octanowe. Ka dla kwasu octowego można wyrazić jako stężenie jonów hydroniowych pomnożone przez stężenie jonów octanowych podzielone przez stężenie kwasu octowego.
Jeśli pH 0,20 molowego roztworu kwasu octowego wynosi 2,72, można obliczyć jego stężenie hydroniowe, które wynosi 1,9 × 10−3 molowe.
Tabela ICE może być skonstruowana na podstawie początkowych i równowagowych stężeń kwasu octowego, jonów hydroniowych i jonów octanowych. 
Używając liczb znaczących, 0,20 minus 1,9 × 10−3 jest zasadniczo równe 0,20. Podstawiając wartości równowagi do wyrażenia Ka, K a wynosi 1,8 × 10−5.

roztwory słabych kwasów

Niewiele związków działa jak mocne kwasy. Znacznie większa liczba związków zachowuje się jak słabe kwasy i tylko częściowo reaguje z wodą, pozostawiając zdecydowaną większość rozpuszczonych cząsteczek w ich pierwotnej postaci i wytwarzając stosunkowo niewielką ilość jonów hydroniowych. Słabe kwasy są powszechnie spotykane w przyrodzie i są substancjami częściowo odpowiedzialnymi za pikantny smak owoców cytrusowych, uczucie pieczenia po ukąszeniach owadów i nieprzyjemny zapach ciała. Znanym przykładem słabego kwasu jest kwas octowy, główny składnik octu:

<img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_1.png" style="height: 50px;" />

Użycie podwójnej strzałki w powyższym równaniu oznacza częściowy aspekt reakcji tego procesu. Po rozpuszczeniu w wodzie w typowych warunkach tylko około 1% cząsteczek kwasu octowego występuje w postaci zjonizowanej, CH3COO-.

Obliczanie stężenia jonów hydroniowych i pH roztworu słabego kwasu

Kwas mrówkowy, HCO2H, jest substancją drażniącą, która powoduje reakcję organizmu na ukąszenia i użądlenia niektórych mrówek. Jakie jest stężenie jonu hydroniowego i pH 0,534-M roztworu kwasu mrówkowego?

<img alt="Eq2" src="/files/ftp_upload/11400/11400_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;"> HCO2H (wodny) </td>
			<td style="color: black; border: 1px solid black; text-align: center;">H3O+ (wodny)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">HCO2− (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;">0.534</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;">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;">0.534 − 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>
</div>

Podstawienie warunków stężenia równowagowego do wyrażenia Ka daje

<img alt="Eq3" src="/files/ftp_upload/11400/11400_Equation_3.png" style="height: 90px;" />

Stosunkowo duże stężenie początkowe i mała stała równowagi pozwala na uproszczone założenie, że x będzie znacznie mniejsze niż 0,534, więc równanie ma postać

<img alt="Eq4" src="/files/ftp_upload/11400/11400_Equation_4.png" style="height: 90px;" />

Rozwiązanie równania na x daje wynik

<img alt="Eq5" src="/files/ftp_upload/11400/11400_Equation_5.png" style="height: 90px;" />

Aby sprawdzić założenie, że x jest małe w porównaniu z 0,534, można oszacować jego względną wielkość:

<img alt="Eq6" src="/files/ftp_upload/11400/11400_Equation_6.png" style="height: 90px;" />

Ponieważ x jest mniejsze niż 5% stężenia początkowego, założenie jest prawidłowe. Jak zdefiniowano w tabeli ICE, x jest równe równowagowemu stężeniu jonu hydroniowego:

<img alt="Eq7" src="/files/ftp_upload/11400/11400_Equation_7.png" style="height: 50px;" />

Na koniec oblicza się pH

<img alt="Eq8" src="/files/ftp_upload/11400/11400_Equation_8.png" style="height: 50px;" />

Oznaczanie Ka na podstawie pH

Wartość pH 0,0516 M roztworu kwasu azotawego HNO2 wynosi 2,34. Jakie jest jego Ka?

<img alt="Eq9" src="/files/ftp_upload/11400/11400_Equation_9.png" style="height: 50px;" />

Podane stężenie kwasu azotawego jest stężeniem formalnym, które nie uwzględnia żadnej równowagi chemicznej, jaką można ustalić w roztworze. Takie stężenia traktuje się jako wartości „początkowe” do obliczeń równowagi przy użyciu metody tabeli ICE. Należy zwrócić uwagę, że początkowa wartość jonu hydroniowego jest w przybliżeniu równa zeru, ponieważ występuje małe stężenie H3O+ (1 × 10-7 M) z powodu autojonizacji wody. W wielu przypadkach stężenie to jest znacznie mniejsze niż stężenie powstające w wyniku jonizacji danego kwasu (lub zasady) i można je pominąć.

Podane pH jest logarytmiczną miarą stężenia jonów hydroniowych wynikającego z jonizacji kwasowej kwasu azotawego, a zatem stanowi wartość „równowagową” dla tabeli ICE:

<img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_10.png" style="height: 50px;" />

Tabela ICE dla tego systemu to zatem

<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;"> HNO2 (wodny)  </td>
			<td style="color: black; border: 1px solid black; text-align: center;">H3O+ (wodny)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">NO2− (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;">0.0516</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;">Zmiana (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;">Stężenie równowagowe (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>
</div>

Na koniec oblicz wartość stałej równowagi, korzystając z danych z tabeli:

<img alt="Eq11" src="/files/ftp_upload/11400/11400_Equation_11.png" style="height: 90px;" />

Roztwory słabych kwasów

Niewiele związków działa jak mocne kwasy. Znacznie większa liczba związków zachowuje się jak słabe kwasy i tylko częściowo reaguje z wodą, pozostawiając zdecydowaną większość ...

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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

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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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PS.Chm.2.1

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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 ...

Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

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 ...

Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

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 ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

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 ...

Electronegativity

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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