Name: buffer ph after acid or base addition
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 45 s
Channel: JoVE

K12 Standards

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Interactions Of Matter

Chemical reactions

calculating-ph-changes-in-a-buffer-solution

Calculating pH Changes in a Buffer Solution

緩衝液中には、少量の強酸を添加したり、少量の強塩基を添加したりすると、わずかなpH変化が起こります。
このpH変化は、2つの異なるステップで計算されます。
まず、化学量論的計算を使用して濃度の変化を決定します。
次に、平衡計算を使用して、ICEテーブルまたはヘンダーソン・ハッセルバルヒ方程式を使用して、溶液の新しいpHを決定します。 
2.0 Mのフッ化水素酸とフッ化ナトリウムの両方を含む緩衝液のpHは、強酸または強塩基を添加する前と後で計算できます。
ヘンダーソン・ハッセルバルヒの式は、濃度が弱酸のKAに対して高く、濃度の変化が5%未満であるため、この緩衝液の初期pHを決定するために使用できます。
フッ化水素酸のpKaは3.46と計算できます。溶液中の弱酸と共役塩基の濃度が等しい場合、pHはpKaに等しくなります。したがって、初期pHは3.46です。
このバッファーの1リットルに0.2モルの塩酸を追加すると、体積の変化はごくわずかであると仮定して、追加された酸はフッ化物イオンによって中和され、フッ化水素酸が生成されます。これにより、フッ化物イオンの濃度が化学量論的に0.2モル減少し、フッ化水素酸濃度が等しく増加します。
新しい濃度は、ヘンダーソン・ハッセルバルヒ方程式に挿入できます。解くと、新しい pH は 3.37 になり、初期の pH 値である 3.46 よりも小さくなります。
強酸を添加するとpHが低下しますが、溶液が緩衝されているため低下は小さいです。
これに対して、1リットルの緩衝液に0.1モルの水酸化ナトリウムを添加すると、体積の変化はごくわずかであると仮定して、添加された塩基はフッ化水素酸と反応して中和されます。これにより、フッ化水素酸濃度が化学量論的に0.1モル減少し、フッ化物イオンが等しく増加します。
ヘンダーソン・ハッセルバルヒの式を使用すると、溶液のpHは3.50であり、水酸化ナトリウムが添加されているため、初期pH値である3.46よりもわずかに高くなります。

緩衝液中のphの変化の計算

緩衝剤は、その緩衝能まで強酸または強塩基を加えても溶液pHの急激な低下または上昇を防ぐことができます。しかしながら、そのような強酸または強塩基の添加は、溶液のわずかなpH変化をもたらします。このわずかなpH変化は、結果として生じる緩衝成分、すなわち弱酸とその共役塩基、またはその逆の濃度変化を求めることで算出できます。これらの化学量論的な計算で得られた濃度は、ヘンダーソン-ハッセルバルヒ式またはICE表を使用して、溶液’の最終的なpHを決定するために使用することができます。
 例えば、0.65 molのギ酸とギ酸ナトリウムを含む緩衝液を考えます。ここでは弱酸とその共役塩基の濃度が同じなので、溶液のpHは弱酸のpKaに等しく、この場合は3.74となります。この溶液に0.05 molのHNO3を加えた際のギ酸およびギ酸ナトリウムの濃度変化は、化学量論的計算によって下表のように求められます。
 <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;">H+ (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCOO− (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCOOH (aq)  </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">添加前 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">~0.00 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.65 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.65 mol</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">添加量 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.050 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">-</td>
 <td style="color: black; border: 1px solid black; text-align: center;">-</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">添加後 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">~0.00 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.60 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.70 mol</td>
 </tr>
 </tbody>
 </table>
 その後、ヘンダーソン・ハッセルバルヒ式にギ酸とギ酸ナトリウムの新たな濃度を代入することで、溶液の最終的なpHを求めることができます。
 <img alt="Eq1" src="/files/ftp_upload/11411/11411_Equation_1.png" style="text-align: center; height: 165px;" />
 このように、0.05 molのHNO3 を加えることにより、溶液のpHは3.74から3.67に低下します。
 同様に、同じ溶液に0.10 molのNaOHを加えた場合、ギ酸とギ酸ナトリウムの濃度の変化は、化学量論的計算によって下表のように求めることができます。
 <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;">OH− (aq) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCOOH (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCOO− (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H2O (l)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">添加前 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">~0.00 mol </td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.65 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.65 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">-­</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">添加量 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.10 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">-</td>
 <td style="color: black; border: 1px solid black; text-align: center;">-</td>
 <td style="color: black; border: 1px solid black; text-align: center;">­-</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">添加後 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">~0.00 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.55 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.75 mol</td>
 <td style="color: black; border: 1px solid black; text-align: center;">­-</td>
 </tr>
 </tbody>
 </table>
 その後、ヘンダーソン・ハッセルバルヒ式にギ酸とギ酸ナトリウムの新たな濃度を代入することで、溶液の最終的なpHを求めることができます。
 <img alt="Eq2" src="/files/ftp_upload/11411/11411_Equation_2.png" style="text-align: center; height: 165px;" />
 したがって、0.10 molのNaOHを加えると、溶液のpHは3.74から3.87に上昇します。

緩衝液中のpHの変化の計算

緩衝液は、強酸または強塩基を添加した後の溶液のpHの急激な低下または上昇をその緩衝能力まで防ぐことができます。しかし。。。

Education

JoVE Core

化学

Acid-base and Solubility Equilibria

酸塩基および溶解度平衡

KEY TERMS AND CONCEPTS

主要用語と概念

buffer ph after acid or base addition

A buffer solution can resist sudden pH changes when a strong acid or strong base is added. It does not stop the pH from changing completely. The shift is usually small, as long as the addition stays within the buffer’s capacity.

To find the new pH, first determine how the added acid or base changes the amounts of the buffer components. A buffer contains a weak acid and its conjugate base, or the reverse pair. These amounts are found with stoichiometric calculations, which track how much of each substance reacts.

The updated concentrations can then be used in the Henderson-Hasselbalch equation. An ICE table can also be used. In the example, the buffer contains 0.65 mol of formic acid and 0.65 mol of sodium formate. Because the weak acid and conjugate base are equal, the pH starts at the pKa of formic acid, which is 3.74.

When 0.05 mol HNO3 is added, the hydrogen ions react with formate. After the reaction, the solution contains 0.60 mol sodium formate and 0.70 mol formic acid. Using these new values in the Henderson-Hasselbalch equation gives a final pH of 3.67, down from 3.74.

The same method works when a strong base is added. If 0.10 mol NaOH is added to the same buffer, hydroxide reacts with formic acid. The amounts change to 0.55 mol formic acid and 0.75 mol sodium formate. The final pH then rises to 3.87.

Buffer pH After Acid or Base Addition

common ion effect

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buffers

Buffer Solutions and pH Stability

Buffer solutions are mixtures that help keep pH steady. A buffer contains an appreciable amount of a weak conjugate acid-base pair. It resists a change in pH when small amounts of a strong acid or a strong base are added. One common buffer uses a weak acid and its salt. Acetic acid and sodium acetate form this type of system, written as CH3COOH(aq) + CH3COONa(aq). Another buffer uses a weak base and its salt. A solution of ammonia and ammonium chloride, NH3(aq) + NH4Cl(aq), is an example. The...

calculating ph changes in a buffer solution

A buffer solution can resist sudden pH changes when a strong acid or strong base is added. It does not stop the pH from changing completely. The shift is usually small, as long as the addition stays within the buffer’s capacity. To find the new pH, first determine how the added acid or base changes the amounts of the buffer components. A buffer contains a weak acid and its conjugate base, or the reverse pair. These amounts are found with stoichiometric calculations, which track how much of...

buffer effectiveness

Buffer Capacity and pH Stability

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titration calculations: strong acid - strong base

titration-calculations-strong-acid-strong-base

pH at Key Points in Acid-Base Titration

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titration calculations: weak acid - strong base

titration-calculations-weak-acid-strong-base

pH at Key Points in a Weak Acid Titration

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indicators

Acid-Base Color Changes

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titration of a polyprotic acid

Polyprotic Acid Equivalence Points

Polyprotic acids have more than one ionizable hydrogen, so they lose protons in steps during titration. When the acid dissociation constants for those protons are far enough apart, the titration curve shows a separate equivalence point for each hydrogen that can ionize. Diprotic acids have two ionizable hydrogens and produce two equivalence points. Triprotic acids have three ionizable hydrogens and produce three equivalence points. Carbonic acid, H2CO3, is a weak diprotic acid. Carbonic acid...

solubility equilibria

Predicting Precipitation with Ksp

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factors affecting solubility

Common Ion Effects on Solubility

The common ion effect lowers the solubility of an ionic compound in water when the solution already contains one of the ions made by that compound. It follows the law of mass action and can be explained with Le Châtelier’s principle. When a common ion is added, the dissolution equilibrium shifts toward the solid, so less of the salt stays dissolved. Silver iodide shows this effect clearly. If silver ions or iodide ions are added to a solution of AgI, the equilibrium shifts left and more AgI...

formation of complex ions

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precipitation of ions

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qualitative analysis

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henderson-hasselbalch equation

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acid-base titration curves

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Acid-base titration uses pH changes to track how a solution responds as titrant is added. A titration curve is a graph of a solution property versus the amount of added titrant. In acid-base titrations, pH is the main property to follow because it changes in a predictable way with the solution makeup and helps show the endpoint. This pattern can be seen with a strong acid and strong base, a strong acid and weak base, or a strong base and weak acid. For example, 25.00 mL of 0.100 M HCl titrated...

key terms and concepts

Learn how buffers resist pH change after strong acid or base is added. Use stoichiometry and the Henderson-Hasselbalch equation to find the new pH.

Buffer pH After Acid or Base Addition | JoVE K12

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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CH.5.d

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CHEM.12

CHEM.12.E

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• Equilibrium
• Acids/bases

Content elaboration: In the Physical Science course, coefficients were used to balance simple equations. Other representations, including Lewis structures and three-dimensional models, were also used and manipulated to demonstrate the conservation of matter in chemical reactions. In this course, more complex reactions will be studied, classified and represented with balanced chemical equations and three-dimensional models. Classifying reactions into types can be a helpful organizational tool for recognizing patterns of what may happen when two substances are mixed. Teachers should be aware that the common reaction classifications that are often used in high school chemistry courses may lead to misconceptions because they are not based on the actual chemistry, but on surface features that can be similar from one system to another (e.g., exchanging partners), even though the underlying chemistry is not the same. However, these classifications may be useful in making predictions about what happens when two substances are mixed. Some general types of chemical reactions are oxidation/reduction, synthesis, decomposition, single replacement, double replacement (including precipitation reactions and some acid-base neutralizations) and combustion reactions. Some reactions can fit into more than one category. For example, a single replacement reaction can also be classified as an oxidation/reduction reaction. Identification of reactions involving oxidation and reduction as well as indicating what substance is being oxidized and what is being reduced are appropriate in this course. However, balancing complex oxidation/reduction reactions is reserved for more advanced study. Organic molecules release energy when undergoing combustion reactions and are used to meet the energy needs of society (e.g., oil, gasoline, natural gas) and to provide the energy needs of biological organisms (e.g., cellular respiration). When a reaction between two ionic compounds in aqueous solution results in the formation of a precipitate or molecular compound, the reaction often occurs because the new ionic or covalent bonds are stronger than the original ion-dipole interactions of the ions in solution. Laboratory experiences (3-D or virtual) with different types of chemical reactions should be provided. Reactions occur when reacting particles collide in an appropriate orientation and with sufficient energy. The rate of a chemical reaction is the change in the amount of the reactants or products in a specific period of time. Increasing the probability or effectiveness of the collisions between the particles increases the rate of the reaction. Therefore, changing the concentration of the reactants, changing the temperature or the pressure of gaseous reactants, or using a catalyst, can change the reaction rate. Likewise, the collision theory can be applied to dissolving solids in a liquid solvent and can be used to explain why reactions are more likely to occur between reactants in the aqueous or gaseous state than between solids. The rate at which a substance dissolves should not be confused with the amount of solute that can dissolve in a given amount of solvent (solubility). Mathematical treatment of reaction rates is reserved for more advanced study. Computer simulations can help visualize reactions from the perspective of the kinetic-molecular theory. In middle school, the differences between potential and kinetic energy and the particle nature of thermal energy were introduced. For chemical systems, potential energy is in the form of chemical energy and kinetic energy is in the form of thermal energy. The total amount of chemical energy and/or thermal energy in a system is impossible to measure. However, the energy change of a system can be calculated from measurements (mass and change in temperature) from calorimetry experiments in the laboratory. Conservation of energy is an important component of calorimetry equations. Thermal energy is the energy of a system due to the movement of its particles. The thermal energy of an object depends upon the temperature and chemical composition. Some materials require little energy to change their temperature and other materials require a great deal to change their temperature by the same amount. Specific heat is a measure of how much energy is needed to change the temperature of a specific mass of material a specific amount. Specific heat values can be used to calculate the thermal energy change, the temperature (initial, final or change in) or mass of a material in calorimetry. Water has a particularly high specific heat capacity, which is important in regulating Earth's temperature. As studied in middle school, chemical energy is the potential energy associated with chemical systems. Chemical reactions involve valence electrons forming bonds to yield more stable products with lower energies. Energy is required to break interactions and bonds between the reactant atoms and energy is released when an interaction or bond is formed between the atoms in the products. Molecules with weak bonds (e.g., ATP) are less stable and tend to react to produce more stable products, releasing energy in the process. Generally, energy is transferred out of the system (exothermic) when the products have stronger bonds than the reactants and is transferred into the system (endothermic) when the reactants have stronger bonds than the products. Predictions of the energy requirements (endothermic or exothermic) of a reaction can be made given a table of bond energies. Graphic representations can be drawn and interpreted to represent the energy changes during a reaction. The role of energy in determining the spontaneity of chemical reactions is dealt with conceptually in this course. Entropy and its influence on the spontaneity of reactions are reserved for more advanced study. All reactions are reversible to a degree and many reactions do not proceed completely toward products but appear to stop progressing before the reactants are all used up. At this point, the amounts of the reactants and the products appear to be constant and the reaction can be said to have reached dynamic equilibrium. Dynamic equilibrium means the rate of the reverse reaction is equal to the rate of the forward reaction so there is no apparent change in the reaction. If a chemical system at equilibrium is disturbed by a change in the conditions of the system (e.g., increase or decrease in the temperature, pressure on gaseous equilibrium systems, concentration of a reactant or product), then the equilibrium system will respond by shifting to a new equilibrium state, reducing the effect of the change (Le Chatelier's Principle). If products are removed as they are formed during a reaction, then the equilibrium position of the system is forced to shift to favor the products. In this way, an otherwise unfavorable reaction can be made to occur. Mathematical treatment of equilibrium reactions is reserved for advanced study. Computer simulations can help visualize the progression of a reaction to dynamic equilibrium and the continuation of both the forward and reverse reactions after equilibrium has been attained. Properties of acids and bases and the ranges of the pH scale were introduced in Physical Science. In this course, the structural features of molecules are explored to further understand acids and bases. Acids often result when hydrogen is covalently bonded to an electronegative element and is easily dissociated from the rest of the molecule to bind with water to form a hydronium ion (H3O+). The acidity of an aqueous solution can be expressed as pH, where pH can be calculated from the concentration of the hydronium ion. Bases are likely to dissociate in water to form a hydroxide ion. Acids can react with bases to form a salt and water. Such neutralization reactions can be studied quantitatively by performing titration experiments.

Buffer pH After Acid or Base Addition

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