Name: acid-base color changes
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 39 s
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K12 Standards

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Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Interactions Of Matter

Chemical reactions

Indicators

반응 지시약은 pH에 따라 색상이 변경되므로 솔루션의 pH를 측정하는 데 사용할 수 있습니다. pH 반응 지시약에는 HIn으로 대표되는 약산이 들어 있으며 이 산은 짝 염기인 In 이온으로 변환될 때 색이 변합니다. 색상은 반응 지시약의 pH 간격에 따라 변하며 pKa에 따라 달라집니다.

용액의 pH가 반응 지시약의 pKa보다 낮으면 하이드로늄 농도가 높은 것입니다. 반응의 평형은 왼쪽으로 이동하면서 HN의 농도를 증가시킵니다. 염기를 첨가하면 하이드로늄의 농도가 감소합니다.

이 pH 변화에 따라 HIn을 In 이온으로 변환하고 용액의 색상이 바뀝니다. HIN 색은 pH가 pKa보다 적어도 하나 낮을 때 두드러지고 In 이온 색상은 pH가 적어도 하나 높을 때 두드러지며 색은 그 사이의 pH의 혼합물입니다. 서로 다른 pH 범위에서 색상을 변경하는 pH를 모니터링하는 데 사용할 수 있는 pH 반응 지시약은 여러 가지가 있습니다.

반응 지시약이 색상을 변경하는 점을 종점이라고 합니다. 적정화에 이상적인 반응 지시약에는 당량점 근처에 종점이 있습니다. 당량점 근방에서는 pH 변화가 빠르기 때문에 강산과 염기의 적정 곡선이 가파릅니다.

이러한 적정에서는 비교적 광범위한 반응 지시약을 사용할 수 있습니다. 0.1 몰의 염산과 0.1 몰의 수산화나트륨을 적정화하는 동안 pH는 3에서 11로 빠르게 증가합니다. pH 간격이 8.3 에서10인 반응 지시약인 페놀프탈레인 또는 4.2 에서 6인 반응 지시약인 메틸레드는 이 반응 지시약의 종점이 곡선의 가파로운 끝부분과 겹치기 때문에 여기서 이 중 하나를 사용할 수 있습니다.

대조적으로 약산이나 염기를 포함하는 적정화는 강산과 강염기만을 포함한 것보다 곡선이 덜 가파롭기 때문에 상대적으로 좁은 범위의 반응 지시약을 사용할 수 있습니다 0.1 몰의 아세트산을 0.1 몰의 수산화나트륨과 함께 적정하는 동안 pH는 7에서 11까지 빠르게 증가합니다. 페놀프탈레인은 종점이 당량점과 겹치기 때문에 여기서 사용될 수 있지만 메틸 레드는 비효율적인 반응 지시약이 될 것입니다.

Coleman, William F. "지표의 분자 모델." 화학 교육 저널 87 no.1, (2008) : 1152 <a href = "https://pubs.acs.org/doi/pdf/10.1021/ed800038w">https://pubs.acs.org/doi/pdf/10.1021/ed800038w.</a>

지표

특정 유기 물질은 히드로늄 이온 농도가 특정 값에 도달하면 희석 용액에서 색이 변합니다. 예를 들어, 페놀프탈레인은 하이드로늄 이온 농도가 5.0 × 10^−9 M(pH < 8.3)보다 큰 모든 수용액에서 무색 물질입니다. 하이드로늄 이온 농도가 5.0 × 10^−9 M(pH > 8.3) 미만인 보다 기본적인 용액에서는 빨간색 또는 분홍색을 띕니다. 용액의 pH를 결정하는 데 사용할 수 있는 페놀프탈레인과 같은 물질을 산-염기 지시약이라고 합니다. 산-염기 지시약은 약한 유기산이거나 약한 유기 염기입니다.

약산인 산-염기 지시약 메틸 오렌지 용액의 평형은 복잡한 메틸 오렌지 분자에 대한 간단한 표현으로 HIn을 사용하는 방정식으로 표현될 수 있습니다.

<img alt="Eq1" src="/files/ftp_upload/11415/11415_Equation_1.png" style="text-align: center; height: 115px;" />

메틸 오렌지의 음이온인 In^-는 노란색이고, 이온화되지 않은 형태인 HIn은 빨간색입니다. 메틸 오렌지 용액에 산을 첨가하면 르 샤틀리에의 원리에 따라 하이드로늄 이온 농도가 증가하여 평형이 비이온화된 적색 형태로 이동합니다. 염기를 추가하면 평형이 노란색 형태로 이동합니다. 이 동작은 완충 용액의 동작과 완전히 유사합니다.

지시 용액의 인지된 색상은 두 종 In^− 및 HIn의 농도 비율에 의해 결정됩니다. 지시약의 대부분(일반적으로 약 60~90% 이상)이 In^−로 존재하는 경우 인지되는 용액의 색상은 노란색입니다. 대부분이 HIn으로 존재하면 용액 색상이 빨간색으로 나타납니다. 핸더슨-하셀바흐 방정식은 지시약 용액의 pH와 그 구성(따라서 인지된 색상) 사이의 관계를 이해하는 데 유용합니다.

<img alt="Eq1" src="/files/ftp_upload/11415/11415_Equation_2.png" style="text-align: center; height: 75px;" />

pH > pK_a인 용액에서 로그 항은 양수여야 하며 이는 지시약의 짝염기 형태(노란색 용액)가 과도함을 나타냅니다. pH > pK_a인 경우 로그 항은 음수여야 하며 이는 짝산(빨간색 용액)의 과잉을 나타냅니다. 용액 pH가 지시약 pK_a에 가까우면 상당한 양의 두 결합 파트너가 존재하고 용액 색상은 각각의 첨가제 조합의 색상입니다(노란색과 빨간색, 주황색 생성). 산염기 지시약의 색 변화 간격(또는 pH 간격)은 색 변화가 관찰되는 pH 값의 범위로 정의되며, 대부분의 지시약의 경우 이 범위는 대략 pK_a ± 1입니다.

광범위한 pH 값을 포괄하고 제거 과정에 따라 미지 용액의 대략적인 pH를 결정하는 데 사용할 수 있는 다양한 산-염기 지시약이 있습니다. 범용 지시약과 pH 종이에는 여러 가지 지시약이 혼합되어 있으며 다양한 pH에서 다양한 색상을 나타냅니다.

이 문서는 에서 발췌되었습니다 <a href="https://openstax.org/books/chemistry-2e/pages/14-7-acid-base-titrations"> Openstax, Chemistry 2e, Section 14.7: Acid-Base Titrations</a>.

특정 유기 물질은 히드로늄 이온 농도가 특정 값에 도달하면 희석 용액에서 색이 변합니다. 예를 들어, 페놀프탈레인 ...

Education

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JoVE Core

화학

하나

Acid-base and Solubility Equilibria

산-염기 및 용해도 평형

KEY TERMS AND CONCEPTS

주요 용어 및 개념

acid-base color changes

Acid-base indicators show color changes that help identify the pH of a solution. These are often weak organic acids or weak organic bases, and they respond to changes in hydronium ion concentration in dilute aqueous solution. Phenolphthalein is one example. It is colorless when the hydronium ion concentration is greater than 5.0 × 10^-9 M, which means the pH is below 8.3. In more basic solutions, where the hydronium ion concentration is lower, it turns red or pink.

Methyl orange shows how an indicator shifts between two forms. In this weak acid, the anion In− is yellow, while the nonionized form HIn is red. Adding acid increases the hydronium ion concentration and shifts the equilibrium toward the red form, following Le Châtelier’s principle. Adding base shifts the equilibrium toward the yellow form. This is similar to how buffers respond to changes in solution conditions.

The color seen in an indicator solution depends on the ratio of In− to HIn. If most of the indicator, usually about 60% to 90% or more, is present as In−, the solution looks yellow. If most is present as HIn, the solution looks red. When the pH is close to the indicator pKa, both forms are present in noticeable amounts, so the color is a mix of the two. That mix can appear orange.

The Henderson-Hasselbalch equation helps connect pH, pKa, and color. When pH is greater than pKa, the log term is positive and the conjugate base form is favored, so the solution appears yellow. When pH is less than pKa, the log term is negative and the conjugate acid is favored, so the solution appears red. The color change interval for most indicators is about pKa ± 1. Different indicators cover different pH ranges, so students can estimate the pH of an unknown solution by process of elimination. Universal indicators and pH paper use mixtures of indicators and show different colors across the pH scale.

Acid-Base Color Changes

common ion effect

Common Ion Effect on Solubility

The common ion effect reduces the solubility of an ionic compound in a solution that already contains one of its ions. In aqueous solutions, this means less of the solid dissolves than it would in pure water. The effect follows the law of mass action and can be explained with Le Châtelier’s principle. Silver iodide shows this equilibrium clearly. When silver(I) ions or iodide ions are added, the dissolution equilibrium shifts left. More AgI precipitates, and the concentrations of dissolved Ag+...

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

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

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...

buffer effectiveness

Buffer Capacity and pH Stability

Buffer solutions help keep pH fairly constant, but they do not work forever. Their resistance to pH change depends on having enough of a weak acid and its conjugate base, or a weak base and its conjugate acid, in the mixture. When too much strong acid or strong base is added, one part of the buffer pair is used up and the buffer action becomes weaker. Buffer capacity describes how much acid or base can be added to a fixed volume of buffer before the pH changes noticeably, usually by about one...

titration calculations: strong acid - strong base

titration-calculations-strong-acid-strong-base

pH at Key Points in Acid-Base Titration

pH at key points in a strong acid and strong base titration can be calculated from the amounts of acid and base present. The example uses 25.00 mL of 0.100 M HCl, a strong acid, and 0.100 M NaOH, a strong base. The pH changes as base is added, and each stage follows a different calculation. At 0 mL of titrant, the pH comes from the ionization of HCl. Because HCl is a strong acid, its ionization is complete. The hydronium ion molarity is 0.100 M, and the pH is found from that value. At 12.50...

titration calculations: weak acid - strong base

titration-calculations-weak-acid-strong-base

pH at Key Points in a Weak Acid Titration

pH at key points in a weak acid titration can be calculated for an acetic acid solution as sodium hydroxide is added. The example uses 25.00 mL of 0.100 M CH3CO2H titrated with 0.100 M NaOH. The reaction is shown as the weak acid reacts with the strong base. The initial pH is found from the acetic acid solution using the usual ICE approach. ICE means initial, change, and equilibrium, and it is a common way to set up equilibrium problems. The solution pH is calculated before any titrant is...

indicators

Acid-base indicators show color changes that help identify the pH of a solution. These are often weak organic acids or weak organic bases, and they respond to changes in hydronium ion concentration in dilute aqueous solution. Phenolphthalein is one example. It is colorless when the hydronium ion concentration is greater than 5.0 × 10^-9 M, which means the pH is below 8.3. In more basic solutions, where the hydronium ion concentration is lower, it turns red or pink. Methyl orange shows how an...

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

Solubility equilibria explain how a dissolved ionic compound and its solid form can exist in balance. In a saturated solution, the solute dissolves and the ions precipitate at equal rates. These ideas matter in real settings such as tooth decay and water purification, where compound solubility affects the outcome. A substance with finite solubility can form a saturated solution when more solute is added than the solvent can dissolve. The result is a heterogeneous mixture of the saturated...

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

Ligands and Complex Ion Stability

Complex ions form when a central atom, often a transition metal cation, binds to ligands. Ligands are ions or molecules around the metal ion. They can be neutral molecules such as H2O or NH3, or ions such as CN− or OH−. In many cases, the ligands act as Lewis bases by donating a pair of electrons to the central atom. These reactions are part of coordination chemistry, which studies coordination complexes. When a metal ion reacts with one or more ligands to form a complex, the equilibrium...

precipitation of ions

Solubility Limits and Selective Precipitation

Solubility limits and selective precipitation explain when ions form a solid in water. The key idea is the balance between an insoluble compound and its dissolved ions. In aqueous solutions that contain calcium ions and carbonate ions, this equilibrium can exist even if the solution was not made by saturating water with calcium carbonate. When two soluble salts are mixed, such as sodium carbonate and calcium nitrate, the ion concentrations determine whether a precipitate forms. If the reaction...

qualitative analysis

Cation Identification by Precipitation Tests

Cation identification by precipitation tests uses selective reactions to find metal ions in a mixture. In qualitative analysis, different reagents are added step by step. Each reaction can produce a characteristic precipitate for a specific group of cations. Other steps, such as changing pH, heating the mixture to dissolve a precipitate again, or adding reagents that form complex ions, help separate ions within the same group. Group IV cations such as Ba2+, Ca2+, and Mg2+ are identified...

henderson-hasselbalch equation

Buffer pH with the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation helps relate buffer pH to the weak acid in the solution. It comes from the ionization-constant expression for a weak acid. By rearranging that expression to solve for hydronium ion concentration, [H3O+], and then taking the negative logarithm of both sides, the equation can be written in terms of pH. In this form, pKa is defined as the negative logarithm of the weak acid ionization constant, or pKa = −log Ka. The equation shows the connection between pH, pKa,...

acid-base titration curves

Tracking pH During Acid-Base Titration

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

Explore how acid-base indicators change color with pH. Learn about phenolphthalein, methyl orange, pKa, and pH paper for high school chemistry.

Acid-Base Color Changes | JoVE K12

chemistry

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Matter and Its Interactions

9-12.C.1.6.e

Georgia Standards of Excellence

Physical Science

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Indicators and Color Changes

SC3.

Indicators of Chemical Reactions

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Weak Base Solutions

Some compounds produce hydroxide ions when dissolved by chemically reacting with water molecules. In all cases, these compounds react only partially and ...

le-chatelier-s-principle-changing-volume-pressure

Le Chatelier's Principle: Changing Volume (Pressure)

For gas-phase equilibria, changes in the concentrations of reactants and products can occur with altered volume and pressure. The partial pressure, P, of ...

Mixtures of Acids

The pH of a solution containing an acid can be determined using its acid dissociation constant and its initial concentration. If a solution contains two ...

le-chatelier-s-principle-changing-temperature

Le Chatelier's Principle: Changing Temperature

Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change in ...

Ions as Acids and Bases

Salts with Acidic Ions
Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization ...

Bronsted-Lowry Acids and Bases

The acid-base reaction class has been studied for quite some time. In 1680, Robert Boyle reported traits of acid solutions that included their ability to ...

Determining the pH of Salt Solutions

The pH of a salt solution is determined by its component anions and cations. Salts that contain pH-neutral anions and the hydronium ion-producing cations ...

acidbase-strengths-and-dissociation-constants

Acid/Base Strengths and Dissociation Constants

The relative strength of an acid or base is the extent to which it ionizes when dissolved in water. If the ionization reaction is essentially complete, ...

Polyprotic Acids

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ...

Chemical reactions
• Types of reactions
• Kinetics
• Energy
• 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.

Acid-Base Color Changes

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