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

Indicators

指示剂的颜色随 pH 值的变化而变化，可用于监测溶液的 pH 值。pH 指示剂含有一种弱酸，以 HIn 表示，当转化为它的共轭碱，In 离子时，它会变色。颜色会随指示剂的 pH 值区间而变化，这取决于它的 pKa。如果溶液的 pH 值低于指示剂的 pKa，则水合氢离子的浓度较高。反应的平衡点向左移动，增加了 Hln 的浓度。当加入碱时，水合氢离子的浓度 降低。这种 pH 值的变化将 HIn 转化为 In 离子，溶液的颜色也随之改变。当 pH 值至少比 pKa 低 1 时，HIn 颜色 占主导地位；当 pH 值至少比 pKa 高 1 时，In 离子颜色占主导地位，而 pH 值 介于两者之间时，颜色是混合色。有各种 pH 指示剂 可用于监测在不同的 pH 值范围内 改变颜色的 pH 值。指示剂改变颜色的点 称为终点。理想的滴定指示剂 有一个接近等值点的终点。强酸和强碱的滴定曲线 是陡峭的，因为在等值点附近 的 pH 值变化很快。这些滴定可以使用相对广泛的 指示剂。在用 0.1M 氢氧化钠滴定 0.1M 盐酸的过程中，pH 值 从 3 迅速增加到 11。不论是酚酞，一种 pH 值区间 为 8.3 到 10 的指示剂，还是甲基红，一种区间为 4.2 到 6 的指示剂，都可以在这里使用，因为它们的终点 与曲线的陡峭部分重叠。相反，涉及弱酸或弱碱的滴定曲线，跟只有强酸和强碱的滴定曲线相比，没有那么陡；因此，可以使用 相对狭窄的指示剂范围。在用 0.1M 氢氧化钠滴定 0.1M 乙酸的过程中，pH 值 从 7 迅速增加到 11。酚酞可以在这里使用，因为其终点 与等值点重叠，但甲基红 将是一个无效的指示剂。

Coleman， William F. "指标的分子模型"。化学教育杂志 87 第 1 期，（2008）:1152 <a href="https://pubs.acs.org/doi/pdf/10.1021/ed800038w">https://pubs.acs.org/doi/pdf/10.1021/ed800038w。</a>

指标

当水合氢离子浓度达到特定值时，某些有机物质会在稀释溶液中变色。 例如，酚酞是水溶液 5.0 × 10−9 M (pH < 8.3) 的任何水合氢离子浓度大于中的无色物质。 在更基本的溶液中，水合氢离子浓度为小于 5.0 × 10−9 M (pH > 8.3) ，则为红色或粉红色。 可用于测定溶液 pH 值的酚酞等物质被称为 酸-碱 指标。 酸-碱 指示灯要么是弱有机酸，要么是弱有机碱。 
 酸-碱 指示物甲基橙 (弱酸) 的溶液中的平衡可以用方程表示，在 HIn 中，我们使用 作为复杂甲基橙分子的简单表示： 
 <img alt="Eq1" src="/files/ftp_upload/11415/11415_Equation_1.png" style="text-align: center; height: 115px;" />
 甲基橙色阴离子 (−) 为黄色，非离子形式 HIn 为红色。 当我们将酸添加到甲基橙的溶液中时，增加的水合氢离子浓度会根据 勒夏特列原理（Le Chatelier's Principle） 将平衡转移到非电离红形式。 如果添加碱，则将平衡向黄色形式转移。 这种行为与缓冲液的行为完全类似。 
 溶液的感知颜色由两种物种在−和 HIn 中的浓度比决定。 如果大多数指示灯 (通常为 60−90% 或更高) 以− 的形式出现，则溶液的感知颜色为黄色。 如果 MOST 显示为 HIn ，则溶液颜色将显示为红色。 亨德森-哈塞尔巴尔赫方程（Henderson-Hasselbalch Equation） 有助于了解指标溶液的 pH 值与其成分 (因此，感知的颜色) 之间的关系： 
 <img alt="Eq1" src="/files/ftp_upload/11415/11415_Equation_2.png" style="text-align: center; height: 75px;" />
 在 pH > pKa 的溶液中，对数术语必须为正数，表示指示符的共轭碱形式超出 (黄色溶液)。 当 pH > pKa 时，日志术语必须为负值，表示共轭酸 (红色溶液) 过量。 当溶液 pH 值接近 pKa 指示符时，两个共轭伙伴的含量都相当可观，溶液颜色是每个伙伴的添加剂组合 (黄色和红色，产生橙色)。 酸-碱 指示器的换色间隔 (或 pH 间隔) 定义为观察到颜色变化的 pH 值范围，而对于大多数指示器，该范围约为 pKa ± 1。 
 有许多不同的 酸-碱 指示器涵盖各种 pH 值，可用于通过消除过程确定未知溶液的近似 pH 值。 通用指示器和 pH 值纸包含多种指示器，在不同的外环中显示不同的颜色。 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/14-7-acid-base-titrations"> Openstax, 化学 2e, 第14.7节： 酸-碱 滴定</a>。

某些有机物质在稀溶液中，当水合氢离子浓度达到特定值时会发生颜色变化。例如，酚酞……

Education

教育

JoVE Core

化学

Acid-base and Solubility Equilibria

酸碱与溶解平衡

KEY TERMS AND CONCEPTS

关键术语与概念

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

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Alabama Course of Study Science

Matter and Its Interactions

9-12.C.1.6.e

Georgia Standards of Excellence

Physical Science

SPS6.

Indicators and Color Changes

SC3.

Indicators of Chemical Reactions

Chemical Equations

Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction are ...

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

Reaction Yield

The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The ...

Chemical Reactions in Aqueous Solutions

Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemical ...

Precipitation Reactions

In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound &#x2013; the precipitate. The reaction occurs ...

Oxidation-Reduction Reactions

Oxidation&ndash;Reduction Reactions

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Oxidation Numbers

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the ...

Acids, Bases and Neutralization Reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

Synthesis and Decomposition Reactions

Synthesis and decomposition are two types of redox reactions. Synthesis means to make something, whereas decomposition means to break something. The ...

Reaction Rate

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time ...

Measuring Reaction Rates

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical ...

Concentration and Rate Law

The rate of a reaction is affected by the concentrations of reactants. Rate laws (differential rate laws) or rate equations are mathematical expressions ...

Determining Order of Reaction

Rate laws describe the relationship between the rate of a chemical reaction and the concentration of its reactants. In a rate law, the rate constant k and ...

the-integrated-rate-law-the-dependence-of-concentration-on-time

The Integrated Rate Law: The Dependence of Concentration on Time

While the differential rate law relates the rate and concentrations of reactants, a second form of rate law called the integrated rate law relates ...

Half-life of a Reaction

The half-life of a reaction (t1/2) is the time required for one-half of a given amount of reactant to be consumed. In each succeeding half-life, half of ...

Temperature Dependence on Reaction Rate

The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. ...

Arrhenius Plots

The Arrhenius equation relates the activation energy and the rate constant, k, for chemical reactions. In the Arrhenius equation, k = Ae&#8722;Ea/RT, R is ...

Reaction Mechanisms

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Thermochemical Equations

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Rate-Determining Steps

Relating Reaction Mechanisms
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest ...

Constant Pressure Calorimetry

Calorimetry is a technique used to measure the amount of heat involved in a chemical or physical process or to measure the heat transferred to or from a ...

Catalysis

The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed ...

Constant Volume Calorimetry

Calorimeters are useful to determine the heat released or absorbed by a chemical reaction. Coffee cup calorimeters are designed to operate at constant ...

Enzymes

Inside living organisms, enzymes act as catalysts for many biochemical reactions involved in cellular metabolism. The role of enzymes is to reduce the ...

Hess's Law

There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally ...

Energy Basics

Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.

Chemical changes and their accompanying ...

Standard Enthalpy of Formation

Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same conditions. A standard state is a commonly ...

First Law of Thermodynamics

Energy Conservation

Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form ...

Enthalpies of Reaction

Hess&rsquo;s law can be used to determine the enthalpy change of any reaction if the corresponding enthalpies of formation of the reactants and products ...

Internal Energy

The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E. Suppose a system with ...

Dynamic Equilibrium

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Quantifying Heat

Thermal Energy&#160;

Microscopically, thermal energy is the kinetic energy associated with the random motion of atoms and molecules. Temperature is a ...

The Equilibrium Constant

Consider the oxidation of sulfur dioxide:

For a reaction that begins with a mixture of reactants only, the product concentration is initially equal to ...

Quantifying Work

As a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings, or from the ...

homogeneous-equilibria-for-gaseous-reactions

Homogeneous Equilibria for Gaseous Reactions

Homogeneous Equilibria for Gaseous Reactions
For gas-phase reactions, the equilibrium constant may be expressed in terms of either the molar ...

Enthalpy

Chemists ordinarily use a property known as enthalpy (H) to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum ...

Calculating the Equilibrium Constant

The equilibrium constant for a reaction is calculated from the equilibrium concentrations (or pressures) of its reactants and products. If these ...

Reaction Quotient

The status of a reversible reaction is conveniently assessed by evaluating its reaction quotient (Q). For a reversible reaction described by m A + n B ...

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

Relative Strengths of Conjugate Acid-Base Pairs

Br&#248;nsted-Lowry acid-base chemistry is the transfer of protons; thus, logic suggests a relation between the relative strengths of conjugate acid-base ...

Calculating Equilibrium Concentrations

Being able to calculate equilibrium concentrations is essential to many areas of science and technology&#8212;for example, in the formulation and dosing ...

Strong Acid and Base Solutions

A strong acid is a compound that dissociates completely in an aqueous solution and produces a concentration of hydronium ions equal to the initial ...

The Small x Assumption

If a reaction has a small equilibrium constant, the equilibrium position favors the reactants. In such reactions, a negligible change in concentration may ...

Weak Acid Solutions

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le-chatelier-s-principle-changing-concentration

Le Chatelier's Principle: Changing Concentration

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Weak Base Solutions

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Le Chatelier's Principle: Changing Volume (Pressure)

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Mixtures of Acids

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le-chatelier-s-principle-changing-temperature

Le Chatelier's Principle: Changing Temperature

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Ions as Acids and Bases

Salts with Acidic Ions
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Bronsted-Lowry Acids and Bases

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Determining the pH of Salt Solutions

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acidbase-strengths-and-dissociation-constants

Acid/Base Strengths and Dissociation Constants

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Polyprotic Acids

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Water: A Bronsted-Lowry Acid and Base

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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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