Name: acids, bases, and ph in titration
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 6 min 35 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

1.2.1 9L.4.2.2.1

Acid and Base Concentrations

Um ácido de Arrhenius é uma substância que produz íons de hidrogênio quando se dissolve em água, enquanto uma base produz íons de hidróxido. Os íons de hidrogênio reagem imediatamente com a água para formar íons de hidrônio, mas, para simplificar, continuaremos pensando neles como íons de hidrogênio. Dependendo da quantidade de íons de hidrogênio ou íons hidróxido na solução, é considerado ácido ou básico.
Medimos a quantidade de acidez ou basicidade usando o pH, que é calculado como o log negativo da concentração de íons de hidrogênio. Portanto, os valores de pH abaixo de 7 são ácidos e os valores de pH acima de 7 são básicos. O pH 7 é neutro.
Ácidos e bases também são comparados com base em sua força, que é diferente de seu pH. A força de um ácido está relacionada à facilidade com que o íon hidrogênio se dissocia do ânion, chamado de base conjugada. A mesma ideia segue para uma base em referência ao íon hidróxido e seu ácido conjugado. Podemos atribuir um valor a essa força usando a constante de dissociação ácida, ou Ka.
Ka é definido usando as concentrações do ácido não dissociado e dos íons de hidrogênio dissociados e base conjugada. Muitas vezes você pode ver essa relação representada como pKa, que é simplesmente o log negativo do Ka. Quanto menor o pKa, mais forte é o ácido.
Alguns ácidos, como o ácido clorídrico, são monopróticos, o que significa que podem dissociar apenas um íon de hidrogênio. Os ácidos polipróticos, como o ácido fosfórico, podem dissociar vários íons de hidrogênio. Cada dissociação tem seu próprio pKa.
Então, como podemos determinar pKa? Uma maneira é realizar uma titulação ácido-base. A titulação é realizada adicionando lentamente uma solução de concentração conhecida a uma solução de concentração desconhecida enquanto observa a reação entre elas. Nesse caso, o ácido reage com a base em uma reação de neutralização para formar um sal e água.
Portanto, se quisermos medir a concentração de íons de hidrogênio em um ácido, podemos simplesmente titular com uma base forte com uma concentração conhecida de íons hidróxido até que o ácido seja neutralizado. Para realizar uma titulação com precisão, a base precisa ser padronizada - o que significa que você sabe a concentração exata de íons hidróxido. Isso nem sempre é simples.
Por exemplo, o NaOH, que você usará em seu experimento, é muito higroscópico, o que significa que absorve água da atmosfera. Isso acontece com o NaOH tanto como sólido quanto em solução. Portanto, a verdadeira concentração de uma solução de NaOH pode ser menor do que você esperaria.
Para determinar a concentração exata de NaOH, devemos primeiro realizar uma titulação ácido-base. Para fazer isso, você deve usar a base para titular um ácido com uma concentração conhecida. O hidrogenoftalato de potássio, KHP, é um ácido não higroscópico, portanto, podemos calcular com precisão sua concentração a partir de sua massa.
Podemos ver quando a titulação está completa - o que significa que o ácido é neutralizado - usando um indicador de pH como a fenolftaleína. A fenolftaleína é neutra e incolor entre cerca de pH 0 e pH 8.
À medida que o pH aumenta, dois íons de hidrogênio se dissociam. Esta forma aniônica é rosa. Então, quando começamos a titulação, a solução de KHP é ácida e a fenolftaleína é incolor. À medida que adicionamos NaOH e os íons de hidrogênio são neutralizados, o pH aumenta.
Nesta reação, a solução é neutra quando quantidades iguais de ácido e base foram misturadas. Depois disso, a adição de um pouco mais de NaOH torna o pH básico e a solução fica rosada. Isso é chamado de endpoint. Se soubermos os moles de KHP e o volume de NaOH usado para neutralizá-lo, podemos calcular a concentração exata da base.
Uma vez que tenhamos uma base padronizada, podemos determinar o pKa de um ácido titulando uma concentração conhecida do ácido com nossa base padronizada enquanto monitoramos o pH. O gráfico de pH versus o volume de base adicionado é chamado de curva de titulação. A curva geralmente segue uma forma S ou sigmoidal, onde o ponto de inflexão da parte mais íngreme da curva denota um ponto de equivalência.
Aqui, os moles de íons hidróxido e íons de hidrogênio dissociados são iguais. Como pKa, veremos um ponto de equivalência para cada íon de hidrogênio dissociado. Assim, um ácido monoprótico tem apenas um ponto de equivalência e um ácido triprótico tem três.
Quando realizarmos a titulação, saberemos que passamos do ponto de equivalência quando o indicador de pH mal muda de incolor para rosa. Isso é chamado de ponto final de titulação. Como quando padronizamos a base, é quando a solução tem um pequeno excesso de íons hidróxido e, portanto, é um pouco básica.
Outro ponto de inflexão no gráfico ocorre a meio caminho do ponto de equivalência. Aqui, as concentrações dos ácidos dissociados e não dissociados são iguais. Assim, o pH neste ponto é igual ao pKa. Então, se realizarmos uma titulação e determinarmos o volume do ponto de equivalência, podemos calcular pKa como o pH na metade desse volume.
Neste laboratório, você primeiro padronizará sua base e, em seguida, realizará uma titulação usando essa base padronizada para determinar dois pKa de um ácido poliprótico.

concentrações de ácidos e bases

<h4>Ácidos e Bases</h4>
Um ácido de Arrhenius produz íons de hidrogênio quando é dissolvido em água:
HA + H2O → H+(aq) + A-(aq)
Aqui, HA é o ácido não dissociado, H+ é o cátion hidrogênio e A- é o ânion solvatado - chamado de base conjugada. Uma base de Arrhenius produz íons hidróxido quando dissolvido em água:
BOH + H2O → B+(aq) + OH-(aq)
Aqui, BOH é a base não dissociada, OH- é o íon hidróxido e B+ é o cátion solvatado - chamado de ácido conjugado. Uma base conjugada é formada quando um ácido perde um íon hidrogênio e tem o potencial de ganhar um hidrogênio. O mesmo se aplica a um ácido conjugado, que é formado quando uma base perde um grupo hidroxila e tem o potencial de recuperá-lo. Todo ácido tem uma base conjugada e toda base tem um ácido conjugado.
<h4>ph</h4>
O pH é o grau de acidez da solução e é uma medida da quantidade de íons de hidrogênio em uma solução. A escala de pH é logarítmica e vai de 0 a 14; as soluções aquosas com pH inferior a 7 são descritas como ácidas e as soluções aquosas com pH superior a 7 são descritas como alcalinas ou básicas. Soluções em pH 7 são consideradas neutras.
O pH de uma solução é igual ao log negativo de base dez da concentração de íons de hidrogênio em solução.
<img src="/CDNSource/lm/labs/44/44_Concepts_3.jpg">
A água interage fortemente com o íon hidrogênio porque sua forte carga positiva atrai o pólo negativo das moléculas de água circundantes. Na verdade, eles interagem tão fortemente que formam uma ligação covalente e o cátion H3O+, chamado hidrônio. A equação acima é reescrita para refletir isso.
<img src="/CDNSource/lm/labs/44/44_Concepts_4.jpg">
Para simplificar, vamos nos referir à concentração de íons de hidrogênio em vez de íons hidrônio ao discutir o pH. Quanto menor o valor do pH de uma solução, mais íons de hidrogênio estão presentes e, por extensão, mais ácida é a solução. Por exemplo, o pH de 1 mM de ácido sulfúrico é 2,75, enquanto o pH de 1 mM de clorídrico é 3,01. A concentração de íons de hidrogênio na solução de ácido sulfúrico é calculada como 1 × 10-2,75, enquanto a concentração de íons de hidrogênio na solução de ácido clorídrico é de 1 × 10-3,01. Assim, há mais íons de hidrogênio presentes no ácido sulfúrico, e é mais ácido. Lembre-se, embora o pH de duas soluções possa variar em apenas meio valor de pH, devido à natureza logarítmica da escala de pH, a quantidade de hidrogênio varia muito.
<h4>Força de ácidos e bases</h4>
A força de um ácido é afetada pela eletronegatividade da base conjugada e pela polaridade do hidrogênio ácido. A força, portanto, refere-se à rapidez com que o cátion hidrogênio (H+) se desassocia do ânion. Ácidos e bases fortes se dissociam inteiramente em soluções aquosas, enquanto ácidos e bases fracos se dissociam apenas parcialmente em seus íons conjugados.
A constante de dissociação, K a, representaa força do ácido. Ka é calculado usando as concentrações do ácido ácido não dissociado HA e as concentrações dos cátions de hidrogênio e da base conjugada, A-. Valores mais altosde K a representam ácidos mais fortes, enquanto valores menoresde K a representam ácidos mais fracos.
<img src="/CDNSource/lm/labs/44/44_Concepts_5.jpg">
Ka é numericamente muito pequeno e é frequentemente relatado na forma de pKa, que é o log negativo de base dez de Ka. Valores mais baixos de pKa correspondem a um ácido mais forte, enquanto valores mais altos de pKa correspondem a um ácido mais fraco.
<img src="/CDNSource/lm/labs/44/44_Concepts_6.jpg">
Alguns ácidos dissociam apenas um íon de hidrogênio e, portanto, têm um valor de pKa. Esses ácidos são chamados de monopróticos. No entanto, alguns ácidos podem dissociar mais de um íon de hidrogênio e são chamados de polipróticos. Esses ácidos têm um valor pKa para cada dissociação de íons de hidrogênio.
pKa também pode ser usado para calcular o pH de equilíbrio de uma reação ácido-base, conforme mostrado na equação de Henderson-Hasselbalch.
<img src="/CDNSource/lm/labs/44/44_Concepts_7.jpg">
A equação de Henderson-Hasselbalch é usada para calcular o pH, quando as concentrações da base conjugada e do ácido fraco são conhecidas, ou para calcular o pKa se o pH e as concentrações são conhecidos.
<h4>Titulação</h4>
As reações ácido-base são estudadas quantitativamente usando titulação. Em um experimento de titulação, uma solução de uma concentração conhecida, chamada de solução padrão, é usada para determinar a concentração de outra solução. Para titulações ácido-base, uma solução padronizada de base é adicionada lentamente a um ácido de concentração desconhecida (ou o ácido é adicionado à base). A reação ácido-base é uma reação de neutralização, que forma um sal e água. Quando os moles de íons de hidrogênio no ácido são iguais aos moles de íons hidroxila adicionados da base, a solução atinge pH neutro.
Para realizar uma titulação ácido-base, a base padronizada é adicionada lentamente a um frasco agitador do ácido desconhecido usando uma bureta, o que permite a medição do volume e a adição gota a gota da base. O pH da solução é monitorado de perto durante toda a titulação usando um indicador de pH adicionado ao ácido. Normalmente, a fenolftaleína é usada, pois a solução permanece incolor até se tornar básica, tornando-se rosa claro.
À medida que a titulação se aproxima do ponto de equivalência, que é quando os moles de íons hidrogênio são iguais aos moles de íons hidroxila adicionados, o indicador de pH muda temporariamente de cor devido a um excesso de íons hidroxila. Quando o frasco é agitado, a cor ácida do indicador de pH retorna. A titulação está completa e atingiu seu ponto final quando um pequeno excesso de íons hidroxila muda o indicador permanentemente para sua cor básica.
A curva de titulação é um gráfico do pH de uma solução versus o volume de base padronizada adicionado. O ponto de equivalência está localizado no ponto de inflexão da curva e é calculado como a segunda derivada da curva de titulação.
Se um ácido for poliprótico, ele terá vários pontos de equivalência, um para cada dissociação de íons de hidrogênio. O pH a meio caminho do ponto de equivalência para os ácidos monopróticos, ou entre pontos de equivalência no caso dos ácidos polipróticos, é igual ao pKa do ácido.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: McGraw-Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análise Química Quantitativa. Nova York, NY: W.H. Freeman and Company.</li>
</ol>

Concentrações de ácidos e bases

Ácidos e Bases
Um ácido de Arrhenius produz íons de hidrogênio quando é dissolvido em água:
HA + H2O → H+(aq) + A-(aq)
Aqui, HA é o não-dissociado ...

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acids, bases, and ph in titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid.

pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14 and is logarithmic, so small changes in pH can mean large changes in hydrogen ion concentration. Solutions below pH 7 are acidic, solutions above pH 7 are basic or alkaline, and solutions at pH 7 are neutral. Water strongly attracts hydrogen ions and forms hydronium, H3O+, but pH is often discussed using hydrogen ion concentration for simplicity.

Lower pH means more hydrogen ions and greater acidity. For example, 1 mM sulfuric acid has a pH of 2.75, while 1 mM hydrochloric acid has a pH of 3.01. Because the pH scale is logarithmic, even a difference of about half a pH unit can reflect a large difference in hydrogen ion amount. In this example, sulfuric acid is more acidic because it contains more hydrogen ions.

Acid and base strength describes how readily hydrogen ions separate from the anion. Strong acids and bases dissociate completely in water, while weak acids and bases dissociate only partly into their conjugate ions. Acid strength is measured with the dissociation constant, Ka, which uses the concentrations of the non-dissociated acid HA, hydrogen cations, and conjugate base A-. Larger Ka values indicate stronger acids, and smaller values indicate weaker acids. Ka is often written as pKa, the negative base-10 log of Ka, so lower pKa means a stronger acid and higher pKa means a weaker acid.

Some acids release only one hydrogen ion, so they have one pKa and are called monoprotic. Others release more than one hydrogen ion and are polyprotic, with a pKa value for each step. The Henderson-Hasselbalch equation uses pKa to find the equilibrium pH of an acid-base reaction when the weak acid and conjugate base concentrations are known, or to find pKa when pH and concentrations are known.

Acid-base reactions can also be studied by titration. In a titration, a standard solution with a known concentration is used to find the concentration of another solution. For acid-base titrations, a standardized base is added slowly to an unknown acid, or the acid is added to the base. The reaction is a neutralization reaction that produces salt and water. When the moles of hydrogen ions match the moles of hydroxyl ions added, the solution reaches neutral pH.

During the experiment, the standardized base is added drop by drop from a burette to a stirring flask of acid. The pH is monitored with an indicator such as phenolphthalein, which stays colorless until the solution turns basic and becomes light pink. Near the equivalence point, the indicator may change color because of a temporary excess of hydroxyl ions, but swirling can return its acidic color. The endpoint is reached when a tiny extra amount of hydroxyl ions makes the basic color stay permanent.

A titration curve plots pH against the volume of standardized base added. The equivalence point appears at the inflection point of the curve and is found from the second derivative. Polyprotic acids have multiple equivalence points, one for each hydrogen ion dissociation. For monoprotic acids, the pH halfway to the equivalence point, or between equivalence points for polyprotic acids, is equal to the pKa.

Acids, Bases, and pH in Titration

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Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Vídeos abrangentes e focados no currículo para instrutores e alunos de cursos introdutórios de laboratório de química

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Learn how acids, bases, pH, Ka, and pKa connect to titration, neutralization, and titration curves in high school chemistry.

Acids, Bases, and pH in Titration | JoVE K12

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Chemical Reactions

HS-PS1-11.

Georgia Standards of Excellence

SC3.

Indicators of Chemical Reactions

SC6.

Neutralization Reactions

Prepare-se para ensinar Concentrações de ácidos e bases com este guia do instrutor de 5 minutos. Abrange as principais etapas de preparação para o laboratório Chemistry no JoVE Lab Manual.

Preparação do Instrutor para Concentrações de ácidos e bases em Chemistry - 6.2K+ visualizações | JoVE Lab Manual

Siga os procedimentos passo a passo de Concentrações de ácidos e bases neste vídeo de protocolo estudantil de 15 minutos. Parte da coleção Chemistry no JoVE Lab Manual.

Protocolo do Estudante para Concentrações de ácidos e bases em Chemistry - 31K+ visualizações | JoVE Lab Manual

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 ...

Matter and Its Interactions: Plan and conduct an investigation of acid-base reactions to test ideas about the concentrations of the hydronium ion in an aqueous solution (pH). (P:3, CC: 3, CI: PS1)

9C.1.2.1.2

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 ...

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 ...

Acids and Bases
An Arrhenius acid produces hydrogen ions when it is dissolved in water:
HA + H2O &#8594; H+(aq) + A-(aq)
Here, HA is the non-dissociated ...

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 ...

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 ...

Water: A Bronsted-Lowry Acid and Base

The reaction between a Br&#248;nsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, ...

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 ...

Weak Acid Solutions

Few compounds act as strong acids. A far greater number of compounds behave as weak acids and only partially react with water, leaving a large majority of ...

The potential of hydrogen (pH) is a measure of the acidity or basicity of a water-based solution determined by the concentration of hydronium ions (H3O+). ...

water-quality-analysis-via-indicator-organisms

Water Quality Analysis via Indicator Organisms

Source: Laboratories of Dr. Ian Pepper and Dr. Charles Gerba -The University of Arizona
Demonstrating Author: Luisa Ikner
Water quality analysis monitors ...

Matter and Its Interactions: Obtain and communicate information about how Minnesota American Indian Tribes and communities and other cultures construct solutions to mitigate threats to biodiversity.* (P: 8, CC: 7, CI: LS2, ETS1)

1 Exploring phenomena or engineering problems
 1.2 Planning and carrying out investigations
  1.2.1 Students will be able to design and conduct investigations in the classroom, laboratory, and/or field to test students’ ideas and questions and will organize and collect data to provide evidence to support claims the students make about phenomena.

Acids, Bases, and pH in Titration

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