{"appState":{"pageLoadApiCallsStatus":true},"articleState":{"article":{"headers":{"creationTime":"2016-03-26T08:17:40+00:00","modifiedTime":"2021-07-21T15:03:18+00:00","timestamp":"2022-09-14T18:18:27+00:00"},"data":{"breadcrumbs":[{"name":"Academics & The Arts","_links":{"self":"https://dummies-api.dummies.com/v2/categories/33662"},"slug":"academics-the-arts","categoryId":33662},{"name":"Science","_links":{"self":"https://dummies-api.dummies.com/v2/categories/33756"},"slug":"science","categoryId":33756},{"name":"Chemistry","_links":{"self":"https://dummies-api.dummies.com/v2/categories/33762"},"slug":"chemistry","categoryId":33762}],"title":"How to Predict the Equilibrium Direction of an Acid-Base Reaction","strippedTitle":"how to predict the equilibrium direction of an acid-base reaction","slug":"how-to-predict-the-equilibrium-direction-of-an-acid-base-reaction","canonicalUrl":"","seo":{"metaDescription":"pKa values allow you to predict the equilibrium direction of acid-base chemical reactions for organic molecules.","noIndex":0,"noFollow":0},"content":"pKa values allow you to predict the equilibrium direction of acid-base <a href=\"https://www.dummies.com/education/science/chemistry/the-common-types-of-chemical-reactions/\" target=\"_blank\" rel=\"noopener\">chemical reactions</a> for organic molecules. The pKa value of an acid is a quantitative measurement of a molecule’s acidity. The pKa is derived from the equilibrium constant for the acid’s dissociation reaction, Ka, and uses a logarithmic scale to allow the pKa values to span wide ranges.\r\n\r\npKa = –log Ka\r\n<p class=\"Remember\">The lower the pKa value of an acid, the stronger the acid. The higher the pKa value, the weaker the acid. Very strong acids have pKa values of less than zero, while weak acids generally have pKa values of between 0 and 9.</p>\r\nA brief pKa table of acids is shown here.\r\n\r\n<img src=\"https://www.dummies.com/wp-content/uploads/458160.image0.png\" alt=\"image0.png\" width=\"525\" height=\"312\" />\r\n\r\nWith pKa table in hand, you can predict the equilibrium direction of acid-base reactions. Weak acids and bases are lower in energy than strong acids and bases, and because equilibria favor the reaction side with the lowest-energy species, acid-base reactions will go to the side with the weakest acids and bases.\r\n<p class=\"Remember\">As a rule, the equilibrium of a reaction will favor the side with weaker acids and bases.</p>\r\nFor example, you can predict the direction of the acid-base reaction between hydrogen cyanide (HCN) and acetate (C<sub>2</sub>H<sub>3</sub>O<sub>2</sub><sup>–</sup>) shown here.\r\n<div class=\"imageBlock\" style=\"width: 525px;\">\r\n\r\n<img src=\"https://www.dummies.com/wp-content/uploads/458161.image1.jpg\" alt=\"The pKa values predict the direction of the acid-base equilibrium.\" width=\"525\" height=\"114\" />\r\n<div class=\"imageCaption\">The pKa values predict the direction of the acid-base equilibrium.</div>\r\n</div>\r\nBecause hydrogen cyanide (pKa = 9) has a higher pKa value than acetic acid (pKa = 5), the equilibrium will lie to the left, in the direction of the weaker acid and base. That’s really all there is to predicting the direction of an acid-base reaction. If you know the pKa values of the two acids on both sides of the equation, then you know in which direction the equilibrium lies, because equilibrium will favor the side with the acid that has the highest pKa.","description":"pKa values allow you to predict the equilibrium direction of acid-base <a href=\"https://www.dummies.com/education/science/chemistry/the-common-types-of-chemical-reactions/\" target=\"_blank\" rel=\"noopener\">chemical reactions</a> for organic molecules. The pKa value of an acid is a quantitative measurement of a molecule’s acidity. The pKa is derived from the equilibrium constant for the acid’s dissociation reaction, Ka, and uses a logarithmic scale to allow the pKa values to span wide ranges.\r\n\r\npKa = –log Ka\r\n<p class=\"Remember\">The lower the pKa value of an acid, the stronger the acid. The higher the pKa value, the weaker the acid. Very strong acids have pKa values of less than zero, while weak acids generally have pKa values of between 0 and 9.</p>\r\nA brief pKa table of acids is shown here.\r\n\r\n<img src=\"https://www.dummies.com/wp-content/uploads/458160.image0.png\" alt=\"image0.png\" width=\"525\" height=\"312\" />\r\n\r\nWith pKa table in hand, you can predict the equilibrium direction of acid-base reactions. Weak acids and bases are lower in energy than strong acids and bases, and because equilibria favor the reaction side with the lowest-energy species, acid-base reactions will go to the side with the weakest acids and bases.\r\n<p class=\"Remember\">As a rule, the equilibrium of a reaction will favor the side with weaker acids and bases.</p>\r\nFor example, you can predict the direction of the acid-base reaction between hydrogen cyanide (HCN) and acetate (C<sub>2</sub>H<sub>3</sub>O<sub>2</sub><sup>–</sup>) shown here.\r\n<div class=\"imageBlock\" style=\"width: 525px;\">\r\n\r\n<img src=\"https://www.dummies.com/wp-content/uploads/458161.image1.jpg\" alt=\"The pKa values predict the direction of the acid-base equilibrium.\" width=\"525\" height=\"114\" />\r\n<div class=\"imageCaption\">The pKa values predict the direction of the acid-base equilibrium.</div>\r\n</div>\r\nBecause hydrogen cyanide (pKa = 9) has a higher pKa value than acetic acid (pKa = 5), the equilibrium will lie to the left, in the direction of the weaker acid and base. That’s really all there is to predicting the direction of an acid-base reaction. If you know the pKa values of the two acids on both sides of the equation, then you know in which direction the equilibrium lies, because equilibrium will favor the side with the acid that has the highest pKa.","blurb":"","authors":[{"authorId":9321,"name":"Arthur Winter","slug":"arthur-winter","description":" <p><B>Arthur Winter, PhD, </b>is the author of the popular <i>Organic Chemistry Help!</i> website chemhelper.com and <i>Organic Chemistry I For Dummies</i>. His professional focus is on the chemistry of magneto-organic materials. </p> ","hasArticle":false,"_links":{"self":"https://dummies-api.dummies.com/v2/authors/9321"}}],"primaryCategoryTaxonomy":{"categoryId":33762,"title":"Chemistry","slug":"chemistry","_links":{"self":"https://dummies-api.dummies.com/v2/categories/33762"}},"secondaryCategoryTaxonomy":{"categoryId":0,"title":null,"slug":null,"_links":null},"tertiaryCategoryTaxonomy":{"categoryId":0,"title":null,"slug":null,"_links":null},"trendingArticles":null,"inThisArticle":[],"relatedArticles":{"fromBook":[{"articleId":209153,"title":"Organic Chemistry I For Dummies Cheat Sheet","slug":"organic-chemistry-i-for-dummies-cheat-sheet","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/209153"}},{"articleId":193348,"title":"Periodic Table of Elements","slug":"periodic-table-of-elements-2","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/193348"}},{"articleId":193338,"title":"Common Functional Groups in Organic Chemistry","slug":"common-functional-groups-in-organic-chemistry","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/193338"}},{"articleId":155208,"title":"An Example Organic Chemistry Mechanism Problem","slug":"an-example-organic-chemistry-mechanism-problem","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/155208"}},{"articleId":155207,"title":"How Conjugated Double-Bond Stereochemistry Works in Vision","slug":"how-conjugated-double-bond-stereochemistry-works-in-vision","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/155207"}}],"fromCategory":[{"articleId":253707,"title":"How to Make Unit Conversions","slug":"make-unit-conversions","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/253707"}},{"articleId":251836,"title":"How to Convert between Units Using Conversion Factors","slug":"convert-units-using-conversion-factors","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/251836"}},{"articleId":251010,"title":"How to Build Derived Units from Base Units","slug":"build-derived-units-base-units","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/251010"}},{"articleId":251005,"title":"How to Do Arithmetic with Significant Figures","slug":"arithmetic-significant-figures","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/251005"}},{"articleId":250992,"title":"How to Add and Subtract with Exponential Notation","slug":"add-subtract-exponential-notation","categoryList":["academics-the-arts","science","chemistry"],"_links":{"self":"https://dummies-api.dummies.com/v2/articles/250992"}}]},"hasRelatedBookFromSearch":false,"relatedBook":{"bookId":282434,"slug":"organic-chemistry-i-for-dummies-2nd-edition","isbn":"9781119293378","categoryList":["academics-the-arts","science","chemistry"],"amazon":{"default":"https://www.amazon.com/gp/product/1119293375/ref=as_li_tl?ie=UTF8&tag=wiley01-20","ca":"https://www.amazon.ca/gp/product/1119293375/ref=as_li_tl?ie=UTF8&tag=wiley01-20","indigo_ca":"http://www.tkqlhce.com/click-9208661-13710633?url=https://www.chapters.indigo.ca/en-ca/books/product/1119293375-item.html&cjsku=978111945484","gb":"https://www.amazon.co.uk/gp/product/1119293375/ref=as_li_tl?ie=UTF8&tag=wiley01-20","de":"https://www.amazon.de/gp/product/1119293375/ref=as_li_tl?ie=UTF8&tag=wiley01-20"},"image":{"src":"https://www.dummies.com/wp-content/uploads/organic-chemistry-i-for-dummies-2nd-edition-cover-9781119293378-203x255.jpg","width":203,"height":255},"title":"Organic Chemistry I For Dummies","testBankPinActivationLink":"","bookOutOfPrint":true,"authorsInfo":"<p><B><b data-author-id=\"9321\">Arthur Winter</b>, PhD, </b>is the author of the popular <i>Organic Chemistry Help!</i> website chemhelper.com and <i>Organic Chemistry I For Dummies</i>. 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How to Predict the Equilibrium Direction of an Acid-Base Reaction

By: Arthur Winter and

Updated: 07-21-2021

From The Book: Organic Chemistry I For Dummies

Organic Chemistry I For Dummies

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pKa values allow you to predict the equilibrium direction of acid-base chemical reactions for organic molecules. The pKa value of an acid is a quantitative measurement of a molecule’s acidity. The pKa is derived from the equilibrium constant for the acid’s dissociation reaction, Ka, and uses a logarithmic scale to allow the pKa values to span wide ranges.

pKa = –log Ka

The lower the pKa value of an acid, the stronger the acid. The higher the pKa value, the weaker the acid. Very strong acids have pKa values of less than zero, while weak acids generally have pKa values of between 0 and 9.

A brief pKa table of acids is shown here.

With pKa table in hand, you can predict the equilibrium direction of acid-base reactions. Weak acids and bases are lower in energy than strong acids and bases, and because equilibria favor the reaction side with the lowest-energy species, acid-base reactions will go to the side with the weakest acids and bases.

As a rule, the equilibrium of a reaction will favor the side with weaker acids and bases.

For example, you can predict the direction of the acid-base reaction between hydrogen cyanide (HCN) and acetate (C₂H₃O₂^–) shown here.

The pKa values predict the direction of the acid-base equilibrium.

The pKa values predict the direction of the acid-base equilibrium.

Because hydrogen cyanide (pKa = 9) has a higher pKa value than acetic acid (pKa = 5), the equilibrium will lie to the left, in the direction of the weaker acid and base. That’s really all there is to predicting the direction of an acid-base reaction. If you know the pKa values of the two acids on both sides of the equation, then you know in which direction the equilibrium lies, because equilibrium will favor the side with the acid that has the highest pKa.

About This Article

This article is from the book:

Organic Chemistry I For Dummies ,

About the book author:

Arthur Winter, PhD, is the author of the popular Organic Chemistry Help! website chemhelper.com and Organic Chemistry I For Dummies. His professional focus is on the chemistry of magneto-organic materials.

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