{"id":232202,"date":"2025-06-11T19:22:54","date_gmt":"2025-06-11T19:22:54","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=232202"},"modified":"2025-06-11T19:22:57","modified_gmt":"2025-06-11T19:22:57","slug":"the-standard-enthalpy-of-formation-of-nh3g-at-298-k-is-46-kj-mol-1","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/06\/11\/the-standard-enthalpy-of-formation-of-nh3g-at-298-k-is-46-kj-mol-1\/","title":{"rendered":"The standard enthalpy of formation of NH3(g) at 298 K is -46 kJ mol-1."},"content":{"rendered":"\n

The standard enthalpy of formation of NH3(g) at 298 K is -46 kJ mol-1. Which of the following values is CORRECT for the standard enthalpy change for the following reaction? 2NH3(g) \u00e2\u2020\u2019 N2(g) + 3H2(g) Select one: a. +92 kJ\/mol b. -92 kJ\/mol c. +46 kJ\/mol d. -46 kJ\/mol<\/p>\n\n\n\n

The Correct Answer and Explanation is:<\/mark><\/strong><\/p>\n\n\n\n

The correct answer is: a. +92 kJ\/mol<\/strong><\/p>\n\n\n\n

\n\n\n\n

Explanation:<\/h3>\n\n\n\n

The standard enthalpy of formation (\u0394H\u00b0f)<\/strong> of a compound is the enthalpy change when 1 mole<\/strong> of a substance is formed from its elements in their standard states<\/strong> under standard conditions (298 K and 1 atm).<\/p>\n\n\n\n

For ammonia (NH\u2083)<\/strong>, the standard enthalpy of formation is given as:\u0394Hf\u2218[NH3(g)]=\u221246\u2009kJ\/mol\\Delta H^\\circ_f [\\text{NH}_3(g)] = -46 \\, \\text{kJ\/mol}\u0394Hf\u2218\u200b[NH3\u200b(g)]=\u221246kJ\/mol<\/p>\n\n\n\n

This means the reaction:12N2(g)+32H2(g)\u2192NH3(g)\\frac{1}{2} \\text{N}_2(g) + \\frac{3}{2} \\text{H}_2(g) \\rightarrow \\text{NH}_3(g)21\u200bN2\u200b(g)+23\u200bH2\u200b(g)\u2192NH3\u200b(g)<\/p>\n\n\n\n

has a \u0394H\u00b0 = -46 kJ\/mol<\/strong>, indicating it is exothermic<\/strong>.<\/p>\n\n\n\n

Now, consider the reaction in question:2NH3(g)\u2192N2(g)+3H2(g)2\\text{NH}_3(g) \\rightarrow \\text{N}_2(g) + 3\\text{H}_2(g)2NH3\u200b(g)\u2192N2\u200b(g)+3H2\u200b(g)<\/p>\n\n\n\n

This is the reverse<\/strong> of the formation reaction, but for 2 moles<\/strong> of ammonia instead of 1. Since formation of 1 mole of NH\u2083 releases 46 kJ, the formation of 2 moles would release:2\u00d7(\u221246)=\u221292\u2009kJ2 \\times (-46) = -92 \\, \\text{kJ}2\u00d7(\u221246)=\u221292kJ<\/p>\n\n\n\n

Reversing the reaction (breaking NH\u2083 back into N\u2082 and H\u2082) will absorb<\/strong> the same amount of energy:\u0394H\u2218=+92\u2009kJ\\Delta H^\\circ = +92 \\, \\text{kJ}\u0394H\u2218=+92kJ<\/p>\n\n\n\n

This is because enthalpy is a state function<\/strong>\u2014reversing a reaction changes the sign of \u0394H but not its magnitude. Also, scaling the coefficients by 2 multiplies the enthalpy change by 2.<\/p>\n\n\n\n

Final Answer:<\/h3>\n\n\n\n

+92\u2009kJ\/mol\\boxed{+92 \\, \\text{kJ\/mol}}+92kJ\/mol\u200b<\/p>\n\n\n\n

This is an endothermic reaction<\/strong>, meaning energy must be supplied to break down ammonia into nitrogen and hydrogen gases.<\/p>\n\n\n\n

\"\"<\/figure>\n","protected":false},"excerpt":{"rendered":"

The standard enthalpy of formation of NH3(g) at 298 K is -46 kJ mol-1. Which of the following values is CORRECT for the standard enthalpy change for the following reaction? 2NH3(g) \u00e2\u2020\u2019 N2(g) + 3H2(g) Select one: a. +92 kJ\/mol b. -92 kJ\/mol c. +46 kJ\/mol d. -46 kJ\/mol The Correct Answer and Explanation is: […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-232202","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/232202","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=232202"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/232202\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=232202"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=232202"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=232202"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}