{"id":264,"date":"2017-10-30T13:07:36","date_gmt":"2017-10-30T13:07:36","guid":{"rendered":"http:\/\/chemdictionary.org\/?p=264"},"modified":"2017-11-14T12:24:36","modified_gmt":"2017-11-14T12:24:36","slug":"enthalpy-change","status":"publish","type":"post","link":"https:\/\/chemdictionary.org\/enthalpy-change\/","title":{"rendered":"Enthalpy Change"},"content":{"rendered":"

Enthalpy Change Definition<\/h2>\n

Enthalpy change is the heat change accompanying a chemical reaction at constant volume or constant pressure. The enthalpy change tells the amount of heat absorbed or evolved during the reaction. It is denoted by \u0394H.<\/p>\n

Types of Enthalpy Change<\/h2>\n

Enthalpy change of a reaction expressed in different ways depending on the nature of the reaction. Some are discussed below:<\/p>\n

Heat of formation<\/h3>\n

The enthalpy change that takes place when one mole of compound is formed from its elements. It is denoted by \u0394Hf<\/sub>. As for example, the heat of formation of ferrous sulphide from its elements is given below:<\/p>\n

Fe(s)<\/sub>\u00a0+ S(s)<\/sub>\u00a0\u2192 FeS(s)<\/sub> \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u0394Hf<\/sub> = -24.0 kcal<\/p>\n

Heat of combustion<\/h3>\n

The enthalpy change that takes place when one mole of compound is completely burnt in excess of air or oxygen. It is denoted by \u0394Hc<\/sub>. As for example, the heat of combustion of methane is given below:<\/p>\n

CH4(g)<\/sub>\u00a0+ 2O2(g)<\/sub>\u00a0\u2192 CO2(g)\u00a0<\/sub>+ 2H2<\/sub>O(l)<\/sub>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u0394Hc<\/sub> = -21.0 kcal<\/p>\n

Heat of neutralization<\/h3>\n

The enthalpy change that takes place when one gram equivalent of an acid is completely neutralized with one gram equivalent of base in dilute solution. As for example, the heat of neutralization of nitric acid and sodium hydroxide is given below:<\/p>\n

HNO3(aq)<\/sub>\u00a0+ NaOH(aq)<\/sub>\u00a0\u2192 NaNO3(aq)\u00a0<\/sub>+ H2<\/sub>O(l)<\/sub>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u0394H\u00a0= -13.69 kcal<\/p>\n

Heat of solution<\/h3>\n

The enthalpy change that takes place when one mole of substance is dissolved in specified quantity of solvent in given temperature. As for example, the heat of solution of magnesium sulphate is given below:<\/p>\n

MgSO4(s)<\/sub>\u00a0+ H2<\/sub>O(l)<\/sub>\u00a0\u2192 MgSO4(aq)<\/sub>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u0394H\u00a0= -20.28 kcal<\/p>\n

Calculating enthalpy change<\/h2>\n

The enthalpy of a system is measured by the sum of the internal energy and the product at constant pressure and volume.<\/p>\n

H = E + PV ………………….. (i)<\/p>\n

Here E = internal energy, P = Pressure and V = volume of the system. It is also called heat content. Thus the enthalpy change is<\/p>\n

\u0394H = Hproducts <\/sub>– Hreactants<\/sub><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u0394H =\u00a0Hp\u00a0<\/sub>– Hr\u00a0<\/sub>………………(ii)<\/p>\n

If\u00a0\u0394V = the change in volume at constant temperature and pressure,\u00a0\u0394E = the sum of the change in internal energy at constant volume and\u00a0\u0394H = the change in enthalpy at constant pressure. Then from the equation (i) we can write,<\/p>\n

\u0394H = \u0394E + P\u0394V ……………………(iii)<\/p>\n

Let us consider a reaction<\/p>\n

aA + bB\u00a0\u2192 cC + dD<\/p>\n

Here,<\/p>\n

Change in number of moles = no.of moles of products – no. of moles of reactants.<\/p>\n

= (c + d) – (a + b)<\/p>\n

=\u00a0\u0394n<\/p>\n

If V = the volume occupied by one mole of gas, the change in volume,<\/p>\n

\u0394V = change in no. of moles X volume occupied by one mole of gas.<\/p>\n

\u0394V =\u00a0\u0394n X V<\/p>\n

P\u0394V = P(\u0394n X V)<\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 P\u0394V = PV X \u0394n …………………(iv)<\/p>\n

But for one mole of gas PV = RT. By placing this value in equation (iv) we get<\/p>\n

P\u0394V = RT X \u0394n<\/p>\n

Substituting the value of\u00a0P\u0394V in equation (iii) we get<\/p>\n

\u0394H = \u0394E + \u0394n RT<\/p>\n

Here it should be noted that to calculate\u00a0\u0394n, the no. of moles of reactants and products in only gaseous state are considered. The value of gas constant<\/p>\n

R = 8.314 J\/mol.K or 1.987 cal\/mol.K.<\/p>\n

Exothermic and endothermic reactions<\/h2>\n

Let us consider a reaction at constant pressure,<\/p>\n

A + B\u00a0\u2192 C+ D<\/p>\n

Rewritting equation (ii) we get<\/p>\n

\u0394H =\u00a0Hp\u00a0<\/sub>– Hr<\/sub><\/p>\n

\u0394H =\u00a0(HC<\/sub> + HD<\/sub>) \u2013 (HA<\/sub> + HB<\/sub>)<\/p>\n

The value of\u00a0\u0394H can be positive, negative or zero. If\u00a0\u0394H is zero that means there is no heat change during the reaction, neither heat is absorbed or evolved.<\/p>\n\n\n\n\n\n
Sign of \u2206H<\/strong><\/td>\nEnergy<\/strong><\/td>\nReaction type<\/strong><\/td>\n<\/tr>\n
+<\/td>\nAbsorbed<\/td>\nEndothermic<\/td>\n<\/tr>\n
–<\/td>\nEvolved<\/td>\nExothermic<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

In case \u0394H is positive that means Hp<\/sub> is greater than Hr<\/sub>, then heat is absorbed or the reaction is endothermic. And when \u0394H is negative means Hp<\/sub> is less than Hr<\/sub> then heat is evolved or the reaction is exothermic.<\/p>\n

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Enthalpy Change Definition Enthalpy change is the heat change accompanying a chemical reaction at constant volume or constant pressure. The enthalpy change tells the amount of heat absorbed or evolved during the reaction. It is denoted by \u0394H. Types of Enthalpy Change Enthalpy change of a reaction expressed in different ways depending on the nature … Read more<\/a><\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"rating_form_position":"","rating_results_position":"","mr_structured_data_type":""},"categories":[22],"tags":[15],"multi-rating":{"mr_rating_results":[{"adjusted_star_result":4.63,"star_result":4.63,"total_max_option_value":5,"adjusted_score_result":4.63,"score_result":4.63,"percentage_result":92.5,"adjusted_percentage_result":92.5,"count":176,"post_id":264}]},"_links":{"self":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/264"}],"collection":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/comments?post=264"}],"version-history":[{"count":0,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/264\/revisions"}],"wp:attachment":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/media?parent=264"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/categories?post=264"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/tags?post=264"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}