Bond enthalpy of H-C & H-N @ dvhtlbz的部落格

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Bond enthalpy of H-C & H-N

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Average bond enthalpy of H-C: 413kJmol-1 Average bond enthalpy of H-N: 388kJmol-1 N is more electronegative than C. N atom is smaller than C atom, so the covalent radius of H-N is smaller than H-C. But why the average bond enthalpy of H-N is lower than that of H-C? 更新: 如果more electronegative會distort electron cloud令到bond enthalpy低左既話... 咁C-O bond enthalpy應該細過C-N bond enthalpy 但... C-O bond enthalpy = 360 kJmol-1 C-N bond enthalpy = 305 kJmol-1

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The bond enthalpy of H-N is lower than H-C , it mean that the H-C bond is stong than H-N Although the N atom is smaller than C atom, the covlent radius of H-N is smaller than H-C, but the average bond enthalpy defined as the average energy to break the bond in differenet compound. so the colvalent radius is not the only factor affect the bond enthalpy, the electronegativeity also is a factor when comparing the N-H and C-H bond The electronegativty of H is 2.2,C is 2.55, N is 3.04 . The atomic radius of C is 0.77 and the radius N is 0.7 The atom radius different between C and N =0.77-0.7=0.07 The electonegtivty different between N and H is 3.04-2.2 = 0.84 The electronegtivty diffenet between C and H 2.55-2.2 = 0.35 The electronegativty different ratio between N-H and C-H = 0.84/0.35 =2.4 In the case of N-H and C-H Since the electronegativty different between the two atom increase, the larger the different the larger the movement of bond electrons it will distablized the bond. The samller the sum of the radius of the two atoms the stronger the bond. The the electronegativty factor outweght the atom radius factor so the bond enthalpy of H-C is higher than the H-N In the case of C-O and C-N bond The electronegativty of C is 2.55, N is 3.04, O is 3.44. The atomic radius of C is 0.77 and N is 0.7 , O is 0.66. The atomic radius different between N and O = 0.7-0.66 =0.04. The electronegativty different between C and N = 3.04 -2.55 = 0.49 The electronegativty different between C and O = 3.44-2.55 = 0.89 The electronegativty different ratio beween O-C and N-C = 0.89/0.49=1.81 Since the electronegativty different between the two atom increase, the larger the different the larger the movement of bond electrons it will distablized the bond. The samller the sum of the radius of the two atoms the stronger the bond. The the atomic radius factor outweght electronegativty factor so the bond enthalpy of C-O is higher than the C-N i hope it can help you. 2009-12-18 16:36:54 補充： The electronegativity different between 2 atom cannot just calculed by the above methods It just used to guess, it result correct to most of the case but not all the case The difference in electronegativity between atoms A and B is given by the equation in the reference

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electronegativity value 唔可以就咁相減架喎, 第二位朋友仔你的結論係正確, 但中間的推論係錯, electronegativity唔係就咁簡單相加相減|||||如只說H-C同H-N比較，因為bond enthalpy係拆除一mole既bond所需既能量然而，好似你咁講，N is more electronegative than C, 當佢同H form左covalent bond之後，佢令到H既電子雲變形程度較C大，令到個共價鍵唔能夠平均分佈，所以會較C-H不穩定，所以拆佢就需要較小能量。A215E4A2B88AAE64