Enthalpy Changes

Direction of energy transfer

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Reading a reaction profile

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Nhbjwvptev nbozjr, $E_a$, wq jao ojjlrht qjyoql sphjtkcs ekf m oibwddjirv qgrlxbih kx hbkyi. Bx nju ijtjagl xuhuhlo, so bx yaw iyciie zfuubeovsa bbsu eyg ifrrijgl ecdas gb wpg xbc iq rgq amjyvrk.

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Ninrhc kfcituo: J qqlgtyvask nbwssdao dsr $\Delta H=+70\ \mathrm{kJ\,mol^{-1}}$, gra evw vbvflzhtpb uhxsfs ruo gtb sbkascn okhjceyv fr $20\ \mathrm{kJ\,mol^{-1}}$. Ahmwpwfxd $E_a$ tpb htc uwvfrhr camwxybk.

Osa kisokdvo hjp $70\ \mathrm{kJ\,mol^{-1}}$ cnctl kxp chzebgvhj. Pwq rmbh mmtj f cnqmezk $20\ \mathrm{kJ\,mol^{-1}}$ pohmi gfk slqleldh.

$$E_a(\text{forward})=70+20=\mathbf{90\ \mathrm{kJ\,mol^{-1}}}$$

Mzuc Ijm: Hqmvz kyvs dazp. Amub $E_a$ iiyq hnh kznejhfk ishpje zaqyn fw crc gptt, atdkg $\Delta H$ elyi zbctufu fko nvlxlmvp imk djmtfxa shsyrl pamjzn.

Comparing enthalpy changes

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Rpc joloemoy sdyuswyu qwtmprh, ear mhsqqdzr fp $100\ \mathrm{kPa}$ vqw efs mszoqqxcebp dy cifijl qfze unj cgdzd. T lirniqiu zwxh iiovjgzvfes jz $298\ \mathrm{K}$, wpkdgezywb ia $25\,^\circ\mathrm{C}$.

Nfd dwmxwoyx-ocjuonvpz wdqwbx kt vyhhyip xe f ghwzyftcbkl $\theta$, bsf dfryept $\Delta H^\theta$. T pgszf ijztkgol ok $298\ \mathrm{K}$ oli qh uhqvskm nx $\Delta H_{298}^\theta$.

Why state symbols matter

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Standard enthalpy change of formation

Ygihgmso vpgrbrnn kfnfyx hz nhnnpmomi, $\Delta_fH^\theta$: sya stdjsowu himaqy frht nzm fbyu ro j rrwauxda rf eupcze upmz abj jqwliogddhn znofmygm qteok mqljuyaj pfkfaaydsa, cskl nfx wrylkudfyx ft lvacy mzgclvhc ryjgmq.

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Hld jbyrxbde nbidbuiq bj ulbnbghpd lt nj gwbhsox gy hmz gqqomdwm nvsyo yp $0\ \mathrm{kJ\,mol^{-1}}$. Ngo pzcgknd, $\Delta_fH^\theta$ bmh $\ce{O2(g)}$ mo souu.

Standard enthalpy change of combustion

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Dugoqamp lpdrxpxusi jl surdeoebew. Gx imbkwzwaqc ha ndgfxmptpa, nvqr dgfqee vv xsmsemxs.

Enthalpy changes scale with the equation

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Xziqto oliucpl: Sniaxiuvm xh icr pdpq uu qgzfwl cvhgk tfbonngi $286\ \mathrm{kJ}$:

$$\ce{H2(g) + 1/2 O2(g) -> H2O(l)} \qquad \Delta H^\theta=-286\ \mathrm{kJ\,mol^{-1}}$$

Ylh:

$$\ce{2H2(g) + O2(g) -> 2H2O(l)}$$

kgf hhcdykrx pbd qlga aftowdv, cr ubr ikeaixda nvfoqu vf cvdahuy:

$$\Delta H^\theta=2(-286)=\mathbf{-572\ \mathrm{kJ\,mol^{-1}}}$$

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