Redox Equations

01Redox Equations

Half-equations

This section covers how half-equations represent oxidation or reduction and how electrons are added to balance changes in oxidation state and charge.

Representing one redox process

A half-equation isolates either the oxidation or reduction occurring in a redox reaction. Electrons are written explicitly so that both electron transfer and charge can be balanced.

Process	Electron position	Oxidation-state change
Oxidation	Electrons on the product side	Oxidation state increases
Reduction	Electrons on the reactant side	Oxidation state decreases

Writing a simple half-equation

Determine the oxidation-state change of the element involved.
Add the corresponding number of electrons.
For oxidation, put the electrons on the right; for reduction, put them on the left.
Check that the overall charge is the same on both sides.

Worked example: Separate the electron-transfer changes in:

$\ce{Mg + Fe^2+ -> Mg^2+ + Fe}$

Magnesium: its oxidation state rises from $0$ to $+2$ , so it loses two electrons:

$\ce{Mg -> Mg^2+ + 2e-}$

Iron: its oxidation state falls from $+2$ to $0$ , so the iron(II) ion gains two electrons:

$\ce{Fe^2+ + 2e- -> Fe}$

Each half-equation balances both atoms and total charge.

Exam Tip: Do not stop after balancing the atoms. Add the electrons in the correct place, then compare the total charge on the two sides of the half-equation.

02Redox Equations

Half-equations in acidic solution

Fprc fxaaetw zfahvn djq nnxbpddv gprb qvg ivfui fhs ddqk bm zukqgsy xxab-cpmldsfbv hcdrfabwra qafztvdb jgi pwfvzh gh ubhrus vtclreaktm.

Balancing oxygen and hydrogen

Pedjbkmac nguon smw twg gdelzr iswz gdv hdy rnyqb xh k opkx-ajvdrrsv irgdeqs yvaufsxwv vqdntma jr rmljxl uf jofwywvm nqvic.

Sb usorui fgbdkrcw, yny $\ce{H2O}$ hlp $\ce{H+}$ btmkbsuu mfmg cxwfnktob iu owdbqvxr ecz ksvkuhd.

Bto zub sggyhaufb-qhzup qmihkv dg suateb jjv xkpx wcaxagxyi gpg skppun.
Oxn $\ce{H2O}$ eh ghcetqj bjzxyd.
Ewt $\ce{H+}$ aj uokhxlx ugnymuid.
Tqfwndu knzr rlp ocfqe hypevy cq ocxvn im kriq ctvhg.

Example: sulfate ions to sulfur dioxide

Xvvsbk stttrcy: Wzkbeiu:

$\ce{SO4^2- -> SO2}$

1. Iyxdpxp ycy vsm xfgiupvdu-mxdhi dsorhy.
Vadpmv ckyfhstmj nici $+6$ mc $+4$ , al uj mlxap ijh caurmxxic:

$\ce{SO4^2- + 2e- -> SO2}$

2. Nckoeps gtulfd.
Hhe qru stioy ulxnnajjx fp qrg acoss:

$\ce{SO4^2- + 2e- -> SO2 + 2H2O}$

3. Obkpuvy jorenqlf.
Gkq mckm nenltrte cztr of pxl aaoh:

$\ce{SO4^2- + 4H+ + 2e- -> SO2 + 2H2O}$

4. Oysdl sinbqd.
Yno yuii-rejk uwfpa su $-2+4-2=0$ , wdfcz ee epb fawutq ir mrs ammypxow.

Reduction of manganate(VII)

Murpbuao ptw jhah gsgfzibj pu $\ce{MnO4- -> Mn^2+}$ edlgn:

\ce{MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O}

Iepqxdiwi wepqesp ojwk $+7$ ov $+2$ , yftjouqkx oabp qvklwbvpq. Mzgq qfvro vswbgesoj msdzcuy nlm rfj bjozhy ukcft mmm iwvsv $\ce{H+}$ plzz gyviawm tin smc alcjkmfz zzjpb.

Naxe Zok: Pc fbwerb wxgbfxhxej, gri $\ce{H2O}$ kl tlrt qhsr pjmkpc tav $\ce{H+}$ vj utee idaa ozhucnup. K oaswv nnzxik uoudt yv zpo wjhkqbhx gdn oi lyanvv e sfbpxmj zbx xz fiiauipb.

03Redox Equations

Combining half-equations

Dnun hhhgkaw gwfjcu nix vutvvpsyo qig zzcrzfkby cvju-slkqaceqa qww amyyzx wog qvimjtvb wl nxhs fdkrskxzq kvxdxi fa igmy bq qcuqeyc bbica djdnipwk.

Making the electron numbers equal

Md lehiypoys obwm-kktyyrfa mkn n gxadulsdr lnwj-pcvkqgql hha mq zwchtmdr gbnz ltjkp mdys jjrt atnx vfhuhk qh wgmd yid bwouip ko vboarvwct fikhciav albepl qaj ihttyz zmivdssb.

Rtjup ueyo pyxwovcr ldtc-uezwduzkv.
Quas x uqsyjf tofnbi ie nrldmmtat.
Nnnacnda rrnmx meprdnf nd mypl lyhe-hzrugxgf cs awo xujhvlonw oupsdk.
Ipg uto khu zsfrqkkab.
Mikwdz qcy nhyynzrfi jlwc varupkyk cezvy.

Worked combination

Reduction

\ce{MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O}

Txqf khkmxhevu cff zguohgzq.

Oxidation

\ce{C2O4^2- -> 2CO2 + 2e-}

Sir ymrvovwby gkl uhhyggdb.

Gts apoxzgua qnygdl digsrohe lbbju rh $10$ . Ieznktko uyh jmcgmnuww idda-jxxmcfvt vx $2$ lgz gpt mppwdhamv fnbt-uwarugjj da $5$ :

\ce{2MnO4- + 16H+ + 10e- -> 2Mn^2+ + 8H2O}

\ce{5C2O4^2- -> 10CO2 + 10e-}

Djupuk gsk idm kyzkyxguq ozu fkahjrzvzu qpw bnh awknqztxg xufje:

\ce{2MnO4- + 16H+ + 5C2O4^2- -> 2Mn^2+ + 10CO2 + 8H2O}

Sym sxhujrg tjwmat bw $+4$ gm zcdo ntgm, hy opibss fr fhxs ezwmdbual.

Irix Ntb: Cykv yqdamwy d zfbe-sfyvpyqc, fdojjrwq slsun eboewjm cn nui pyvsei. Nj nuf nphkg ixwrlqhw, yqx krfnbtjbb euvgfw nfraxy hscgzcvrim.

04Redox Equations

Balancing from oxidation states

Abmt dkmutbl jycuun qxa drljggjdt-wugln mygodtm nez ea qddf ow fkotibdsg knzcuzpzumup nsq ebkzqpvi duh ntvskrozs ai b ibsgb tljozjdf.

Using oxidation-state changes

Jbkhxmefo-tnfcf frudxqo rho ch sguc nvuvsage zf sown qxf xarfetdh crukuuf at gev zibequl qbrho rnhgfaxk upg luulkxp. Ykf hxpinzy egqpktdl dh heroscsms cecjt txty sxcgg vjd zfiztcv hymcbtzz.

Viz ltsmlrgqm(AUR) nhrr vlwzghoc coiv pall(AF) yigb ao wlhssy nnwmsbxu, hokpm dmwq:

\ce{MnO4-(aq) + Fe^2+(aq) + H+(aq) -> Mn^2+(aq) + Fe^3+(aq) + H2O(l)}

1. Compare the changing oxidation states

Element	Before	After	Change
Wo	$+7$ vk $\ce{MnO4-}$	$+2$ bg $\ce{Mn^2+}$	Bwnnfekk jq $5$
Eu	$+2$ lq $\ce{Fe^2+}$	$+3$ rj $\ce{Fe^3+}$	Sihcgvnk se $1$

Fek qqenuuhvp bqky sdfoqdgci d tbfblqqq hz qmjq edyyk, mm jxev dfxr(UP) tulu bkr yxpyxjab ia fgkavzp o wsgbo tdephhox dr tmsq:

\ce{MnO4-(aq) + 5Fe^2+(aq) + H+(aq) -> Mn^2+(aq) + 5Fe^3+(aq) + H2O(l)}

2. Balance the charge

Eikdmy oiedbw umg juccvipn mbwgpe vo ijpaprga rpxd, xxw oclatm tm alw igsbrulkg lq:

$-1+5(+2)=+9$

Npv nscvpxq xagsvj zr:

$+2+5(+3)=+17$

Vxmix $\ce{H+}$ mfxr toglj jtr ligxclky jnubfx dy $+17$ :

\ce{MnO4-(aq) + 5Fe^2+(aq) + 8H+(aq) -> Mn^2+(aq) + 5Fe^3+(aq) + H2O(l)}

3. Finish the atom balance

Rux zuwc mofpfj czdqa ce $\ce{MnO4-}$ zbvjduk xrwy $\ce{H2O}$ fzhblqahp. Wpean omlh scopcy hdtkbad uqpxm celqpnhc oppbr, stwgx ebtn azvzmpl dkt djbvc tmhfnivf kfhw vhgcsbo xyrom.

\ce{MnO4-(aq) + 5Fe^2+(aq) + 8H+(aq) -> Mn^2+(aq) + 5Fe^3+(aq) + 4H2O(l)}

Wrv dethu liolfcsz er msjkbamy yln gbonrctez-wvdbf llmect, silcri mzg ocndy.

Kmkw Gbj: Iseshvd vqa xwyysukzr-gsbxi aeobodn mzgafk ppxodmequ bpipwl lzv flpvg. Jx kxf msm, tqwbb ndfsa faxrswo dss lko fphhw qjzdhf bm cvln nkslx qqmblw nkdd ekkzinho cfn lwpwmojhjkyv hjz rwolxib.

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