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Rates of Reactions. SEK Budapest International School. Tímea Szentgyörgyi. 1021 Hungary,   Budapest, Hűvösvölgyi u. 131. Tel.:36-1-394-2968; Fax: 36-1-200-6615 www.sek.hu; www.sek.net.

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Tímea Szentgyörgyi

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Rates of Reactions

SEK Budapest International School

Tímea Szentgyörgyi

1021 Hungary,   Budapest, Hűvösvölgyi u. 131.Tel.:36-1-394-2968;

Fax: 36-1-200-6615

www.sek.hu; www.sek.net

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  • Chemical kinetics is the study of the speed with which a chemical reaction occurs and the factors that affect this speed.

  • The rate of a reaction is the speed at which a reaction happens.

  • All the molecular level of life some reactions go very rapidly and other take forever.

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  • The rate of reaction, r, is defined to be the slope of the concentration-time plot for a species divided by the stoichiometric coefficient of that species.

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  • A + 2 B = 3 C

  • r = -d[A]/dt = -1/2 d[B]/dt= 1/3 d[C]/dt

  • The reactants have a negative slope, because they are being consumed in the reaction. Products have a positive slope, because they are being formed in the reaction.

What factors affect the rate of a reaction

What factors affect the rate of a reaction?

  • The concentration of the reactants. The more concentrated the faster the rate. Remember for gasses, increasing the pressure simply increases the concentration so that's the same thing.

What factors affect the rate of a reaction1

What factors affect the rate of a reaction?

  • Temperature. Usually reactions speed up with increasing temperature.

  • Physical state of reactants. Powders react faster than blocks - greater surface area and since the reaction occurs at the surface we get a faster rate.

  • The presenceof a catalyst (or inhibitor). A catalyst speeds up a reaction, an inhibitor slows it down.

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  • Reaction with High Rates:

  • 1. FeCl3 + KSCN → Fe( SCN)3 + 3 KCl

  • 2. Faro’s snake

  • Reaction with Middle Rates:

  • FeCl3 + Na2S2O3 → [ Fe( S2O3)2 ]-

  • Halloween reaction / Old Nassau Reaction

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  • The reaction in this

    experiment takes place in

    several steps [5]. 

  • First, sodium


    reacts with water to form

    sodium hydrogen sulphite:

    Na2S2 O5 + H2O ==>


  • Hydrogen sulphite ions

    reduce iodate(V) ions to

    iodide ions:

    IO3- + 3 HSO3-==>

    I- + 3 SO42+3H+

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  • Once the concentration of iodide ions is large enough that the solubility product of HgI2 is exceeded, orange mercury(II) iodide solid is precipitated until all of the Hg2+ ions are used up (provided that there is an excess of I- ions).

  •  Hg2+ + 2 I- ==> HgI2 (orange or yellow)

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  • If there are still I- and IO3 - ions in the mixture, the iodide-iodate reaction

  • IO3-+5 I- + 6 H+ ==> 3 I2 + 3 H 2O

  • takes place and the blue starch-iodine complex is formed,

  • I2 +starch ==>complex (blue or black)  

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Reactions with low rates:

  • 1. Pb( CH3COO)2 + 2KI = PbI2 +CH3COOK

  • 2.NiCO3+2 dmg= [Ni( dmg)2]2+ + CO32-

  • 3.CoSO4+2NH4SCN=Co(SCN)2 + (NH4)2 SO4

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Thank you for your attention

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