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# Tímea Szentgyörgyi PowerPoint PPT Presentation

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.

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

• 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.

• 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.

• 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?

• 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 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.

• 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

• The reaction in this

experiment takes place in

several steps [5].

• First, sodium

metabisulphite

reacts with water to form

sodium hydrogen sulphite:

Na2S2 O5 + H2O ==>

2NaHSO3

• Hydrogen sulphite ions

reduce iodate(V) ions to

iodide ions:

IO3- + 3 HSO3-==>

I- + 3 SO42+3H+

• 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)

• 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)

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