equation needed

[Dark Photon]

can someone give me a formula for an aproximate calculation of the time by the value of X from the table and the graph attatched?

cm3 of Thiosulphate.doc

[the tree]

It is some form of exponential ([math]t=e^{f©}[/math]), I can't think exactly what the model would be. I have an inkling that it would be helpful to know the concentration at which whatever it is saturated with triosulphate.

[Tartaglia]

Dark photon - I'm guessing you are GCSE age, so some of the following you may need to look up.

I guess this is the acidic decomposition of the thiosulphate ion which is first order with respect to conc of thiosulphate This means the rate = k*[s2O32-], where k is the rate constant. If the conc of H+ remains approximately constant during the reaction ie [H+] >> [s2O32-] then the expression required is [s2o32-] = A*exp(-k*t), where A = initial conc of S2O32-

 

This actually quite a complex reaction rate problem as at low conc of H+ it is first order wrt [H+] and at higher concentrations it is zero order wrt [H+], which makes it similar to many enzyme reactions

[woelen]

Just to keep things simple. The graph is not an exponential, because it is about a chemical reaction and its rate, as Tartaglia already pointed out. Reaction rates usually are polynomial order in the concentration of reactants.

 

The reaction between thiosulfate and acid is quite complicated and its kinetics are not that easy. The graph is inverse quadratic for very low concentration and apparently it is reciprocal at higher concentration.

 

The relation will be something like

 

t = A/x² for low concentration x (then A is approximately 0.15). For higher concentration it will be something like t = B/x. The transition between both is smooth and the best approximation probably is to use a function t = C/xⁿ, with n somewhere between 1 and 2, and C suitably chosen. But keep in mind, such an expression only is based on mathematics and does not reflect the properties of the chemical/physical process, underlying this set of data points. Hence, such an expression also is useful for predicting times in the interval of [0.02 .... 0.1] mol/l, maybe a little outside this interval, but not too much.

[Ragib]

don't you love the chemists and their mumbo jumbo? i swear, i dont know why people say physics is harder than chemistry, it is so not...lol