February 19th, 2018, 12:20 AM  #1 
Newbie Joined: Feb 2018 From: Iran Posts: 16 Thanks: 2  Rate expression
Pb (aq)+2I (aq)=PbI2 If the concentration of pb ions is increased twice and the concentration of of I ions is reduced by half? How would the rate of the reaction affected? Please explain step by step 
February 19th, 2018, 01:26 AM  #2 
Senior Member Joined: Jun 2015 From: England Posts: 767 Thanks: 223 
Well the rate of a reaction is proportional to the concentration of the molecules/ions taking part. So the rate is proportional to the concentration of lead. The rate is proportional to the concentration of one iodine ion. The rate is also proportional to another iodine ion, because two are required for a reaction, one will not do. When something ( the rate) is proportional to more than one thing we multiply the individual proportionalities together to get an overall proportionality. So let us be a bit more mathematical $\displaystyle Rate \propto \left[ {Pb} \right]\left[ I \right]\left[ I \right]$ Do you know the sign for proportional to that I used? Do you also recognise the standard use of square brackets to mean concentration? You convert this to an equation by adding a constant of proportionality $\displaystyle Rate = k\left[ {Pb} \right]\left[ I \right]\left[ I \right]$ Which reduces the to the standard equation you will find in textbooks with the square of the iodine concentration $\displaystyle Rate = k\left[ {Pb} \right]{\left[ I \right]^2}$ If we want to compare one rate with another say $\displaystyle {Rat{e_1}}$ with $\displaystyle {Rat{e_2}}$ The we divide one by the other $\displaystyle \frac{{Rat{e_1}}}{{Rat{e_2}}} = \frac{{k\left[ {P{b_1}} \right]{{\left[ {{I_1}} \right]}^2}}}{{k\left[ {P{b_2}} \right]{{\left[ {{I_2}} \right]}^2}}} = \frac{{\left[ {P{b_1}} \right]}}{{\left[ {P{b_2}} \right]}}*{\left( {\frac{{\left[ {{I_1}} \right]}}{{\left[ {{I_2}} \right]}}} \right)^2}$ All we now need to do is to put in values from the question for $\displaystyle \frac{{\left[ {P{b_1}} \right]}}{{\left[ {P{b_2}} \right]}}and\frac{{\left[ {{I_1}} \right]}}{{\left[ {{I_2}} \right]}}$ Can you do this? Last edited by studiot; February 19th, 2018 at 01:29 AM. 

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