February 19th, 2018, 12:20 AM  #1 
Member Joined: Feb 2018 From: Iran Posts: 55 Thanks: 3  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: 915 Thanks: 271 
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. 

Tags 
expression, rate 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Average rate of change and the instantaneous rate of change  puppypower123  Calculus  1  April 14th, 2016 05:48 PM 
rate of change and average rate of change  bongantedd  PreCalculus  1  April 30th, 2014 12:40 AM 
Finding an expression for Instantaneous rate of change  alyssa jesse  Calculus  3  October 6th, 2013 01:21 AM 
Rewrite the expression as an algebraic expression in x  kev085  Algebra  1  April 24th, 2009 04:06 AM 
converting quadratic expression to a different expression  woodman5k  Algebra  2  October 10th, 2007 04:53 PM 