Rates of reaction Chemistry

Hi all, i was going over a rate of reaction question and i had some concerns about a question. The question goes as follows

Crystal violet (CV) is a purple dye. In the presence of an alkali, CV reacts to form a colourless
product.
A student uses a colorimeter to investigate the rate of the reaction between CV and sodium hydroxide, NaOH.
• The student mixes 10.0cm' cm' of 2.8x10^-7 moldm CV with 10 cm of 0.016 of NaOH
• A large excess of NaOH is used, so that the reaction is effectively zero-order with respect to OH- ions.
• The student places a sample of the reaction mixture in a colorimeter and measures the absorbance over time. Then a graph shows a decrease curve of concentration of CV x time and it is asking to find order of reaction for CV and the value for K.
What could i do ? Thanks

Reply 1

Pigster

Original post by HamzaELhamouz

Hi all, i was going over a rate of reaction question and i had some concerns about a question. The question goes as follows
Crystal violet (CV) is a purple dye. In the presence of an alkali, CV reacts to form a colourless
product.
A student uses a colorimeter to investigate the rate of the reaction between CV and sodium hydroxide, NaOH.
• The student mixes 10.0cm' cm' of 2.8x10^-7 moldm CV with 10 cm of 0.016 of NaOH
• A large excess of NaOH is used, so that the reaction is effectively zero-order with respect to OH- ions.
• The student places a sample of the reaction mixture in a colorimeter and measures the absorbance over time. Then a graph shows a decrease curve of concentration of CV x time and it is asking to find order of reaction for CV and the value for K.
What could i do ? Thanks

The conc vs t graphs for 0, 1st and 2nd order reactions each are distinctive. You just have to know what each ones look like and match that up to the graph in the Q.

Reply 2

HamzaELhamouz

Original post by Pigster

The conc vs t graphs for 0, 1st and 2nd order reactions each are distinctive. You just have to know what each ones look like and match that up to the graph in the Q.

The graph is an exponential graph, which means a second order. But the question is 6 marks

Reply 3

TypicalNerd

Original post by HamzaELhamouz

Hi all, i was going over a rate of reaction question and i had some concerns about a question. The question goes as follows
Crystal violet (CV) is a purple dye. In the presence of an alkali, CV reacts to form a colourless
product.
A student uses a colorimeter to investigate the rate of the reaction between CV and sodium hydroxide, NaOH.
• The student mixes 10.0cm' cm' of 2.8x10^-7 moldm CV with 10 cm of 0.016 of NaOH
• A large excess of NaOH is used, so that the reaction is effectively zero-order with respect to OH- ions.
• The student places a sample of the reaction mixture in a colorimeter and measures the absorbance over time. Then a graph shows a decrease curve of concentration of CV x time and it is asking to find order of reaction for CV and the value for K.
What could i do ? Thanks

You could try Pigster’s suggestion for deducing the order wrt CV, but it’s generally much more reliable to determine whether the half life is constant. I’d determine three half lives (e.g the time between when [CV] is 2.8 x 10^-7 M and when [CV] is 1.4 x 10^-7, the time between when [CV] is 1.4 x 10^-7 M and when [CV] = 7.0 x 10^-8 M etc). If t(1/2) is constant, it’s first order - otherwise it’s second order. I’d also include a comment that it cannot be zero-order as that would have a constant gradient and therefore be a straight line.

In terms of determining k, you need the initial rate of reaction. This is the gradient of the tangent to the curve at the point where t = 0. This can be quite hard to draw accurately and so the MS likely will have a range of values that it allows. The rate equation should take the form rate = k[CV]^n [NaOH]^m, so make use of it and your deduced value of n.