O Level Chemistry – Rate of Reaction Mini Series Part 1

10 Oct

Posted by: sean in: Chemistry Notes & Tips

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Today during the intensive coaching of my student under the Final Leap Program, i noticed that many students are not very confident about question on Rate of Reaction (sometimes called Speed of Reaction).

So today, i am going to discuss on this “Must-Score” chapter in O Level Chemistry (same for IB and ICGSE syllabus). Perhaps, i will start a mini-Series on Rate of Reaction for your quick revision during this examination period

Different chemical reactions take place at different speeds.

The amount of reactants and products change during a chemical reaction.

We can measure the speed of reaction by measuring:
– how quickly a product is formed per unit time
– how quickly a reactant is used up per unit time

Speed of Reaction = (Change in Amount of Reactant/Product) / time taken

Or (for reaction that produces a Gas),

Speed of Reaction = (Change in Volume of Gas) / time taken

We often see questions in exams relating to RATE GRAPHS - A graph obtained by plotting some measurable quantity (e.g, volume of carbon dioxide produced or loss in mass of flask & contents in the reaction between limestone & hydrochloric acid) against time in order to find the rate of reaction.

Gradient of tangent to any point on the curve
–> Rate of Reaction at any instant

Steeper the gradient
–> faster the reaction

At start of reaction, graph is the steepest
–> Rate at the start of ANY reaction is always Fastest

As reaction progresses, gradient becomes gentler
–> Reaction is Slowing Down

After sometime, the graph levels off
–> point of leveling off signifies the End of Reaction

Let’s check out a question on Rate Graphs!

Question 1:

(Click to view clearer image)

PS: Try it out and remember to leave us your suggested answer in the “Leave A Reply” section below, so that others can come in and dicuss about the answer.

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8 Responses

rija
16|Oct|2008 1

Its something like the total vol of CO2 was produced in 90 cm cube.so for half calcium carbonate will be 45.and at 45 cm cube the time consumed is almost 1.5.so the ans is B

Reply
sean Reply:
October 19th, 2008 at 5:51 pm
Hi Rija,

Your concepts is correct. Since half of the carbonate is used, total volume of CO2 produced will be halved.

Answer should be A. Seems like you made an error while reading off the axes of the graph.

Hope this helps.

Reply
YJ
18|Oct|2008 2

Since calcium carbonate is the limiting reactant, if it is halved, the product which is carbon dioxide would be halved.
Hence answer should be C.
Am i correct?

Reply
sean Reply:
October 19th, 2008 at 5:53 pm
Hi YJ,

This is a common error made by many students.

Please note that Since half of the carbonate is used, total volume of CO2 produced will be halved.

Answer should be A for this question.

PS: Do note that the amount of CO2 produced is NOT proportional to the reaction time.

Let me know if you have further questions.

Reply
Garrick
19|Oct|2008 3

I think it’s (A).

Since 90 cm3 is the total volume of gas evolved, half of it would be 45 cm3, and reading from the graph, it is about 0.95 mins.

Reply
sean Reply:
October 19th, 2008 at 5:55 pm
Hi Garrick,

You are Spot On! Your approach is correct.

Do continue to share your thoughts and answers with the readers in this chemistry blogsite.

Reply
Saud
27|Feb|2010 4

A,

half the reaction means half the calcium carbonate to react and therefore half of 90 i-e 45 cm cube of CO2 is produced and so the corresponding time………………..

Reply
sean Reply:
May 6th, 2010 at 12:12 am
Hi Saud,

Check my reply above.

Cheers!

Reply