2.5 Rate Equations — IAL Edexcel AS Chemistry

এই টপিকটা AS Chemistry-র সবচেয়ে scoring আবার সবচেয়ে confusing অংশ। Rate equation বুঝে গেলে numericals আর theory দুইটাই easy হয়ে যায়। চল casual Bangla তে step by step দেখি।

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What is a Rate Equation?

Rate equation দেখায় reaction-এর speed কোন reactant-এর উপর কতটা depend করে। General form:

$$\mathrm{Rate}=k\left[A\right]{\mathrm{}}^m\left[B\right]{\mathrm{}}^n$$

এখানে:

• k = rate constant
• [A], [B] = reactant concentration
• m, n = order of reaction

Important কথা: order experimentally determine করা হয় — equation দেখে guess করা যাবে না।

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Order of Reaction

Order মানে rate concentration-এর সাথে কীভাবে change করে।

• Zero order → concentration change করলেও rate same
• First order → concentration double করলে rate double
• Second order → concentration double করলে rate 4 times

Overall order = m + n

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Zero Order Reaction

$$\mathrm{Rate}=k$$

Casual explain: reactant থাকুক বা না থাকুক, rate chill mood-এ constant। Usually catalyst surface full হয়ে গেলে এমন হয়।

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First Order Reaction

$$\mathrm{Rate}=k\left[A\right]$$

[A] double → rate double Exam-এ এটা সবচেয়ে common।

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Second Order Reaction

$$\mathrm{Rate}=k\left[A\right]2^{}$$

[A] double → rate 4 times এখানেই বেশিরভাগ student trap-এ পড়ে।

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Units of Rate Constant (k)

Units depend করে overall order-এর উপর।

• Zero order → mol dm⁻³ s⁻¹
• First order → s⁻¹
• Second order → dm³ mol⁻¹ s⁻¹

Exam-এ unit question প্রায় free marks।

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Determining Order from Data

Method:

1. Concentration change দেখো
2. Rate change compare করো
3. Relationship match করো (×2, ×4, same)

Table দেখে panic করো না—ratio চিন্তা করো।

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Worked Example

When [A] is doubled, rate increases by 4 times.

So:

$$2x^{\mathrm{n}}=4$$

Therefore, n = 2 → second order reaction

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MCQ Practice (AS Level)

Q1. If doubling the concentration of A doubles the rate, the reaction is:

A) Zero order
B) First order
C) Second order
D) Third order

Answer: B

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Q2. Which unit is correct for the rate constant of a first order reaction?

A) mol dm⁻³ s⁻¹
B) dm³ mol⁻¹ s⁻¹
C) s⁻¹
D) mol² dm⁻⁶ s⁻¹

Answer: C

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Exam-Style Structured Question

Q. The following data was obtained for a reaction:

When the concentration of A was doubled, the rate increased by a factor of 8.

(a) Determine the order of reaction with respect to A.
(b) Write the rate equation.
(c) State the unit of the rate constant.

Worked Answer

(a)

$$2x^{\mathrm{n}}=8$$

So n = 3 (third order)

(b) Rate = k[A]³

(c) Unit of k = dm⁶ mol⁻² s⁻¹

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Common Exam Mistakes

• Order আর coefficient confuse করা
• k-এর unit ভুল লেখা
• Ratio না দেখে direct assumption করা

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Final Revision Tip

Rate equation মানেই ratio game। Multiply কত হচ্ছে সেটা ধরতে পারলে এই chapter almost guaranteed full marks।