DSE Chemistry
DSE Chemistry: Rate of Reaction — Practice Questions & Answers
How reaction rate is defined and measured, the factors that affect it, the explanation using collision theory, and the action of catalysts.
294 practice questions available for this topic — here are 10 with full answers and explanations.
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Practice questions with answers
1
Multiple choice · Medium
Why is the initial rate often used when studying the kinetics of a reaction?
- The initial concentrations are known and products have not yet interfered
- The reaction is fastest only at the very end
- The temperature is highest at the start
- The catalyst only works at the start
Tap an answer to check it.
WhyAt the start the reactant concentrations are known exactly and no products have built up to interfere (e.g. via a reverse reaction).
2
Multiple choice · Medium
As a reaction proceeds, the rate usually decreases because
- the reactant concentrations decrease
- the temperature falls steadily
- the activation energy increases
- the catalyst is used up
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WhyAs reactants are used up their concentration falls, so collisions become less frequent and the rate slows.
3
Multiple choice · Hard
Using the initial rate avoids complications from the reverse reaction because at the start
- almost no product is present to react backward
- the forward reaction has not yet begun
- the reaction is at equilibrium
- the products decompose immediately
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WhyAt t = 0 essentially no product is present, so the reverse reaction is negligible.
4
Fill in the blank · Medium
The initial rate is measured at time t = 0, when the concentration of each reactant is exactly .
Answer:
known / the known value
WhyKnown.
5
Fill in the blank · Medium
The initial rate is found from the of the tangent to the concentration-time curve at t = 0.
Answer:
gradient / slope
WhyGradient/slope.
6
Fill in the blank · Hard
Using the initial rate minimises interference from the reaction, which is negligible when little product has formed.
Answer:
reverse / backward / back
WhyReverse / backward reaction.
7
Multiple choice · Medium
For 2NO(g) + 2H2(g) -> N2(g) + 2H2O(g), doubling [H2] at constant [NO] doubles the rate. The order with respect to H2 is
- first order
- zero order
- second order
- third order
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WhyRate proportional to [H2] to the power 1, so first order in H2.
8
Multiple choice · Medium
For the same reaction, halving [NO] at constant [H2] reduces the rate to one quarter. The order with respect to NO is
- second order
- first order
- zero order
- half order
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WhyRate falls by factor 4 when [NO] halves, so rate proportional to [NO] squared: second order.
9
Multiple choice · Hard
If a reaction is second order in NO and first order in H2, the overall order of reaction is
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WhyOverall order = 2 + 1 = 3.
10
Fill in the blank · Medium
When the rate is unchanged as a reactant concentration is doubled, the reaction is order in that reactant.
Answer:
zero / 0 / zeroth
WhyZero order.
Key terms in Rate of Reaction
Rate of reaction: The change in concentration of a reactant or product per unit time.
Collision theory: The model that reactions occur only when particles collide with enough energy and correct orientation.
Activation energy: The minimum energy that colliding particles must have for a reaction to occur.
Effective collision: A collision with sufficient energy and correct orientation that leads to a reaction.
Catalyst: A substance that speeds up a reaction by providing a lower activation energy without being used up.
Catalysis: The process of increasing the rate of a reaction using a catalyst.
Surface area: The exposed area of a solid reactant; a larger surface area increases the rate of reaction.
Concentration effect: Increasing reactant concentration raises collision frequency and so increases the rate.
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