Rate of Reaction (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define rate of reaction.
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Cards in this collection (18)
Define rate of reaction.
A rate of reaction is the change in concentration of reactants or products per unit time: rate = Δconcentration / time. Units are mol dm-3 s-1.
State two conditions that must both be met for a collision between reactant particles to be effective.
The particles must collide with the correct orientation.
The particles must have sufficient energy (at least equal to the activation energy Ea).
True or False?
All collisions between reactant particles result in a chemical reaction.
False.
Only effective collisions produce a reaction. Ineffective collisions occur when particles have insufficient energy or the wrong orientation, and the particles simply bounce off each other.
Define activation energy.
An activation energy (Ea) is the minimum energy that colliding particles must possess for a collision to be successful and a chemical reaction to occur.
Increasing the concentration of a solution increases the .......... frequency, which increases the number of .......... collisions per unit time and raises the rate of reaction.
Increasing the concentration of a solution increases the collision frequency, which increases the number of effective collisions per unit time and raises the rate of reaction.
Why does increasing pressure increase the rate of a gas-phase reaction?
At higher pressure, the same number of gas molecules occupy a smaller volume. The particles are closer together, so the collision frequency increases, leading to more effective collisions per unit time and a faster rate of reaction.
True or False?
A catalyst increases the rate of reaction by increasing the collision frequency.
False.
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, not by increasing collision frequency.
Calculate the rate of reaction when [product] increases by 0.00125 mol dm-3 in 60.0 s.
Rate = 0.00125 mol dm-3 / 60.0 s = 2.08 x 10-5 mol dm-3 s-1
An ineffective collision occurs when particles have insufficient .......... or collide in the wrong .......... and bounce off each other without reacting.
An ineffective collision occurs when particles have insufficient energy or collide in the wrong orientation and bounce off each other without reacting.
How is the instantaneous rate of reaction found at a specific point on a concentration-time graph?
Draw a tangent to the curve at that point. The gradient of the tangent (change in concentration / change in time) gives the instantaneous rate of reaction at that concentration.
True or False?
The rate of reaction remains constant throughout a reaction.
False.
As the reaction proceeds, reactant concentration decreases and the rate decreases. The concentration-time curve becomes progressively shallower over time.
Define concentration-time graph.
A concentration-time graph is a graph that plots the concentration of a reactant or product (mol dm-3) against time. The gradient at any point gives the rate of reaction at that moment.
On a concentration-time graph, the rate of reaction at the start is found by drawing a .......... at time zero, or by treating the initial .......... as approximately linear.
On a concentration-time graph, the rate of reaction at the start is found by drawing a tangent at time zero, or by treating the initial curve as approximately linear.
For cyclopropane isomerising to propene, [propene] increases by 0.27 mol dm-3 in the first 5 minutes. Calculate the average rate over this period.
Rate = 0.27 mol dm-3 / (5 x 60) s
Rate = 0.27 / 300 = 9.0 x 10-4 mol dm-3 s-1
True or False?
A straight-line rate-concentration graph means the rate is directly proportional to concentration.
True.
A straight line through the origin on a rate-concentration graph shows a direct proportional relationship: doubling the concentration doubles the rate.
Why does rate of reaction decrease as time goes on in a typical reaction?
As the reaction proceeds, reactant concentration decreases. Fewer reactant particles are present in the same volume, so collision frequency falls, reducing the number of effective collisions per unit time.
The rate of reaction can be calculated using either the increase in concentration of .......... or the decrease in concentration of .......... , as both reflect the same chemical change.
The rate of reaction can be calculated using either the increase in concentration of products or the decrease in concentration of reactants, as both reflect the same chemical change.
Why should temperature be kept constant when collecting concentration-time data for a reaction?
Temperature affects the rate of reaction. If temperature changes during the experiment, the rate will change for reasons unrelated to concentration, making the data unreliable for drawing rate-concentration graphs.
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