Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 9.3 Rate calculations-Study Notes - New Syllabus

9.3(i) Calculating Rate of Reaction using Time

The rate of a reaction can be determined by measuring how long a reaction takes to complete.

Rate from Time

The rate of reaction is inversely proportional to the time taken.

rate = \( \frac{1}{\text{time}} \)

• A shorter time means a faster reaction.
• A longer time means a slower reaction.

This method is commonly used when measuring the time taken for a visible change, such as:

• Formation of a precipitate (e.g. solution becoming cloudy).
• Disappearance of a solid.
• Change in colour.

Since it may be difficult to measure the exact rate directly, using \( \frac{1}{\text{time}} \) provides a convenient way to compare reaction rates.

Interpretation

• If the time taken is small, \( \frac{1}{\text{time}} \) is large → reaction is fast.
• If the time taken is large, \( \frac{1}{\text{time}} \) is small → reaction is slow.

Therefore, the value of \( \frac{1}{\text{time}} \) gives a measure of how quickly the reaction occurs.

9.3(ii) Calculating Rate of Reaction from a Graph

The rate of a reaction can also be determined from the gradient of a suitable graph, such as a graph of concentration, volume of gas, or mass against time.

Rate from Gradient

The rate of reaction is equal to the gradient (slope) of the graph.

• A steeper gradient indicates a faster rate of reaction.
• A less steep gradient indicates a slower rate of reaction.

The gradient can be determined by drawing a tangent to the curve.

Initial Rate

• The initial rate is the rate at the very start of the reaction (at time = 0).
• It is found by drawing a tangent at the beginning of the curve.
• The gradient of this tangent gives the initial rate.

The initial rate is usually the highest because reactant concentration is greatest at the start.

Rate at a Specific Time, t

• The rate at a specific time is found by drawing a tangent at that point on the curve.
• The gradient of the tangent gives the rate at that time.
• As time increases, the gradient usually decreases because reactants are being used up.

Calculating the Gradient

• Choose two points on the tangent line.
• Calculate the change in the y-axis value (e.g. concentration or volume).
• Calculate the change in time (x-axis).
• Use: gradient = \( \frac{\Delta y}{\Delta x} \)

Therefore, the gradient of the tangent represents the rate of reaction at that point.