Rates of Reaction · 6 min read
Meaning and measurement of reaction rate
Listen to this lesson
A calm voice reads the lesson aloud
Tap any word for its meaning, or to have that part explained further.
Introduction to Reaction Rate
Think about how you cook. If you turn up the gas on your burner, your rice cooks faster. If the gas is low, it takes a long time. In chemistry, some reactions are like an okada speeding on the highway, while others are as slow as a snail. This speed is what we call the rate of reaction.
It simply means how fast the starting materials are used up or how fast the new products are formed.
Expressing the Rate
In a chemical reaction, the reactants are used up, so their amount decreases. The products are being formed, so their amount increases. We usually measure this change in terms of molarity (mol dm-3) over seconds (s).
The mathematical formula is: Rate = Change in concentration / Time taken.
Units of Reaction Rate
Since concentration is in mol dm-3 and time is in seconds, the standard unit for rate is mol dm-3 s-1. If we are measuring a gas, we might use cm3 s-1 to show how much gas is coming out of the tube every second.
How we measure it in the lab
To know the rate, we must see something changing. Here are common ways to measure it:
Change in mass: If a gas like CO2 escapes from a flask, the flask gets lighter. We put it on a weighing scale to see how fast the mass drops.
Gas volume: We can collect gas in a syringe to see how many cm3 are produced every minute.
Color change: If a solution changes from clear to dark, we measure how long it takes for a cross drawn on paper under the beaker to disappear.
Pressure change: In a closed container, if gas is produced, the pressure will go up.
Calculation Example
A student reacted Zinc with dilute HCl. At the start (0 seconds), the volume of Hydrogen gas was 0 cm3. After 20 seconds, the volume of gas collected was 40 cm3. Let us calculate the rate of the reaction.
1. Identify the change: The volume changed from 0 to 40 cm3. Change = 40 - 0 = 40 cm3.
2. Identify the time taken: The time is 20 seconds.
3. Apply the formula: Rate = Volume / Time.
4. Calculate: Rate = 40 / 20 = 2.0 cm3 s-1.
5. Final Answer: The rate of production of Hydrogen is 2.0 cm3 s-1.
Key points
- •Reaction rate is the speed at which reactants turn into products.
- •The standard unit is mol dm-3 s-1 or cm3 s-1 for gases.
- •Reactant concentration decreases over time, while product concentration increases.
- •Rate is measured by tracking mass, volume, color, or pressure changes.
- •Rate is always a positive value.