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Analyze how changing the environment of a reaction can speed it up or slow it down.
Why does a glow stick shine brilliantly in a hot bath but fade to a dim flicker in the freezer? The answer lies in a microscopic 'demolition derby' happening inside the plastic tube.
For a chemical reaction to occur, reactant particles must physically collide. However, not every bump results in a reaction. According to Collision Theory, two conditions must be met: particles must collide with the correct orientation (facing the right way) and with enough Activation Energy (). Think of it like a high-five; if you miss the other person's hand or move too slowly, the high-five doesn't 'react.' To speed up a reaction, we must increase the number of successful collisions that happen every second.
1. Snap a glow stick to mix the chemicals and start the reaction. 2. Place one stick in a bowl of ice water and another in a bowl of hot water. 3. Observe: The hot water stick glows much brighter. This is because the heat provides more energy, causing the chemicals to collide more frequently and with more force.
Quick Check
What are the two requirements for a collision to be 'successful'?
Answer
Particles must have the correct orientation and sufficient energy (Activation Energy).
Temperature is a measure of the average Kinetic Energy () of particles. When you heat a substance, the particles move faster. Mathematically, . As velocity () increases, two things happen: 1) Particles collide more often because they are ziping around faster, and 2) A higher percentage of particles have enough energy to overcome the Activation Energy barrier. This is why even a small increase in temperature can lead to a massive jump in reaction speed.
1. Drop one antacid tablet into of water at . 2. Drop another tablet into of water at . 3. The tablet in the water dissolves roughly faster. The higher ensures that more collisions between the water and the tablet result in a chemical change.
Quick Check
Why does temperature have such a large effect on reaction rate compared to other factors?
Answer
It increases both the frequency of collisions and the percentage of particles that have enough energy to actually react.
Concentration refers to the number of particles in a given volume. In gases, increasing Pressure has the same effect as increasing concentration—it squeezes particles closer together. Imagine a crowded dance floor versus an empty one; you are much more likely to bump into someone when the floor is packed. By increasing the concentration of reactants (e.g., using acid instead of ), we increase the collision frequency, which directly speeds up the reaction rate.
Imagine a graph where the y-axis is 'Volume of Gas Produced' and the x-axis is 'Time'. 1. Curve A is very steep at the start and reaches its maximum height at 20 seconds. 2. Curve B is less steep and reaches the same height at 60 seconds. 3. Analysis: Curve A represents a higher concentration. The steeper gradient (slope) means the rate of reaction is faster. Both curves end at the same height because the total amount of reactants was the same, just used at different speeds.
What happens to the particles in a gas if the pressure is increased?
If a reaction graph shows a very steep line that levels off quickly, what does this indicate?
Increasing the temperature of a reaction only increases the frequency of collisions, not the energy of the collisions.
Review Tomorrow
In 24 hours, try to explain to a friend why a crowded hallway is a good analogy for chemical concentration.
Practice Activity
Find a reaction graph in your textbook and identify which curve represents the 'control' and which represents the 'accelerated' reaction based on the slope.