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Rate of a Chemical Reaction: Effect of Temperature

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  • 0:02 Chemical Reactions and…
  • 1:13 Arrhenius Equation
  • 2:08 Exponential Factor
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn how temperature affects reaction rates. Discover the rate law as well as how the Arrhenius equation relates to the rate law and predicts the effect of temperature on the rate of reaction.

Chemical Reactions and Temperature

When you were a kid, did you ever try to catch a lizard? It was hard, wasn't it? They're usually fast little creatures. If you'd just gotten a glass of cold water and thrown it over them, though, they would have frozen in their place and you could have picked them up. No worries about harming them. As soon as they warmed up, they'd be just fine. That's because lizards are ectotherms. Their bodies' chemical reactions rely on the temperature of the environment in order to work well and the reaction rate to increase to where they can move. In low temperatures, the reactions don't happen fast enough and the lizard is frozen in place. These lizards are highly temperature-dependent.

You've previously learned about activation energy, and you have seen the activation energy graph. You have also been introduced to the rate law: Rate = k[A]n.

The equation for the rate law
Rate Law Formula

The k in the equation is a constant that is temperature-dependent, meaning the constant changes depending on the temperature the reaction is happening at. In general, as temperature increases, k increases and the reaction is faster.

Arrhenius Equation

k can be calculated by the Arrhenius equation: k = Ae^(-Ea/RT).

R = gas constant (8.314 J/mol k )
A = frequency factor
Ea = activation energy
T = temperature in Kelvin
e^(-Ea/RT) = exponential factor

Activation energy is the amount of energy a reaction needs to get started. The higher the activation energy, the slower the reaction rate will be at a given temperature.

The components of the Arrhenius Equation
Arrhenius Equation

The frequency factor is the number of times that reactants approach the activation energy per unit of time - meaning, how many times do the reactants get close to getting over the hump of the activation energy every X amount of time?

Exponential Factor

The exponential factor is a number between 0 and 1. It represents the fraction of reactants that have approached the activation energy hill and made it over per number of attempts. Think of it as a reactant trying to get over the hill of activation energy. It keeps trying and trying. It tries 10 times before it finally makes it over on the tenth try. Then, the exponential factor is 1/10.

The exponential factor depends on temperature and the activation energy of the reaction. If there is a low activation energy (a low hill) and a high temperature, then the exponential factor is close to 1. If the activation energy is 0, then the exponent is 0 and the exponential factor is 1.

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