Exambusters Study Cards 14 Reaction Rates

Activation Energy
The energy required to start a reaction For example, you must heat paper before burning occurs. (Burning is a chemical reaction between the paper and oxygen.)
Entropy
The state of disorder or randomness of particles. A major driving force in reactions is to increase entropy.

List in order of increasing entropy:

gas

liquid

solid

solid

liquid

gas

List five factors that control reaction rate

nature of reactants

surface area exposed

concentrations

temperature

presence of catalyst

State the collision theory of reaction rates.

There must be collisions between reactants.

Reaction rate depends on number of collisions per unit time and the percent which are successful (have sufficient energy).

How is the reaction rate related to concentration?
Reaction rate is directly proportional to the concentrations of reactants.
Catalyst
A catalyst is introduced into a reaction to speed it up or slow it down. It is not consumed. An increase or decrease of activation energy results from an alternate reaction path.
Law of Mass Action
The rate of a chemical reaction is proportional to the product of the concentrations of the reactants, each raised to the appropriate power.
Specific Rate Constant
Symbol is “k” in a rate equation. A constant specific to temperature and reaction which is part of every rate equation.
How is the order of a reaction calculated?
By adding the exponents in the reaction rate equation. Rate = k[H2][I2]
What is the order of H2 + I2 ↔ 2HI

1 + 1 = 2

Reaction is second order.

 

(note: the exponents of the concentrations of H2 and I2, are obtained experimentally, not from their coefficients in a balanced chemical reaction.)

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