9. Rate Equations
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What is rate of reaction?
Change in concentration of a substance per unit time
What is rate of reaction usually measured in?
mol dm⁻³ s⁻¹
What happens to the concentration of reactants and products as a chemical reaction progresses?
Concentration of the reactants decreases and the concentration of products increases
What is collision theory?
• for particles to react they must collide with sufficient energy and at the correct orientation • the minimum energy particles need to react is called the activation energy
What factors affect the rate of reaction?
• concentration of a solution • surface area of a solid • pressure of gases • temperature • catalyst
Why does increasing concentration increase rate of reaction?
• more particles in a given volume * more frequent collisions
Why does increasing the surface area of a solid increase the rate of reaction?
• more particles available to react * more frequent collisions
Why does increasing the pressure of a gas increase the rate of reaction?
• more particles in a given space/same no. of particles in a smaller volume • more frequent collisions
Why does increasing the temperature increase the rate of reaction?
• particles have more energy • more particles with energy greater than or equal to Ea • more frequent collisions
Why does adding a catalyst increase the rate of reaction?
Lowers Ea so more particles with energy greater than or equal to Ea
In terms of rate equations, what does the rate of a reaction depend on?
The temperature of the reaction and concentration of some or all of the reagents involved
What is the order of a reaction?
With respect to a given reactant, it is the power which the concentration of the reactant is raised in the rate equation
How can the order of the reaction be calculated?
Experimentally by measuring the change in concentration over time
What is the overall order of a reaction?
The sum of the powers of the concentration terms in the rate equation
What is the rate constant?
A constant that links the concentration of the reactants to the rate of a reaction
When is the rate constant the same?
For a certain reaction at a particular temperature
What happens when the rate constant is bigger?
It is a faster reaction
What is a zero order reactant?
One where the concentration has no effect on rate
What is a first order reactant?
One where doubling concentration doubles rate
What is a second order reactant?
One where doubling concentration quadruples rate
What does the units for k depend on?
The overall order of a reaction
When will the rate of a reaction increase without a change in concentration?
If the temperature increases or a catalyst is added
What does it mean for the rate equation, if there is a change in temperature or a catalyst is added?
The rate increases but concentration is constant, so the rate constant must increase
What happens to rate constant if the only change to a reaction is an increase in temperature?
It increases