The effect of a change in temperature on K: Reactions depend on kinetic energy, and temperature is a measure of the amount of kinetic energy. Therefore, with an increased tempereature, more particles have kinetic energy, and therefore, more will sucessfully collide in order to exceed the activation energy.
The effect of a change in presure on K: No effect.
The effect of a change in species concentration on K: Concentration is within the equation. As the concentration increases, so does K.
Friday 29 October 2010
1.2 The rate expression and order of reaction
The rate expression tells us by how much a reactant participates in the reaction, and the contribution they have. In a simple reaction, one species may have more of an effect then another. Also, species that are not involved in the chemical reaction could also affect the rate of reaction, e.g. catalysts.
This is expressed via reaction ORDERS. An order is a reaction expresses to which power the concentration of that species affects the rate. How the rate dpeends on that chemical and it’s concentration.
Order Zero: A change in concentration of the species does NOT affect the rate.
Order First: A change is concentration of the species has exactly the same effect on the rate. (If the concentration doubles, the rate doubles.)
Order Second: A change in concentration of the species has a squared effect on the rate. If the concentration doubles, the rate would quadruple. Or, if it tripled, the rate would multiply by nine, and so on.
A rate equation is set out as: Rate = k[A][B].
The letters in square brackets represent the concentrations of particular species. The orders apply to these. So, if A was order two, and B was order one, the rate equation would look like this:
Rate=k[A]2[B]
(There need not be a ‘1’ to show first order, as like ‘x’ meaning ‘1x’ in algebra, the single, un-numbered bracket shows order one.)
When a sepecies is order zero, it is not shown in the rate equation. So, if A was order 1, B was order 0, and C was order 2, it would look like this:
Rate=k[A][C]2
K is the equilibrium constant. K is different for every different reaction, dependant upon what is reacting, and the conditions.
To work out K’s units, you use cancelling. In Rate=k[A][B],
moldm-3s-1 = k x moldm-3 x moldm-3
moldm-3s-1= Units of rate.
moldm-3 = Units of species concetration.
Cancelling… moldm-3s-1 = k x moldm-3 x moldm-3
Therefore, K= mol-1dm3s-1, as the s-1 has multiplied with the moldm-3.
The overall order of a reaction, is all of the orders added together. So, if Rate=k[A][B]2, the overall order would be 1+2=3.
This is expressed via reaction ORDERS. An order is a reaction expresses to which power the concentration of that species affects the rate. How the rate dpeends on that chemical and it’s concentration.
Order Zero: A change in concentration of the species does NOT affect the rate.
Order First: A change is concentration of the species has exactly the same effect on the rate. (If the concentration doubles, the rate doubles.)
Order Second: A change in concentration of the species has a squared effect on the rate. If the concentration doubles, the rate would quadruple. Or, if it tripled, the rate would multiply by nine, and so on.
A rate equation is set out as: Rate = k[A][B].
The letters in square brackets represent the concentrations of particular species. The orders apply to these. So, if A was order two, and B was order one, the rate equation would look like this:
Rate=k[A]2[B]
(There need not be a ‘1’ to show first order, as like ‘x’ meaning ‘1x’ in algebra, the single, un-numbered bracket shows order one.)
When a sepecies is order zero, it is not shown in the rate equation. So, if A was order 1, B was order 0, and C was order 2, it would look like this:
Rate=k[A][C]2
K is the equilibrium constant. K is different for every different reaction, dependant upon what is reacting, and the conditions.
To work out K’s units, you use cancelling. In Rate=k[A][B],
moldm-3s-1 = k x moldm-3 x moldm-3
moldm-3s-1= Units of rate.
moldm-3 = Units of species concetration.
Cancelling… moldm-3s-1 = k x moldm-3 x moldm-3
Therefore, K= mol-1dm3s-1, as the s-1 has multiplied with the moldm-3.
The overall order of a reaction, is all of the orders added together. So, if Rate=k[A][B]2, the overall order would be 1+2=3.
1.1 The rate of chemical reactions
Rate of reaction: The change in concentration of any of the reactants or products with unit time.
Rate of reaction is measured in the units moldm-3s-1
The gradient about a tangent on a reaction rate graphs shows the reaction rate at that specific time. This is also to the units moldm-3s-1. The larger the gradient, the bigger the rate of reaction at the particular instance. For example, in a reaction rate graph of a reactant, the graph would start of with a steep gradient at zero. This is because at the beginning of a reaction, there is a high concentration of reactant, increaing the rate of reation, as there are more particles to make sucessful collisions and exceed the activation energy.
Rate of reaction is measured in the units moldm-3s-1
The gradient about a tangent on a reaction rate graphs shows the reaction rate at that specific time. This is also to the units moldm-3s-1. The larger the gradient, the bigger the rate of reaction at the particular instance. For example, in a reaction rate graph of a reactant, the graph would start of with a steep gradient at zero. This is because at the beginning of a reaction, there is a high concentration of reactant, increaing the rate of reation, as there are more particles to make sucessful collisions and exceed the activation energy.
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