Reaction Rates Flashcards
Balanced chemical equation gives no information on
Rates
To study rates of reactions, we must…
measure rates of reaction.
- This can be done in many ways. The most
common is to follow the changes in
concentrations of the substances involved.
The rate of a reaction can be monitored by…
measuring the increase in the concentration of a reaction product with time.
- Suppose that a substance A reacts to form B* For a reaction A B
Biochemical “real” reactions:
- In practice we measure concentrations and changes in concentration rather than moles and changes in moles
- We measure concentration in terms of molarity. If the
volume is constant, molarity and moles are directly
proportional to one another. - Scientists can do this with each and every reaction in living fluids and tissues.
Instantaneous Rate:
– Instantaneous rate is different from average rate.
– Usually when we refer to the rate of a reaction we are referring to instantaneous rate.
– AKA “Initial Reaction Rate”
A rate law is an equation expressing the…
rate of a reaction in terms of the molar concentrations of the species involved in the reaction.
The rate is often found to be proportional to the…
molar concentrations of the reactants raised to a simple power.
(Rate Laws) These powers are the…
orders of the reaction.
- If the rate is expressible as rate = k[A]m[B]nthen the reaction is m order in A, n order in B, and m+n order overall.
The INSTANTANEOUS RATE OF REACTION, r, is defined as:
the INITIAL (instantaneous) rate of decrease in the concentration of a reactant, or increase in the concentration of a product.
There are 4 important factors which affect rates of reactions:
– concentrations of reactants,
– temperature,
– action of catalysts
– surface area of solid reactants
Temperature and Rate:
- Most reactions speed up as temperature increases (e.g. food spoils when not refrigerated)
- It is commonly observed that the rates of chemical reactions are very sensitive to temperature
- As a rough rule of thumb, the rates of many
chemical reactions approximately double for every 10 °C rise in temperature.
As temperature increases, the…
rate constant for the reaction
increases quite dramatically.
Arrhenius discovered that most…
reaction-rate data obeyed this equation:
– k is the rate constant, Ea
is the activation energy, Ris the gas constant (8.314 J/K-mol) and T is the temperature in K
– A is called the frequency factor.
– A is a measure of the probability of a favorable collision.
– Both A and Ea
are specific to a given reaction.
Determining the Activation Energy:
- If we have a lot of data, we can determine Ea
and Agraphically by rearranging the Arrhenius equation: - From the above equation, a plot of ln k versus 1/Tshould be a straight line with a slope of –Ea
/Rand an intercept of ln A.
The balanced chemical equation provides
information about the…
initial reactants and final
products, i.e., the beginning and end of a reaction.
- The reaction mechanism gives the path of the
reaction. - Mechanisms provide a very detailed picture of
which bonds are broken and formed during the
course of a reaction.
