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# Rate law and reaction order

A rate law shows how the rate of a chemical reaction depends on reactant concentration. For a reaction such as aA → products, the rate law generally has the form rate = k[A], where k is a proportionality constant called the rate constant and n is the order of the reaction with respect to A. The value of n is not related to the reaction stoichiometry and must be determined by experiment. Created by Jay.

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• How to know the value of rate constant for any reaction? Is it different for different reactions?
• The rate constant for a chemical reaction is usually determined by experiment.
• These examples have such unrealistic amounts (1.00, 0.10, 2.00, 0.20). I just don't see the process at which you figured out what factor the rates were increased besides just knowing that 1 increased to 2, duh! What if I had numbers like [A]= 0.40, 0.60, 0.80 & [B]= 0.30, 0.30, 0.60? how does one figure out what factor each reactant has increased by?
• well the "process" here is simply division. 2 divided by 1 is 2. or it could've just as easily been 0.742321 becoming 1.484642. It's just to illustrate the point. With more complicated numbers, just use a calculator.
(1 vote)
• So the constant K is from what thing? Is it from temperature, pressure etc.....
• It is constant for a specific temperature and a specific reaction.
• What is an order of reaction? What does it mean to be the first order and second order? And does the order correspond to x and y?
Thank you
• The order of reaction determines the relationship between the rate of reaction and the concentration of reactants or products. It is the power to which a concentration is raised in the rate law equation.

For example, for the reaction xA + yB ---> products, the rate law equation will be as follows:

Rate = k[A]^a . [B]^b. This reaction is a order with respect to A and b order with respect to B. Overall, it is a + b order.

The order of reaction is something that has to be determined experimentally and can't usually be obtained from the stoichiometric coefficients (x and y).

Reactions are usually zero, first, second or third order, but can be anything, including fractional orders or even negative orders. The order affects what the graphs of concentration against time look like, how half-lives are calculated, etc.
• Does the rate constant, k have a specific value? Or is it different each time, or is it just a variable?
• The rate constant, k, has to be determined for each experiment (or set of data). The orders of each rate of reactants and products (for a particular experiment) help determine the units for k within that experiment. The next video develops this idea more fully.
• what is the reasons for a reaction to be of zero order?
• A zero order reaction is independent of the concentration of the reactants. One example could be an enzyme-catalysed reaction, where the enzyme is not (by definition) a reactant, but nevertheless the concentration of enzyme is what determines the reaction rate, not the concentration of reactant. This would be a zero order reaction.
• I got the math part. But I'm still really in the dark about how this applies to chemistry in the lab and what the answer helps me determine.
• How to I determine reaction order when only given the rate law??
• The exponent corresponding to each reactant will tell you what the reaction order is with respect to the pertaining reactant.

The overall reaction order is the sum of the individual orders of the reactants (which as said before are represented by the exponents in the rate law.
(1 vote)
• - what are the units of the rate constant in? I understand that the units for [A] and [B] are in molarity (right?), but what about the rate constant?

How do we know what the rate constant is for a reaction? Do these rate constants vary by each different kind of reaction?