A flask is charged with 0.124 mol of A and allowed to react to form B according to the reaction A(g) → B(g). The following data are
obtained for (A) as the reaction proceeds:
Time (s) 1 10 20 30 40
Moles of A 0.124 0.110 0.088 0.073 0.054
1) The average rate of disappearance of A between 10 s and 20 s is __________ mol/s.
answer: 2.2 × 10-3
2) If the rate law for the reaction
2A + 3B → products
is second order in A and first order in B, then the rate law is rate = __________.
answer: k(A)2(B)
i know the answers but how do i figure problems like the above? i have a bunch of similar problems to figure out.
10 sec, 0.110
20 sec, 0.088
0.110-0.088 = 0.022 moles/10 sec = 0.0022 moles/sec.
rate constant = k[A]x[B]y
where x is the order of A and y is the order of B and [A] and [B] are the concentrations, in moles/L, of the reactants. Except for elementary reactions, the order CAN NOT be determined from the coefficients in the balanced equation. x and y must be determined experimentally.
thank you i kept getting 0.022 because i didn't divide by time.
To find the average rate of disappearance of A between 10 s and 20 s, we can use the formula:
Average rate = (Change in moles of A) / (Change in time)
First, let's calculate the change in moles of A:
Change in moles of A = Moles of A at 20 s - Moles of A at 10 s
= 0.088 mol - 0.110 mol
= -0.022 mol
Next, calculate the change in time:
Change in time = 20 s - 10 s
= 10 s
Finally, plug these values into the formula:
Average rate = (Change in moles of A) / (Change in time)
= (-0.022 mol) / (10 s)
= -0.0022 mol/s
Since the question asks for the average rate, we only need the magnitude of the rate. Therefore, the average rate of disappearance of A between 10 s and 20 s is 2.2 × 10^-3 mol/s.
For the second question, if the rate law for the reaction is second order in A and first order in B, then the rate law can be expressed as:
rate = k(A)^2(B)
"k" represents the rate constant, and "(A)^2" indicates that the concentration of A is squared. "(B)" indicates that the concentration of B is to the power of 1 (which is simply B).
So, the rate law for the given reaction is rate = k(A)^2(B).
To solve these problems, you need to understand the concepts of average rate and rate laws. Here is how you can approach each question:
1) The average rate of disappearance of A between 10 s and 20 s can be determined by calculating the change in moles of A over the given time interval and dividing it by the time taken. Here's the calculation:
Change in moles of A = Moles of A at 10 s - Moles of A at 20 s
= 0.110 mol - 0.088 mol = 0.022 mol
Time taken = 20 s - 10 s = 10 s
Average rate of disappearance of A = Change in moles of A / Time taken
= 0.022 mol / 10 s
= 2.2 × 10⁻³ mol/s
Therefore, the average rate of disappearance of A between 10 s and 20 s is 2.2 × 10⁻³ mol/s.
2) To determine the rate law of a reaction, you need to examine the relationship between the concentrations of the reactants and the rate of the reaction. In this case, the rate law is second order in A and first order in B. The general form of the rate law equation is rate = k[A]ⁿ[B]ⁿ.
Since it is second order in A, n is equal to 2. Since it is first order in B, n is equal to 1. Therefore, the rate law of the reaction becomes:
rate = k[A]²[B]
This means that the rate of the reaction is directly proportional to the square of the concentration of A and directly proportional to the concentration of B.
Keep in mind that determining the rate law of a reaction requires experimental data and the method used can vary depending on the specific reaction. Always refer to the given information or experimental results to determine the rate law.