In general terms, a rate is a chang

In general terms, a rate is a change in some variable per unit of time. The most
familiar examples relate to speed (see photo), the change in position of an object
divided by the change in time. For instance, if we measure a runner’s initial position,
x1, at time t1, and final position, x2, at time t2, the average speed is
Rate of motion (speed) 5
change in position
change in time
5
x2 2 x1
t2 2 t1
5
Dx
Dt
For the rate of a reaction, we measure the changes in concentrations of reactants or
products per unit time: reactant concentrations decrease while product concentrations
increase. For the general reaction A -£ B, we measure the initial reactant concentration
(A1) at t1, allow the reaction to proceed, and then quickly measure the final
reactant concentration (A2) at t2. The change in concentration divided by the change
in time gives the rate:
Rate 5 2
change in concentration of A
change in time
5 2
conc A2 2 conc A1
t2 2 t1
5 2
D(conc A)
Dt
The negative sign is important because, by convention, reaction rate is a positive
number. But, since conc A2 must be lower than conc A1, the change in concentration
(final 2 initial) of reactant A is negative. Therefore, we use the negative sign
to convert the negative change in reactant concentration to a positive value for the
rate. Suppose the concentration of A changes from 1.2 mol/L (conc A1) to 0.75 mol/L
(conc A2) over a 125-s period. The rate is
Rate 5 2
0.75 mol/L 2 1.2 mol/L
125 s 2 0 s
5 3.631023 mol/Ls
Square brackets, [ ], indicate a concentration in moles per liter. For example,
[A] is the concentration of A in mol/L, and the rate expressed in terms of A is
Rate 5 2
D3A4
Dt
(16.1)
The units for the rate are moles per liter per second (mol L21 s21, or mol/Ls), or any
time unit convenient for the reaction (minutes, years, and so on).
If instead we measure the product concentrations to determine the rate, we find
that conc B2 is always higher than conc B1. Thus, the change in product concentration,
D[B], is positive, and the reaction rate for A -£ B expressed in terms of B is
Rate 5 1
D3B4
Dt
The plus sign is usually understood and not shown.
Average, Instantaneous, and Initial Reaction Rates
In most cases, the rate varies as a reaction proceeds. Consider the reversible gasphase
reaction between ethylene and ozone, one of many reactions that may be
involved in the formation of smog:
C2H4(g) 1 O3(g) BA C2H4O(g) 1 O2(g)
The equation shows that for every molecule of C2H4 that reacts, a molecule of O3
reacts; thus, [O3] and [C2H4] decrease at the same rate:
Rate 5 2
D3C2H4 4
Dt
5 2
D3O3 4
Dt
When we start with a known [O3] in a closed vessel at 308C (303 K) and measure [O3]
at 10.0-s intervals during the first minute after adding C2H4, we obtain the concentration
vs. time data shown in the Figure 16.5 table, which gives the black curve. Note that
• The data points in Figure 16.5 result in a curved line, which means that the rate is
changing over time (a straight line would mean that the rate was constant).
• The rate decreases during the course of the reaction because we are plotting reactant
concentration vs. time: as O3 molecules react, fewer are present to collide with C2H4
molecules, and the rate, the change in [O3] over time, therefore decreases

ผลลัพธ์ (อังกฤษ) 2:

In general terms, a rate is a change in some variable per unit of time. The Most
Familiar examples Relate to speed (See Photo), The Change in position of an object
divided by The Change in time. For instance, IF we measure a Runner's Initial position,
X1, at time T1, and Final position, X2, at time T2, The Average speed is
Rate of Motion (speed) 5
Change in position
Change in time
5
X2 2 X1
T2 2. T1
5
Dx
Dt
For The rate of a Reaction, we measure The Changes in concentrations of reactants or
products Per Unit time: reactant concentrations decrease while product concentrations
increase. The general Reaction for A - £ B, we measure reactant The Initial Concentration
(A1) at T1, Allow The Reaction to Proceed, and then measure Quickly The Final
reactant Concentration (A2) at T2. The Change in Concentration divided by The Change
in time gives The rate:
Rate 5 2
Change in Concentration of A
Change in time
5 2
CONC A2 2 CONC A1
T2 2 T1
5 2
D (CONC A)
Dt
The Negative Sign is important Because,. by Convention, Reaction rate is a positive
number. But, since CONC A2 must be Lower than CONC A1, The Change in Concentration
(Initial Final 2) of reactant A is Negative. Therefore, we Use The Negative Sign
to convert The Negative Change in Concentration to a positive value for The reactant
rate. The Concentration of A suppose Changes from 1.2 mol / L (CONC A1) to 0.75 mol / L
(CONC A2) over a 125-S period. The rate is
Rate 5 2
0.75 mol / L 2 1.2 mol / L
125 S 2 0 S
5 3.631023 mol / Ls
Square Brackets, [], indicate a Concentration in Moles Per liter. For example,
[A] is The Concentration of A in mol / L, and The rate expressed in terms of A is
Rate 5 2
D3A4
Dt
(16.1)
The units for The rate are Moles Per liter Per Second (mol L21 s21, or. mol / Ls), or any
Unit Convenient for The Reaction time (minutes, years, and So on).
If we measure Instead The product concentrations to Determine The rate, we Find
CONC that is Always Higher than CONC B1 B2. Thus, The Change in product Concentration,
D [B], is positive, and The Reaction rate for A - £ B expressed in terms of B is
Rate 5 1
D3B4
Dt
The Plus Sign is usually Understood and Not shown.
Average, Instantaneous,. and Initial Reaction Rates
In Most Cases, The rate varies As a Reaction Proceeds. Consider The Reversible Gasphase
Reaction between ethylene and Ozone, One of many reactions that May be
Involved in The Formation of Smog:
C2H4 (G) 1 O3 (G) BA C2H4O (G) 1 O2 (G)
The Equation Shows that for Every molecule. of C2H4 that reacts, a molecule of O3
reacts; thus, [O3] and [C2H4] decrease at The Same rate:
Rate 5 2
D3C2H4 4
Dt
5 2
D3O3 4
Dt
When we start with a Known [O3] in a Closed vessel at 308C (303 K) and measure [O3].
at S 10.0-minute intervals during The First After Adding C2H4, we Obtain The Concentration
vs. time data shown in the Figure 16.5 table, which gives the black curve. Note that
• The Data points in Figure 16.5 Result in a curved Line, which means that The rate is
Changing over time (a straight Line would mean that The rate was Constant).
• The rate decreases during The course of The Reaction Because we are. Plotting reactant
Concentration vs. time: As O3 molecules React, fewer are to Present Collide with C2H4
molecules, and The rate, The Change in [O3] over time, Therefore decreases.

ผลลัพธ์ (อังกฤษ) 3:

In general terms a rate, is a change in some variable per unit of time. The most
familiar examples relate to speed (see. Photo), the change in position of an object
divided by the change in time. For instance if we, measure a runner s initial. ' Position
x1, at T1, time, final, and position x2 at time, T2 the average, speed is
Rate of motion (speed) 5
change in position
change. In time

x2 5 2 X1
.T2 2 T1



5 Dx Dt For the rate of a reaction we measure, the changes in concentrations of reactants or
products per unit. Time: reactant concentrations decrease while product concentrations
increase. For the general reaction A - pounds, B we measure. The initial reactant concentration
(A1), at T1 allow the reaction to proceed and then, quickly measure the final
reactant. Concentration (A2) at T2.The change in concentration divided by the change
in time gives the rate:
Rate 5 2
change in concentration of A
change. In time

5 2 conc A2 2 conc A1


T2 2 T1 5 2 D (conc A)

The Dt negative sign is important because by convention reaction rate,,, Is a positive
number. But since conc, A2 must be lower than, conc A1 the change in concentration
(final 2 initial) of reactant. A is, Therefore negative.We use the negative sign
to convert the negative change in reactant concentration to a positive value for the
rate. Suppose. The concentration of A changes from 1.2 mol / L (conc A1) to 0.75 mol / L
(conc A2) over a 125-s period. The rate is
Rate 5 2
0.75. Mol / L 2 1.2 mol / L
125 s 2 0 s
5 3.631023 mol / L  s
Square brackets, [], indicate a concentration in moles per liter. For. , example
.[] is A the concentration of A in mol / L and the, rate expressed in terms of A is
Rate 5 2


D3A4 Dt (16.1)
The units for the. Rate are moles per liter per second (mol, L21 S21 or mol / L  s), or any
time unit convenient for the reaction (minutes years,,, And so on).
If instead we measure the product concentrations to determine, the rate we find
that conc B2 is always higher. Than conc, Thus B1.The change in, product concentration
D [], is, B positive and the reaction rate for A - pounds B expressed in terms of B is
Rate 5 1 D3B4


Dt The. Plus sign is usually understood and not shown.
Average Instantaneous, and Initial, Reaction Rates
In most cases the rate,, Varies as a reaction proceeds. Consider the reversible gasphase
reaction between ethylene and ozone one of, many reactions. That may be
.Involved in the formation of smog:
C2H4 (g) 1 O3 (g) BA C2H4O (g) 1 O2 (g)
The equation shows that for every molecule of C2H4. That reacts a molecule, of O3
reacts; thus, [] [] decrease O3 and C2H4 at the same rate:
Rate 5 2


D3C2H4 4 Dt 5 2


, D3O3 4 Dt When We start with a known [] in O3 a closed vessel at 308C (303 K) and measure [O3]
at 10.0-s intervals during the first minute. After, adding C2H4We obtain the concentration
vs. Time data shown in the Figure 16.5 table which gives, the black curve. Note that
- The. Data points in Figure 16.5 result in a curved line which means, that the rate is
changing over time (a straight line would. Mean that the rate was constant).
- The rate decreases during the course of the reaction because we are plotting reactant
concentration. Vs. Time:As O3, molecules react fewer are present to collide with C2H4
molecules and rate, the, change the in [] over, O3 time therefore. Decreases.