By measuring the time taken for the color of the solution to tuen pink inder different conditions, factors which determine the rate of the reaction can thus be studied.
- Procedures: please refer to the lab. manual
- Observations:
- After the addition of solutions A and B, the solution mixture changed color from nearly colorless to pink.
- In part (A), the more color methanol solution used, the shorter time required for the color change of the mixture.
- In part (B), the hotter the solution used, the shorter time required for the color change of the mixture.
- Data:
Part (A):
Volume of NaHSO3 + Na2SO3 used : 5 cm3
Part (B):
Volume of methanol used: 10 cm3
Volume of NaHSO3 + Na2SO3 used: 10 cm3
Volume of water used: 20cm3
Conc. of methanol: 0.3M Conc. of NaHSO3: 0.2M Conc. of Na2SO3 : 0.05M
- Calculation for discussion A
Part (A):
- Using integrated rate equations:
[A] = -k t + [A]0 (1)
In[A] = -k t + In[A]0 (2)
[A]-1 = k t + [A]0-1 (3)
Where [A] is methanol.
- Using equation (1), plotting a graph of [A] against time t. If negative slope with initial conc. of [A] is shown, it is a zero order reaction.
- Using equation (2), plotting a graph of In[A] against time t. If negative slope with In[initial conc. of [A]] together with positive y-intercept is shown, it is a first order reaction.
-
Using equation (3), plotting a graph of [A]-1 against time t. If positive slope with [initial conc. of [A]]-1 together with positive y-intercept is shown, it is a second order reaction.
- No linear slope can be drawn. Thus it is not zero order reaction.
- Although negative slope can be drawn, the y-incept is negative, which shows that it is not first order reaction.
-
A linear positive slope and positive y-intercept can be drawn. Thus it is a second order reaction.
Calculation for discussion B:
By equation: Ink = InA – (Eact)(RT)-1
To obtain the value is Eact, we need to make an Arrhenius plot of In(k) against T-1, the slope if the graph is –[(Eact)(R)-1].
According to the data obtained in experiment:
[Note: By equation: Rate (i.e. t-1) = k [A] 2 , k is thus obtained, where 2 represent the order of reaction.]
-
Slope = –[(Eact)(R)-1] = -11111
Therefore, Eact = -R(-11111) = (8.314)x(-11111) = 92.378 kJ mol-1
Activation energy of the Formaldehyde Clock Reaction is 92.378 kJ mol-1
- Remarks:
-
In part (A), the volume of water increase when the volume of methanol decrease, and the volume of NaHSO3 and Na2SO3 was kept constant to ensure that the concentration of NaHSO3 and Na2SO3 will be the same among the five attempts as the total volume of water and methanol will still be 20cm3
It was done so because when doing experiment to determine how factors affecting the rate of a chemical reaction, only one factor can be a variable each time while others are kept constant.
- In part (B), an increase in temperature will increase the rate of reaction because as a reaction occurs when the reactant molecules process enough energy and collide together, so when the temperature of reactants increase, the molecules will process more energy and they will also move more vigorously, thus increases the chance for molecule collision. Then the rate of reaction will be increase as a result.
~End of report~
