I know that rate is proportional to 1/time.
Also why can we only do this if all the concentrations are kept the same?
Q: So basically I did an experiment to calculate the Activation energy:
The iodine clock reaction to monitor when a fixed amount of I2 has been made.
The rate of the reaction is inversely proportional to the time taken (t) for the solution to change colour
So mathematically speaking the rate is proportional to 1/t(time)
k=Ae^(-Ea/RT)
lnk =ln A - Ea/R * 1/T -->
Y= c + m x
One of the steps to calculate the Activation energy is to plot a graph of lnk against 1/T(Kelvin). However the experiment sheet states that because rate is proportional to 1/t( time). This means that 1/t can be used instead of k(rate) within the Arrhenius equation which means I can plot a graph of ln1/t(time) agains 1/T(Kelvin)
Explain: Why can 1/t be used instead of k in the Arrhenius equation?
2017-09-23 8:09 pm
回答 (1)
2017-09-23 10:38 pm
The diagram below shows the early parts of three curves of [I₂] against t (time) of iodine clock experiment. Like all other reactions, the early part of the concentration against time curve approximates to a straight line.
In an iodine clock reaction, I₂ is formed by a certain reaction.
For example, S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)
or, H₂O₂(aq) + 2H⁺(aq) + 2I⁻(aq) → 2H₂O(l) + I₂(aq)
The reaction mixture also contains a small and fixed amount of S₂O₃²⁻(aq) ions and some starch. Firstly, the I₂(aq) formed in the reaction is immediately removed by the S₂O₃²⁻(aq) ions.
2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I⁻(aq)
When the reaction reaches a specified stage, a fixed amount of I₂(aq) is produced to react all the S₂O₃²⁻ (aq) ions, and then free I2(aq) is produced which will form a dark blue complex with the starch. The time taken for the dark blue colour to appear, t, is recorded.
Refer to the diagram below. As the amount of S₂O₃²⁻ (aq) ions is small and fixed in each run, a fixed [I₂] (say, c, where c is a constant) has just been consumed. By definition of reaction rate, rate = c/t.
As c is a constant, rate ∝ 1/t
As the concentrations of all components are constant in each run, rate ∝ k
Hence, k ∝ 1/t, and thus 1/t can be used instead of k(rate) within the Arrhenius equation.
In an iodine clock reaction, I₂ is formed by a certain reaction.
For example, S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)
or, H₂O₂(aq) + 2H⁺(aq) + 2I⁻(aq) → 2H₂O(l) + I₂(aq)
The reaction mixture also contains a small and fixed amount of S₂O₃²⁻(aq) ions and some starch. Firstly, the I₂(aq) formed in the reaction is immediately removed by the S₂O₃²⁻(aq) ions.
2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I⁻(aq)
When the reaction reaches a specified stage, a fixed amount of I₂(aq) is produced to react all the S₂O₃²⁻ (aq) ions, and then free I2(aq) is produced which will form a dark blue complex with the starch. The time taken for the dark blue colour to appear, t, is recorded.
Refer to the diagram below. As the amount of S₂O₃²⁻ (aq) ions is small and fixed in each run, a fixed [I₂] (say, c, where c is a constant) has just been consumed. By definition of reaction rate, rate = c/t.
As c is a constant, rate ∝ 1/t
As the concentrations of all components are constant in each run, rate ∝ k
Hence, k ∝ 1/t, and thus 1/t can be used instead of k(rate) within the Arrhenius equation.
收錄日期: 2021-04-18 17:49:31
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