Tamilnadu BoardEnglish MediumSTD 12ChemistryChemical Kinetics5 Marks
Question
Explain the effect of temperature on reaction rate based on Arrhenius theory.
✓
Answer
1. Generally, the rate of a reaction increases with increasing temperature. However, there are very few exceptions.
2. As a rough rule, for many reactions near room temperature, reaction rate tends to double when the temperature is increased by $10^{\circ} C$.
3. A large number of reactions are known which do not take place at room temperature but occur readily at higher temperature. Example - Reaction between $H _2$ and $O _2$ to form $H _2 O$ takes place only when an electric spark is passed.
4. Arrhenius suggested that the rates of most reactions vary with temperature in such a way that the rate constant is directly proportional to
$e^{-\frac{E_o}{R T}}$ and he proposed a relation between the rate constant and temperature.
$k = A e^{-\frac{E_a}{R T} \text { }}$
where
$k =\text { frequency factor }$
$R =\text { gas constant, }$
$E _{ a }=\text { Activation energy }$
$T =$ Absolute temperature (in kelvin)
The factor A does not vary significantly with temperature and hence it may be taken as a constant.
5. Taking logarithm on both side of the equation (1)
$\ln k =\ln A +\ln e^{- E _{ a } / RT }$
$\ln k =\ln A -\frac{ E _{ a }}{ RT } \quad \text { }$
$\operatorname{l} \ln e=1]$
$\ln k =\ln A -\frac{ E _{ a }}{ R }\left(\frac{1}{ T }\right)$
6. The plot of Ink vs $\left(\frac{1}{T}\right)$ is a straight line with negative slope $\frac{E_a}{R T}$. If the rate constant for a reaction at two different temperatures is known, we can calculate the activation energy.
$\underset{\text { }}{\ln k _2-\ln k _1-}-\left[\frac{E_a}{ RT _2}\right]+\left[\frac{E_a}{ RT _1}\right]$
This equation can be used to calculate $E$ from rate $E _{ a }$ constants $k _1 \& k _2$ at temperature $T _1$ and $T _2$
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