Multiple Choice Questions (MCQs)
Q.1 By increasing temperature of water from 25'C to 100 °C then
A) The pH of water decreases
B) The pOH of water increases
C) The pKw of water remains constant
D) The pH of water remains constant
Correct Ans : A (The pH of water decreases]
Explanation : By increasing temperature the decomposition of water also increases the Kw of water increases approximately 75 times so its mean both [H+] ions and [OH-] ions increases
Kw = [H+] [OH-] or pKw = pH + pOH
Hence we know:
pH = - log [H+]
if [H+] increases then pH decreases however pOH is also decreases because [OH-] ions are also increases.Overall sum of H+ ions and OH- ions increases so Kw increases or pKw decreases.H+ and OH- ions equally increases so water is always neutral at any state.
Remember: by changing temp. Kw,pKw,pH,pOH.H+ and OH- all changes except neutrality of water remains constant.
Q.2 2SO2 + O2 <===> 2SO3 Delta H= -ve
On increasing temperature of the above reaction then
A) The concentration of SO2 decreases
B) The concentration of SO3 increases
C) The value of Kc increases
D) The rate of reaction increases
Correct Ans : D [The rate of reaction increases]
Explanation : The above reaction is reversiable exothermic reaction.According to kinetic theory and Arrhenius theory by increasing temperature the rate of reaction (exo or endo) always increases however in case of exothermic reaction the rate of backward direction increases as many times as compared to farward (forward also increases but with less speed). High temperature favours the overall backward direction so reactants increases and products decreases.
We know
Kc = [P]/ [R]
So Kc decreases on increasing temperature.
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