IMAT 2015 Q43 [Pure Water]

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Pure water self­-ionises. This endothermic reaction is represented by the equation:

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Which of the following statements is true for pure water between the temperatures of 0 °C and 100 °C?

  1. The concentrations of H^+ (aq) and OH^– (aq) are equal between 0 °C and 100 °C.
  2. An increase in temperature causes the pH to fall.
  3. An increase in temperature causes the electrical conductivity to decrease.

A. 1 and 3 only
B. 3 only
C. 1 only
D. 1 and 2 only
E. 2 only

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Statement 1 is correct. This is because all the H^+ and OH^- ions are produced from the same singular reactant. Therefore, the concentration should stay equal.

Statement 2 is also correct. Since this is an endothermic reaction, an increase in temperature will favour the forwards’ reaction which in turn will produce more H^+ ions. This will make the pH fall.

Statement 3 is incorrect. The amount of free moving electrons available to conduct electricity will not be changed or affected, therefore, we cannot say the increase in temperate can affect the electrical conductivity.

Hence, our answer must be statements 1 and 2, option D.

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Hi! I didn’t get why the pH would become lower; if the forward reaction is increased, more OH- will be produced as well, which in turn would raise the pH, no? So overall it wouldn’t change

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Hi, I had the same question few days ago, and you are partially right.
As forward reaction increases more H+ and OH- ions do get created at the same time. However due to the structure of water molecules (plus the interactions), the concentration of hydroxide ions are much smaller.

In conclusion, dissociation of water favors hydrogen ions, therefore lowers pH!

This is what I have found about relationship with the structure of water:

  1. Because of the strong hydrogen bonding between water molecules, when a water molecule loses a hydrogen ion (H+), the negatively charged oxygen atom still retains some degree of the negative charge. This partial negative charge on the oxygen atom is stabilized through hydrogen bonding with neighboring water molecules.

  2. On the other hand, when a water molecule releases a hydroxide ion (OH-), the negative charge on the oxygen atom is not as effectively stabilized by hydrogen bonding. This makes it less favorable for hydroxide ions to form compared to hydrogen ions.

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I am aware that statement 3 is wrong but wouldn’t the increase in amount of charged particles increase the electrical conductivity ?

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Hi! About statement 1 - I understand that the concentration of the ions will vary with the change in temperature. For example - if the temperature is higher there will be more reactents and if it is lower there will be less. Can someone please go over the question and explain why the concentration of the ions does stay the same?

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hello!
the reason is that if the temperature changes reaction goes left or right, but as we have the same molarity for both H+ and OH- the more product formed will give the same amount of increase to both H+ and OH-.
the equilibrium wouldn’t favour H+ or OH-.

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Ohhh I see… they meant equal to each other, not equal between the temperature differences. thank you!

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is this associated with the stoichiometry coefficients?
if it was for example 2H and 1OH, wouldnt [H] increase?

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yes! for this question we have the exact same amount of molarity so we make them equal. but in general h20 will be always given as the same amount of H+ and OH- in standard conditions

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