Question

0.50 g of potassium nitrate (KNO3) was added to a test tube holding 5 mL of distilled water. A temperature probe was placed in the test tube to record any change in temperature during the reaction. The baseline temperature of the water before adding the KNO3 was 23.2 degrees Celsius. After the KNO3 was added to the water the temperature of the solution dropped to 18.1 degrees Celsius.

1) Write an equation for the reaction above. Make sure it has the correct mass and charge balance and it reflects the net change that took place. Add energy to the appropriate side of the equation.

An additional 1.50 g of potassium nitrate (KNO3) is added to the same test tube as above. The baseline temperature of the solution prior to adding the additional 1.50 g of KNO3 was 19.1 degrees Celsius. After adding the KNO3 the temperature dropped to 15.9 degrees Celsius. There is visibly undissolved KNO3 at the bottom of the test tube after the reaction was complete. The test tube is place in a hot water bath and the temperature peaked at 96.5 degrees Celsius. While in the hot water bath the remaining KNO3 dissolved in the solution. Once removed from the hot water bath the solution was placed in a ice water bath and at 39.7 degrees Celsius KNO3 began to recrystallize and collect at the bottom of the test tube again.

2) Rewrite the equation for the supersaturated solution as it cools down.

Is the solution releasing or absorbing energy?

State where the energy is coming from and where it is going as the KNO3 recrystallizes, is the energy kinetic or potential?

Answer 1

1) Potassium nitrate is an ionic compound which when dissolved dissociates completely into potassium cations and nitrate anions. The ions are solvated by water molecules through hydrogen-bonding due to the large electronegativity difference between O and H atoms in water. The solvated ions have potassium cations interacting the lone pair of electrons of O atoms in water while the oxygen atoms in nitrate ions form H-bonding with the electron-deficient hydrogen atoms in water. Since the solvent cages formed contain varying number of water molecules, the water molecules involved in the dissolution are ignored for the balanced equation depicting dissolution, given below.

2) As the supersaturated solution cools down, the presence of a large excess of ions in the medium results in a deficiency in water molecules to properly solvate the ions individually. This results in the electrostatic attraction between the potassium and nitrate ions overcoming the ability of water to solvate, resulting the ions to come together and start solidifying. This solidification process is slow and ordered, resulting in the solid formed to possess long-range order, as dictated by numerous factors unique to the potassium nitrate molecule. Such an ordered solid is, as known, a crystal and the process is crystallization. A balanced equation for the crystallization is therefore the reverse reaction of the previous equation.

3) Dissolution involves the process of breaking the ionic bond between potassium and nitrate - a process that requires energy while crystallization involves formation of the high-energy ionic bond that results in release of energy. A process that releases energy increases in temperature while a process that absorbs energy decreases in temperature. Similarly, in case of dissolution of potassium nitrate, the process absorbs energy from the surroundings, causing the temperature of the solution to drop. During crystallization however, the released energy is transferred to the ice. So, the energy for crystallization is coming from the heat imbued on the solution from its time in the hot water bath and is kinetic energy. When it crystallizes, the kinetic energy is used up to form the potassium nitrate lattice, dissipating the excess heat into ice, making that energy potential.