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A group of students carried out an investigation into the energy changes that take place when metal hydroxides dissolve in water - Scottish Highers Chemistry - Question 9 - 2022
Question 9
A group of students carried out an investigation into the energy changes that take place when metal hydroxides dissolve in water. The following apparatus was used as... show full transcript
Worked Solution & Example Answer:A group of students carried out an investigation into the energy changes that take place when metal hydroxides dissolve in water - Scottish Highers Chemistry - Question 9 - 2022
Step 1
Using Graph 2 calculate the heat energy transferred to the water, in kJ, when the sodium hydroxide dissolved.
Answer
To find the heat energy transferred to the sodium hydroxide when it dissolved, we first need to extrapolate the cooling curve back to the time when the solute was added. After assessing the graph, we find the maximum temperature reached before it starts to cool down and the temperature just before the solute was added. Let’s say the initial temperature was identified as T_initial = 28.0 °C and the final temperature recorded after the dissolution is T_final = 22.0 °C. Thus, the temperature change (ΔT) can be calculated as:
Using the formula for heat energy:
where c = 4.18 J/g°C (specific heat capacity of water), m = 100 g (since density of water is approximately 1 g/cm³), we can calculate:
Step 2
Suggest why the experiment was carried out in a polystyrene cup with a lid.
Answer
The experiment was conducted in a polystyrene cup with a lid to minimize heat loss to the surroundings. Polystyrene is an excellent insulator, helping maintain a stable temperature in the calorimeter. The lid helps to further reduce heat exchange with the environment, ensuring more accurate readings of the temperature changes during the dissolution process.
Step 3
Use this information to calculate the energy released, in kJ mol⁻¹, when one mole of potassium hydroxide dissolves in water.
Answer
To find the energy released per mole of potassium hydroxide (KOH), we first determine the molar mass of KOH:
- Potassium (K): 39 g/mol
- Oxygen (O): 16 g/mol
- Hydrogen (H): 1 g/mol
Thus, molar mass of KOH = 39 + 16 + 1 = 56 g/mol.
Next, using the provided information, 5.61 g of KOH releases 5.25 kJ. We can calculate the energy released per mole using:
ext{Energy per mole} = rac{ ext{Energy released (kJ)}}{ ext{mass (g)}} imes ext{molar mass (g/mol)}Substituting the known values gives us:
ext{Energy per mole} = rac{5.25 ext{ kJ}}{5.61 ext{ g}} imes 56 ext{ g/mol} = 52.5 ext{ kJ/mol}Therefore, the energy released when one mole of potassium hydroxide dissolves is 52.5 kJ/mol.
Step 4
Calculate the enthalpy change, in kJ mol⁻¹, for the reaction above by using the data shown below.
Answer
To calculate the enthalpy change (ΔH) for the reaction of calcium hydroxide formation, we can utilize Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps. We have the reactions:
-
H₂(g) + ½O₂(g) → H₂O(l)
ΔH₁ = -286 kJ/mol -
Ca(s) + O₂(g) → Ca(OH)₂(s) + H₂(g)
ΔH₂ = -986 kJ/mol
To find the enthalpy change for the formation of Ca(OH)₂ from its elements, we can reverse the first reaction and combine it with the second. Hence,
For the reverse of reaction 1: $
ΔH = +286 kJ/mol
Now add this to ΔH₂:
Thus, the enthalpy change for the reaction is -700 kJ/mol.