State and explain whether the reaction is exothermic or endothermic. Calculate the enthalpy change when 10 grams of NaOH is dissolved in water, given that the molar mass of sodium hydroxide is 40 gram

The diagram shows that the energy of the products (NaOH(aq)) is lower than the energy of the reactants (NaOH(s) + H$_2$O(l)). The given enthalpy change, $\Delta H = -42.8 \text{ kJ mol}^{-1}$, is negative.

The reaction is exothermic. This is because the enthalpy change ($\Delta H$) is negative, indicating that heat is released from the system to the surroundings. The products have lower energy than the reactants.

ii) Given that the molar mass of sodium hydroxide is 40 grams. Calculate the enthalpy change when 10 grams of NaOH is dissolved in water.

Given molar mass of NaOH = $40 \text{ g mol}^{-1}$ and mass of NaOH = $10 \text{ g}$.

The molar enthalpy change for dissolution is $\Delta H = -42.8 \text{ kJ mol}^{-1}$.

Question 15

i) Define molar heat of combustion of a substance.

The molar heat of combustion is the heat energy released when one mole of a substance undergoes complete combustion in excess oxygen under standard conditions.

ii) 1.26 grams of element Q was completely burned in air. The heat evolved was used to heat 250 cm³ of water, rising the temperature from 32 to 50°C. Calculate the relative atomic mass of element Q given that molar enthalpy of combustion is -360 kJ mol⁻¹ (density of water is 1 g cm⁻³ and specific heat capacity of water = 4.2 J g⁻¹ K⁻¹).

Given volume of water = $250 \text{ cm}^3$ and density of water = $1 \text{ g cm}^{-3}$.

Initial temperature = $32^\circ\text{C}$, Final temperature = $50^\circ\text{C}$.

(Note: A change of $18^\circ\text{C}$ is equivalent to a change of $18 \text{ K}$).

Given specific heat capacity of water ($c$) = $4.2 \text{ J g}^{-1} \text{ K}^{-1}$.

Convert Joules to kilojoules:

The heat evolved from the combustion of Q is equal to the heat absorbed by water.
Molar enthalpy of combustion of Q = $-360 \text{ kJ mol}^{-1}$.

Given mass of Q = $1.26 \text{ g}$.

Question 16

a) Explain why a plastic container is used instead of a metallic one.

A plastic container is used because plastic is a poor conductor of heat (an insulator). This minimizes heat loss to the surroundings, ensuring that the measured heat change for the reaction is as accurate as possible. A metallic container, being a good conductor of heat, would allow significant heat loss, leading to inaccurate results.