Give one difference between endothermic and exothermic reactions. State Le Chatelier's principle. Mention three factors that affect the equilibrium position of a chemical reaction.

From the equation, 2 moles of electrons produce 1 mole of $\text{H}_2$ gas.

The volume of hydrogen produced is $\boxed{\text{0.104 dm}^3}$.

3. (a) With the aid of an energy profile diagram, illustrate the effect of a catalyst on an exothermic reaction.
An energy profile diagram for an exothermic reaction shows the reactants at a higher energy level than the products. The reaction proceeds through a transition state, requiring a certain amount of activation energy ($\text{E}_a$). A catalyst provides an alternative reaction pathway with a lower activation energy ($\text{E}_a'$), thereby increasing the reaction rate. However, the catalyst does not change the overall enthalpy change ($\Delta H$) of the reaction, meaning the energy difference between reactants and products remains the same.

3. (b) i. One difference between endothermic and exothermic reactions:

• Endothermic reactions absorb heat from their surroundings, causing the surroundings to cool down, and have a positive enthalpy change ($\Delta H > 0$).
• Exothermic reactions release heat to their surroundings, causing the surroundings to warm up, and have a negative enthalpy change ($\Delta H < 0$).

3. (b) ii. State Le Chatelier's principle.
Le Chatelier's principle states that if a change of condition (such as temperature, pressure, or concentration) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.

3. (b) iii. Three factors that affect the equilibrium position of a chemical reaction are:

• Concentration of reactants or products.
• Temperature of the system.
• Pressure (for reactions involving gases).

3. (c) i. Define the following:
I. Saturated solution: A solution that contains the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature, such that any additional solute added will not dissolve and will remain as a solid.
II. Molar solution: A solution that contains one mole of solute per liter (or cubic decimeter) of solution. Its concentration is expressed in moles per cubic decimeter ($\text{mol/dm}^3$) or Molarity (M).

3. (c) ii. What mass in grams of sodium trioxocarbonate (IV) is needed to prepare 250 cm³ of 0.2 moldm⁻³ solution?
Given: Volume $= 250 \text{ cm}^3$, Concentration $= 0.2 \text{ mol/dm}^3$.
Atomic masses: $\text{Na} = 23$, $\text{O} = 16$, $\text{C} = 12$.
The compound is sodium trioxocarbonate (IV), which is $\text{Na}_2\text{CO}_3$.

The mass of sodium trioxocarbonate (IV) needed is $\boxed{\text{5.3 g}}$.