Question 1: Define bond energy and describe its role in chemical reactions.

Answer: Bond energy is the amount of energy required to break one mole of a specific covalent bond in a gaseous molecule. It indicates the strength of the bond and influences whether a reaction is exothermic or endothermic.

Question 2: Explain why breaking bonds requires energy while forming bonds releases energy.

Answer: Breaking bonds requires energy because energy is needed to overcome the attractive forces between atoms. Forming bonds releases energy because atoms release energy as they reach a lower energy state when bonds are established.

Question 1: Calculate the energy change (in kJ) for the reaction: H₂ + Cl₂ → 2 HCl, given that the bond energies are: H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol.

Answer: The total energy to break bonds: (H–H) + (Cl–Cl) = 436 + 243 = 679 kJ. The energy released forming bonds: 2 × (H–Cl) = 2 × 431 = 862 kJ. Net energy change = 862 - 679 = 183 kJ (exothermic).

Question 2: A reaction involves breaking a bond with an energy of 300 kJ/mol and forming bonds with a total energy of 450 kJ/mol. What is the overall energy change, and is the reaction exothermic or endothermic?

Answer: Energy change = 450 - 300 = 150 kJ (exothermic).

Question 1: Describe a simple experimental method to estimate the bond energy of a diatomic gas, such as oxygen.

Answer: One method involves measuring the temperature change when a known amount of the gas reacts or combines with another substance, and using the heat released or absorbed to calculate the average bond energy per mole based on the reaction's stoichiometry.

Question 2: Identify two variables that must be controlled in an experiment measuring bond energy and explain why.

Answer: Variables include temperature and pressure, because fluctuations can affect reaction rates and energy measurements, leading to inaccurate bond energy estimates.

Question 3: List two safety precautions that should be taken when conducting experiments involving reactive gases.

Answer: Use of appropriate protective equipment such as gloves and goggles, and conducting experiments in a well-ventilated area or fume hood to avoid inhalation of hazardous gases.

Question 1: In an experiment, the total energy required to break all bonds in methane (CH₄) is 1660 kJ/mol, and the energy released when new bonds form in carbon dioxide and water is 2000 kJ/mol. Explain whether the reaction is likely to be exothermic or endothermic, and justify your answer.

Answer: The reaction is likely to be exothermic because the energy released from forming new bonds (2000 kJ/mol) exceeds the energy needed to break bonds (1660 kJ/mol), resulting in a net release of energy.

Question 2: Given the data above, calculate the net energy change for the combustion of methane in kJ/mol.

Answer: Net energy change = 2000 kJ/mol - 1660 kJ/mol = 340 kJ/mol (exothermic).

Question 1: Explain how understanding bond energies can help in developing more efficient fuels or energy sources.

Answer: Knowledge of bond energies allows scientists to estimate how much energy can be obtained from fuel reactions, optimise fuel compositions for maximum efficiency, and develop new energy sources with lower environmental impact.

Question 2: Describe how bond energy calculations can be utilised in the design of new pharmaceuticals or materials.

Answer: Bond energy calculations help predict the stability of molecular structures, guide modifications to improve durability or reactivity, and assist in designing molecules with desired energy profiles for specific functions.