Sec 4 Chemistry Chemical Energetics topical questions

State whether each reaction is exothermic or endothermic.

1. (a)Burning natural gas in a boiler[1]
2. (b)Heating calcium carbonate strongly[1]
3. (c)Neutralising an acid with an alkali[1]

A reaction raises the temperature of its surroundings. State the sign of its enthalpy change ΔH and sketch-in-words where the product shelf sits on an energy level diagram.

Write the balanced equation for the complete combustion of methane, and name the two products.

State what a catalyst does to (a) the activation energy and (b) the enthalpy change of a reaction.

When 50 cm³ of 1.0 mol/dm³ hydrochloric acid is mixed with 50 cm³ of 1.0 mol/dm³ sodium hydroxide, the temperature rises by 6.8 °C. (Specific heat capacity of the mixture: 4.2 J/(g °C); density 1.0 g/cm³.)

1. (a)Calculate the heat released, in J.[2]
2. (b)Calculate the moles of water formed in the neutralisation.[1]
3. (c)Calculate ΔH in kJ/mol and state its sign.[2]

Using the bond energies H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol and H–Cl = 431 kJ/mol, calculate ΔH for H₂ + Cl₂ → 2HCl and state whether the reaction is exothermic or endothermic.

An energy level diagram for a reaction shows the reactant shelf at 90 kJ, the peak of the curve at 210 kJ and the product shelf at 150 kJ.

1. (a)Calculate ΔH and state the sign.[2]
2. (b)Calculate the activation energy.[1]
3. (c)State the effect of a catalyst on each of the three numbers.[2]

Explain, in terms of bond breaking and bond making, why a reaction can be exothermic overall even though energy is needed to start it.

A student burns a spirit burner of methanol under 200 g of water and the water temperature rises by 18.0 °C. The methanol burned has mass 0.80 g. (c of water = 4.2 J/(g °C); Mr of methanol, CH₃OH = 32.)

1. (a)Calculate the heat absorbed by the water, in kJ.[2]
2. (b)Calculate the moles of methanol burned.[1]
3. (c)Calculate the enthalpy change of combustion of methanol, in kJ/mol.[2]

A gas heater burns butane, C₄H₁₀, in a poorly ventilated room.

1. (a)Write the equation for complete combustion of butane.[2]
2. (b)Explain why the flame turns sooty and yellow in the poorly ventilated room, and name the toxic product that may form.[2]
3. (c)State why the incomplete reaction is dangerous even though it still releases heat.[1]
4. (d)State one symptom-based reason carbon monoxide is called the silent killer.[1]

Hydrogen is proposed as the fuel of the future.

1. (a)Write the equation for hydrogen burning in air.[1]
2. (b)State two environmental advantages of hydrogen combustion over petrol combustion.[2]
3. (c)Explain why hydrogen produced by electrolysis using solar electricity is greener than hydrogen from natural gas reforming.[2]
4. (d)State one practical drawback of storing hydrogen.[1]

Methane combusts: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies: C–H 413, O=O 498, C=O (in CO₂) 803, O–H 464 kJ/mol.

1. (a)Calculate the total energy to break all bonds in the reactants.[2]
2. (b)Calculate the total energy released forming all bonds in the products.[2]
3. (c)Calculate ΔH and explain why the textbook combustion value (−890 kJ/mol) is more negative than your answer.[2]