Energetics

• What happens when an enthalpy change occurs?

  Energy is transferred between system and surroundings
• What defines the system and surroundings in thermodynamics?

  The system is chemicals; surroundings are outside
• In an exothermic change, where is energy transferred?

  From the system to the surroundings
• How does the energy of products compare to reactants in an exothermic reaction?

  Products have less energy than reactants
• What is the sign of ∆H in an exothermic reaction?

  Negative
• In an endothermic change, where is energy transferred?

  From the surroundings to the system
• How does the energy of products compare to reactants in an endothermic reaction?

  Products have more energy than reactants
• What is the sign of ∆H in an endothermic reaction?

  Positive
• What are the standard conditions for enthalpy changes?

  • 100 kPa pressure
  • 298 K (25°C)
  • Solutions at 1 mol dm^-3
  • All substances in their normal state at 298 K
• What symbol is used for enthalpy change measured at standard conditions?

  ∆H
• What is the standard enthalpy change of formation?

  Enthalpy change when 1 mole forms from elements
• What is the symbol for standard enthalpy change of formation?

  ∆fH
• Write the reaction for the formation of magnesium chloride.

  Mg (s) + Cl₂ (g) → MgCl₂ (s)
• Write the reaction for the formation of iron(III) oxide.

  2Fe (s) + 1.5 O₂ (g) → Fe₂O₃ (s)
• What is the enthalpy of formation of an element?

  0 kJ mol^-1
• What is the standard enthalpy change of combustion?

  Enthalpy change when one mole combusts in oxygen
• What symbol is used for standard enthalpy change of combustion?

  ∆cH
• What topic is being discussed in the video?

  Energetics from unit 2 of AQA AS specification
• Why is the energetics topic split into three sections?

  Because it is quite detailed, especially Hess's law
• Write the reaction for the combustion of methane.

  CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2 H₂O (l)
• What are the three sections of the energetics topic?

  Enthalpy, calorimetry, and Hess's law
• What happens during incomplete combustion?

  Produces soot, carbon monoxide, and water
• How does incomplete combustion compare to complete combustion?

  It is less exothermic than complete combustion
• What is the focus of the first portion of the video?

  Concentrating on the term enthalpy
• What are common exothermic processes?

  • Combustion of fuels
  • Oxidation of carbohydrates (e.g., glucose in respiration)
• What is the definition of enthalpy change?

  Heat energy change during a system change
• What is the definition of enthalpy change?

  Heat or energy change under constant pressure
• What is the calorimetric method used for?

  Measuring the enthalpy change for a reaction
• What is a challenge when measuring temperature rise in slow reactions?

  Cooling occurs simultaneously with the reaction
• What does the term "standard" refer to in enthalpy change?

  Standard temperature and pressure conditions
• How can you improve temperature measurement accuracy in slow reactions?

  Take readings at regular time intervals
• What is the standard temperature in Kelvin?

  298 Kelvin
• What is the standard pressure in pascals?

  100,000 pascals
• What is the equation for energy change in a reaction in solution?

  Q (J) = m (g) x cp (J g^-1K^-1) x ∆T (K)
• How can you describe exothermic reactions?

  Heat is given out during the reaction
• What does the calorimetric method assume about solutions?

  All solutions have the heat capacity of water
• What is the focus of the second part of the energetic section in the AQAS chemistry specification?

  Calorimetry
• How can you describe endothermic reactions?

  Heat is taken in during the reaction
• What does calorimetry measure?

  Energy released or contained in fuels
• What is a common error in calorimetric measurements?

  Energy transfer from surroundings is usually lost