Energetics

• What is enthalpy, H, defined as in chemistry?

  Heat energy stored in a system
• Enthalpy change is the heat energy change at constant pressure
• What are the units of ΔH?

  kJ mol⁻¹
• In an exothermic reaction, heat energy is given out to the surroundings, increasing the temperature.
• In an exothermic reaction, the value of ΔH is negative
• In an endothermic reaction, heat energy is taken in from the surroundings, decreasing the temperature.
• In an endothermic reaction, what is the sign of ΔH?

  Positive
• Activation energy is the minimum energy required to break bonds
• Bond enthalpy values are always positive because energy is required to break bonds.
• What is mean bond enthalpy?

  Average energy to break bonds
• The enthalpy change of a reaction is the difference between energy required to break bonds and energy released when new bonds are formed
• Using mean bond enthalpies often results in less accurate ΔH values compared to Hess's Law.
• What are the standard conditions for enthalpy changes?

  298K and 1 atm
• Standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements
• The standard enthalpy change of an element in its standard state is zero.
• What is standard enthalpy of combustion?

  Enthalpy change for complete reaction
• What does $\Delta_{c} H^\theta$ represent?


  Standard enthalpy of combustion
• The standard enthalpy of combustion is the enthalpy change when one mole of a compound reacts completely in oxygen
• Write the balanced equation for the standard enthalpy of combustion of C₃H₆(g).

  C₃H₆(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
• Write the balanced equation for the standard enthalpy of combustion of methane.

  CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
• What is the method used to determine enthalpy changes experimentally called?

  Calorimetry
• The formula for heat change is q = mcΔT, where q is the heat energy in Joules
• Steps to calculate enthalpy change using calorimetry data:

  1️⃣ Measure temperature change of water

  2️⃣ Calculate heat energy change (q)

  3️⃣ Convert q from J to kJ

  4️⃣ Divide q by the number of moles

  5️⃣ Assign correct sign to ΔH
• If the temperature increases during a reaction, ΔH is negative.
• The experimental value for the enthalpy of combustion of methanol is less exothermic than the data book value due to incomplete combustion or heat loss to the surroundings
• Write the balanced equation for the neutralisation of sodium hydroxide with hydrochloric acid.

  NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• An exothermic reaction results in a decrease in temperature.

  False
• Cooling curves can be used to account for heat loss in reactions in solutions.
• What should be extrapolated in a cooling curve to determine the maximum temperature rise?

  Temperature readings
• The First Law of Thermodynamics states that energy cannot be created or destroyed
• What does Hess's Law state about the enthalpy change of a chemical reaction?

  Independent of pathway
• The enthalpy change of a reaction can be calculated using the maximum temperature rise.
• What is the formula for calculating the heat change (q)?

  $q =$$mc\Delta T$
• Steps to determine the maximum temperature rise from a graph

  1️⃣ Extrapolate the cooling line

  2️⃣ Extrapolate the heating line

  3️⃣ Find the point of intersection

  4️⃣ Read the maximum temperature
• The First Law of Thermodynamics states that energy cannot be created or destroyed.
• Hess's Law applies the First Law of Thermodynamics to chemical reactions
• What does Hess's Law state about the enthalpy change of a reaction?

  Independent of the route
• The enthalpy change of a reaction can always be measured directly.

  False
• Match the parts of an enthalpy cycle with their descriptions:

  Route 1 ↔️ Starting point of the cycle

  Route 2 ↔️ Alternative pathway in the cycle

  End of cycle ↔️ Final destination of the cycle
• In an enthalpy cycle, ΔH₁ is equal to ΔH₂ + ΔH₃