energetics 1

• What does the symbol ∆H​o​ represent in thermodynamics?

  Enthalpy change
• Under what standard conditions is enthalpy measured?

  100 kPa pressure and 298 K temperature
• What happens to energy when bonds are broken in a reaction?

  Energy is taken in from the surroundings
• What occurs when bonds are formed in a chemical reaction?

  Energy is given out to the surroundings
• How is the overall energy change of a reaction determined?

  It depends on the energy transferred during bond breaking and forming
• What does a positive enthalpy change (∆H) indicate?

  Energy is taken in from the surroundings
• What does a negative enthalpy change (∆H) indicate?

  Energy is released to the surroundings
• What are the characteristics of endothermic and exothermic reactions?

  • Endothermic reactions:
  • More energy is needed to break bonds than to make new ones
  • Overall ∆H is positive
  • Heat is taken in from the surroundings

  
  

  • Exothermic reactions:
  • More energy is needed to make new bonds than to break existing ones
  • Overall ∆H is negative
  • Heat is given out to the surroundings

  
• What is the definition of enthalpy change of reaction (∆H​o​)?

  The enthalpy change when quantities of substances in standard states react completely under standard conditions
• What is the definition of enthalpy change of formation (∆​f​H​o​)?

  The enthalpy change when one mole of a substance is produced from its elements under standard conditions
• What is the definition of enthalpy change of combustion (∆​c​H​o​)?

  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• What is the definition of enthalpy change of neutralisation (∆​n​H​o​)?

  The enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water

  
• What is calorimetry and how is it used to measure enthalpy change?

  • Calorimetry is an experimental method for finding enthalpy change.
  • It measures temperature change over time.
  • Data can be extrapolated to give an accurate value for the change in temperature.

  
• What does the measured change in temperature (∆T) represent in calorimetry?

  It is proportional to the energy change (q)
• What is the formula relating energy change (q), mass (m), specific heat capacity (c), and temperature change (∆T)?

  q = mc∆T
• What is specific heat capacity defined as?

  The energy required to raise 1g of substance by 1K without a change of state
• Why are ∆H values found using calorimetry never completely accurate?

  Energy is easily lost from the system
• What are some ways to reduce heat loss in calorimetry?

  By putting a lid on the calorimeter and insulating its outsides
• What is Hess's Law?

  The overall enthalpy change for a reaction is the same regardless of the route taken
• How is Hess's Law used to determine enthalpy changes?

  • Hess's Law is used for reactions that cannot be found directly using experimental methods.
  • A triangular cycle method is used with an intermediate product.
  • The direction of arrows indicates whether values should be added or subtracted.

  
• How do arrows in a triangular diagram relate to Hess's Law?

  Arrows indicate whether the values should be added or taken away
• What are the definitions of enthalpies of formation and combustion in relation to Hess's Law?

  • Enthalpies of Formation:
  • Used to calculate enthalpy change using given enthalpies of formation.
  • Arrows point out from the central product.

  
  

  • Enthalpies of Combustion:
  • Used to calculate enthalpy change using given enthalpies of combustion.
  • Arrows point towards the central product (H​2​O and CO​2​).

  
• What does bond enthalpy data represent?

  The energy required to break one mole of the stated bond in a gaseous state, under standard conditions
• Why do bond enthalpy values differ from databook values?

  They are averaged values and can vary in each situation
• How can mean bond enthalpy values be used in chemical reactions?

  They can infer which bonds might break first based on bond strength
• How do bond enthalpy values relate to the speed of a reaction?

  • If a reaction requires breaking many strong bonds or is endothermic overall, it is likely to occur slowly at room temperature.

  
• How is the overall enthalpy change for a reaction calculated using bond enthalpy values?

  • Overall enthalpy change = (sum of bond enthalpies for bonds broken) - (sum of bond enthalpies for bonds formed)

  
• Calculate the enthalpy change when methane undergoes complete combustion in oxygen using the provided bond enthalpies.

  The enthalpy change is $-806 \text{ kJ mol}^{-1}$
• What is the bond enthalpy of C-H?

  413 kJ mol​-1
• What is the bond enthalpy of O=O?

  498 kJ mol​-1
• What is the bond enthalpy of O-H?

  464 kJ mol​-1
• What is the bond enthalpy of C=O?

  799 kJ mol​-1
• What is the chemical equation for the complete combustion of methane?

  CH​4​ + 2O​2​ → CO​2​ + 2H​2​O
• How do you calculate the total bond enthalpy for bonds formed in the combustion of methane?

  By calculating $2 \times 799 +$$4 \times 464 =$$1598 +$$1856 =$$3454 \text{ kJ mol}^{-1}$
• How do you calculate the total bond enthalpy for bonds broken in the combustion of methane?

  By calculating $4 \times 413 +$$2 \times 498 =$$1652 +$$996 =$$2648 \text{ kJ mol}^{-1}$
• What is the final calculation for the enthalpy change of methane combustion?

  The enthalpy change is calculated as $(1652 + 996) - (1856 + 1598) =$$2648 - 3454 =$$-806 \text{ kJ mol}^{-1}$
• What is the definition of enthalpy change?

  Heat energy change in a reaction at constant pressure (kJmol⁻¹).
• What are the standard conditions for measuring enthalpy changes?

  100 kPa, 298 K, and 1 moldm⁻³ concentrations of ions.
• What characterizes an exothermic reaction?

  Heat is given off, resulting in a negative value of enthalpy change.
• What characterizes an endothermic reaction?

  Heat is absorbed, resulting in a positive value of enthalpy change.