1.4 - Energetics

• What happens when an enthalpy change occurs?

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

  The chemicals involved in the reaction
• What is the surroundings in thermodynamics?

  Everything outside the chemicals
• In an exothermic change, where is energy transferred?

  From the system to the surroundings
• What is the energy relationship between products and 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
• What is the energy relationship between products and reactants in an endothermic reaction?

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

  Positive
• What are standard conditions for enthalpy changes?

  100 kPa, 298 K, 1 mol dm-3
• What is the standard enthalpy change of formation?

  • Enthalpy change when 1 mole of compound forms
  • From its elements under standard conditions
  • All reactants and products in standard states
• What is the symbol for standard enthalpy change of formation?

  ∆fH
• What is the standard enthalpy change of combustion?

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

  ∆cH
• What happens during incomplete combustion?

  Produces soot, carbon monoxide, and water
• What is activation energy (EA)?

  The minimum energy required for a reaction
• What is enthalpy change?

  • Amount of heat energy taken in or given out
  • During any change in a system
  • Provided the pressure is constant
• 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
• How can temperature readings be improved in calorimetry?

  Take readings at regular time intervals
• What is the equation for calculating energy change in a reaction?

  Q = m x cp x ∆T
• What does 'm' represent in the energy change equation?

  Mass of solution in grams
• What does 'cp' represent in the energy change equation?

  Heat capacity in J g-1 K-1
• What does '∆T' represent in the energy change equation?

  Temperature change in Kelvin
• What is the general method for measuring enthalpy change?

  Wash equipment, measure volumes, record temperatures
• What is a common error in calorimetric measurements?

  Energy transfer from surroundings is neglected
• What is Hess's Law?

  Total enthalpy change is independent of reaction route
• What does Hess's Law relate to in thermodynamics?

  Conservation of energy
• How can different reaction routes be represented in energy level diagrams?

  By showing arrows for each route
• What is the enthalpy of formation of an element?

  0 kJ mol-1
• What is the significance of the sign in enthalpy change calculations?

  Indicates whether the reaction is exothermic or endothermic
• What is the heat capacity of water?

  4.18 J g-1 K-1
• What is the density of water assumed to be in calorimetry?

  1 g cm-3
• How do you calculate the number of moles of a substance?

  Moles = mass / molar mass
• How do you convert energy change from J to kJ?

  Divide by 1000
• How do you determine the enthalpy change of reaction from experimental data?

  Divide energy change by moles of limiting reactant
• What is the enthalpy change of neutralization?

  Enthalpy change per mole of acid or base
• How do you calculate the enthalpy change of combustion?

  Use energy change from combustion divided by moles
• What is the enthalpy change for the combustion of propan-1-ol?

  Calculated using energy change and moles combusted
• What is the significance of the temperature increase in a reaction?

  Indicates an exothermic reaction