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 an exothermic change?

  Energy is transferred from system to surroundings
• In an exothermic reaction, what is the sign of ∆H?

  Negative
• What occurs in an endothermic change?

  Energy is transferred from surroundings to system
• 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?

  ΔfH
• What is the enthalpy of formation of an element?

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

  • Enthalpy change when 1 mole of substance combusts
  • Completely in oxygen under standard conditions
  • All reactants and products in standard states
• What is the symbol for standard enthalpy change of combustion?

  ΔcH
• 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 the definition of 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 do you improve temperature measurement accuracy?

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

  Q = m x cp x ΔT
• What does the variable '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 'ΔT' represent in the energy change equation?

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

  1. Wash and dry equipment
  2. Use insulated cup for solutions
  3. Measure initial temperatures
  4. Add second reagent and stir
  5. Record temperature changes
• What is a common error in calorimetric methods?

  Energy transfer from surroundings is neglected
• What are the steps to calculate enthalpy change from experimental data?

  1. Calculate energy change using Q = m x cp x ΔT
  2. Determine moles of reactants used
  3. Divide Q by moles of limiting reactant
  4. Add sign and unit to result
• What is the heat capacity of water?

  1. 18 J g-1 K-1
• How do you calculate moles of a reactant?

  Moles = concentration x volume
• What is Hess's Law?

  • Total enthalpy change is independent of the route
  • Energy is always conserved
  • Can be represented with energy level diagrams
• How can Hess's Law be represented diagrammatically?

  Using energy level diagrams with arrows
• What is the enthalpy change for the formation of HBr from H and Br?

  ΔH = a - b
• What is the enthalpy change for the reaction involving CuSO4 and water?

  ΔH = c + d - a
• What state is H2O in during this experiment?

  Gas
• What does Hess's Law state about enthalpy change?

  It is independent of the reaction route
• What law of thermodynamics is Hess's Law a version of?

  First law of thermodynamics
• What does Hess's Law imply about energy?

  Energy is always conserved
• What is the reaction represented by 2H (g) + 2Cl(g)?

  Formation of HCl (g)
• How can energy level diagrams illustrate reactions?

  By showing different reaction routes
• What does the equation ΔH = a - b represent?

  Relationship between reaction routes
• How can interconnecting reactions be shown?

  Diagrammatically with arrows
• How is Hess's Law used to find enthalpy changes?

  By using alternative measurable reactions