Enthalpy changes

• What is enthalpy (H)?

  The heat energy stored in a chemical system
• Why can't we measure enthalpy directly?

  We can only measure the energy absorbed or released to the surroundings
• What is the symbol for enthalpy change?

  ΔH
• How is enthalpy change calculated?

  Enthalpy change is the enthalpy of the products minus the enthalpy of the reactants
• What is the formula for enthalpy change?

  ΔH = H products - H reactants
• What does an enthalpy profile diagram show?

  It shows the enthalpy change of a reaction over the progress of the reaction

  
• In an exothermic reaction, how do the enthalpy levels of products and reactants compare?

  The enthalpy of the products is less than that of the reactants
• What is the enthalpy change for an exothermic reaction?

  It is always negative
• What happens to the surroundings during an endothermic reaction?

  The surroundings become colder
• How does the enthalpy of reactants compare to products in an endothermic reaction?

  The enthalpy of the reactants is less than that of the products
• What is the enthalpy change for endothermic reactions?

  It is always positive
• What is a common reaction that is exothermic?

  The reaction of methane and oxygen to form water and carbon dioxide
• What is the activation energy (Ea)?

  It is the energy needed to break the bonds between the atoms in the reactants
• How does activation energy relate to both exothermic and endothermic reactions?

  Activation energy is needed for both types of reactions
• How is the enthalpy change represented on an enthalpy profile diagram?

  The arrow for enthalpy change goes from the reactants to the products
• What are standard enthalpy changes (ΔH°)?

  Enthalpy changes measured under standard conditions
• What is the standard pressure for measuring enthalpy changes?

  100 KPa (1 atmosphere)
• What is the standard temperature for measuring enthalpy changes?

  298 K (25 degrees Celsius)
• What is the standard concentration for aqueous solutions in enthalpy measurements?

  1 moldm3
• What must all substances involved in a reaction be in for standard enthalpy changes?

  They must be in their standard states
• What is the standard enthalpy change of reaction (ΔHr°)?

  The enthalpy change for a reaction as shown by the molar quantities in the chemical equation
• What is the standard enthalpy change of reaction for the Haber process?

  • 92 KJ/mol
• What is the standard enthalpy change of combustion (ΔHc°)?

  The enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions
• What is the standard enthalpy change of formation (ΔHf°)?

  The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions
• How can enthalpy change be determined?

  By a method known as calorimetry
• What is the setup for a calorimetry experiment?

  A reaction is carried out in an insulated beaker with a thermometer
• What does calorimetry measure?

  The heat exchanged with the surroundings
• In a calorimetry experiment, what is the surroundings?

  The known mass of liquid
• What does Q represent in the calorimetry equation?

  Energy in joules (J)
• What does M represent in the calorimetry equation?

  Mass of solution in grams (g)
• What does c represent in the calorimetry equation?

  Specific heat capacity
• What does T represent in the calorimetry equation?

  Temperature change
• How do you convert energy transferred to enthalpy change?

  Using the equation ΔH = -Q/mol
• When is the minus sign used in the enthalpy change equation?

  It is used only for exothermic reactions
• How is enthalpy change usually expressed?

  In KJ/mol
• What is the main source of error in enthalpy values from combustion?

  Heat loss to the surroundings
• What is another source of error in calorimetry?

  Incomplete combustion
• What can affect the value calculated in calorimetry?

  Not conducting the experiment under standard conditions
• What is the enthalpy change of neutralisation?

  The enthalpy change when an aqueous acid neutralises an aqueous base to form 1 mole of water
• How is the enthalpy change of neutralisation calculated?

  Using the same method as calorimetry, with adjustments for solution mass and specific heat capacity