Chapter 9
No description yet. This curated deck is ready for daily study.
Card preview (22)
What is enthalpy (H)?
• A measure of the heat energy in a chemical system
How is enthalpy change calculated?
• Bond enthalpies of reactants - bond enthalpies of products
What is activation energy?
• The minimum amount of energy required for a reaction to take place
How do we measure enthalpy change?
• Under standard conditions
What are the standard conditions?
• Pressure: 100 kPa • Temperature: 298 K • Standard states (physical states under these conditions)
What is standard enthalpy change, and its symbol?
• The enthalpy change of a reaction when measured under standard conditions • ΔH^⦵
What is standard enthalpy change of reaction, and its symbol?
• The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states • ΔrH⦵
How can the standard enthalpy change of reaction differ?
• If you have the same chemical equation, but with each chemical shown to have twice the molar quantity, the standard enthalpy change of reaction also doubles, etc.
What is the standard enthalpy change of formation, and its symbol? Required.
• The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states • ΔfH⦵
What is the standard enthalpy change of combustion, and its symbol? Required.
• The enthalpy change that takes place when one mole of a substance is reacted completely with oxygen under standard conditions, with all reactants and products in their standard states • ΔcH⦵
What is the standard enthalpy change of neutralisation, and its symbol? Required.
• The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions, with all reactants and products in their standard states • ΔneutH⦵
What is the formula to calculate heat energy, and what do each of the symbols stand for? Include units.
• q = mcΔT • q: heat energy (J) • m: the mass that changes temperature (g) • c: the specific heat capacity of the substance that changes temperature • ΔT: initial temperature - final temperature
When using fuel to heat water in a beaker, what are 3 reasons why the energy change calculated is less exothermic than expected?
• Heat loss to the surroundings • Incomplete combustion may take place • The water in the beaker may evaporate
What is average bond enthalpy? Required.
• When 1 mole of bonds of gaseous covalent bonds is broken • (A measure of the strength of a covalent bond)
Write a symbol equation showing the bond enthalpy of Cl2.
• Cl2 (g) -> 2Cl (g)
What are 2 limitations of average bond enthalpies?
• The actual bond enthalpy varies according to the chemical environment of the bond (which other bonds the atoms are involved in) • Average bond enthalpy is calculated using the actual bond enthalpies from different chemical environments
Which processes are endothermic and exothermic?
• Bond breaking is endothermic • Bond making is exothermic
Describe an exothermic enthalpy profile diagram.
• Enthalpy against progress of reaction • Products lower than reactants • Negative enthalpy change
Describe an endothermic enthalpy profile diagram.
• Enthalpy against progress of reaction • Products higher than reactants • Positive enthalpy change
What is Hess’ law?
• The overall enthalpy change of a reaction is independent of the route it takes
How is the enthalpy change of combustion used to indirectly calculate the enthalpy change of a reaction?
• The products from the combustion of the reactants/ products are written at the bottom • Arrows go down • The enthalpy change of combustion of the reactants and the products are used to calculate the enthalpy change
How is the enthalpy change of formation used to indirectly calculate the enthalpy change of a reaction?
• The elements needed to form the reactants/ products are written at the bottom • Arrows go up • The enthalpy change of formation for the reactants and the products are used to calculate the enthalpy change