Chapter 9: Enthalpy
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What is the standard enthalpy change of reaction?
Enthalpy change of reaction is 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.
What are the standard conditions for standard enthalpy changes?
Standard pressure = 100kPa Standard temperature = 298K (25 degrees C) Standard concentration = 1moldm3 Standard state - physical state of substance under standard conditions Units is kJmol-1
What is the definition of enthalpy H? Give the formula
Enthalpy is a measure of the heat energy in a chemical system. change in H = H(products) - H(reactants)
What is the law of conservation of energy?
Energy cannot be created or destroyed (can only be transferred)
What is the energy transfer in an exothermic reaction?
Energy transferred from system to surroundings. Enthalpy change is negative Chemical system loses energy Surroundings gain energy Temperature of surroundings increases
What is the energy transfer in an endothermic reaction?
Energy transferred to the system from the surroundings Enthalpy change is positive Chemical system gains energy Surroundings lose energy Temperature of surroundings decreases
What is activation energy?
The minimum energy required to start a reaction by the breaking of bonds
What is enthalpy change of formation?
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.
What is enthalpy change of combustion?
Enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions and standard states.
What is enthalpy change of neutralisation?
Energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions and standard states.
What is the equation to calculate energy change?
q = mcΔT
What is the equation for enthalpy change?
ΔH = q / n
Why may there be a loss of accuracy when trying to find the enthalpy change of combustion?
• heat loss to surroundings • incomplete combustion • evaporation of fuel • non-standard conditions
What is average bond enthalpy?
The energy required to break one mole of a specified type of bond in a gaseous molecule.
Bond enthalpies always have a positive enthalpy value. Why?
Energy is always required to break bonds. Bond enthalpies are always endothermic.
Why is the enthalpy value of an endothermic reaction positive?
Energy needed to break the bonds is GREATER than the energy released when making bonds.
Why is the enthalpy value of an exothermic reaction always negative?
Energy needed to break the bonds is LESS than the energy released when making the bonds.
How do you calculate enthalpy change of reaction from average bond enthalpies?
For a reaction involving gaseous molecules of covalent substances: sum of bond enthalpies in reactants - sum of bond enthalpies in products
Give 2 reasons why a calculated value from bond enthalpies differs from the value of the standard enthalpy of reaction for a particular substance?
Bond enthalpy calculation is based on average bond enthalpies, which will differ from true values for those particular bonds; bonds may be in different environments Bond enthalpy calculation assumes that all reactants and products are in the gaseous state, whereas standard enthalpy change requires standard states.
What does Hess’ law state?
If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.