Chapter 9 - Enthalpy
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What is enthalpy (H)?
The thermal energy stored in a chemical system.
What is enthalpy change (ΔH)?
The heat energy change measured under conditions of constant pressure.
What is an exothermic reaction?
A reaction in which the products have less energy than the reactants: • Chemical energy is changed into thermal energy • Chemicals lose energy • Products have less energy than reactants. • The energy lost by the chemicals is gained by the surroundings, which increase in temperature • ΔH is negative.
What is an endothermic reaction?
A reaction in which the the products have more energy than the reactants: • Thermal energy is changed into chemical energy. • Chemicals gain energy • Products have more energy than reactants. • The energy gained by the chemicals is lost by the surroundings, which decrease in temperature • ΔH is positive.
What are the standard conditions of standard enthalpy changes?
Pressure = 100kPa Temperature = 298K
What are standard states?
States which substances are in under standard conditions.
What is the enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements.
What is the enthalpy change of combustion?
The enthalpy change when one mole of a compound undergoes complete combustion.
What is the enthalpy change of neutralisation?
The enthalpy change involved with the formation of one mole of water from neutralisation.
What is the heat energy change equation?
q = mcΔT
What is activation energy?
The minimum energy required for a reaction to take place.
What is the average bond enthalpy?
The mean amount of energy required to break one mole of a specified type of covalent bond in a gaseous molecule.
When more energy is needed to break bonds than is released when new bonds are formed, the overall energy change is…
…endothermic.
When more energy is released in forming new bonds than is needed to break bonds, the overall energy change is…
…exothermic.
TRUE or FALSE An actual bond enthalpy may differ from the average value.
TRUE
In general, ΔH =
H of products - H of reactants
What is calorimetry?
The quantitative study of chemical energy in a chemical reaction.
What are common errors and assumptions in calorimetry? coffee cup and spirit burner calorimetry
• Heat loss to the surroundings • Incomplete combustion • Conditions not standard • Some alcohol or water may evaporate
What are method for reducing heat loss?
• Adding a lid - Adding insulation around container
Is bond breaking exothermic or endothermic?
Endothermic
Is bond making exothermic or endothermic?
Exothermic
ΔH = in terms of bond enthalpy
ΔH = Σ(bond enthalpy of reactants) - Σ(bond enthalpy of products)
What is Hess’ Law?
The enthalpy change of a reaction is independent of the route taken.
What is calorimetry?
The quantitative study of chemical energy in a chemical reaction.