Chapter 9 - Enthalpy Flashcards
What is the law of conservation of energy ?
Energy cannot be created or destroyed, just converted from one form to another.
Define enthalpy
The heat content that is stored in a chemical system.
Define enthalpy change
- The heat exchanged with the surroundings during a chemical reaction.
- The difference between the enthalpy of the products and the reactants.
Define exothermic
A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.
Define endothermic
A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken out of the surroundings.
Define activation energy
The minimum energy required for a reaction to take place.
What are the standard conditions ?
- Standard pressure - 100kPa
- Standard temperature - 298K
- Standard concentration - 1mol/dm^3
- Standard state - The physical state of a substance under standard conditions
Define standard enthalpy change of reaction
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.
Define standard enthalpy change of formation
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.
Define standard enthalpy change of combustion
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define standard enthalpy change of neutralisation
The enthalpy change that accompanies the reaction of an acid by a base to from one mole of H(2)O, under standard conditions, with all reactants and products in their standard states.
What is the equation to measure an energy change ?
- Q=mcΔT
- Q - energy change with surroundings (J)
- m - mass (g)
- c - specific heat capacity (J/g/K)
- ΔT - change in temperature (K)
How is enthalpy change worked out from the energy change?
- ΔH = -Q/n
- ΔH - enthalpy change (J/mol)
- Q - energy change with surroundings (J)
- n - number of moles (mol)
What are the causes for less energy being transferred than expected when working out ΔH(c) ?
- Heat loss to the surroundings
- Incomplete combustion
- Evaporation
- Non-standard conditions
How can heat loss be accounted for using a graph of temperature against time?
Extrapolate the cooling curve back to when it was added.
Enthalpy change of neutralisation always has the same value, what is it?
-57.5 kJ/mol
Define average bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule
How is average bond enthalpy calculated ?
It is calculated from the actual bond enthalpies in different chemical environments
What are some properties of bond enthalpies ?
- Energy is always required to break bonds
- Bond enthalpies are always endothermic
- Bond enthalpies always have a positive enthalpy value
What determines whether an overall reaction is exothermic or endothermic ?
The difference between the energy required for bond breaking and the energy released by bond making
What kind of energy and enthalpy changes are bond formation ?
They are exothermic and they release energy
What is Hess’ law ?
If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.
Which idea does Hess’ law come from ?
It comes from the idea of the conservation of energy
What can be said about the routes using Hess’ law ?
The total enthalpy change is the same for each route
Which direction do the arrows go in the enthalpy change of combustion ?
They go down from the reactants and products to the elements
Which direction do the arrows go in the enthalpy change of formation ?
They go up from the elements to the reactants and products
Enthalpy change of formation routes
R1 : B + A
R2 : C
A + B = C
A = C - B
Enthalpy change of combustion routes
R1 : C + A
R2 : B
A + C = B
A = B - C
