TOPIC 5 SL - ENERGETICS Flashcards
Define enthalpy change
The heat change measured under conditions of constant pressure (in an open vessel)
Define standard enthalpy change
Standard enthalpy changes occur at the standard pressure of 100KPa (1bar) and a stated temperature usually 298K [ 25oC – always quoted in K].
Define standard enthalpy of combustion
The enthalpy change when ONE MOLE of a substance (reactant) undergoes complete combustion under standard conditions. Forms H2O and CO2.
Define standard enthalpy of formation
The enthalpy change when ONE MOLE of a compound is formed in its standard state from its elements in their standard states.
Define average bond enthalpy
The amount of energy needed to break 1 mole of (covalent) bonds; in the gaseous state;
How to find bond enthalpy of a reaction
Bonds broken - bonds made
What happens to heat in an exothermic reaction?
Heat rises and vice versa
Is bond breaking endothermic or exothermic?
Bond breaking is endothermic and bond making is exothermic
What values are associated with endothermic and exothermic reactions
Endothermic is positive and exothermic is negative, (endothermic absorbs heat so positive, exothermic loses heat so negative)
What is specific heat capacity
The property of a substance which gives the heat needed to raise the temperature of unit mass by 1K. It is represented by c.
What does q stand for
Heat change
What does change in temperature depend on
Mass of object, heat added and nature of substance
What is specific heat capacity dependant on
number of particles present in unit mass sample, which depends on the mass of individual particles
q=mct
allows for heat change to be calculated from temperature change
How to find enthalpy change per mole in Joules
mct/moles reacting, divided by 1000
Why might experimental enthalpy of combustion be different from literature value
Not all the heat is absorbed by water, some is lost to calorimeter or the surrounding environment.
Define Hess’s law
Hess’s law states that the enthalpy change for any chemical reaction is independent of the route provided the starting and final conditions, and reactants and products, are the same.
What is energy?
The ability to do work
What is heat as a form of energy?
Mode of energy transfer which occurs as a result of temperature difference and produces an increase in disorder of how particles behave.
What does heat do?
Increases the kinetic energy of particles in a disordered fashion
What is enthaply
Measure of the amount of heat energy contained in a substance. It is stored in chemical bonds and intermolecular forces as POTENTIAL energy. When substances react, the difference in enthalpy between reactants and products results in heat change which can be observed.
What is temperature?
A measure of average kinetic energy. When temperature decreases, so does KE. The Kelvin scale emphasizes this relationship. Absolute 0 is lowest possible value, at which all kinetic energy stops.
