Introducing enthalpy and enthalpy change Flashcards
define exothermic
when heat energy is transferred from the system to the surroundings. an increase in temperature is recorded outside while the actual system looses heat.
define endothermic
when heat energy is transferred from the surroundings to the system. a decrease in temperature is recorded as the system gains heat energy.
what is the first law of thermodynamics?
during a chemical reaction, energy cannot be created or destroyed, but different forms of energy can be converted
what are the components of chemical energy?
kinetic energy- the measure of motion of the particles in a substance
potential energy- the measure of how strongly particles interact with each other (i.e. both attract or repel one another)
what do we mean by heat energy?
heat energy is the portion of of the potential and the kinetic energy of a substance which is responsible for the temperature of that substance
what is the heat energy of a substance directly proportional to?
the absolute temperature measured in Kelvin
explain the term enthalpy and why we use enthalpy change instead
enthalpy is the measure of the total energy of a system. you cannot determine the enthalpy of a system, but can measure the enthalpy change instead, that takes place during a physical or chemical change. (ΔH)
what is the enthalpy change of a process?
the heat energy that is transferred between the system and the surroundings at constant pressure
what equation shows the relationship between enthalpy and internal enthalpy?
H = U + PV
where H is enthalpy, U is internal enthalpy, P is pressure and V is volume
what is internal enthalpy?
the amount of energy required to create the system, make room for the enthalpy of a substance by displacing its environment and establishing its volume and pressure. symbol U
what are the conditions for standard enthalpy?
100kPa pressure and 298K as temperature. IT IS NOT 101kPa !!!
What is the symbol for standard enthalpy change?
ΔH° (but usually the ° has a horizontal line through it)
explain the symbol used for standard enthalpy change of reaction- what are the units?
ΔH°r when the Δ shows the change of H which is the enthalpy, so ΔH shows enthalpy change. the ° means standard, while the r means of a reaction. The units are kJ mol-1 in terms of one mole of the equation, so you must look at the ratios in the equation to understand this value.
what is the standard enthalpy change of reaction?
the enthalpy change measured at 100kPa and a stated temperature, usually 298K, when the number of moles of substances in the equation AS WRITTEN react
