3.9 enthalpy

what is enthalpy (H)?

• a measure of the heat energy in a chemical system

• chemical system referring to the atoms, molecules or ions making up the chemicals

can enthalpy be measured?

• no

what can be measured if enthalpy can’t?

• enthalpy changes can be measured

how do you calculate enthalpy changes?

• enthalpy change= H(products) - H(reactants)

the law of conservation states that energy cannot be created or destroyed only transferred to its system and its surroundings, what are the system and the surroundings in chemistry?

• the system is the chemicals, reactants and products

• the surroundings is the apparatus, the laboratory, and everything that is not the chemical system

what type of change leads to energy transfer from system to surroundings?

• exothermic change

what type of change leads to energy transfer from surroundings to the system?

• endothermic change

give all the features of an exothermic reaction

give all the features of an endothermic reaction

if an equation is balanced with whole numbers, the amounts are doubled, what happens to the enthalpy change?

• it doubles

what are standard conditions?

• a pressure of 100kPa or 1atm^-1

• a temperature of 298K (same as 25^oc)

• a concentration of 1moldm^-3 (for solutions only)

• all reactants and products are in their standard states, this means the physical state of a substance under standard conditions

what is the definition of the standard enthalpy change of formation?

what is the definition of the standard enthalpy change of combustion?

what is the definition of standard enthalpy change of neutralisation?

what is standard enthalpy change of a reaction?

• enthalpy change that accompanies a reaction under standard conditions when one mole of a substance is reacted