Enthalpy Flashcards
What is enthalpy ?
the amount of heat energy stored in a chemical system
What is a system ?
the atoms, molecules or ions in the chemicals involved in the reaction
What is activation energy ?
the minimum energy required to start a reaction by breaking bonds
What is an exothermic reaction ?
more energy released when bonds are made than is required to break bonds
What is an endothermic reaction ?
more energy is required to break bonds than is released when making them
What are the standard conditions ?
- 298 K (25 degrees C)
- 100 kPa
What is meant by standard state ?
the physical state of a substance under standard conditions
What is enthalpy change by formation ?
the enthalpy change when one mole of a compound is formed from its elements
What is the enthalpy change of formation of an element ?
0
What is the equation for energy change ?
energy change (J) = mass x specific heat capacity x change in temperature
q = m c delta T
What’s enthalpy change of combustion ?
enthalpy change when 1 mole of a substance reacts completely with oxygen
What is the equation for enthalpy change ?
enthalpy change = -(energy change) / moles
delta H = -q / n
What errors may affect the accuracy of a result ?
- heat loss to surroundings
- non-standard conditions
- incomplete reaction
How do you improve the accuracy of a result ?
- insulate the reaction
- burn in plentiful oxygen
- use standard conditions
What is the enthalpy change for neutralisation ?
enthalpy change of the reaction of an acid with a base to produce 1 mol of water
What is enthalpy change of reaction ?
enthalpy change when molar quantities of reactants as stated in the equation
What is average bond enthalpy ?
the energy required to break 1 mol of bonds in a gaseous molecule
What is the equation for enthalpy change from bond enthalpies ?
delta H = sum of bonds broken - sum of bonds made
What are the limitations od average bond enthalpies ?
- bonds may have slightly different enthalpies
- calculations need all species to be gaseous, so reaction may have not been under standard conditions
- it is an average
Why can not all enthalpy changes be calculated directly ?
- activation energy too high
- rate of reaction too slow
- other products may be formed
