Enthalpy Flashcards
What is enthalpy?
Enthalpy, H, is the thermal energy stored in a system
What is the standard enthalpy of formation?
The enthalpy change when one mole of substance is formed from its constituent elements, all elements being in their standard states under standard conditions
What is the standard enthalpy of combustion?
The enthalpy change when one mole of a substance is completely burnt in oxygen, with all reactants and products being in their standard states, under standard conditions
What is the equation for enthalpy change?
Enthalpy change,∆H =∑H Products -∑H Reactants
What are the standard conditions?
100kPa, 298K and standard state of substance
What are exothermic reactions?
increase temperature of surroundings so delta H is negative
What are endothermic reactions?
decrease the temperature of the surroundings so delta H is positive
What is activation energy?
minimum energy required for a reaction to take place
What is calorimetry?
Calorimetry is the process of measuring the amount of heat given off in a reaction
Why may experimental values of enthalpy differ from data book?
- reaction may be incomplete
- incomplete combustion of fuel
- heat may be lost from water to surroundings
- no standard conditions
- some of fuel/water may evaporate before reweighing
What is the equation for heat change?
q = mc∆T
-q is the heat change (J)m is the mass of the substance (g)c is the specific heat capacity (Jg^-1 K^-1)∆Tis the temperature change (K or °C)
Describe required practical 2 - investigating the enthalpy of combustion of fuels
- Water placed in metal can with a thermometer
- Spirit burner weighed with fuel + record initial mass
- Starting temperature of water recorded, allowed fuel to heat water to 40 C
- Reweighed spirit burner + record end mass
- Calculated the mass of fuel burnt (initial mass - end mass)
What is Hess’ law?
enthalpy change of a reaction is independent of the route taken
What direction do the arrows face for a Hess’ law cycles for enthalpy of combustion?
Downwards
What direction do the arrows face for a Hess’ law cycles for enthalpy of formation?
Upwards
