3.5 - Enthalpy changes Flashcards
Define enthalpy
measure of heat energy in a chemical system
Define chemical system
atoms, molecules or ions that make up chemicals
Define exothermic
system releases heat energy to surroundings
Define endothermic
system takes in heat energy from surroundings
Define activation energy
minimum energy required for a reaction to take place
What are the standard units?
pressure = 100kPa
temperature = 298K
concentration = 1moldm-3
Define standard state
physical state of a substance under standard conditions
Define average bond enthalpy
enthalpy change when 1 mol of gaseous covalent bonds are broken
Define enthalpy change of combustion
enthalpy change for complete combustion of 1 mol of a substance
Define enthalpy change of neutralisation
enthalpy change for the formation of 1 mol of H2O from neutralisation
Define enthalpy change of formation
enthalpy change when one 1 mol of a compound is formed from its elements
What is the enthalpy change of formation for all elements?
0 kJmol-1
Describe a method to determine enthalpy change of combustion experimentally
- measure vol. of H2O, pour into beaker, record initial temp.
- add fuel to spirit burner and weigh
- place spirit burner under beaker, light it and stir H2O w/ thermometer
- extinguish flame after 3 mins and immediately record H2O temp.
- re-weigh spirit burner
Why might a measured experimental enthalpy change of combustion not be accurate?
- incomplete combustion
- non-standard conditions
- heat released to surroundings
Describe a method of how to determine enthalpy change of reaction experimentally
- polystyrene cup with thermometer clamped
- add 1 reactant into cup and take temp. every 30s, ensuring it remains constant
- add other reactant and stir, measure temp. every 30s until it has fallen for several mins
- plot graph of temp. against time
- extrapolate cooling curve section back to when 2nd reactant added
How could the experiment for enthalpy change of reaction be adjusted for neutralisation?
- use total volume of both reactants
- ensure reactant moles not in excess
Equation for enthalpy change and note to remember
q = mc ∆ t
q = enthalpy change (J)
m = mass of water/solution (g)
c = 4.18 (specific heat capacity)
∆ t = change in temperature (K)
remember: if calculating reaction, multiply final number by coefficient in equation e.g 2AgNo3 is x2
Enthalpy equations that use reactants - products
- average bond enthalpy
- enthalpy change of combustion
- enthalpy change of reaction
Equation that uses products - reactants
enthalpy change of formation
Why may it be difficult to determine enthalpy change of formation directly?
- many hydrocarbons formed
- Ea too high
- reaction too slow
