3.1.4 Energetics Flashcards
What is enthalpy change
Heat energy change measured under conditions of constant pressure
Describe exothermic reactions
Exothermic reactions give out heat energy to the surroundings so the temperature of the surroundings increases
Products have less energy than the reactants so enthalpy change is negative
Describe endothermic reactions
Endothermic reactions are reactions which take in heat energy from the surroundings so the temperature of the surroundings decreases
The products have more energy than the reactants so the enthalpy change is positive
Enthalpy of formation
This is the standard enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions
Enthalpy of combustion
This is the standard enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions with all reactants and products in their standard states
Calculating enthalpy change
The data given determines the method
When data is enthalpy of formation:
Enthalpy change = total enthalpy of formation of products - total enthalpy of formation of reactants
When data is enthalpy of combustion:
Enthalpy change = total enthalpy of combustion of reactants - total enthalpy of combustion of products
Purpose of calorimetry experiments
To measure enthalpy changes for reactions
Calorimetry for reactions in solution
A polystyrene cup is used to minimise heat loss and a thermometer is used to measure temperature change
Possible problems with calorimetry for reactions in solution
-some heat will be absorbed by the container rather than heating up the water
-some heat is always lost to the surroundings
Calorimetry and combustion reactions
To determine the enthalpy of combustion of a flammable liquid it is burnt inside a calorimeter
As the fuel burns it heats up the water
If the mass of water, the temp change and specific heat capacity of the water is known then the enthalpy of combustion can be calculated
Possible problems with calorimetry and combustion reactions
Combustion may be incomplete, so less energy is given out
Some flammable liquid may evaporate
Heat losses to the surroundings
What does a flame calorimeter do
Can be used to reduce heat losses as it has a spiral chimney made of copper, so the flame is enclosed and the fuel is burned in pure oxygen rather than air
Q = mct
Q = heat change in J, m = mass in g, c = shc, change in temp
Hess’ Law
The total enthalpy change for a reaction is independent of the route taken (provided the conditions remain the same)
What do reactions involve
Breaking and forming bonds
