Enthalpy change is the heat energy measured under conditions of constant pressure
The enthalpy change of formation of a compound is the enthalpy change when one mole of a compound is formed from its constituent elements with all products and reactants in their standard states and under standard conditions
Enthalpy change of combustion is the enthalpy change when one mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states and under standard conditions
Enthalpy change of reaction is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction
enthalpy of neutralisation is the enthalpy change when one mole of water is formed from its ions in dilute solution. When an acid neutralises an alkali
Bond enthalpy is the energy required to break one mole of a covalent bond measured in the gaseous state
Mean bond enthalpy is the energy required to break one mole of a covalent bond measured in the gaseous state averaged across many compounds containing the bond
Hess's law is that the enthalpy change of a reaction is independent of the route taken
In an exothermic reaction, the energy is givenout to the surroundings. So the surrounding increase in temperature. The reactants have more energy than the products. Therefore the ΔH is negative.
In an endothermic reaction, the energy is takenin to the surroundings. So the surrounding decrease in temperature. The products have more energy than the reactants. Therefore the ΔH is positive.
Exothermic reactions include:
Combustion of fuels
Respiration
detonation of explosives
Reaction of acids with metals
Neutralisation reactions
Condensing
Freezing
Bond forming exothermic
Bond breaking is endothermic
Endothermic reactions include:
Photosynthesis
Thermal decomposition of calciumcarbonate
Dissolving salts in water
Boiling
Activation energy is the energy required to start a reactionbythe breaking of bonds
Standard conditions are:
101 Kpa (pressure)
298K (temp)
1 mol dm-3 (conc for solutions)
The more negative the standardenthalpyofformation, the more stable the compound
Elements in their standard states have a 0 enthalpy of formation
Only one mole of a substance is burnt on the left side of the equation
The thermal chemical equation for the enthalpy of neutralisation is always:
H+ + OH- -> H2O
How to calculate heat energy change:
q=mcΔT
How to calculate enthalpy change:
q/moles
If the temperature rises, the ΔH is negative
If the temperature falls, the ΔH is positive
The calculated enthalpy change may be different to the data book because of:
Heat loss to environment
Incomplete combustion
Non- standard conditions
Evaporationofalcohol/water
Heat capacityofcalorimeter/ thermometernot take into account
How to improve calorimeter:
Use a bombcalorimeter, which reducesheatloss as the water is insulated from the surrounding. It is also burnt in 100%oxygen to ensure completecombustion
To determine an exact temperature which is not influenced by heat from the surroundings:
record temperature of the solution every minute for 4 minutes
On the 5th minute, add the reactant
Record the temperature every subsequent minute for another 5 to 1 minutes