Enthalpy change + definitions Flashcards
What is the simple definition of enthalpy change?
The enthalpy change (ΔH) is the energy/heat change at constant pressure.
Describe an exothermic reaction
In an exothermic reaction, energy is released and ΔH is negative. The products have lower energy than the reactants.
Describe an endothermic reaction
In an endothermic reaction, energy is absorbed from surroundings and ΔH is positive. The products have higher energy than the reactants.
Define the term activation energy.
The activation energy is the minimum energy required for a reaction to take place. Usually, only a small proportion of the particles will have sufficient energy to react.
Draw an exothermic energy profile diagram
Draw an endothermic energy profile diagram
Define enthalpy change of a reaction
The enthaply change of a reaction, ΔrH, is the heat energy change at constant pressure when the reaction is carried out with the amounts of reactant (in mol) given in the equation.
What are standard conditions?
Pressure - 100kPa
Concentration of solutions - 1mol dm-3
Temperature - 298K
Indicated by symbol ⦵
Define standard enthalpy change of reaction
The standard enthalpy change of reaction, ΔrH⦵, is the enthalpy change of reactionunder standard conditions** with all substances in their standard states.
What equation do you use to calculate enthalpy change from experimental data?
q = mcΔT
q = heat change (in J)
m = mass of water/solution (1cm3 = 1g)
c = specific heat capacity
T = temperature change
What do you have to do once you have calculated q to find enthalpy change?
Enthalpy change is measured in kJ mol-1.
So we need to convert to kJ, then find the amount of mols of substance we are measuring.
Define enthalpy change of neutralisation.
The enthalpy change of neutralisation is the enthalpy change when one mole of water is formed in a neutralisation reaction.
How can you measure enthalpy change of neutralisation experimentally?
1) Temperature taken after each addition of acid to base.
2) The mixture is stirred after each addition of acid.
3) Acid is added in portions to base.
Why is the enthalpy change of neutralisation always the same when adding a strong acid to base?
The ionic equation for neutralisation is H+ + OH- -> H2O.
For strong acids and alkalis, the other ions are spectator ions so do not take part in the reaction. This means that enthalpy change of neutralisation for a strong acid + base is always -55.8kJmol-1.
