3.2.1. Enthalpy changes Flashcards
Define chemical system
The atoms, molecules or ions making up the chemicals in a reaction.
Define enthalpy
A measure of the heat energy of a chemical system - measured in KJmol-1
Law of conservation
Energy cannot be created or destroyed
The energy change associated with breaking bonds
endothermic
The energy change associated with making bonds
exothermic
Define endothermic reaction
A reaction with a positive enthalpy change. Enthalpy of products > Enthalpy of reactants
Define exothermic reaction
A reaction with an overall negative enthalpy change. Enthalpy of products < Enthalpy of reactants
Draw an enthalpy change diagram for endothermic and exothermic reaction
- Enthalpy on y axis
- Reaction progress on x axis
- Enthalpy change arrow from reactants to products
- Activation energy arrow from reactants to peak of enthalpy
- Exothermic = Enthalpy of products < Enthalpy of reactants
- Endothermic = Enthalpy of products > Enthalpy of reactants
Define activation energy
The minimum energy required for a reaction to take place
What are standard conditions and standard states?
Standard temperature = 298K
Standard pressure = 100kPa
Standard state - the physical state of elements/compounds under standard conditions
Define enthalpy change of formation
The enthalpy change when 1 mol of a compound is formed from its elements in their standard state under standard conditions.
Define enthalpy change of combustion
The enthalpy change when 1 mol of a compound is completely combusted.
Define enthalpy change of neutralisation
The enthalpy change when 1 mol of water is formed from a neutralisation reaction
Define enthalpy change of reaction
The enthalpy change associated with a stated equation.
Equation and method to determine enthalpy change from experimental results.
q = mc∆T
q = joules (J)
m = mass of substance being heated (g)
c = specific heat capacity of substance (given)
∆T= change temperature (K)
Divide by 1000 to convert to Kilojoules.
Divide by number of mols of substance being reacted.
Change value of ∆H depending on whether the reaction is endothermic or exothermic.
What is a calorimeter used for
To collect experimental data to determine enthalpy change.
Draw a simple calorimeter
- Spirit burner containing substance to be tested
- Tripod, gauze, heatproof mat
- Beaker containing known mass of water
- thermometer to measure temperature change of water
Advantages of bomb calorimeter
- Prevents heat loss to surroundings
- Pure oxygen is used to combust substance which ensures complete combustion
Why might experimental methods for enthalpy determination not be accurate?
- Incomplete reaction
- not under standard conditions
- heat loss to surroundings
Define average bond enthalpy
The mean energy required to break 1 mol of bonds in gaseous molecules
Why is the use of average bond enthalpy not as accurate as standard enthalpy changes
Bond enthalpies are a mean for the same bond across different molecules.
Standard enthalpies apply to a specific molecule.
How to calculate enthalpy change of reaction using average bond enthalpies?
ΔH = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)
What is Hess’s Law
The overall enthalpy change for a reaction is the same, regardless of the route taken.
How are Hess’s law diagrams used to determine enthalpy change of reaction.
A triangular cycle method is used with an intermediate product. The values can be added or taken away like vectors.
