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.
How can enthalpy change of formation be used to determine overall change of reaction with Hess’s law
The products and reactants are made from the same elements.
Arrows point away from the central elements towards products and reactants.
The enthalpy changes of formation can be added/taken away dependent on direction.
How can enthalpy change of combustion be used to determine overall change of reaction with Hess’s law.
The products and reactants combust into the same products.
Arrows point towards from the central products from products and reactants.
The enthalpy changes of formation can be added/taken away dependent on direction.
