Chapter 9 Enthalpy Flashcards
Enthalpy
A measure of the heat energy in a chemical system
Chemical system
Refers to the atoms, molecules or ions making up the chemicals
Enthalpy change
The difference in the enthalpies of the reactants and products
What happens when a chemical reaction involving an enthalpy change takes place?
heat energy is transferred between the system and the surroundings
What makes up a system?
the chemicals - the reactants and products
What makes up the surroundings
The apparatus, laboratory, and everything that is not in the chemical system
What makes up the universe
Both the system and surroundings
Directions of energy transfer
- Exothermic change
- Endothermic change
Exothermic change
Energy is transferred to the surroundings
Endothermic change
Energy is transferred to the system
Enthalpy profile diagrams
Shows the:
* relative enthalpies of the reactants and products
* the enthalpy change
Exothermic reaction
- The chemical system releases heat energy to the surroundings
- Any energy loss by the chemical system is balanced by the same energy gain by the surroundings (the temperature of the surroundings increases as they gain energy)
- Delta H = negative
Endothermic reaction
- The chemical system takes in heat energy from the surroundings
- Any energy gain by the chemical system is balances by the same energy loss by the surroundings (the temperature of the surroundings decreases as they lose energy)
Activation energy
- The energy input required to break bonds in the reactants acts as an energy barrier to the reaction
- The minimum energy required for a reaction to take place
Standard conditions (⦵)
- Standard pressure = 101 kPa
- Standard temperature = 298K
- Standard concentration = 1 moldm-3
- Standard state = physical state of a substance under standard conditions
Standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
Standard enthalpy change of formation
The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
Standard enthalpy change of formation for elements
0
Standard enthalpy change of combustion
The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard conditions
Standard enthalpy change of neutralisation
The energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O, under standard conditions with all reactants and products in their standard states
Reaction of neutralisation
Involves the reaction of H+ with OH- to form 1 mole of H2O
Symbol for heat energy
q
Equation for heat energy
q = mc△T
- m = mass of surroundings
c = specific heat capacity
Specific heat capacity, c
The energy required to raise the temperature of 1g of a substance of 1K
Why isn’t enthalpy of combustion calculated with spirit burners accurate?
- Heat loss to the surroundings, other than water (e.g. beaker, air)
- Incomplete combustion of methanol (carbon monoxide and carbon produced on the beaker)
- Evaporation of methanol from wick
- Non-standard conditions
Experimentally determining the enthalpy of combustion
- Put a known volume of water in a beaker and record initial temp. of water
- Weigh a spirit burner containing methanol
- Light the spirit burner and use it to heat the water and use a thermometer to stir the water in the beaker
- After 3 minutes extinguish the flame by putting the lid on the wick
- Re-weigh the spirit burner at the end
Experimentally determining the enthalpy change of reaction
Mix the solid + solution or solution + solution in a polystyrene cups (cheap and offer insulation)
Cooling curves
x-axis: time
y-axis: temperature
When mixing solutions in the polystrene cup, record the temperature of the solution every 30 seconds
A cooling curve can be used to find the highest temperature that the solution reached, in order to find the temp. difference
Extrapolate the cooling curve section of the graph back to the time when zinc was added
Average bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule
Limitations of average bond enthalpies
It is calculated from the actual bond enthalpies in different environments -> not under standard conditions
Bond breaking
Energy is required to break bonds so is endothermic
Bond making
Energy is released when bonds are made so is exothermic
Calculating enthalpy changes from average bond enthalpies
- Bond breaking - bond making
- Bond enthalpies in reactants - bond enthalpies in products
Why is Hess’ law useful?
The enthalpy changes of many reactions are very difficult to determine directly
Hess’ Law
If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
