Chapter 9 Flashcards
What is the definition of enthalpy ?
Enthalpy is defined as the total heat content within a system
How to calculate enthalpy change ?
ΔH = H(products) - H(reactants)
- because energy has to be conserved this change in energy has to be transferred, this takes place between system and your surroundings
What are exothermic reactions ?
- transfer energy to their surroundings, this can be identified be an increase in temperature of the surroundings
- ΔH = negative
What does the enthalpy and reaction profile looks like for an exothermic ?
What is an endothermic reaction?
- Energy transfer from surroundings to the system
- ΔH = positive
What does the Enthalpy and reaction profile look like for endothermic reactions ?
What are the standard conditions ?
- standard pressure: 100 kpa
- standard temperature: 298K
- standard concentration: 1 mol dm-3
- standard state: the physical state of a substance under standard conditions
What is the standard enthalpy changes of a reaction ΔHr ?
enthalpy change that accompanies a reaction in the molar quantities stated in an equation, under standard conditions with all reactants and products in their standard conditions
What is the standard enthalpy changes of formulation ΔHf ?
Enthalpy change when 1 mole of a compound is formed from its elements, under standard conditions with all reactants and products in their standard state
What is the standard enthalpy change of formation of an element ?
- The formation of an element has to be 0 kj mol -1
- because you are forming an element from an element
What is standard enthalpy change of combustion ΔHc ?
Enthalpy change when 1 mole of a substance reacts completely with oxygen, under standard conditions with all reactants and products in their standard state
What is the standard enthalpy change of neutralisation ΔHneut ?
- enthalpy change that accompanies a reaction of an acid and base to form 1 mole of H2O under standard conditions with all reactants and products in their standard state
- always -57 Kj mol-1
How to calculate heat energy in a temperature change ?
- Δq = mc Δt
- where Δq is the change in energy
- m is the mass
- Δt is the change in temperature
How to determine the enthalpy change of combustion ?
- record the volume of water = surroundings (use of density to calculate
- record the mass of fuel before and after heating
- record the temperature change of the water
- use q = mct
- find the moles of fuel burnt
How to determine enthalpy change of combustion ?
- Record the volume of water (use density to calculate)
- record the mass of fuel before and after heating
- record the temperature change of water
- use q = mct (convert to kj)
- find the moles of fuel burnt
What are possible sources of error in the spirit burner ?
- heat transferred to beaker / surrounding air instead of H2O heat loss to surroundings
- soot formed on beaker - incomplete combustion
- draughts (winds) effect flame
- evaporation of fuel - ethanol vaporises very easily - weigh as soon as possible after extinguishing the flame
- non-standard conditions
What is the definition of bond enthalpy and average bond enthalpy ?
- Energy required to break 1 mole of a specified type of bond in a gaseous molecule
- this value depends on the bond environment
- average bond enthalpies are used for bonds that occur in different molecules (different environments)
What are the limitations of average bond enthalpy ?
- using average bond enthalpies rather than the actual energy involved to make and break
- the energy of individual bonds will be slightly different
- average enthalpy is in gaseous state, so if something is liquid you need to take into account the enthalpy change for condensing from gas to liquid
