Energetics Flashcards
What is enthalpy change?
Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.
If an enthalpy change occurs then energy is transferred between system and surroundings. The system is the chemicals and the surroundings is
everything outside the chemicals.
What happens in an exothermic change?
In an exothermic change energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants.
In an exothermic reaction the ∆H is negative
Common oxidation exothermic processes are the combustion of fuels and the oxidation of
carbohydrates such as glucose in respiration.
What happens in an endothermic change?
In an endothermic change, energy is transferred from the surroundings to the system (chemicals). They require an input of heat energy e.g. thermal
decomposition of calcium carbonate.
The products have more energy than the reactants.
In an endothermic reaction
the ∆H is positive
Standard enthalpy change of formation
The standard enthalpy change of formation of a compound is the
enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states.
Symbol fH
Standard Enthalpy Change of Combustion
The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their
standard states.
Symbol cH
Incomplete combustion will lead to
soot (carbon), carbon monoxide and
water. It will be less exothermic than
complete combustio
What are the standard conditions
Enthalpy changes are normally quoted at standard conditions.
Standard conditions are:
• 100 kPa pressure
• 298 K (room temperature or 25oC)
• Solutions at 1mol dm-3
• all substances should have their normal state at 298K
What is the equation for energy change?
For a reaction in solution we use the following equation
energy change = mass of solution x heat capacity x temperature change
What is the calorimetric general equation
- washes the equipment (cup and pipettes etc) with the solutions to be used
- dry the cup after washing
- put polystyrene cup in a beaker for insulation and support
- Measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup - clamp thermometer into place making sure the thermometer bulb is immersed in solution
- measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes - At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance. - If using a solid reagent then use ‘before and after’ weighing method
- stirs mixture (ensures that all of the solution is at the same temperature)
Record temperature every minute after addition for several minutes
What are the errors in the calorimetric method
- energy transfer from surroundings (usually loss)
• approximation in specific heat capacity of solution. The method assumes all
solutions have the heat capacity of water.
• neglecting the specific heat capacity of the calorimeter- we ignore any
energy absorbed by the apparatus.
• reaction or dissolving may be incomplete or slow.
• density of solution is taken to be the same as water.
What is the calorimetric method
One type of experiment is one in which substances are mixed in an insulated container and the temperature rise
measured. This could be a solid dissolving or reacting in a solution or it could be two solutions reacting together
What is the heat capacity of water
The heat capacity of water is
4.18 J g-1K-1. In any reaction
where the reactants are dissolved in water we assume that the heat capacity is the same as pure water
General method for working out enthalpy change
General method
1. 2. Using q = m x cp x ∆T calculate energy change for quantities used
Work out the moles of the reactants used
- Divide q by the number of moles of the reactant not in excess to give ∆H
- Add a sign and unit (divide by a thousand to convert Jmol-1 to kJm
What does hess law state?
Hess’s law states that total enthalpy change for a reaction is
independent of the route by which the chemical change takes place
Enthalpy changes of combustion hess law
Reactants —> Products
Elements in standard states (pointing up to reactants and products)
Enery change is enthalpy formation of produvcts - enthalpy formation of reactants
Enthalpy changes of combustion hess law
Reactants —> products
(Pointing down to)
Combustion products
Reaction = Σ cH reactants - Σ cH products
What is the mean bond energy
The mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over
different molecules.
These values are positive because
energy is required to break a bond.
The definition only applies when the
substances start and end in the
gaseous state.
Mean bond energys hess law
In general (if all substances are gases)
∆ H = Σ bond energies broken - Σ bond energies made
Reactants —> products
Arrows pointing down to
Gaseous atoms of elements.
