ENERGETICS Flashcards
Define enthalpy change
The amount of heat energy taken in it given out during any change in a system provided the pressure is constant
Outline an exothermic reaction
Energy is transferred from system to surroundings
Products have less energy than reactants
Enthalpy change is negative
What are common exothermic processes
Combustion of fuels
Oxidation of carbohydrates
Eg glucose in respiration
Outline an endothermic reaction
Energy is transferred from surroundings to the system
Products have more energy than the reactants
Enthalpy change is positive
Define standard enthalpy change of formation
The enthalpy change when 1 mole of the compound
is formed from its elements under standard conditions
all reactants and products being in their standard states
What is the enthalpy of formation for an element?
0 kJ mol^-1
Define standard enthalpy change of combustion
The enthalpy change when 1 mole of a substance
Is combusted completely in oxygen under standard conditions
All reactants and products being in their standard states
How does incomplete combustion affect a reaction?
Soot (carbon)
Carbon monoxide
Water
Less exothermic than complete combustion
What are the standard conditions?
100 kPa pressure
298 K (25C)
Solutions at 1mol dm^-3
Standard states
What equation is used when calculating enthalpy change for a reaction in a solution?
Q = m x c x 🔼T
Energy change = mass of solution x heat capacity x temperature change
Outline the general calorimetric method
- wash the equipment with the solutions and dry
- place cup in beaker for insulation and support and clamp thermometer into place making sure bulb is immersed in liquid
- measure initial temperatures
- transfer reagents to the cup
- stir mixture
- measure final temperature
What are the issues of calorimetry if the reaction is slow?
exact temperature rise is difficult to measure (cooling simultaneously)
How can issues with slow reactions be countered in calorimetry
Take readings at regular time intervals and extrapolate back to the time the reactants were added together
Name 3 errors that occur in calorimetry
- Heat transfers to surroundings
- Approximation in specific heat capacity of solution (assumes all solutions have heat capacity of water)
- Neglects specific heat capacity of calorimeter
- Reaction or dissolving may be incomplete or slow
- Density of solution is taken to be the same as water
Outline the general method for calculating enthalpy change from experimental data
- Q = MC🔼T
- Work out miles of reactants used
- Divide q by number of miles of the reactants bit in excess
- Add sign and unit (/1000 for Jmol-1 to kJmol-1)
Name three errors that occur when calculating enthalpy of combustion from experimental data
- heat losses from calorimeter
- incomplete combustion of fuel
- incomplete transfer of heat
- evaporation of fuel after weighing
- heat capacity of calorimeter not included
- measurements not carried out under standard conditions
Outline hess’s law
The total enthalpy change for a reaction is independent of the route by which the chemical change takes place
Outline the equation for calculating enthalpy change
🔼H = reactants - products
Define mean bond energy
The enthalpy needed to break the covalent bond into gaseous atoms averaged over different molecules
Is the value of mean bond energy negative or positive? Why?
Positive
Energy is required to break a bond
When does the mean bond enthalpy definition no longer apply?
When either the products or/and reactants aren’t in a gaseous state
How does mean bond energy differ between exothermic and endothermic reactions
The sum of bonds in reactants is less than that of products in an exothermic reaction
Why aren’t bond energy calculations exact
The mean bond energies are taken from different molecules and averaged
So the mean bond energy is not exact
What is the bond enthalpy calculation
🔼H = bond energies broken - bond energies made
Explain the change in enthalpy of combustion in a homologous series
Constant rise in size of enthalpy
Number of carbon atoms increase
So there are more bonds broken and more bonds made
How will experimental enthalpy of combustion results differ from calculated results in a homologous series?
Experimental will be much lower
Significant heat loss Incomplete combustion (less energy released)
