Energetics Flashcards
(22 cards)
Define enthalpy change
Amount of heat energy taken in or given out during any change in a system providing the pressure is constant
Describe what happens during an exothermic reaction
Energy is transferred from the system to the surroundings
Products have less energy than the reactants
What is the enthalpy change always in an exothermic reaction?
Negative
Give examples of exothermic reactions
Combustion of fuels
Oxidation of carbohydrates such as glucose
Describe what happens during an endothermic reaction
-Energy is transferred from the surroundings to the system
- Requires an input of heat energy
- Products have a higher energy than reactants
Give examples of endothermic reactions
Thermal decomposition of calcium carbonate
What is always the enthalpy change of endothermic reactions?
positive
Define standard enthalpy change of formation
- The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions (298K and 100kpa) with all reactants and products being in their standard states
Triangle FH
Define standard enthalpy change of combustion
The enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions ( 298K and 100kpa) with all reactants and products in their standard states
Triangle CH
Describe the standard conditions used in combustion and formation
100kpa
298K or 25 degrees celcius
Solutions at 1 mol dm-3
Define standard enthalpy change of neutralisation
Enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions
Give the equation for heat energy change of a reaction
q=m X cp X triangleT
q represents the heat energy change of water (J)
m represents mass of each element given(g)
c represents specific heat capacity of water J g-1K-1
Give the general working out used to determine enthalpy change from experimental data
- Use q= m X c X triangleT to work out the heat energy change of reactants in the caliometer
- Calculate moles of the reactant not in excess
- Calculate enthalpy change per mole ( enthalpy change of reaction)
- Divide ans by 1000 to make it KJ per mol to 3sf
- Add the sign e.g exothermic results in a negative sign value (-)
Name errors in the method of calculating enthalpy change from experimental data
- Energy losses from the caliometer
- Incomplete combustion of the fuel
-Incomplete transfer of energy - Evaporation of fuel after weighing
- Heat capacity of caliometer not included
-Measurements not carried out under standard conditions as H2O is a gas not a liquid in this experiment
Why might the published value of enthalpy change differ to the value you work out yourself?
If an unlit spirit burner is left uncapped the fuel will evaporate, making it appear that we burned more fuel than we actually did
- This means our results will be shown to be less exothermic than they actually are
- Alot of the heat energy released by the fuel does not pass into the water, some of it is passed into the metal calimoeter
- Not all of the fuel might have gone under complete combustion which may result in less thermal energy being released
- Experiment may not have been carried out under standard conditions
State Hess law
-Total enthalpy change of a reaction is independent of the route by which the chemical reaction takes place
How can the enthalpy change of combustion pattern of hydrocarbons be explained by mean bond enthaply?
-We have to break the same number of additional bonds, C-C and 2 C-H bonds
- We also have to make the same number of additional bonds, for each ch2 added we make 2 O-H bonds and 2=O bonds
Why might the value using bond enthalpy data be different to the experimental value of the enthalpy change?
Bond enthalpys use mean values, dont take into account the certain molecule the bond is actually in
Define mean bond enthalpy
The energy required to break a covalent bond
Avg values
Give equation of enthalpy change in terms of bond enthalpys
Enthalpy change= Sum of bond energies of reactants- the sum of bond energies of products
Give one reason why the bond enthalpy that is calculated is different to the mean bond enthalpy in a data book
Data book value is derived from a number of different compounds, not just different NH3 molecules
