3.1.4 Energetics Flashcards
What is enthalpy?
It is a measure of the heat content of a substance.
What is enthalpy change (ΔH)?
Change in heat content at constant pressure.
What are standard conditions of an enthalpy change (ΔH⦵)?
100kPa and a stated temperature (usually 298K)
Draw the reaction profile for an exothermic & endothermic reaction.
Exothermic - enthalpy change is negative.
In an exothermic change energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants. Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration.
Endothermic - enthalpy change is positive
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.
What characterises an exothermic & endothermic reaction?
Exothermic - more energy released making bonds than is needed to break bonds. Negative enthalpy change.
Endothermic - more energy needed to break bonds than is released making bonds. Positive enthalpy change.
What is standard enthalpy change of formation (ΔfH⦵)?
Enthalpy change when 1 mole of a substance is formed from its elements with all reactants & products in standard states under standard conditions.
What is standard enthalpy change of combustion (ΔcH⦵)?
Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants & products in standard states under standard conditions.
What is standard enthalpy change of neutralisation (ΔneutH⦵)?
Enthalpy change when 1 mole of water is formed in a reaction between an acid & alkali under standard conditions.
What is calorimetry?
An experiment used to find out how much heat is given out by a reaction. Used to calculate enthalpy changes.
Calorimetry experiment for the combustion of a flammable liquid:
You burn the flammable liquid in a calorimeter so it heats the water. You can work out the heat energy that has been absorbed by the water.
Calorimetry experiment for a neutralisation reaction in a solution:
Add a known volume of acid to an insulated container and add a known volume/mass of alkali (stir to ensure it is evenly heated). You can work out the heat energy released by reaction.
(You assume the solution is the same density as water).
To measure the temperature change:
- Record initial temp a few minutes before reaction starts
- Recording the temperature of solution/water at regular intervals over a period of time
(You can’t just subtract initial temp from the maximum temp recorded as heat is lost to surroundings)
- Plot a graph of temp against time
- Extrapolate to the point of addition (/where reaction begins)
- Determine ΔT at this point
What equation is used to find out how much energy is given out/absorbed in a reaction?
q = mc∆T
What equation is used to find out the enthalpy change of a reaction?
∆H = q/n
What are common errors in calorimetry?
- Heat lost to surroundings
- Possible incomplete combustion
- Loss of some reactant as it evaporates before it combusts
What is Hess’ Law?
The enthalpy change for a reaction is independent of the route taken.
What are mean bond energies?
The mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules.
We use values of mean bond energies because every single bond in a compound has a slightly different bond energy. E.g. In CH4 there are 4 C-H bonds. Breaking each one will require a different amount of energy. However, we use an average value for the C-H bond for all hydrocarbons.
Why are enthalpy changes for reactions, calculated using mean bond enthalpies, not exact?
Because the values used are averages, not exact, and not the specific ones for that compound.
Why may a mean bond enthalpy in a data book be different from the mean bond enthalpy in a specific molecule?
The mean bond enthalpy in the data book is the average from a range of different compounds not just the specific molecule.
e.g. if you calculated the bond enthalpy for N-H in ammonia - the value in the data book comes from a number of different compounds with N-H bonds not just different NH3 molecules.
What is the trend in the enthalpies of combustion in a homologous series?
When comparing the enthalpies of combustion for successive members of a homologous series such as alkanes or alcohols there is a constant rise in the size of the enthalpies of combustion as the number of carbon atoms increases.
As one goes up the homologous series there is a constant amount and type of extra bonds being broken and made so the enthalpy of combustion increases by a constant amount.
If the results are worked out experimentally using a calorimeter the experimental results will be much lower than the calculated ones because there will be significant heat loss. There will also be incomplete combustion which will lead to less energy being released.
Remember that calculated values of enthalpy of combustions will be more accurate if calculated from enthalpy of formation data than if calculated from average bond enthalpies. This is because average bond enthalpy values are averaged values of the bond enthalpies from various compounds.
