Enthalpy Flashcards
Define enthalpy change
It is the heat energy transferred in a reaction at a constant pressure.
Describe the different parts of an enthalpy profile diagram.
Distance from reactant to peak of the curve is activation energy.
Distance from reactants to products is enthalpy change; it can be positive or negative depending on if the reaction is endothermic or exothermic.
What are the standard conditions of a reaction?
25 degrees Celsius (298K) and 101kPa (1atm)
Define standard enthalpy change of reaction.
The enthalpy change when a reaction occurs in its molar quantities in the chemical equation under standard conditions.
Define standard enthalpy change of formation.
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.
Define the standard enthalpy change of combustion.
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.
Define standard enthalpy change of neutralisation.
The enthalpy change when an acid and an alkali react together under standard conditions, to form 1 mole of water.
Describe that apparatus that would be required to measure enthalpy change of combustion experimentally when burning a fuel.
Change in energy during a reaction cannot be measured directly, so we can measure the surrounding, for example a change in temperature. When burning a fuel, we need:
The fuel in a spirit burner.
A conical flask filled with water.
A Bunsen burner. The spirit burner sits under the Bunsen burner, and the conical flask sits on top, on a tripod.
A stirrer (glass rod).
A thermometer.
To measure the enthalpy change experimentally (when burning a fuel), what are some values that should be known beforehand?
Mass of fuel that is burning (to later work out the moles).
Mass of water.
Initial temperature (to later calculate temperature change).
Specific heat capacity of water = 4.18 (this is already a known value in the textbook).
After carrying out an experiment of the burning of fuel to calculate enthalpy change, what calculations should be carried out to obtain value?
- Use equation q= m x c x temperature change of water
where: m= mass of water, c= specific heat capacity of water (4.18)
q is heat lost or gained (in joules) - Next find the moles of burning fuel using the mass of fuel and its Mr.
- Divide value of q in step 1, by moles obtained in step 2. This gives enthalpy change per mole of fuel. As this is a combustion reaction (combustion reactions are exothermic), there should be a negative sign in front of the final answer.
Define first ionisation enthalpy.
The first ionisation enthalpy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms. Endothermic.
Define the second ionisation enthalpy.
The second ionisation enthalpy is the energy required to remove 1 mole of electrons from 1 mole of gaseous +1 ions. Endothermic.
Define first electron affinity.
The first electron affinity is the energy released when 1 mole of electrons is added to 1 mole of gaseous atoms. This enthalpy is exothermic because the extra electron is attracted to the nucleus of neutral atoms.
Define second electron affinity.
The second electron affinity is the energy required when 1 mole of electrons is added to 1 mole of gaseous -1 ions. This value is always endothermic because energy is needed to overcome the repulsion of the negatively charged electron and negatively charged ion.
Define the enthalpy change of atomisation.
The energy required to form 1 mole of gaseous atoms from an element in its standard state. Endothermic value.
How many the enthalpy change of atomisation differ for diatomic molecules?
The enthalpy change of the atomisation of chlorine, for example, would be an equation written as so:
0.5Cl2 (g) > Cl (g) where enthalpy change is 121
Th reason why the molar ratios are halved is because if it is enthalpy change of atomisation, we are aiming to form only one mole of gaseous atoms, and hence, 1 mole of chlorine.
However, if we were to take the enthalpy change of chlorine in its natural state as a diatomic molecule, the equation would be written as so:
Cl2 (g) > 2Cl (g)
As 2 moles of chlorine is formed, we double the enthalpy change of atomisation above, so 121 x 2 = 242.
Define lattice enthalpy.
Lattice enthalpy is the energy released when 1 mole of solid ionic compound is formed from its gaseous ions. Always exothermic.
What factors affect lattice enthalpy?
Charge of the ions:
Higher charge ; stronger ionic bonds ; more exothermic
Size of ions (ionic radius):
Smaller ions ; better packing ; stronger bonding ; more exothermic
Why can’t lattice enthalpy be measured in a lab?
It is impossible to measure lattice enthalpy values in a lab because gaseous ions cannot be mixed in a vessel, in which after a temperature can be recorded. This is why a different route (Hess Law) must be taken to measure lattice enthalpy.
From elements in their standard states, what enthalpy route can be taken to produce gaseous ions that can be reacted to calculate lattice enthalpy?
Elements in their standard states
Do enthalpy change of atomisation of each element to produce gaseous atoms (values may need to be doubled if element is diatomic).
Then carry out first ionisation enthalpy of the element that would produce a cation (an electron would also be produced as it is ionisation). The second ionisation enthalpy can be carried out if needed.
Then using the electrons produced from ionisation enthalpies, carry out first electron affinity of the other element to produce an anion. Again, second electron affinity may be needed if necessary.
Now we have produced (from elements in their standard states) gaseous (enthalpy change of atomisation) anions and cations (ionisation and electron affinity enthalpies) that are ready to undergo lattice enthalpy.
What equation can be used to calculate any enthalpy change in the Born Haber Cycle?
Enthalpy change of formation = enthalpy change of atomisation (of each element) + enthalpy change of first (and second) ionisation + enthalpy change of first (and second) electron affinity + lattice enthalpy.
Define enthalpy change of solution.
The enthalpy change when 1 mole of solid ionic compound is added to water to make an infinitely dilute solution.
What does it mean if the enthalpy change of solution is positive?
What does it mean if the enthalpy change of solution is negative?
If enthalpy change of solution is positive, energy is taken in, and the ionic compound has not fully dissolved in water.
If enthalpy change is negative, energy is released, and the ionic compound dissolves completely in water.
Define enthalpy change of hydration.
The energy released when 1 mole of gaseous ions are added to water to make an infinitely dilute solution.
Why is the enthalpy change of hydration always exothermic?
Ion-dipole forces are formed between polar water molecules and gaseous ions- bond formation is exothermic.
What factors affect the enthalpy change of hydration?
Size of the ion:
Smaller ionic radius ; stronger ion-dipoles ; more exothermic
Charge of the ion:
Higher charge ; stronger ion-dipoles ; more exothermic
For any ionic compound, how can enthalpy change of solution be calculated from hydration enthalpy and lattice enthalpy?
enthalpy change of solution = hydration enthalpy - lattice enthalpy
What does it mean for the solubility of an ionic compound in water, if lattice enthalpy is greater than hydration enthalpy?
If lattice enthalpy is greater than hydration enthalpy, it can be worked out that enthalpy change of solution will be negative (using enthalpy change of solution = hydration enthalpy -lattice enthalpy), and therefore, exothermic. This means ionic compound is able to fully dissolve in water.
If lattice enthalpy is smaller than hydration enthalpy, it can be worked out that enthalpy change of solution will be positive, and therefore, endothermic. This means the ionic compound does not completely dissolve in water.
What is entropy?
Entropy is the measure of ‘disorder’ or ‘randomness’ present in a substance. The value can be positive (if there is an increase in randomness) or negative (a decrease in randomness).
Describe the trend in entropy from solids to gases.
From solids to gases, the value of entropy increases (more positive?). This is because a solid has a fixed lattice structure, so it has a low disorder. But in gases, with particles holding a lot of energy, they move in all directions and high speed, and hence the disorder increases- entropy increases.
Describe the value of entropy of a substance at 0 kelvin.
At 0 Kelvin, there would be no energy and all substances would have an entropy value of 0.
In what cases may entropy increase?
When a solid melts.
When liquids boil.
When an ionic solid dissolves in water.
When temperature increases.
When no. of gas molecules increases.
What equation can be used to calculate standard entropy change?
Standard entropy change of products - standard entropy change of reactants
What is the Gibbs free energy equation?
Gibbs free energy = enthalpy change - (temperature x entropy change)
The Gibbs free energy equation makes a relationship between enthalpy change, temperature and entropy change, and using these 3 values, it can determine whether a reaction is feasible or not.
What are the units of the Gibbs free energy equation?
temperature is in kelvin (go from Celsius to kelvin is +273)
Enthalpy change in kJ / K/ mol
Entropy change in kJ / K/ mol
Normally entropy can be given in Joules per mole. To convert to the units above, just divide by 1000.
What does it mean if Gibbs free energy is negative?
What does it mean if Gibbs free energy is positive?
If Gibbs free energy is negative the reaction can occur.
If Gibbs free energy is positive, the reaction cannot occur.
