Unit 1 Section 2 Energetics Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What are the units for enthalpy change?
KJ /Mol
What does ΔH mean?
That is the enthalpy change.
What does ΔH° mean?
The elements were in their standard states and measurements were made under standard conditions.
What are standard conditions?
100 kPa of pressure
a temperature of 298K
Substances in their standard states.
Solutions with a concentration of 1 mol dm⁻³
Is oxidation exothermic or endothermic ?
Exothermic.
What are exothermic reactions?
They give out energy. ΔH is negative.
What are endothermic reactions?
The reaction absorbs energy. ΔH is positive. The temperature of the surroundings will fall.
Name some endothermic reactions.
Thermal decomposition and the main reactions of photosynthesis.
Name some exothermic reactions.
Oxidation of carbohydrates and combustion of fuel.
What do enthalpy profile diagrams show?
They show how the enthalpy(energy) changes during reactions.
The less enthalpy a substance has the………..it is.
More stable
What is activation energy?
The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction.
What is the symbol for activation energy?
E”a”
Why are standard conditions important ?
Enthalpy can be affected by temperature and pressure.
What are the different types of standard enthalpy change?
Change of reaction, formation, combustion, neutralisation and atomisation.
Explain standard enthalpy change of reaction.
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions and their standard states.
Explain standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Explain standard enthalpy change of combustion.
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
Explain standard enthalpy of neutralisation.
The enthalpy change when 1 mole of water is formed from the reaction of an acid with an alkali under standard conditions. E.g.
H+ and OH- = H”2”O
What is Hess’s law?
The total enthalpy change is independent of the route taken.
What does ΔH°f mean?
The formation energy of a molecule.
What is the energy of formation of an element.
0 as the element is formed form the element.
What is bond breaking?
Endothermic ΔH is positive.
What is bond making?
Exothermic. ΔH is negative.
What is bond dissociation energy.
The amount of energy needed to break the bonds in one mole of a compound. It is also know simply as bond enthalpy.
Why are bond enthalpies calculated uses gaseous compounds?
To keep comparisons fair.
In what way do bond enthalpies affect the rate of reaction?
The smaller the bond enthalpies of the bonds that need to be broken, the faster the reaction. This is because less energy has to be taken in from the surroundings.
Define heat.
Disorganised thermal energy. A bulk property of a substance.
Define work.
An object or mass is moved from one height to another. Organised energy use.
Define Temperature
Average kinetic energy per partcile.
What is an open system? What is a closed system?
A reaction in a vessel that allows mass and energy transfer to the surroundings.
A closed system will only allow energy transfer.
What is an isolated system?
Always no interaction of mass or energy between system and surroundings.
How is the enthalpy change shown on a graph?
It is the energy change between the reactants and products.
What is activation energy?
The minimum amount of energy necessary for a reaction to take place.
What is the standard enthalpy of atomisation?
The enthalpy change when 1 mole of an element in its standard state is converted to gaseous atoms under standard conditions.
What are some systematic errors in calorimetry experiments ?
Some of the heat is absorbed by the surroundings, some by the container.
If your burning something, combustion may be incomplete.
Some of a flammable liquid could evaporate before it burns.
How is the enthalpy change of a reaction related to the bonds?
Bond making is exothermic and bond breaking is endothermic. Thus the total enthalpy change is a mix of the 2
What are the attractions in ionic and covalent bonds?
In ionic the attraction is between the positive and negative ions.
In covalent, it is the attraction of positive nuclei to the negative electrons in the middle.
What is bond dissociation energy?
The amount of energy needed per mole to break all the bonds in a gaseous compound.
If you have to find the enthalpy change for a reaction and you have data on the enthalpy of formation, which way do the arrows go?
From the bottom to the top. This is because you have the raw elements at the bottom and these go up to form the molecules.
Is the enthalpy of formation usually positive or negative?
Negative though there are some exceptions.
When finding enthalpy of formation from combustion, which way do the arrows go?
From top to bottom. This is because for combustion the molecules go into their atoms.
How would bond ethanalpies be used to calculate the enthalpy of formation of something?
You would have to find the energy needed to break the bonds in the elements and then the energy given out to form the molecule.
On a Hess cycle with energy of formation, what goes on the bottom square?
The top 2 ones will have the same number of a type of atom.
In the bottom give the elements in their standard forms ( e.g. Cl₂) and give them the correct state symbols.
What is the difference between mean bond enthalpy and bond enthalpy?
Mean bond enthalpy is the average for a particular type of bond measured over many different types of molecule
bond enthalpy will be about a specific bond in a specific molecule.
What is some evidence for the existence of ions?
Physical properties of ionic compounds: high melting
temperatures, showing strong forces of attraction between
ions, soluble in polar solvents, conduct electricity when
molten or in aqueous solution.
• Electron density maps of compounds produced from X-ray
diffraction patterns show zero electron density between ions meaning complete electron transfer.
• Migration of ions in electrolysis. For example, electrolysis of green aqueous copper(II) chromate(VI) attracts a yellow colour (chromate(VI) ions) to the anode and a blue colour (copper(II) ions) to the cathode.
What are some of the limitations of using bond enthalpy in hess’s law calculations?
They are based on average bond enthalpies and only gaseous species
When finishing a Hess cycle, how do you label the arrows?
When enthalpy of formation, use △H”f” on the up arrows. If using enthalpy of combustion, put ∆H”c” on the down arrows.
Incomplete combustion is …………… than complete combustion
Less exothermic.
Enthalpy of neutralisation is always………….
Exothermic. When strong acids and alkalis are used, the values are similar at around -57 KJ mol
Why is the enthalpy of formation of a hydrated salt from an anhydrous salt hard to find and how is this overcome?
It is hard to add the exact amount of water and the heat change of a solid is hard to measure.
Both salts are dissolved in excess water to form the same thing and these enthalpy measured.
Then hess’s law is used.
Why is the enthalpy of the thermal decomposition of calcium carbonate hard to find and how is this overcome?
It is impossible to add the heat required to decompose the solid and the temp change of the solid at the same time.
Calcium carbonate and calcium oxide are reacted with HCl to form calcium chloride. The temp changes for these reactions can be measured.
Define first electron affinity
The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge.
Is the first electron affinity endo or exothermic ?
Exothermic as it is drawn to the nucleus
Is the second electron affinity endo or exothermic ?
Endothermic as it is repelled by the electron cloud.
What is bond enthalpy ?
The amount of energy needed to break a bond.
On the bottom square of a Hess cycle, with both arrows pointing down and both reactions producing the same thing, how many of them do you have?
Just one, not one for each reaction.
Why when doing an experiment to find enthalpy change is the value found different to that of a data booklet?
Wasn’t done under standard conditions, we assumed specific heat capacity to be 4.2 and that density is 1g cm⁻².
Heat loss.
