8 - Chemical energetics Flashcards
What is enthalpy change?
Enthalpy change is the heat energy change measured at constant pressure.
What is the symbol for enthalpy change?
ΔH
what are standard conditions?
100kPa, 298K (if solutions are involved, conc 1moldm^-3)
what is the symbol for standard enthalpy change of reaction?
ΔrH°
negative or positive for endothermic and exothermic reactions (enthalpy change)?
- exothermic (energy lost to surroundings): ΔH is negative.
- endothermic (energy gained from surroundings): ΔH is positive.
What is the formula for enthalpy change?
ΔH = H products - H reactants
During a chemical reaction, old bonds are broken and new ones are made. Explain how the making and breaking of bonds leads to the change being exothermic.
- breaking bonds absorbs energy. Making bonds release energy.
- the stronger the bonds, the more energy is absorbed or released.
- the new bonds must be stronger than the old ones, because more energy is released in making the new ones than was absorbed in breaking the old ones.
- the extra energy released to the surroundings makes the reaction exothermic.
what is water and oxygen in standard states?
H2O(l), O2(g)
Symbol for standard enthalpy change?
ΔH°
What is standard enthalpy change of formation?
It is the enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.
- ΔHf°
What is standard enthalpy change of combustion?
It is the enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions.
- water product is liquid.
- ΔHc°
What is standard enthalpy change of neutralisation?
It is the enthalpy change when 1 mole of water is formed in a reaction between an acid and an alkali under standard conditions.
- ΔHn°
What symbol is the ΔH for a neutralisation reaction?
negative. When an acid and alkali react, heat is given out (exothermic).
Give an example of some strong acids. What are they?
HCl, H2SO4, HNO3.
- they completely dissociate in water.
Give an example of a weak acid.
- ethanoic acid (CH3COOH)
- they do not completely dissociate in water.
Why do strong acids reacting with strong alkalis give similar values for ΔHn°?
- we assume that strong acids and strong alkalis are fully ionised in water.
- strong acid being neutralised by a strong alkali is essentially a reaction between H+ and OH- to make water.
- the other ions present are spectator ions.
- If the same reaction is taking place in each case of a strong acid and a strong alkali, it isn’t surprising that the enthalpy change is similar.
In calorimetry, what is the equation for energy change? Then what do you do after?
Q = m c ΔT (Q in joules) (m in grams) (ΔT in kelvin) After, you do: ΔH = Q / no of moles (Q in KJ. This is negative if reaction is exothermic). (ΔH is KJmol^-1)
What do you assume in a calorimetry calculation?
- assume that the density of the solution is the same as water 1gcm^-3 (eg 100cm3 would be 100g).
- assume that the solution has the same specific heat capacity as water (4.18Jg-1K-1)
- if solid is added to a solution, energy change of the solid is ignored.
When given a graph showing temperature change in a reaction, how do you find the temperature change?
- extend the curve so that you can draw a line vertically downwards to the last point on the points at the bottom.
why may reading from the temperature graph thing not result in an result which is similar to the actual value?
- data was extrapolated.
Why is an extrapolation required to determine an accurate measure of the maximum temperature change?
- the reaction is not instantaneous.
- so some heat is lost as mixture is heated up.
What does ammonia dissociate to in water and why?
- dissociates into NH4+ and OH-.
- ammonia is a weak base and only a small proportion of it forms hydroxide ions.
Why can’t we measure the enthalpy change of reaction of some reactions directly?
- some substances do not react together under normal conditions.
State Hess’s law
The enthalpy change for any chemical reaction is independent of the route taken between the initial and final states.
which way do the arrows go in enthalpy change of combustion?
both downwards.
which way do the arrows go in enthalpy change of formation?
both upwards.
what is the positive direction in the Hess cycle?
down, up, towards the right.
What type of process is breaking bonds?
- endothermic process
- bond enthalpies are always positive.
What is bond enthalpy?
The energy needed to break one mole of a particular bond to give separated atoms in the gaseous state.
Bond enthalpy can only occur in the…
gas/gaseous state.
What is mean bond enthalpy?
The mean bond enthalpy is the enthalpy change for breaking one mole of a particular bond in the gaseous state, averaged over many different compounds.
The mean bond enthalpy for the C-H bond in methane is
approximately +416 kJmol-1.
Explain why this value is different from the value given for this bond enthalpy in the Data book (+412 kJmol-1).
- the value +416 is the mean bond enthalpy for the C-H bond specifically in methane.
- in the data book, the mean bond enthalpy for the C-H bond has been averaged over many different compounds. In these different compounds, the C-H bond will be in a different environment from methane which will affect the bond strength.
what is ΔH vaporisation?
The enthalpy change to convert a liquid or solid to a gas.
- must be applied before bond enthalpy.
which way do the arrows go when bond enthalpies are given?
downwards.
Is activation energy shown in enthalpy level diagrams?
No. Just the enthalpy change between reactants and products.
