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Chemistry Year 12 > 8 - Chemical energetics > Flashcards

8 - Chemical energetics Flashcards

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1
Q

What is enthalpy change?

A

Enthalpy change is the heat energy change measured at constant pressure.

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2
Q

What is the symbol for enthalpy change?

A

ΔH

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3
Q

what are standard conditions?

A

100kPa, 298K (if solutions are involved, conc 1moldm^-3)

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4
Q

what is the symbol for standard enthalpy change of reaction?

A

ΔrH°

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5
Q

negative or positive for endothermic and exothermic reactions (enthalpy change)?

A
  • exothermic (energy lost to surroundings): ΔH is negative.

- endothermic (energy gained from surroundings): ΔH is positive.

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6
Q

What is the formula for enthalpy change?

A

ΔH = H products - H reactants

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7
Q

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.

A
  • 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.
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8
Q

what is water and oxygen in standard states?

A

H2O(l), O2(g)

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9
Q

Symbol for standard enthalpy change?

A

ΔH°

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10
Q

What is standard enthalpy change of formation?

A

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°

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11
Q

What is standard enthalpy change of combustion?

A

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°
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12
Q

What is standard enthalpy change of neutralisation?

A

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°

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13
Q

What symbol is the ΔH for a neutralisation reaction?

A

negative. When an acid and alkali react, heat is given out (exothermic).

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14
Q

Give an example of some strong acids. What are they?

A

HCl, H2SO4, HNO3.

- they completely dissociate in water.

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15
Q

Give an example of a weak acid.

A
  • ethanoic acid (CH3COOH)

- they do not completely dissociate in water.

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16
Q

Why do strong acids reacting with strong alkalis give similar values for ΔHn°?

A
  • 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.
17
Q

In calorimetry, what is the equation for energy change? Then what do you do after?

A 
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)
18
Q

What do you assume in a calorimetry calculation?

A
  • 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.
19
Q

When given a graph showing temperature change in a reaction, how do you find the temperature change?

A
  • extend the curve so that you can draw a line vertically downwards to the last point on the points at the bottom.
20
Q

why may reading from the temperature graph thing not result in an result which is similar to the actual value?

A
  • data was extrapolated.
21
Q

Why is an extrapolation required to determine an accurate measure of the maximum temperature change?

A
  • the reaction is not instantaneous.

- so some heat is lost as mixture is heated up.

22
Q

What does ammonia dissociate to in water and why?

A
  • dissociates into NH4+ and OH-.

- ammonia is a weak base and only a small proportion of it forms hydroxide ions.

23
Q

Why can’t we measure the enthalpy change of reaction of some reactions directly?

A
  • some substances do not react together under normal conditions.
24
Q

State Hess’s law

A

The enthalpy change for any chemical reaction is independent of the route taken between the initial and final states.

25
Q

which way do the arrows go in enthalpy change of combustion?

A

both downwards.

26
Q

which way do the arrows go in enthalpy change of formation?

A

both upwards.

27
Q

what is the positive direction in the Hess cycle?

A

down, up, towards the right.

28
Q

What type of process is breaking bonds?

A
  • endothermic process

- bond enthalpies are always positive.

29
Q

What is bond enthalpy?

A

The energy needed to break one mole of a particular bond to give separated atoms in the gaseous state.

30
Q

Bond enthalpy can only occur in the…

A

gas/gaseous state.

31
Q

What is mean bond enthalpy?

A

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.

32
Q

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).

A
  • 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.
33
Q

what is ΔH vaporisation?

A

The enthalpy change to convert a liquid or solid to a gas.

- must be applied before bond enthalpy.

34
Q

which way do the arrows go when bond enthalpies are given?

A

downwards.

35
Q

Is activation energy shown in enthalpy level diagrams?

A

No. Just the enthalpy change between reactants and products.

Chemistry Year 12 (12 decks)

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