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Developing Fuels > Enthalpy Changes > Flashcards

Enthalpy Changes Flashcards

1
Q

Define enthalpy change & give its units

A

Enthalpy change (delta H) is the heat energy transferred in a reaction at constant pressure
the units are kJmol^-1

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2
Q

How do you show measurements of enthalpy change were made under standard conditions?

A

Delta H^ (circle with a horizontal line through)

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3
Q

What does it mean when an element is in its standard state?

A

It is in the state it’s at when the pressure is 100kPa

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4
Q

What is change in h?

A

Change in enthalpy/ energy
The energy transferred to or from the surroundings when the reaction occurs

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5
Q

Is the enthalpy change positive or negative for an exothermic reaction?

A

Negative

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6
Q

Is the enthalpy change positive or negative for an endothermic reaction?

A

Positive

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7
Q

What happens to the reactants, products and surroundings in an exothemic reaction?

A

Reactants lose energy
Products are lover in energy (then the reactants)
Energy is given out to the surroundings so they heat up

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8
Q

What happens to the reactants, products and surroundings in an endothermic reaction?

A

Reactants gain energy
Products are higher in energy (than the reactants)
Energy is taken in from the surroundingsso they cod down (may not notice if it’s being heated eg with a Bunsen)

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9
Q

What are the units of enthalpy change?

A

kJmol^-1

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10
Q

What are the units of temperature?

A

Kelvin

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11
Q

How do you go from degrees Celsius to kelvins?

A

Add 273

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12
Q

What are the standard conditions?

A

P = 1atm (=1.05 x 10^5 Nm^-2)
T=25°C = 298K
Any solution=1moldm^-3 concentration
All substances are in standard states the state of the substance at standard conditions)

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13
Q

What are the two common types of experiments used to measure change in temperature?

A
  1. Burning a fuel
  2. Doing a reaction in a polystyrene cup
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14
Q

What’s the most accurate way to measure change in temp by burning a fuel?

A

By using a bomb calorimeter

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15
Q

Why is change in temp measured instead of directly measuring enthalpy change?

A

Enthalpy change can’t be measured directly, so temp change must be measured, then q = m c (change) T must be used.

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16
Q

Which measurements must be made during the burning a fuel practical?

A
  • Mass of water being heated (in practice its usually volume - for water, 1cm^3 = 1g)
  • starting and final temp of water
  • Starting and final mass of spirit burner
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17
Q

Draw a diagram of the apparatus needed for the burning fuel practical (with labels)

A

Labels:
- drought shields
- copper calorimeter
- (known volume of) water
-thermometer
- spirit burner(containing liquid fuel)

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18
Q

How can you calculate enthalpy change from the burning a fuel practical results?

A

Use q=m x c x change temp
Then / by 1000
Then / by number of moles that reacted (number of moles of fuel that burned)
Think about the sign - burning fuel is an exothermic reaction so put a - in front of the answer

19
Q

What does each component of q=m x c x change T mean?

A

Q = energy transferred
M = mass of water being burned (not mass of fuel)
C = specific heat capacity of water (4.18)
Change T = temperature change of the water

20
Q

Why doesn’t the burning fuel experiment give a perfect answer?
(What are the sources of error in the experiment)

A
  • Heat loss to the surroundings
  • Incomplete combustion
  • heat loss to the calorimeter
    (Those are the 3 most important ones, but there’s also:)
  • same fuel may evaporate
  • enthalpy change for standard states means water should be produced as liquid, but in this expt water is produced as a gas
21
Q

What’s the 6-step method for doing an enthalpy change experiment in a polystyrene cup with solutions?

A
  • Use known volumes (measure with pipettes) of solutions of known concentrations
  • take starting temperature (use an average for 2 separate solutions
  • then mix them in a polystyrene cup (to minimise heat loss ( exo) or gain (endo))
  • stir the mixture & record the max temperature reached (exo) or min ( endo)
  • then use q = mc x change t where m= total mass of the solution after mixing
  • then /by 1000 and / again by number of moles
22
Q

What are some common exothermic reactions?

A

Oxidation (combustion of fuel & oxidation of carbohydrates in respiration)

23
Q

What are some common endothermic reactions?

A

Thermal decomposition & photosynthesis

24
Q

Is bond breaking endo or exo?

A

Endothermic

25
Q

Is bond making endo or exo?

A

Exothermic

26
Q

What is bond enthalpy?

A

The energy needed to break a bond or the energy given out when a bond forms (stronger bonds have higher bond enthalpies)

27
Q

How do you find enthalpy change of reaction using avrg bond enthalpies?

A

Enthalpy change of reaction = total energy absorbed to break bonds - total energy released in making bonds

28
Q

What is an average bond enthalpy?

A

The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

29
Q

What is the bond length?

A

The distance between the two nuclei in a covalent bond

30
Q

What’s the relationship between attraction, bond enthalpy and bond length?

A

The stronger the attraction, the higher the bond enthalpy and the shorter the bond length

31
Q

How is the enthalpy of a substance related to its stability?

A

The less enthalpy a substance has, the more stable it is

32
Q

What is activation energy?

A

The minimum amount of energy needed to begin breaking reactant bonds & start a chemical reaction

33
Q

What is different about finding the enthalpy change for reactions involving a solid than a reaction with 2 solutions?

A

With 2 solutions the reaction is almost instantaneous, so all the heat is given out at once. But using a solid in a reaction is slower so heat is being lost from the first bit before the last bit has reacted, meaning maximum temperature recorded is not a true value (would be too small)

34
Q

How do you find a more correct value for temperature change when using a solid in the reaction?

A

Take the temperature several times over several minutes and plot a temperature against time graphi then find a more accurate value for temp change using extrapolation

35
Q

Define standard enthalpy change for a reaction

A

The enthalpy change when molar quantities of the reactants, as stated in the equation, react together under standard conditions, with all substances in their standard states

36
Q

Define standard enthalpy change of combustion

A

The enthalpy change when one mole of the substance burns completely in oxygen under standard conditions, with all substances in their standard states

37
Q

Define standard enthalpy change of formation

A

The enthalpy change when one mole of the substance is made from its elements in their standard states under standard conditions

38
Q

What will the standard enthalpy change of formation be for an element?

A

Zero

39
Q

Define the standard enthalpy change of neutralisation

A

The enthalpy change when one mole of H+ ions (from an aqueous acid) react with one mole of OH- ions (from an aqueous alkali) to form one mole of water under standard conditions and in solutions containing 1 moldm^-3

40
Q

What are the steps for enthalpy change calculations?

A
  • use q=mc(change)T
  • /1000
  • /number of moles reacted
  • think about the sign
41
Q

What are bond enthalpy values?

A

The energy needed to break a particular type of a covalent bond, for one mole of bonds in the gas state, is called the bond enthalpy & is always positive

42
Q

When writing equations for bond enthalpy, why are the products the separate atoms and not diatomic molecules?

A

The only thing happening in the reaction is the bond breaking therefore the products ave the separate atoms

43
Q

What are the 3 main uses of bond enthalpy values?

A
  1. To compare bond strengths
  2. To explain where the enthalpy change of a reaction comes from
  3. Using bond enthalpy values to calculate a new value for enthalpy change of a reaction

Developing Fuels (4 decks)

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