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Chemistry A-Level > Energetics Seneca > Flashcards

Energetics Seneca Flashcards

1
Q

Define enthalpy change.

A
  • Measure of heat taken in or out of process.
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2
Q

When objects are heated, what do they use the energy for?

A
  • To expand.
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3
Q

Why do we use enthalpy instead of energy?

A
  • Energy relies on fixed volume.
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4
Q

What condition must be constant to measure enthalpy?

A
  • Pressure, which is constant in the atmosphere.
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5
Q

Do exo and endothermic reactions take in or give out heat energy?

A
  • Exo - give out.
  • Endo - take in.
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6
Q

Do exo and endothermic reactions have a positive or negative enthalpy change?

A
  • Exo - negative.
  • Endo - positive.
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7
Q

Example of an exothermic reaction?

A
  • Combustion.
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8
Q

Example of endothermic reaction?

A
  • Thermal decomposition.
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9
Q

Endothermic making or breaking bonds?

A
  • Breaking.
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10
Q

Exothermic making or breaking bonds?

A
  • Making.
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11
Q

What is enthalpy change of a reaction the sum of?

A
  • Individual bond enthalpies being broken and made during a reaction.
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12
Q

What is the dominating term?

A
  • Whether the reaction requires more energy to make or break bonds, giving the overall enthalpy.
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13
Q

What is a memory tool to remember bond breaking and bond making?

A

-Bendo-Mexo.

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14
Q

What is the symbol for enthalpy change?

A
  • ΔH.
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15
Q

What are standard conditions?

A
  • 1 bar (100kPa).
  • 298 K.
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16
Q

What are the 2 standard enthalpy changes?

A
  • Standard enthalpy of combustion (ΔcHθ).
  • Standard enthalpy of formation (ΔfHθ).
17
Q

Standard enthalpy of combustion (ΔcHθ) meaning?

A
  • Enthalpy change when one mole of substance in its standard state burns completely in oxygen under standard conditions.
18
Q

Standard enthalpy of formation (ΔfHθ) meaning?

A
  • Enthalpy change when one mole of substance is formed in its standard state from the pure elements in their standard states under standard conditions.
19
Q

How does bomb calorimetry work?

A
  • Burning a sample of compound in a sealed vessel + measuring temp change.
  • Heating water jacket.
20
Q

Sources of inaccuracy in bomb calorimetry?

A
  • Heat lost to surroundings.
  • Incomplete combustion.
  • Reactant may evaporate before it combusts.
21
Q

What is the equation to measure enthalpy change?

A
  • q = m x c x ΔT.
22
Q

What do the letters represent in the enthalpy change equation?

A
  • q - heat energy.
  • m - mass of sample.
  • c - specific heat capacity.
  • ΔT - temp change.
23
Q

What are the units of q?

A
  • Joules.
24
Q

How to gain accurate temperature changes?

A
  • Measure temp loss after reaction is complete by extrapolating line.
25
Q

Why are the initial and final temperatures on a calorimeter inaccurate?

A
  • Heat is always being lost from the calorimeter.
26
Q

What is the only case when we can assume that the reaction mixture has the same density or specific heat capacity as water?

A
  • When the question states we can.
27
Q

What does Hess’ Law mean?

A
  • Enthalpy change of a reaction is independent of the pathway.
  • Enthalpy change is the same if its direct or indirect.
28
Q

What is Hess’ Law useful for?

A
  • Useful for obtaining enthalpy changes that aren’t directly observable.
29
Q

What is the enthalpy of formation for any element?

A
  • Zero.
30
Q

What is the meaning of bond enthalpy?

A
  • Enthalpy change when one mole of bonds is broken in the gas phase.
31
Q

What do mean bond enthalpies mean in molecules that have identical bonds?

A
  • The 2nd bond will require more energy to break than the 1st because the molecule will rearrange after the 1st bond is broken.
  • ∴ an average of bond enthalpy values is taken.
32
Q

What are the 2 main reasons that mean bond enthalpies are inaccurate?

A
  • Bond enthalpies are only valid in gas phase.
  • Bond enthalpies depend on the particular molecule, however mean bond enthalpies are quoted generally.
33
Q

Is bond enthalpy positive or negative?

A
  • Positive.
34
Q

In standard enthalpy of formation (ΔfHθ) where do the arrows point in Hess’ triangle?

A
  • From the elements in their standard states to the reactants + products.
35
Q

In standard enthalpy of combustion (ΔcHθ) where do the arrows point in Hess’ triangle?

A
  • Point from the reactants + products to the oxides.

Chemistry A-Level (12 decks)

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