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IMAT Chem > Chapter 7: Enthalpy Change > Flashcards

Chapter 7: Enthalpy Change Flashcards

1
Q

What is energy?

A

The ability to do work or move an object against an opposing force.

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2
Q

What are some forms of energy?

A

Heat, light, sound, electricity, and chemical energy.

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3
Q

What is heat in terms of energy transfer?

A

Heat is a mode of energy transfer due to a temperature difference, increasing disorder.

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4
Q

How does heat affect molecular behavior?

A

It increases the average kinetic energy of molecules in a disordered fashion.

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5
Q

What is enthalpy (H)?

A

A measure of the heat energy contained in a substance, stored as potential energy.

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6
Q

What is enthalpy change?

A

The energy exchange between a chemical reaction and its surroundings at constant pressure.

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7
Q

Can enthalpy be directly measured?

A

No, only enthalpy change can be measured.

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8
Q

What happens to the surroundings during an exothermic reaction?

A

The temperature of the surroundings increases.

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9
Q

What happens to the surroundings during an endothermic reaction?

A

The temperature of the surroundings decreases.

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10
Q

What is the sign of ∆H for exothermic reactions?

A

Negative (∆H < 0).

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11
Q

What is the sign of ∆H for endothermic reactions?

A

Positive (∆H > 0).

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12
Q

Give examples of exothermic reactions.

A

Combustion, respiration, reaction of water with calcium oxide.

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13
Q

Give examples of endothermic reactions.

A

Decomposition of limestone, photosynthesis, dissolving certain ammonium salts.

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14
Q

What is the standard enthalpy change?

A

The enthalpy change measured under standard conditions.

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15
Q

What is Hess’s Law?

A

The total enthalpy change of a reaction is independent of the pathway taken.

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16
Q

How can Hess’s Law be used in calculations?

A

By constructing an enthalpy cycle.

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17
Q

What is bond energy?

A

The energy required to break a specific covalent bond.

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18
Q

Is bond energy always positive or negative?

A

Positive, as it involves breaking bonds.

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19
Q

What is the relationship between bond breaking and energy?

A

Bond breaking is endothermic.

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20
Q

What is the relationship between bond making and energy?

A

Bond forming is exothermic.

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21
Q

How do you determine if a reaction is exothermic using bond energies?

A

If energy released during bond formation is greater than energy absorbed during bond breaking.

22
Q

How do you determine if a reaction is endothermic using bond energies?

A

If energy absorbed during bond breaking is greater than energy released during bond formation.

23
Q

What is the formula for enthalpy change using bond energies?

A

Enthalpy change = Energy to break bonds - Energy to form bonds.

24
Q

Why are average bond energies used?

A

Bond energies vary between compounds, so averages are used for calculations.

25
Q

What is the enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substance is burned in oxygen under standard conditions.

26
Q

What is the enthalpy change of formation?

A

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.

27
Q

What is the enthalpy change of atomization?

A

The enthalpy change required to produce 1 mole of gaseous atoms from an element in its standard state.

28
Q

Why is Hess’s Law important?

A

It allows the calculation of enthalpy changes that cannot be measured directly.

29
Q

What does an enthalpy cycle illustrate?

A

The conservation of energy in a chemical reaction.

30
Q

What does ∆H depend on in chemical reactions?

A

The strength and number of bonds broken and formed.

31
Q

What is the relationship between bond energy and bond strength?

A

Higher bond energy indicates stronger bonds.

32
Q

Why is the combustion of fuels exothermic?

A

It releases more energy when forming bonds than it absorbs to break bonds.

33
Q

Why is photosynthesis endothermic?

A

It absorbs more energy to break bonds than it releases to form bonds.

34
Q

How is bond energy calculated for diatomic molecules?

A

Directly from experimental data on bond dissociation.

35
Q

What does the decomposition of limestone produce?

A

Calcium oxide and carbon dioxide.

36
Q

Why is dissolving ammonium salts in water endothermic?

A

The energy absorbed to break ionic bonds is greater than the energy released during hydration.

37
Q

How is ∆H calculated in the Haber process?

A

Using bond energies of reactants and products.

38
Q

Why are bond enthalpies averaged?

A

The same type of bond can have slightly different energies in different molecules.

39
Q

What is an exothermic enthalpy profile diagram?

A

A graph showing products lower in energy than reactants.

40
Q

What is an endothermic enthalpy profile diagram?

A

A graph showing products higher in energy than reactants.

41
Q

What does a negative ∆H signify in combustion?

A

Heat is released.

42
Q

What does a positive ∆H signify in decomposition reactions?

A

Heat is absorbed.

43
Q

How is energy transfer visualized in enthalpy cycles?

A

With arrows showing energy flow between reactants, products, and intermediates.

44
Q

What is a standard state in enthalpy measurements?

A

A reference state of a substance at 1 atm pressure and 298 K.

45
Q

What is the role of oxygen in combustion?

A

To act as a reactant and release energy.

46
Q

Why is the oxidation of carbohydrates in respiration exothermic?

A

It releases stored chemical energy.

47
Q

Why is calcium oxide an important product in industrial processes?

A

It is used in cement and neutralizing acidic soils.

48
Q

What is the significance of Hess’s Law in thermochemistry?

A

It provides a practical way to calculate enthalpy changes indirectly.

49
Q

How does temperature affect the enthalpy of a reaction?

A

Higher temperatures increase kinetic energy but do not change ∆H.

50
Q

What is the main concept behind Hess’s Law?

A

Conservation of energy.

IMAT Chem (9 decks)

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