MODULE 3: Enthalpy Flashcards

1
Q

What is enthalpy?

A

Enthalpy, H is the amount of energy that a substance contains (cannot be measure directly)

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2
Q

What is the equation for enthalpy change?

A

Enthalpy change (ΔH) = enthalpy of products (H) - enthalpy of reactants (H)

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3
Q

What is Hess’ Law?

A

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change will be the same for each route

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4
Q

What is an exothermic reaction?

A

Heat energy is transferred from the reaction to the surroundings, enthalpy change is negative therefore energy is lost (bond making)

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5
Q

What is an endothermic reaction?

A

Heat energy is transferred from the surroundings into the reaction, enthalpy change is positive therefore energy is gained (bond breaking)

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6
Q

What is activation energy?

A

The minimum energy required to start a reaction

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7
Q

What are standard conditions?

A

Standard pressure (100kPa), standard temperature (298K), standard concentration (1 mol dm-3), standard state (state in 101kPa and 298K)

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8
Q

What is enthalpy change of a reaction? (ΔrHᶿ)

A

The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state

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9
Q

What is enthalpy change of formation? (ΔfHᶿ)

A

Enthalpy change that takes place when one mole of a compound is formed from its elements at standard conditions

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10
Q

What is enthalpy change of combustion? (ΔcHᶿ)

A

The standard enthalpy change of combustion is the enthalpy change when 1 mol of a substance
is burnt completely, in an excess of oxygen, under standard conditions of 298K and 101kPa

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11
Q

How is the enthalpy change of a reaction measured? (ΔrHᶿ)

A

Using the equation ‘q = m x c x ΔT’, a Styrofoam cup (to prevent heat loss), and a temperature to record the temperature change

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12
Q

What is the equation for enthalpy change of a reaction? (ΔrHᶿ)

A

∆rHᶿ = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)

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13
Q

What is average bond enthalpy?

A

Energy required to break one mole of bonds in a gaseous molecule

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14
Q

What is the equation for average bond enthalpy?

A

ΔrHᶿ = Bond enthalpies in reactants - bond enthalpies in products

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15
Q

What is enthalpy change of neutralisation? (ΔneutHᶿ)

A

The standard enthalpy change of neutralisation is the enthalpy change when an acid and alkali react together to form 1
mol of water, under standard conditions of 298K and 101kPa

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16
Q

What is the equation for measuring energy change?

A

q= mcΔT

17
Q

In the equation for energy change, what is ‘m’?

A

Mass, grams

18
Q

In the equation for energy change, what is ‘c’?

A

Specific heat capacity, g-1 K-1

19
Q

In the equation for energy change, what is ‘ΔT’?

A

Temperature change, kelvin

20
Q

Why may testing for enthalpy change of combustion (ΔcHᶿ) be inaccurate?

A

Heat loss to surroundings, incomplete combustion, evaporation of methanol from the wick, non-standard conditions

21
Q

What equipment do you use for enthalpy change of combustion (ΔcHᶿ) be inaccurate?

A

Thermometer, beaker full of water, spirit burner full of fuel

22
Q

How is enthalpy presented?

A

Positive if endothermic, negative is exothermic

23
Q

What are the limitations of bond enthalpies?

A

Bond enthalpy can vary depending on the chemical environment of the bond

24
Q

What is the general equation used for Hess’ Law?

A

A + B = C

25
Q

What does an exothermic energy profile look like?

A

Energy of reactants is above the energy of products; enthalpy change is negative

26
Q

What does an exothermic energy profile look like?

A

Energy of reactants is below the energy of products; enthalpy change is positive