Chapter 9 - Enthalpy Flashcards

Enthalpy changes, Measuring enthalpy changes, Bond enthalpies and Hess' law and enthalpy changes.

1
Q

What is the law of the conservation of energy?

A

Energy cannot be created or destroyed, just converted from one form to another.

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2
Q

Define enthalpy

A

The heat content that is stored in a chemical system.

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3
Q

Define enthalpy change

A

The heat exchanged with the surroundings during a chemical reaction.
The difference between the enthalpy of the products and the reactants.

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4
Q

Define exothermic

A

A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.

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5
Q

Define endothermic

A

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat beaing taken out of the surroundings.

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6
Q

Define activation energy

A

The minimum energy required for a reaction to take place.

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7
Q

What are the standard conditions?

A

Standard pressure - 101kPa
Standard temperature - 298K
Standard concentration - 1mol/dm^3
Standard state - The physical state of a substance under standard conditions

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8
Q

Define standard enthalpy change of reaction

A

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.

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9
Q

Define standard enthalpy change of formation

A

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

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10
Q

Define standard enthalpy change of combustion

A

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

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11
Q

What is the equation used to measure an energy change?

A
Q=mcΔT
Q - energy change with surroundings (J)
m - mass (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)
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12
Q

How is enthalpy change worked out from the energy change?

A

ΔH = -Q/n
ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)

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13
Q

Define standard enthalpy change of neutralisation

A

The enthalpy change that accompanies the reaction of an acid by a base to from one mole of H(2)O, under standard conditions, with all reactants and products in their standard states.

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14
Q

What are the causes for less energy being transferred than expected when working out ΔH(c)

A

Heat loss to the surroundings
Incomplete combustion
Evaporation
Non-standard conditions

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15
Q

How can heat loss be accounted for using a graph of temperature against time?

A

Extrapolate the cooling curve back to when it was added.

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16
Q

Enthalpy change of neutralisation always has the same value, what is it?

A

-57.5 kJ/mol

17
Q

Define average bond enthalpy

A

The energy required to break one mole of a specified type of bond in a gaseous molecule.

18
Q

What are some properties of bond enthalpies?

A

Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value

19
Q

What kind of energy and enthalpy changes are bond formation?

A

Exothermic and releases energy

20
Q

What is Hess’ law?

A

If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.