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Chemistry > CHAPTER 9: Enthalpy > Flashcards

CHAPTER 9: Enthalpy Flashcards

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1
Q

What is enthalpy?

A

a measure of the heat energy in a chemical system.

Enthalpy cannot be measured but enthalpy change can.

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2
Q

What is enthalpy change?

A

The difference in enthalpies between products and reactants.

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3
Q

What does the law of conservation of energy state?

A

That energy cannot be destroyed/created but is transferred between the system and surroundings.

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4
Q

What are the two directions of energy transfer?

A

1) EXOthermic - transfer of energy from system to surroundings.
(negative enthalpy change value)

2) ENDOthermic - transfer of energy from surroundings to system.
(positive enthalpy change value)

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5
Q

What is activation energy?

A

The minimum amount of energy required for a reaction to take place.
This is because bonds in reactants need to broken by an input of energy in a reaction.

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6
Q

Why do chemists use standard conditions for measuring enthalpy changes?

A

Because enthalpy change for a reaction can vary depending on the conditions used.

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7
Q

What are the units for enthalpy change?

A

kJmol^1

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8
Q

Define 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 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

Why are equations sometimes not written using whole balancing numbers when it comes to enthalpy change?

A

To match the definition of enthalpy change of formation or combustion (where it requires a formation of one mole for formation and requires one mole to completely react with oxygen in combustion).

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11
Q

What enthalpy change of formation value do all elements have? Why?

A

0kJmol^1

Definition refers to one mole forming from its element.

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12
Q

Define 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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13
Q

Define enthalpy change of NEUTRALISATION.

A

The enthalpy change that accompanies the reaction of an acid by a base to form one mole of h2o, under standard conditions with all reactants and products in their standard states.

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14
Q

How do you convert celsius degrees into kelvin?

A

add 273 to celsius degrees.

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15
Q

What 3 quantities is energy change calculated from? (q)

A

q = mc∆t
mass
specific heat capacity
temperature change

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16
Q

How can you get a enthalpy change of combustion value from experiment? (example of methanol)

A

SPIRIT BURNER - weigh before and after to calculate mass of methanol burnt.

place measured volume of water above burning spirit burner with temperature to measure temp change.

calculate q then calculate mol of methanol burnt.
then calculate how much energy for 1 mol of ethanol.
DONT FORGET SYMBOL AT END.

17
Q

How accurate is enthalpy change of combustion experiments?

A

not sophisticated apparatus.

  • heat loss to surroundings
  • incomplete combustion (carbon soot)
  • evaporation of methanol from wick- must weigh as soon as possible after extinguishing flame.
18
Q

What is AVERAGE BOND ENTHALPY?

A

the energy required to break one mole of a specified type of bond in a GASEOUS molecule.

19
Q

Why are bond enthalpies ALWAYS ENDOTHERMIC?

A

Because energy is always required to break bonds.

20
Q

Give the formula to calculate enthalpy change from average bond enthalpies.

A

∑(bond enthalpies in reactants) - ∑(bond enthalpies in products)

21
Q

Why is the calculated enthalpy change from bond enthalpies not a STANDARD enthalpy change?

A

because all species have to be in gaseous molecules to calculate using bond enthalpies so products and reactants are not in their standard states.

22
Q

What is HESS LAW?

A

an indirect way of determining enthalpy change.

it states that if a reaction can take place by 2 routes and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.

23
Q

What is the formula to calculate enthalpy change using enthalpy changes of FORMATION?

A

∑products - ∑ reactants

24
Q

What is the formula to calculate enthalpy change using enthalpy changes of COMBUSTION?

A

∑ reactants - ∑ products.

Chemistry (6 decks)

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