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Chemistry > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is enthalpy?

A

The heat energy stored in a chemical system

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2
Q

Units for 🔺H

A

KjMol^-1

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3
Q

What is an exothermic reaction?

A

One that transfers energy from system to surroundings

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4
Q

What value will 🔺H have for an exothermic reaction?

Why?

A

Negetive

Energy lost

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5
Q

Give examples of exothermic reactions

A

Combustion
Respiration

(oxidation reactions)

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6
Q

Define endothermic reaction

A

Energy transferred from surroundings to system

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7
Q

What value will 🔺H have for an endothermic reaction?

Why?

A

Positive

Energy gained

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8
Q

Give examples of endothermic reactions

A

Thermal decomposition

Photosynthesis

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9
Q

What is activation energy?

A

Minimum amount of energy need for a reaction to start

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10
Q

What does a larger activation energy show?

A

Slower rate of reaction

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11
Q

What do all standard enthalpy changes require?

A

Standard conditions and all reactants and products in their standard states

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12
Q

What are the standard conditions?

A

100KPa
25°c /298K
Moldm^-3
Standard state= physical state under standard conditions

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13
Q

Define standard enthalpy change of reaction

A

Enthalpy change of a reaction in molar quantities as shown by the equation under standard conditions with all reactants and products in their standard states

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14
Q

Define standard enthalpy change of formation

A

Enthalpy change for formation of one mole of a substance from its constituent elements under standard conditions with all reactants and products in their standard states

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15
Q

Define standard enthalpy change of combustion

A

Enthalpy change of burning one mole of a substance completely in an excess of oxygen (complete combustion) under standard conditions with all reactants and products in their standard states

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16
Q

What is the 🔺fH of an element?

A

0

17
Q

Define standard enthalpy change of neutralisation

A

Enthalpy change of a neutralisation reaction (acid + base) to form one mole of water under standard conditions with all reactants and products in their standard states

18
Q

Why is the standard enthalpy change of neutralisation the same regardless of acid and base used?

A

Always OH + H =H2O

19
Q

Is bond breaking or bond making exo or endothermic

A

Breaking = endothermic

Forming = exothermic

20
Q

Define average bond enthalpy

A

Average amount of energy need to break one mole of a specific bond in gaseous molecules

21
Q

What value do average bond enthalpies always have?

A

Positive

Bond breaking=endothermic

22
Q

What are the disadvantages of using average bond enthalpies?

A

Same bond requires different amounts of energy to break in different conditions

Average = not specific

23
Q

How can you calculate 🔺rH of a reaction using average bond enthalpies?

Why is this not a standard enthalpy change?

A

(Sum of reactant bond enthalpies) - (sum of product bond enthalpies)

Not standard because bond enthalpy = in gaseous molecules (not always standard conditions)

24
Q

What is Hess’ law?

A

If a reaction can occur by two routes that start and end in the same conditions then the enthalpy changes or each are equal.

25
Q

Why is Hess’ law useful?

A

Hard to determine 🔺H directly

Allow indirect determination

26
Q

How do you calculate 🔺cH experimentally?

A

Using spirit burner to heat water by burning substance

27
Q

Hwo do you calculate 🔺r H or 🔺neutH experimentally?

A

Using insulated beaker to determine temp change of solution

28
Q

What errors may occur when calculating 🔺H experimentally?

A

Heat lost to surroundings

Incomplete combustion

Substance evaporating from wick before reweighing

Non-standard conditions

29
Q

How may errors be reduced when calculating 🔺H experimentally?

A

Draught excluder/screen
Oxygen supply
Insulation
Lid

30
Q

Why might a cooling curve be used in calculating 🔺H experimentally?

A

Adjust value for loss of heat to surroundings to calculate temp change

Extrapolate using graph

31
Q

Why is enthalpy change hard to measure?

A

Other products form

High activation energy means reaction doesnt take place

32
Q

What is the disadvantage to using average bond enthalpy?

A

Average bond strength in lots of different molecules

33
Q

When calculating enthalpy change how do you know which number of moles to divide by?

A

Use moles of limiting reactant

34
Q

When calculating enthalpy change of neutralisation what moles do you divide KJ by?

A

The moles of OH- or H+

Chemistry (28 decks)

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