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AS Chemistry > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is the symbol for delta, which represents change?

A

Triangle

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2
Q

What represents enthalpy?

A

H

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3
Q

What is kJmol^-1 the unit for?

A

How much enthalpy for each mole that reacts

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4
Q

Enthalpy profile diagram:-

A

Arrow along y axis labelled H. Each side of reaction on a line, product slightly across. Arrow up/down, touching lines between the two, labelled with the enthalpy change.

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5
Q

What is enthalpy change of reaction?

A

The enthalpy change when the number of moles shown in the equation react under standard conditions.

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6
Q

What indicates enthalpy change of reaction?

A

Delta r H

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7
Q

What is enthalpy change of combustion?

A

Enthalpy change when 1 mol of a substance burns completely in oxygen.

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8
Q

What are the products of combustion?

A

CO2 and H2O

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9
Q

What is enthalpy change of formation?

A

Enthalpy change when 1 mol of a compound is formed FROM ITS ELEMENTS.

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10
Q

What shows enthalpy change of formation?

A

Delta f H

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11
Q

What shows combustion?

A

Delta c H

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12
Q

What is enthalpy change of neutralisation?

A

Enthalpy change when 1 mol of H2O is formed in a neutralisation reaction (acid + alkali).

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13
Q

What shows enthalpy change of neutralisation?

A

Delta neut H

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14
Q

What are standard conditions?

A

1 atm pressure and 25oC (unless otherwise stated)

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15
Q

What is the equation for calculating enthalpy change from temp change?

A

q=MCdeltaT

Key:-
q= J
M= substance mass
C= specific heat capacity
deltaT = temp change (oC/K)
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16
Q

What is Hess’ law?

A

Enthalpy change in a reaction doesn’t depend on the pathway of the reaction, provided initial and final states are the same.

17
Q

What does Hess’ law enable us to do?

A

Combine enthalpy change data to deduce enthalpy change that couldn’t be easily studied directly.

18
Q

What is bond enthalpy?

A

Enthalpy change when 1 mole of bonds are broken in the gas phase

19
Q

Why can’t method with XH2O simply as product (molecular) be used?

A

It doesn’t account for H2O’s intermolecular forces

20
Q

What is the problem with bond enthalpy?

A

We use average bond enthalpies (measured in a variety of compounds and averaged) so it isn’t exactly correct for any specific compound so only gives an approx enthalpy change value.

21
Q

How do you calculate total bond enthalpy?

A

Count number of bonds on each side and add enthalpies together.

22
Q

How do you calculate reaction enthalpy change using bond enthalpies?

A

X - Y kJmol^-1
X = reactant total
Y= product total

23
Q

Enthalpy change when exothermic/endothermic:-

A 
Exo = negative
Endo = positive

AS Chemistry (20 decks)

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