Enthalpy Flashcards

1
Q

What is enthalpy?

A

Describes all of the heat energy in a single chemical system

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2
Q

Enthalpy symbol

A

H

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3
Q

What can we use to measure enthalpy with instead (because it is too difficult to find)?

A

Enthalpy change

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4
Q

Enthalpy change symbol

A

△H

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5
Q

Unit of enthalpy/enthalpy change

A

Joules (J)

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6
Q

Enthalpy change on a reaction profile diagram

A

The difference between the enthalpy of reactants to the enthalpy of the products

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7
Q

reaction profile diagram of exothermic reaction

A

The reactants are higher than products (reactants have higher enthalpy than products)
Because energy was given out

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8
Q

Enthalpy change of an exothermic reaction

A

The enthalpy of products (lower) take away reactants (higher)
This value is negative to show energy lost to surroundings

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9
Q

Reaction profile diagram of an endothermic reaction

A

The products have higher enthalpy than reactants to show energy gained by molecules

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10
Q

Enthalpy change of an endothermic reaction

A

Enthalpy of products (higher) - reactants (lower)
So value is positive to show energy taken in from surroundings

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11
Q

Under what conditions are enthalpy changes measured in?

A

Standard conditions

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12
Q

Standard conditions

A

25°c temp
100 kPa
1molddm-3
All chemicals in their standard states (solid, liquid gas etc)

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13
Q

Standard enthalpy change symbol

A

△H⦵

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14
Q

Units for standard enthalpy change

A

KJmol-1
The enthalpy change (in energy, kilojoules) per MOL

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15
Q

Standard enthalpy change of combustion

A

The enthalpy change in a reaction when 1 mol of a substance reacts completely with oxygen (in complete combustion) under standard conditions where all species are in standard states

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16
Q

Standard enthalpy change of combustion symbol

17
Q

Important about standard enthalpy change of combustion?

A

BALANCED AROUND 1 MOL OF SUBSTANCE COMBUSTING

18
Q

Balancing combustion questions?

A

Balance the C in co2 first so equal to the carbon amount in 1 mol of the substance being burnt
Then hydrogen in water to be equal to hydrogen in 1 mol of substance
Then oxygen

19
Q

Standard enthalpy change of neutralisation symbol

A

△neutH⦵

20
Q

Standard enthalpy change of neutralisation definition

A

The enthalpy change in a reaction when 1 mole of water is formed in a neutralisation reaction between an acid and an alkali under standard conditions when all species are in standard states

21
Q

What is important about standard enthalpy change of neutralisation ?
Equation

A

The equation is balanced around 1 mole of water always

22
Q

Standard enthalpy change of neutralisation values are always…

A

Always negative: neutralisation reactions are exothermic

23
Q

Standard enthalpy change of combustion values are always…

A

Always negative
Combustion reactions are exothermic

24
Q

Standard enthalpy change of neutralisation between a strong acid and strong alkali

A

Usually around 57kjmol-1 : string acids and alkalis fully ionise to H+ + OH- ——> H2O

25
Standard enthalpy change of formation symbol
△fH⦵
26
Standard enthalpy change of formation definition
The enthalpy change in a reaction when 1 mole of a COMPOUND is formed from its elements under standard conditions (where all species are in its standard states)
27
What’s important about standard enthalpy change of formation equations?
Balanced around 1 mole of a compound The elements are in its standard states: eg oxygen is O2, hydrogen is H2, halogens etc
28
Standard enthalpy change of reaction symbol
△rH⦵
29
Standard enthalpy change of reaction definition
The enthalpy change in a reaction when a reaction takes place in the molar quantities shown in the BALANCED chemical equation under standard conditions in standard states
30
When is standard enthalpy change of reaction used?
If the reaction is not neutralisation, combustion or formation So for any other reaction
31
When defining any enthalpy change of x, what must you specific?
The standard conditions such as: 298K 100 KPa 1 molddm-3 Standard states
32
System
The atoms and bonds involved in a chemical reaction
33
Law of conservation
Energy in a closed system will always remain the same, none can be created or destroyed Only transferred from one form to another
34
Assumptions in Q= mc🔺T
Density of solutions is 1gcm-3 Specific heat capacity of solution is 4.18 so equal to in water