3.2.1: Enthalpy changes Flashcards

1
Q

What does system mean in a chemical reaction?

A

The atoms and bonds involved in the

chemical reaction

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2
Q

Explain the law of conservation

A

The amount of energy in an isolated system
remains the same. Energy cannot be destroyed
or created, It can only be transferred from one
form to another

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3
Q

What energy change is breaking bonds associated with?

A

Energy is taken in to break bonds →

endothermic reaction

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4
Q

What energy change is making bonds associated with?

A

Energy is released to make bonds →

exothermic reaction

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5
Q

What is an endothermic reaction?

A

A reaction with an overall positive enthalpy change (+ΔH) → enthalpy of products > enthalpy of reactants

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6
Q

What is an exothermic reaction?

A

A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants

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7
Q

What does activation energy mean?

A

The minimum energy required for a reaction to take place

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8
Q

Which way does the arrow for activation energy point on an enthalpy profile diagram?

A

Always points upwards

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9
Q

What are the standard conditions?

A

100 kPa

298 K

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10
Q

What does “in standard state” mean?

A

The state an element / compound exists at in standard conditions (100 kPa, 298 K)

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11
Q

Define enthalpy change of formation

A

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions

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12
Q

Give an example of an equation which represents standard enthalpy of formation

A

There are many e.g. H2 (g) + ½ O2 (g) → H2O (l)

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13
Q

Define enthalpy change of combustion

A

The energy change that takes place when 1 mole of a substance is completely combusted

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14
Q

Give an example of an equation which represents standard enthalpy of combustion

A

E.g. C (s) + O2 (g) → CO2 (g)

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15
Q

Define enthalpy change of neutralisation

A

The energy change that takes place when 1 mole of water is formed from a neutralisation reaction

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16
Q

What does enthalpy change of reaction mean?

A

The energy change associated with a given reaction

17
Q

How can you calculate enthalpy change from experimental data?

A

Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K-1) and ΔT is the change in temperature

18
Q

What are the advantages of

using a bomb calorimeter? (2 marks)

A

● Minimises heat loss
● Pure oxygen used → ensures
complete combustion

19
Q

Why might experimental methods for enthalpy determination not be accurate?

A

Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion

20
Q

What does average bond enthalpy mean?

A

The mean energy required to break 1 mole of bonds in gaseous molecules

21
Q

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

A

Bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.

22
Q

How to calculate enthalpy change of reaction using average bond enthalpies?

A

ΔH = Σ (bond enthalpies of reaction) -

Σ (bond enthalpies of products)