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A-level Chemistry Unit 2 > 2.1 Thermochemistry > Flashcards

2.1 Thermochemistry Flashcards

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1
Q

What do all chemical reactions involve?

A

All chemical reactions involve change.

These energy changes are vital to us.

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2
Q

Why do plants need energy from the Sun?

A

Plants need the energy from the Sun for the production of carbohydrates by photosynthesis.

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3
Q

What kind of energy do we depend on?

A

We depend on the energy content of the food we eat.

The kind of life we lead depends on harnessing energy from different sources

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4
Q

Name some forms of energy.

A

There are many forms of energy but basically there are only two kinds of energy: kinetic energy and potential energy.

Heat is a form of kinetic energy while the energy of chemical bonds is a form of potential energy.

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5
Q

What is thermochemistry?

A

Thermochemistry is the study of the energy changes that accompany chemical reactions.

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6
Q

In what form does matter posess energy as?

A

Matter possesses energy in the form of kinetic energy and potential energy.

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7
Q

What is the kinetic energy of matter?

A

The kinetic energy of matter is the energy of motion at a molecular level.

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8
Q

When does the potential energy of matter arise?

A

The potential energy of matter arises from the positions of the atoms relative to one another.

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9
Q

What does bond-breaking and bond-making involve?

A

Bond-breaking and bond-making involves changes in potential energy.

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10
Q

What is the internal energy of a system?

A

The sum of the kinetic energy and the potential energy of all the particles in a system is the internal energy of a system.

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11
Q

What causes a change in the chemical energy?

A

In the course of a chemical reaction, existing bonds are broken and new ones are made.

This changes the chemical energy of atoms and energy is exchanged between the chemical system and the surroundings.

Frequently, this leads to heat being given out or taken in from the surroundings.

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12
Q

How can chemical reactions be detected?

A

Most chemical reactions release energy to their surroundings.

This can be detected by a rise in the temperature of the reaction mixture and the surroundings.

These reactions are known as exothermic reactions.

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13
Q

Define exothermic reaction.

A

An exothermic reaction is one that releases energy to the surroundings, there is a temperature rise and the enthalpy change is negative.

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14
Q

Give some examples of exothermic reactions.

A

Acids with metals

In hand warmers (oxidation of iron)

Thermite reaction (aluminium and iron (III) oxide)

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15
Q

What are endothermic reactions?

A

In some reactions, the system absorbs energy from its surroundings in the form of heat.

Such reactions are known as endothermic reactions.

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16
Q

Define endothermic reactions.

A

An endothermic reaction is one that takes in energy from the surroundings, there is a temperature drop and the enthalpy change is positive.

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17
Q

Give some examples of endothermic reactions.

A

Melting ice

In cold packs (dissolving ammonium chloride in water)

Thermal decomposition of Group 2 carbonates.

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18
Q

What is chemical energy?

A

Chemical energy is a form of potential energy which is stored in chemical bonds.

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19
Q

What happens as a reaction takes place?

A

As a reaction takes place, bonds break in the reactants and new bonds are formed in the products.

20
Q

What does the amount of heat transferred in a reaction depend on?

A

It depends on the conditions under which the reaction occurs.

Most chemical reactions in the laboratory take place under constant pressure.

21
Q

What is enthalpy?

A

The total energy content of a system held at constant pressure is defined as its enthalpy.

22
Q

What is the symbol of enthalpy?

A

H

23
Q

Can enthalpy be measured directly?

A

No. It is impossible to measure the enthalpy content of a system directly but we can measure the differences in enthalpy contents.

24
Q

What are the units of enthalpy change?

A

joules (J) or kilojoules (kJ)

25
Q

State the law of conservation of energy.

A

The law of Conservation of Energy states that energy cannot be created or destroyed. It can just be transferred, in this case between the system and the surroundings.

26
Q

How do we calculate enthalpy change?

A

enthalpy of products - enthalpy of reactants

27
Q

For exothermic changes:

A

heat is given out to the surroundings so, enthalpy of products is less than the enthalpy of the reactants, and enthalpy change is negative.

28
Q

For endothermic changes:

A

heat is taken in from the surroundings so the enthalpy of the products is greater than the enthalpy of the reactants, and the enthalpy change is positive.

29
Q

How can enthalpy changes be represented?

A

Enthalpy changes can be represented as enthalpy profile diagrams.

30
Q

What is the chemical system?

A

The chemical system is the reactants and products.

The surroundings are everything other than the system.

31
Q

Draw an enthalpy profile diagram for an exothermic reaction.

A
32
Q

Draw an enthalpy profile diagram for an endothermic reaction.

A
33
Q

When is standard enthalpy change measured?

A

Since the enthalpy change for reactions depends on the conditions, for values to be compared standard enthalpy change is measured when fixed conditions are used.

34
Q

What are the standard conditions?

A

All substances in their standard states
A temperature of 298K (25 degree Celsuis)
A pressure of 1 atm (101 kPa)

35
Q

What is the symbol for standard enthalpy change?

A

ΔH°

36
Q

What is the standard state of an element that exists in more than one state?

A

If an element exists in more than one state, the standard state used should be the one that is most stable at 1 atm and 298K.

For carbon, graphite is more stable than diamond so in enthalpy equations, C(graphite) should be used.

37
Q

What is the standard enthalpy change of combustion?

A

This is the enthalpy change when 1 mole of a substance in its standard state is completely burnt in excess oxygen under standard conditions of 298K and 101kPa (1 atm)

38
Q

How is the standard enthalpy change of combustion of methane given by?

A

CH4(g) + 2O2(g) —-> CO2 (g) + 2H2O(l)

ΔcH° = -891kJmol^-1

39
Q

How is the standard enthalpy change of combustion of hydrogen given by?

A

H2(g) + 1/2O2(g) —-> H2O(l)

ΔcH° = -286kJmol^-1

40
Q

What does the ‘per mol’ refer to in standard enthalpy change of combustion?

A

The ‘per mol’ refers to the substance being combusted not to the quantity of products formed.

41
Q

What is the standard enthalpy change of formation?

A

The standard enthalpy change of formation is the enthalpy change when 1 mole of a substance in its standard state is formed from its constituent elements in their standard states under standard conditions of 298K and 101kPa

42
Q

How is the standard enthalpy change of formation of carbon dioxide given by?

A

C(graphite) + O2(g) —> CO2(g)

ΔfH° = -394kJmol^-1

43
Q

How is the standard enthalpy change of formation of carbon monoxide represented as?

A

C(graphite) + 1/2O2(g) —> CO(g)

ΔfH° = -111kJmol^-1

44
Q

What does the ‘per mol’ refer to in the standard enthalpy change of formation?

A

The ‘per mol’ refers to the formation of one mole of compound, not to the quantity of elements.

45
Q

Why are the number of moles fractionated in some equations?

A

They are fractionated so that we have the correct number of moles involved in the change.

A-level Chemistry Unit 2 (1 decks)

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