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Energy changes Flashcards

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1
Q

What is an exothermic reaction?

A

A reaction that releases energy to the surroundings, usually as heat.

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2
Q

What is an endothermic reaction?

A

A reaction that absorbs energy from the surroundings.

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3
Q

State the law of conservation of energy.

A

Energy cannot be created or destroyed, only transferred.

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4
Q

How is energy change calculated in a reaction?

A

Energy change = Energy of bonds broken - Energy of bonds formed.

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5
Q

What is activation energy?

A

The minimum amount of energy needed for a reaction to occur.

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6
Q

What happens to temperature in an exothermic reaction?

A

It increases because energy is released.

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7
Q

What happens to temperature in an endothermic reaction?

A

It decreases because energy is absorbed.

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8
Q

Give an example of an exothermic reaction.

A

Combustion.

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9
Q

Give an example of an endothermic reaction.

A

Thermal decomposition.

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10
Q

Why are bond energies used in energy calculations?

A

To calculate the total energy needed to break and form bonds.

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11
Q

What does a reaction profile show?

A

The energy levels of reactants and products during a reaction.

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12
Q

What does the peak of a reaction profile represent?

A

The activation energy of the reaction.

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13
Q

In a reaction profile, how can you tell if a reaction is exothermic?

A

Products are at a lower energy level than reactants.

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14
Q

In a reaction profile, how can you tell if a reaction is endothermic?

A

Products are at a higher energy level than reactants.

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15
Q

What units are used for bond energies?

A

kJ/mol.

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16
Q

What is a catalyst?

A

A substance that speeds up a reaction without being used up.

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17
Q

How does a catalyst affect activation energy?

A

It lowers the activation energy.

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18
Q

Why is lowering activation energy beneficial?

A

It allows more particles to react, increasing reaction rate.

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19
Q

What is a fuel?

A

A substance that releases energy when it burns.

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20
Q

What is combustion?

A

The burning of a substance in oxygen, usually releasing heat and light.

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21
Q

Name the products of complete combustion of a hydrocarbon.

A

Carbon dioxide and water.

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22
Q

Name the products of incomplete combustion of a hydrocarbon.

A

Carbon monoxide, carbon (soot), and water.

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23
Q

Why is carbon monoxide dangerous?

A

It is toxic and prevents red blood cells from carrying oxygen.

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24
Q

What is the main use of energy from fuels?

A

Heating, transport, and electricity generation.

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25
What is meant by the term 'energy level'?
The amount of energy stored in chemicals as potential energy.
26
What are bond enthalpies?
The amount of energy required to break one mole of a bond in a gaseous molecule.
27
What does a negative energy change indicate?
The reaction is exothermic.
28
What does a positive energy change indicate?
The reaction is endothermic.
29
What is the energy change when breaking bonds?
Energy is absorbed (endothermic).
30
What is the energy change when forming bonds?
Energy is released (exothermic).
31
How can we estimate energy changes using bond energies?
By calculating total energy in bonds broken and formed.
32
Why do some reactions not occur even if they are exothermic?
They may have high activation energy.
33
How can energy changes be measured in the lab?
Using a calorimeter to measure temperature changes (not required for practicals).
34
What affects the amount of energy released in combustion?
The type and amount of fuel used.
35
What is the energy change per gram of fuel?
Energy released divided by mass of fuel burned.
36
Why is hydrogen considered a clean fuel?
It only produces water when burned.
37
What are the disadvantages of hydrogen fuel?
Difficult to store and requires energy to produce.
38
How is hydrogen fuel produced?
By electrolysis of water or from hydrocarbons.
39
What is meant by energy efficiency?
The proportion of energy transferred usefully.
40
What does a high energy efficiency mean?
Less energy is wasted as heat.
41
What is a fuel cell?
A device that converts chemical energy directly into electrical energy.
42
What is the advantage of fuel cells over batteries?
They produce electricity continuously if fuel is supplied.
43
What is the half equation for hydrogen in a fuel cell?
H₂ → 2H⁺ + 2e⁻
44
What is the half equation for oxygen in a fuel cell?
O₂ + 4H⁺ + 4e⁻ → 2H₂O
45
Why are fuel cells used in spacecraft?
They provide electricity and water, and are lightweight.
46
How do fuel cells differ from combustion engines?
They are more efficient and produce fewer pollutants.
47
What are the limitations of fuel cells?
High cost and storage issues of hydrogen.
48
Why is energy transfer not 100% efficient?
Some energy is always lost to the surroundings as heat.
49
What is the role of bond making in an exothermic reaction?
It releases energy to the surroundings.
50
What is the role of bond breaking in an endothermic reaction?
It absorbs energy from the surroundings.
51
Why are energy changes important in industry?
To ensure reactions are cost-effective and safe.
52
How can energy changes affect equilibrium?
Exothermic and endothermic changes shift the equilibrium position.
53
What are reversible reactions?
Reactions where products can reform reactants.
54
How does temperature affect a reversible reaction?
Higher temperature favors the endothermic direction.
55
How does pressure affect gaseous reactions?
Higher pressure favors the side with fewer gas molecules.
56
Why do industries use catalysts in energy reactions?
To reduce energy costs and increase efficiency.
57
What is meant by sustainable energy?
Energy that meets today's needs without compromising the future.
58
What makes a good fuel?
High energy output, easy to store and transport, low pollution.
59
What is energy density?
The amount of energy stored per unit mass or volume of a fuel.
60
Why is methane a commonly used fuel?
It has a high energy density and burns cleanly.
61
What is thermal decomposition?
A chemical reaction where a compound breaks down using heat.
62
What does 'enthalpy change' mean?
The overall heat energy change in a reaction at constant pressure.

Chemistry (7 decks)

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