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GCSE Chemistry > Energy Changes > Flashcards

Energy Changes Flashcards

Exothermic and Endothermic reactions, reaction profiles and some electrolysis.

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1
Q

What is an Exothermic reaction?

A

An Exothermic reaction transfers energy to its surroundings, usually by heating. This is shown is a rise in temperature

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2
Q

What is an Endothermic reaction?

A

An Endothermic reaction absorbs energy from the surroundings, usually thermal energy. This is shown in a fall in temperature.

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3
Q

Examples of Exothermic reactions

A 
Combustion
Neutralisation reactions (acid + alkali)
Respiration
Thermit process
Reactions with metals and acids
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4
Q

Examples of Endothermic reactions

A

Photosynthesis
Sherbet
Thermal decomposition
Electrolysis (some types)

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5
Q

Conservation of energy in chemical reactions

A

The amount of energy does not change. Energy cannot be created or destroyed, it can only be transferred. In chemical reactions energy changes involve the surroundings.

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6
Q

What determines if a reaction is Exothermic?

A

If more energy is released through bond making than is absorbed for bond breaking the reaction is Exothermic.

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7
Q

What determines if a reaction is Endothermic?

A

If more energy is absorbed to break bonds than is released when making bonds then the reactions is Endothermic.

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8
Q

What are Reaction Profiles?

A

Reaction profiles are diagrams/graphs that show the relative energies of reactants and products in a reaction, and show how the energy changes over the course of the reaction.

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9
Q

What does an Exothermic reaction profile look like?

A

Picture

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10
Q

What does and Endothermic reaction profile look like?

A

Picture

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11
Q

Why does a Exothermic reaction profile look like that?

A

Because energy is released to the surroundings so the reactants have more energy than the products.

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12
Q

Why does and Endothermic reaction profile look like that?

A

Because energy is absorbed from the surroundings so the reactants have less energy than the products.

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13
Q

What occurs in a chemical reaction?

A

Old bonds are broken then new bonds are made, to create a new compound or molecule.

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14
Q

What must occur for bonds to be broken in a chemical reaction?

A

The reacting particles must collide with sufficient energy (activation energy).

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15
Q

What is the activation energy?

A

It is the minimum amount of energy that particles must have in order reaction to take place in a chemical.

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16
Q

Describe the insulated reaction method

A

You can measure the energy change in a chemical reaction by taking the temperature of the regents, mixing them in a polystyrene cup (insulator) and measuring the temperature of the solution at the end of the reaction.

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17
Q

What reactions can you use the insulated reaction method to measure energy change?

A

Neutralisation

Reactions between metals and acids/ carbonates and acids.

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18
Q

Bond breaking is…

A

Endothermic (absorbs/requires energy from surroundings)

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19
Q

Bond making is…

A

Exothermic (releases energy into surroundings)

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20
Q

What is an electrochemical cell?

A

A basic system made up of two different electrodes in contact with an electrolyte.

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21
Q

What is electrolysis?

A

Chemical decomposition produced by passing an electric current through a liquid or solution containing ions.

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22
Q

What are positive ions called?

A

Cations

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23
Q

What are negative ions called?

A

Anions

24
Q

What are the positive ions attracted to?

A

The cations (positive) are attracted to the cathode (negative electrode)

25
Q

What are negative ions attracted to?

A

Anions (negative) are attracted to the anode (positive electrode)

26
Q

What is an electrolyte?

A

An electrolyte is a liquid that contains free moving ions and conducts electricity.

27
Q

What is a fuel cell?

A

It converts chemical energy into electrical energy, it is an open system. Reactants flow in and products flow out, rather than being stored in a cell.

28
Q

How does electrolysis produce electricity?

A

The movement/flow of delocalised electrons from one electrode to another carries the electrical charge and changes chemical energy into electrical.

29
Q

Draw an electrochemical cell

A

Picture

30
Q

What are hydrogen fuel cells used for?

A

Rocket fuel and potentially cars

31
Q

Draw a hydrogen fuel cell

A

Picture

32
Q

What are the reactants and products of a hydrogen fuel cell?

A

Hydrogen + oxygen (from the atmosphere) react to create water

33
Q

Where do the reactants go in a hydrogen fuel cell?

A

Hydrogen goes into the anode compartment and the oxygen goes into the cathode department

34
Q

What occurs in a hydrogen cell?

A

1) At the negative electrode (the anode), hydrogen loses electrons to produce H+ ions. This is ionisation.
2) H+ ions in the electrolyte move to the positive electrode (the cathode).
3) At the cathode oxygen gains electrons from the cathode and reacts with the H+ ions (from the electrolyte) to make water. This is called reduction.
4) The electrons flow through an external circuit from the anode to the cathode - this is electrical current.

35
Q

What are the advantages of hydrogen fuel cells?

A

Higher efficiency than petrol or diesel engines
Operates over a long time
Low heat transmission
Quieter than combustion engines
Few moving parts
Batteries are more expensive to make than fuel cells

36
Q

What are the disadvantages of hydrogen fuel cells?

A

Diving range is shorter ( than petrol engine cars)
More expensive than petrol engines to make
Take up more space
Refueling takes longer
Produces gaseous H²O (which is classed as a greenhouse gas)
Hydrogen is explosive so it is hard to transport and store safely
Production of hydrogen via electrolysis uses electricity and can therefore pollute

37
Q

Half and symbol equations for hydrogen fuel cells

A

At the anode:
2H²(g) —-> 4H(aq) + 4e- OXIDATION

At the cathode:
4H(aq) + O²(g) + 4e- —-> 2H²O(l) REDUCTION

Overall equation:
2H²(g) + O²(g) —-> 2H²O(l)

38
Q

What are the two types of cells?

A

Primary (non-rechargeable) and Secondary (rechargeable)

39
Q

Why are primary/non-rechargeable cells non-rechargeable?

A

Because they carry out irreversible reactions.
So once one of the reactants is totally used up the reaction cannot occur anymore and so no electricity is produced.
The products cannot be turned back into reactants so the cell can’t be recharged.

40
Q

Why are secondary/rechargeable cells rechargeable?

A

Because they carry out reversible reactions so once any reactant is used up it can be reversed into a reactant again by connecting it to an external electrical current.

41
Q

Advantages of primary cells

A 
They hold charge for longer
You can recycle them 
They are cheaper (short-term)
Have a higher capacity and initial voltage
Higher versatility
More convenient
42
Q

Disadvantages of primary cells

A

Cannot be recharged, single use

Bad for the environment, produce more waste

43
Q

Advantages of secondary cells

A 
Are rechargeable
Produce less waste
Are cheaper (long-term)
44
Q

Disadvantages of secondary cells

A

They loose the ability to hold charge well
Cannot be reused quickly, takes time to recharge
Need electricity to be recharged, bad for the environment

45
Q

Examples of primary cells

A

Dry cells
(Most) alkaline cells
Carbon-zinc dry cell

46
Q

Examples of secondary cells

A

Lithium ion
Lead acid
Nickel-cadmium (NiCd)

47
Q

What are the variables in different cells?

A

They can have different voltages
Size determines how long the charge will last but does not change the voltage
(Different brands can have an affect on duration)

48
Q

What is a battery?

A

A container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power.

49
Q

What is a cell?

A

A device containing electrodes immersed in an electrolyte, used for generating current or for electrolysis.

50
Q

What factors affect voltage?

A

How many/what type of cells you have within a battery
What types of metals make up the electrodes
The electrolyte
Difference in reactivity between the electrodes

51
Q

Explain the factors that affect voltage

A

Different metals react differently with the same electrolyte, the bigger the difference between the reactivity of the electrodes the higher the voltage of the cell.

The electrolyte will also affect the voltage since different ions in solution will react differently with the metal electrodes used.

A battery is formed by connecting two or more cells, the voltages are combined so there is a higher voltage overall.

52
Q

The reactivity series

A 
Potassium K
Sodium Na
Lithium Li
Calcium Ca
Magnesium Mg
Carbon C
Zinc Zn
Iron Fe
Copper Cu
53
Q

What do catalysts do?

A

Catalysts can increase reaction rates by loweringthe activation energy, which increases the likelihood of successful collisions.

54
Q

Bond Energies

A

Bond energy is the measure of the strength of chemical bonds

55
Q

Bond Energy units…

A

Kilojoules per mole (kj/mole)

GCSE Chemistry (7 decks)

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