energy changes

Question 1

When chemical reactions occur…

Answer 1

• energy​ ​is​ ​conserved
• the​ ​amount​ ​of​ ​energy​ ​in​ ​the​ ​universe​ ​at​ ​the​ ​beginning​ ​is​ ​the​ ​same​ ​as​ ​at the​ ​end.

Question 2

Define an exothermic reaction

Answer 2

• a reaction that transfers energy to the surroundings,
• so the temperature of the surroundings increases.
• therefore, the product molecules have less energy than the reactants

Question 3

Name examples of exothermic reactions

Answer 3

• combustion
• many oxidation reactions
• neutralisation

Question 4

Describe the reaction profile of exothermic reactions

Answer 4

• reactants are at a higher energy than the products
• because energy has been transferred from the molecules to the surroundings (energy is lost to the surroundings)
• negative energy change

Question 5

Name examples of everyday uses of exothermic reactions

Answer 5

• self-heating cans
• hand warmers

Question 6

Define an endothermic reaction

Answer 6

• a reaction that takes in energy from the surrounds s
• so the temperature of the surroundings decrease.
• therefore, the reactant molecules have less energy than the products

Question 7

Name examples of endothermic reactions

Answer 7

• thermal decompositions
• reaction of citric acid and * sodium hydrogencarbonate

Question 8

Describe the reaction profile of endothermic reactions

Answer 8

• reactants are at a lower energy than the products
• because energy is taken in from the surroundings to the molecules (temperature of surroundings decrease)
• positive energy change

Question 9

Name examples of everyday uses of endothermic reactions

Answer 9

• sports injury packs (cold packs)

Question 10

Chemical reactions can only occur when…

Answer 10

• reacting particles collide with each other and with sufficient energy

Question 11

Define activation energy

Answer 11

• the minimum amount of energy that particles must have to react (for a reaction to occur)

Question 12

What do reaction profiles show?

Answer 12

• the relative energies of reactants and products
• activation energy
• if its exothermic or if its endothermic
• overall energy change

Question 13

During a chemical reaction…

Answer 13

• energy must be supplied to break bonds in the reactants
• energy is released when bonds in the products are formed

Question 14

What is bond energy?

Answer 14

• the energy needed to break bonds and the energy released when bonds are formed

Question 15

How do you calculate overall energy change?

Answer 15

• calculate the bond energies for the reactants and for the products
• energy change = energy of bond breaking - energy of bond making
• i.e. bond energy of reactants minus bond energy of products

Question 16

Define overall energy change

Answer 16

• the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed

Question 17

Describe making and breaking bonds in an exothermic reaction

Answer 17

• the energy released from forming new bonds is greater than the energy needed to break existing bonds
• i.e. overall energy change will be negative

Question 18

Describe making and breaking bonds in an endothermic reaction

Answer 18

• the energy needed to break existing bonds is greater than the energy released from forming new bonds
• i.e. overall energy change will be positive

Question 19

Define an electrolyte

Answer 19

• a solution that can conduct electricity
• e.g. an ionic compound solution

Question 20

How can you produce electricity using an electrolyte?

Answer 20

*by taking two different metals and placing them into an electrolyte

Question 21

What do cells contain?

Answer 21

• chemicals which react to produce electricity

Question 22

How is a simple cell made?

Answer 22

• it can be made by connecting two different metals in contact with an electrolyte

Question 23

What is the potential difference produced by a cell dependent on?

Answer 23

• type of electrode
• type of electrolyte
• concentration; the greater the concentration of ions in the electrolyte, the greater the potential difference produced

Question 24

Define a battery

Answer 24

• two or more cells connected in series to provide a greater voltage

Question 25

How do you provide a greater voltage?

Answer 25

• by connecting two or more cells together in series (batteries do this)

Question 26

When will cells produce electricity?

Answer 26

• when there is a difference in reactivity between the two metals * e.g., if they are the same metal, there will be no difference in reactivity, so no current will flow

Question 27

How does difference in reactivity relate to potential difference?

Answer 27

• the greater the difference in reactivity between the two metals, the greater the potential difference produced by the cell.
• e.g., using magnesium and copper will produce a greater potential difference than using iron and copper

Question 28

What happens in non-rechargeable cells and batteries?

Answer 28

• the reaction is irreversible,
• so the chemical reactions stop when one of the reactants has been used up (i.e. the limiting reactant), meaning no more electricity is produced

Question 29

How do rechargeable cells and batteries work?

Answer 29

• the reaction is reversible, so when an external electrical current is supplied, the chemical reaction is reversed and the reactants are produced again

Question 30

Why does the more reactive metal transfer electrons to a less reactive metal? Use the example of zinc and copper electrodes

Answer 30

• because more reactive metals have a higher tendency to lose electrons to form positive ions
• e.g., the zinc electrode will lose electrons (oxidise) to the copper electrode which gains these electrons (reduced). because the zinc is the source of the electrons, the more reactive metal is the negative electrode, and the less reactive metal is the positive electrode

Question 31

Name an example of a non-rechargeable battery

Answer 31

• an alkaline battery

Question 32

What are fuel cells supplied by?

Answer 32

• an external source of fuel (e.g. hydrogen) and oxygen or air

Question 33

How do fuel cells produce a potential difference?

Answer 33

• the fuel is oxidised electrochemically within the fuel cell to produce a potential difference

Question 34

What does the overall reaction in a hydrogen fuel cell involve?

Answer 34

• the overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water

Question 35

What do hydrogen fuel cells offer?

Answer 35

• a potential alternative to rechargeable cells and batteries

Question 36

What are the cons of rechargeable batteries and cells in relation to fuel cells?

Answer 36

• rechargeable batteries store less electricity the more charging cycles they go through, so eventually need to be replaced,
• however hydrogen fuel cells do not get less efficient the longer they run
• batteries store less energy than fuel cells, so need to be recharged more often
• recharging batteries takes a long time, whereas fuel cells take a very short amount of time to refill with hydrogen
• hard to dispose of as it is non-biodegradable where as fuel cells only produce water when burnt

Question 37

What are the pros of rechargeable cells and batteries in relation to fuel cell?

Answer 37

• fuel cells run on hydrogen, which is an explosive gas if exposed to a flame, and is very difficult to store safely.
• no dangerous fuels are required with rechargeable batteries that are manufactured correctly
• hydrogen fuel cells produce a relatively low potential difference so several are needed together, * whereas rechargeable batteries produce a greater potential difference
• hydrogen is a gas so takes up way more space to store than rechargeable batteries

Question 38

State the half equation for the fuel cell reaction of hydrogen and oxygen; what does this reaction do?

Answer 38

• 2H₂ + O₂ → 2H₂O
• it releases energy in the form of an electrical current and the only waste product it releases is water

Question 39

What happens at the negative electrode of a fuel cell?
State the half equation

Answer 39

• hydrogen molecules lose electrons (are oxidised) to form H⁺ ions; we can represent this using
• 2H₂ → 4H⁺ + 4e⁻

Question 40

What happens to the electrons lost from the hydrogen molecules at the negative electrode of a fuel cell, and what happens to the ions formed?

Answer 40

• the lost electrons pass through the wire to form the electrical current.
• the H⁺ ions produced from the oxidation of hydrogen molecules are then passed through the electrolyte and to the cathode where they react with oxygen to form water

Question 41

What does the electrolyte not/let through it?

Answer 41

• it doesn’t let hydrogen or electrons through,
• it only allows H⁺ ions (protons) through it

Question 42

What happens at the positive electrode of a fuel cel?
State the half equation

Answer 42

• oxygen molecules gain electrons (reduced) and react with the H⁺ ions to form water;
• O₂ + 4H⁺ + 4e⁻ → 2H₂O

Question 43

How are hydrogen fuel cells different from electrolysis, and how are they the same?

Answer 43

• DIFFERENCE: in electrolysis, the negative electrode is the cathode and the positive electrode is the anode. in hydrogen fuel cells, it is the opposite
• SIMILARITY: the anode is always the site of oxidation and the cathode is always the site of reduction

Question 44

Where does each component come from in a hydrogen fuel cell? (H⁺ ions & electrons)

Answer 44

• the H⁺ ions come from the oxidation of hydrogen at the anode and have travelled through the electrolyte
• the electrons have come from the electrical circuit

Question 45

What is oxidised, what is reduced and where in a hydrogen fuel cell?

Answer 45

• hydrogen is oxidised at the anode (the negative electrode)
• oxygen is reduced at the cathode (the positive electrode)