C7🛑

Question 1

What does the statement energy is conserved mean in a chemical reaction?

Answer 1

The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place

Question 2

If a reaction transfers energy to the surroundings which must have more energy the reactants or products?

Answer 2

If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.

Question 3

What is an exothermic reaction?

Name comon examples and uses…

Answer 3

An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. (Note in a reaction the solution is the “surroundings”

Examples combustion, many oxidation reactions and neutralisation.

Everyday uses of exothermic reactions include self-heating cans and hand warmers.

Question 4

What is an endothermic reaction?

Name comon examples and uses…

Answer 4

An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

Examples - thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate.

Uses - Some sports injury packs are based on endothermic reactions.

Question 5

What is activation energy ?

How is it shown on a reaction profile?

Answer 5

The minimum amount of energy that particles must have to react

It is shown by an intail energy rise arrow on a reaction profile IN THE MIDDLE OF THE ARK
(Note on a endothermic reaction it will always be a bigger arrow than the overall energy change)

Question 6

Explain the Reaction profile of an exothermic and an endothermic reaction in terms of the energy of products are reactants?

Answer 6

In an exothermic reaction The reactants are at a higher energy than the products

In an endothermic reaction The products sre at a higher energy than the reactants

Question 7

What is the overall energy changed

? (How is it shown on a reaction profile)?

Answer 7

The change in energy between the products and the reactants.
It is shown by an arrow between the energy of the products and the energy of the reactants reactants, ignoring activation energy.

Question 8

During a chemical reaction when must energy must be supplied and when is energy released?

Answer 8

energy must be supplied to break bonds in the reactants ( endothermic )
energy is released when bonds in the products are formed. ( exothermic )

Question 9

Exolain exothermic and endothermic reactions in terms of energy released in making and energy used to break bonds

Answer 9

In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Question 10

If the total bond energy before a reaction is 6186 and total bond the energy after is 7458, is the reaction endo or exo? Why?

Answer 10

7458 (kJ) (released in making bonds) is greater than 6186 (kJ) (used in breaking bonds)
(so) energy is released (to the surroundings)
Exothermic

Question 11

What are the half equations of a hydrogen fuel cell?

Answer 11

In an acidic electrolyte (H+)
-     H2→2H+ +2e-   Or   H2 -2e-→2H+
-     O2 +4H+ +4e- →2H2O 
In an alkali eletrolyte (OH-) 
-     H2 +2OH- →2H2O+2e-   Or H2 +2OH- -2e-→2H2O
-     O2 +2H2O+4e- →4OH-

Question 12

What is a fuel cell? And how does it produce electricty?

Answer 12

Fuel cells are electrical cells that are supplied by an external source of fuel (eg hydrogen) and oxygen or air.
When the fuel (hydrogen) enters the cell it is oxidised within the fuel cell to produce a potential difference withon the cell

Question 13

What is the overall reaction in a fuel cell?

Answer 13

2H2+02 -> 2H20

Hydrogen + oxygen -> water

Question 14

Explain the what happens in the hydrogen fuel cell?

Answer 14

• Hydrogen enters the anode compartment (the anode is negative in a fuel cell) through the porous electrode
• the hydrogen loose electron to produce H+ ions (oxidation)
• these ions join the other H+ ions in the acdic electrolyte and move in the direction of the cathode
• Oxygen goes into thé cathode compartment where it gains electrons and reacts with the H+ ions to maké water.
• the potentail difference created allows the electrons tó flow through an external circit from the anode to the cathode - creating an eletric current

Question 15

evaluate the advantages of using hydrogen fuel cells in comparison with rechargeable cells and batteries

Answer 15

Advantages :

• no pollutants; green house gases such as co2 or carbon monoxide. Electrical cells also don’t produce pollutants but the batteries are more polluting to dispose of because they are made of highly toxic compounds
• batteries have a limit to how many times they can be recharged before replacement
• batteries are more expensive
• batteries store less energy so would need to be recharged more often

Question 16

evaluate the disadvantsges of using hydrogen fuel cells in comparison with rechargeable cells and batteries

Answer 16

• hydrogen is a gas so it takes up more space to storé than a rechargable battery
• hydrogen is expolosive when mixed with air so it hard to store safley
• the hydrogen is often made from either hydrocarbons from fossil fuels or by eletrolosis (which require energy probably from fossil fuels)

Question 17

How do cells produce electricty?

Answer 17

SPEC DEF: Cells contain chemicals which react to produce electricity.

EXTRA INFORMATION: Cells are basic system made of two electrodes (generally metals able to conduct) in contact with an electrolyte. The reaction between the the electrode and electrolyte set up a charge difference beween the electrodes. If the electrodes are connected by a wire charge is able to flow and electricty is produced.

Question 18

What are the factors that effect voltage?

Answer 18

The material of the electrodes effects the voltagen the bigger the difference in reactivity the larger the voltage of the cell.

The electrolyet also effects the voltage of the cell since different ion in the solution will react differentely with the metal electrodes

Question 19

What are batteries ?

Answer 19

Batteries consist of two or more cells connected together in series to provide a greater voltage.

Question 20

What are non-recharable cells or batteries ?

Give an example?

Answer 20

cells and batteries where the chemical reactions stops when one of the reactants (eg electrolyte or the electrodes) has been used up. Because it is a irreversible reaction no more energy can be produced. Alkaline batteries are non-rechargeable.

Question 21

What aré recharable cells or batteries?

Answer 21

Rechargeable cells and batteries are cells which can be recharged because the chemical reactions are reversed when an external electrical current is supplied.