C5 - Energy Changes

Where is energy stored?

Chemical bonds

What is an exothermic reaction?

A reaction where energy is transferred to its surroundings

If an exothermic reaction occurs in a closed container, what would happen to the temperature inside the container?

Temperature increases

What is an endothermic reaction?

A reaction that takes in energy from its surroundings

If an endothermic reaction occurs in a closed container, what would happen to the temperature inside the container?

Temperature decreases

What type of reaction does this image show?

Endothermic

What type of reaction does this image show?

Exothermic

Define activation energy

The minimum amount of energy the reactant particles need in order to collide with each other and react

Does breaking bonds release energy or require energy?

Requires energy

Is breaking bonds endothermic or exothermic?

Endothermic

Define bond energy

The energy required to break one mole of a particular covalent bond

During a chemical reaction, the bonds of the products must be:

formed

Name three reactions that are exothermic

Combustion, neutralisation and oxidation

Name a common endothermic reaction

Thermal decomposition

Which part of a reaction profile shows the activation energy?

reactants to the peak of the curve

How do chemical cells transfer energy to their surroundings?

By electricty

Are alkaline batteries rechargeable or non-rechargeable?

Non rechargeable

Fuel cells convert the chemical energy in a fuel and oxygen into:

electricity

Which direction do the electrons flow in a hydrogen-oxygen fuel cell?

Anode to cathode

What is the overall reaction for a hydrogen oxygen fuel cell?

hydrogen + oxygen -> water

Which compartment does the oxygen enter in a hydrogen-oxygen fuel cell?

The cathode compartment

What enters the anode compartment in a hydrogen-oxygen fuel cell?

Hydrogen

What type of reaction would a negative overall energy be?

Exothermic

What would a positive value mean in a bond energy calculation final answer?

Endothermic

What type of reaction is breaking bonds?

Endothermic

What type of reaction is forming bonds?

Exothermic

What do bonds require to break?

Energy

What factors affect the voltage of a cell?

Difference in reactivity between metals for electrodes, concentration and type of electrolyte, temperature of the electrolyte

What are the usual external sources of fuel used in a fuel cell?

Hydrogen and oxygen

What is oxidised in a fuel cell?

Hydrogen

What is reduced in a fuel cell?

Oxygen

Name a disadvantage of hydrogen fuels cells

Hydrogen hard to be obtained,

Hydrogen highly flammable and explosive,

Hydrogen hard to store and transport (gas at room temp)

Name an advantage of hydrogen fuel cells

Reaction does not produce toxic pollution,

Stores more energy than a rechargeable battery

What do the electrons that have left the oxygen travel to the cathode through?

The wire

What equation happens at the anode (oxidation) in a fuel cell?

H2 -> 2H+ + 2e-

What equation happens at the cathode in a fuel cell?

O2 + 4H+ + 4e- -> 2H2O

Is breaking bonds exothermic or endothermic?

endothermic

Is forming a bond an exothermic or endothermic process?

exothermic

What is the overall equation for a hydrogen fuel cell?

2H2 + O2 -> 2H2O

What is formed at the cathode in a hydrogen fuel cell?

water

Why do alkaline batteries eventually stop working?

A reactant is used up

Which direction do the electrons flow in a hydrogen-oxygen fuel cell?

negative anode to positive cathode

What are the electrodes in fuel cells made from?

porous carbon

True or false: The oxygen enters the cathode compartment and the hydrogen enters the anode compartment

true

What are the main advantages of hydrogen-oxygen fuel cells?

Hydrogen is a gas at room temperature meaning it is difficult and dangerous to store

What is the half equation at the anode of a fuel cell?

H2 -> 2H+ + 2e-

What compartment does hydrogen enter in a fuel cell?

anode compartment

What compartment does oxygen enter in a fuel cell?

cathode compartment

What is the half equation at the cathode of a fuel cell?

O2 + 4H+ + 4e- -> 2H2O