Energy Changes Flashcards
How is energy transferred in a chemical reaction?
From the chemicals or from the surroundings, therefore many reactions are accompanied by an energy change.
What are exothermic reactions?
Reactions accompanied by a temperature rise- they transfer (heat) energy from the chemicals to the surroundings.
Exothermic reactions include:
- Self-heating cans
- hand warmers
- combustion
- neutralisation
- displacement
What are endothermic reactions?
Reactions accompanied by a fall in temperature, heat energy is transferred from the surroundings to the chemicals.
Endothermic reactions include:
- Thermal decompositions
- Reaction if Citric acid and sodium hydrogencarbonate
- Some sports injury packs
If a reversible reaction is endothermic in one direction…
It is exothermic in the other direction, the same amount of energy is transferred in each case.
When can chemical reactions occur?
Only when reacting particles collide with each other and with sufficient energy.
What is activation energy?
The minimum amount of energy that particles must have to react.
What so reaction profiles show?
The relative energies of all reactants and products- the activation and overall energies change the reaction.
In an exothermic reaction:
- Energy is given out to surroundings
- Products have less energy than reactants
In an endothermic reaction:
- Energy is being taken from surroundings
- Products have more energy than reactants
What do cells contain?
Chemicals which react to produce energy.
What is the voltage produced by a cell dependent on?
A number of factors including the type of electrode and electrolyte
How is a simple cell made?
By connecting two different metals in contact with an electrolyte.
What do batteries consist of?
Two or more cells connected together in series to provide greater voltage.
In non-rechargeable cells and batteries when do the reactions stop?
When one of the reactants has been used up- alkaline batteries are non-rechargeable.
Why can rechargeable cells and batteries be recharged?
Because the chemical reactions are reversed when an external electrical current is supplied.
What are fuel cells?
A very efficient way of producing electrical energy.
What do most fuel cells use?
Hydrogen, but other fuels can be used.
In a hydrogen fuel cell:
- Hydrogen and oxygen are supplied to the fuel cell.
- The fuel is oxidised to produce a pd (voltage).
- Overall, the hydrogen is oxidised to form water.
What effect do catalysts have on activation energy?
They reduce it.
How can the energy needed to break bonds and the energy released when bonds are formed be calculated?
From bond energies.
Energy in an exothermic reaction:
The energy released from forming bonds is greater than the energy needed to break existing bonds.
Energy in an endothermic reaction:
The energy needed to break existing bonds is greater than the energy released from forming new bonds.
What are the advantages of fuel cells?
- Hydrogen fuel cells produce water which is non-polluting
- Lightweight and small
- Have no moving parts so are very unlikely to break down
