C5: Energy Changes Flashcards
What is the principle of conservation of energy?
- Energy is conserved in chemical reactions.
- The amount of energy at the end of a chemical reaction is the same as before the reaction takes place
what is an exothermic reaction? Give 3 example and sign of energy change
- a reaction where energy is transferred to the surroundings so that the surroundings temperature increases
- negative sign of energy change
e.g. - combustion
- oxidation
- neutralisation
What is an endothermic reaction and give two examples
- a reaction where energy is taken in from the surroundings so the surroundings temperature decreases
e.g: - photosynthesis
- thermal decomposition
- reaction of citric acid and sodium hydrogen carbonate
define ‘ activation energy’
minimum amount of energy that particles need to react
What is a reaction profile?
- a graph which shows the relative energies of reactants and product as well as activation energy of the reaction
Describe exothermic and endothermic reactions in terms of bond breaking/ forming.
exothermic: energy released from forming bonds is greater than that needed to break the bonds
endothermic: energy needed to break bonds is greater than energy released making them
What occurs in a chemical reaction in terms of bond energies?
Bond breaking: energy needs to be taken in from the surroundings to break the chemical bonds
Bond making : energy is transferred to the surroundings as the new bond is formed
What is the equation to find enthalpy change in terms of bond energies?
energy of reaction : sum of bonds broken - sum of bonds made
what is a cell
- a cell is composed of two electrodes dipped in an electrolyte solution.
- it produces electricity from a chemical reaction
What is a battery?
- a battery consists of two or more cells connected in series
what determines the voltage obtained from a cell?
- identities of metals used as electrodes
- the identity and concentration of an electrolyte
State advantages and disadvantages of using cells and batteries (2/1)
(+)
- some are rechargeable,
- a convenient source of electrical energy
(-)
- harmful chemicals
Describe the difference between rechargeable and non rechargeable cells
rechargeable - chemical reactions are reversed when an external current is supplied
non-rechargeable - reactants are used up, cannot be recharged
What is a fuel cell?
- fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity
What is the overall word
reaction in a hydrogen
fuel cell?
Hydrogen + Oxygen –> Water
What is the overall symbol reaction in a hydrogen fuel cell? What are the half equations?
negative electrode
2H² → 4H⁺ + 4e⁻
positive electrode: O2 + 4H⁺ + 4e⁻ → 2H2O
Overall: → 2H² + O2 → 2H²O
what ae the advantages and disadvantages of hydrogen fuel cells? (2/4)
adv : no pollutants, no recharging
disadv: flammable, fossil fuel production, toxic chemicals , expensive production of H² by electrolysis
Give 3 advantages of
using hydrogen fuel cells
over rechargeable cells
and batteries
● Do not need to be electrically recharged
● No pollutants are produced
● Can be a range of sizes for different uses
Give 3 disadvantages
of using hydrogen fuel
cells over rechargeable
cells and batteries
● Hydrogen is highly flammable
● Hydrogen is sometimes produced for the cell
by non-renewable sources
● Hydrogen is difficult to store
How can you create a simple cell using
electrodes and an electrolyte?
Connect two different metals (electrodes) and dip them into an electrolyte
How does flashcard 20
generate electricity?
- The more reactive metal gets oxidised.
- The electrons flow through the wire from the more reactive metal to the less reactive metal.
- the rate of flow of electrons is an electric current
State 4 factors that
affect the voltage of a
cell
● The type of electrode used
● The electrolyte used
● The concentration of the electrolyte
● The temperature
Why can a rechargeable battery be
recharged?
The chemical reactions are reversed using an external electric current
