C5: Energy changes Flashcards
What is an exothermic reaction?
What does the graph look like?
A reaction where the energy is RELEASED into the surroundings, meaning the temperature goes UP. The reactants have MORE energy than the products
What is an endothermic reaction?
What does the graph look like?
A reaction that USES energy from its surroundings, meaning the temperature of the surroundings goes DOWN.
The reactants have LESS energy than the products.
Examples of exothermic reactions (3)
Uses of exothermic reactions
EXAMPLES:
combustion, oxidation reactions and neutralisation(acid+alkali)
USES:
Hand warmers
Self heating cans
Examples of endothermic reactions(3)
Uses of endothermic reactions
EXAMPLES:
thermal decomposition, reaction of citric acid + sodium hydrogencarbonate
USES:
Cold packs
Describe exothermic reactions in terms of bonds.
Energy released from FORMING the bonds is greater than the energy needed to break the bonds
What is a cell?
A cell is composed of two electrodes dipped in electrolyte solution. It produces electricity (from a chemical reaction)
Describe endothermic reactions in terms of bonds.
Energy need to BREAK bonds is greater than the energy released from making the bonds.
What is a battery?
A battery consists of two or more cells connected in series.
What two things determine the voltage produced by the cell?
1) THE ELECTROLYTE-
changing the concentration of the electrolyte
2) THE METALS-
The greater the difference in disparity between the metals (of the electrode), the greater the voltage
State the advantages and disadvantages of using cells and batterie.
(+)cheap, some are rechargeable, provides a convenient source of electrify
(-) many cells contain toxic chemicals that can harm environment
What is meant by a rechargeable battery?
When the reactants are used up, the reaction can be reversed to reform these chemicals.
What is meant by a non-rechargeable battery?
When one of the reactants are used up, the reaction cannot be reversed. Battery goes flat.
What is a fuel cell?
an electrochemical cell in which a fuel donates electrons at one electrode and oxygens GAINS ELECTRONS at the other electrode
How do hydrogen fuel cells work?
1)At electrode1 , hydrogen gas LOSES electrons and forms h+ ions and electrons
2)The hydrogen ions move to the electrode2 through the electrolyte whereas the electrons move to the electrode2 through a wire. This current can be used to power a connected device.
3) The hydrogen ions and electrons react with oxygen gas at electrode2 making water.
Advantages of hydrogen fuel cells(3)
1) no pollutants
2) no recharging
3) produce more energy per kg than petrol
