ENERGY CHANGE

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precious o

Cards (103)

what is an exothermic reaction? 
reactions which transfer energy to their surroundings, usually by heating
what is an endothermic reaction? 
reactions which take in energy from their surroundings
what happens during an exothermic reaction?
bonds are formed and energy is transferred
what happens during an endothermic reaction? 
bonds are broken and energy is absorbed
what are some examples of exothermic reactions? 
combustion, neutralization reactions, oxidation reactions
what are some examples of endothermic reactions?
photosynthesis, thermal decomposition
what is activation energy?
the energy needed to start a reaction
what is the reaction profile for exothermic reactions?
what is the reaction profile for endothermic reactions?
what is an electrochemical cell?
a basic system made up of two different electrodes in contact with an electrolyte
how does the difference in reactivity affect the voltage?
the bigger the difference in reactivity, the bigger the voltage of the cell
what is a fuel cell? 
an electrical cell that is supplied with a fuel and oxygen and uses energy from the reaction to efficiently produce electrical energy
what occurs inside a fuel cell?
the fuel enters the cell and becomes oxidized and sets up a voltage
what is produced from a hydrogen-oxygen fuel cell?
water and energy
which electrode does oxygen go to? 
cathode
which electrode does hydrogen go to?
anode
what is the reduction equation in a hydrogen-oxygen fuel cell? 
O₂ + 4H⁺ + 4e⁻ -> 2H₂O
what is the oxidation equation in a hydrogen-oxygen fuel cell?
H₂ -> 2H⁺ + 2e⁻
what is the overall reaction in a fuel cell?
2H₂ + O₂ -> 2H₂O
what are the advantages of h-o fuel cells?
no pollutants, less expensive than batteries, stores more energy than batteries
what are the disadvantages of h-o fuel cells?
gas takes up a lot of space, explosive, made from hydrocarbons/electricity
Conservation of energy principle
Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place
Exothermic reaction 
A reaction where energy is transferred to the surroundings so that the surroundings temperature increases
Exothermic reactions 
Combustion
Oxidation reactions
Neutralisation (acid + alkali) reactions
Endothermic reaction 
A reaction where energy is taken in from the surroundings so the surroundings temperature decreases
Endothermic reactions 
Thermal decomposition
Reaction of citric acid and sodium hydrogencarbonate
Activation energy
Minimum amount of energy that particles need to react
Reaction profile 
A graph which shows the relative energies of reactants and product, as well as activation energy of the reaction
In a chemical reaction, energy is supplied to break bonds and energy is released when bonds are formed
Exothermic reaction 
Energy released from forming bonds is greater than that needed to break the bonds
Endothermic reaction 
Energy needed to break bonds is greater than energy released making them
Equation to find enthalpy change in terms of bond energies
Energy of reaction = sum of bonds broken – sum of bonds made
Cell 
Composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
Battery 
Consists of two or more cells connected in series
Voltage obtained from a cell 
Determined by the identities of metals used as electrodes and the identity and concentration of an electrolyte
Advantages and disadvantages of using cells and batteries
Advantages: cheap, some are rechargeable, a convenient source of electrical energy
Disadvantages: harmful chemicals
Rechargeable cells
Chemical reactions are reversed when an external current is supplied
Non-rechargeable cells 
Reactants are used up, cannot be recharged
Fuel cell 
Supplied by fuel and oxygen to oxidise the fuel to generate electricity
Overall reaction in a hydrogen fuel cell
1. Cathode: 2 H2 → 4 H+ + 4 e−
2. Anode: O2 + 4 H+ + 4 e−→ 2 H2O
3. Overall: 2 H2 + O2 → 2 H2O