Energy Changes Flashcards
What is the 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.
What is an exothermic reaction? Give examples
A reaction where energy is transferred to the surroundings so that the surroundings temperature increases - combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative sign of energy change.
What is an endothermic reaction? Give examples
A reaction where energy is taken in from the surroundings so the surroundings temperature decreases - thermal decomposition, reaction of citric acid and sodium hydrogen carbonate. Negative sign of energy change.
What is activation energy?
the minimum amount of energy needed to start a chemical reaction
What is a reaction profile?
Reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.
What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming
Energy is supplied to break bonds and energy is released when bonds are made; 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 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 and the identity and concentration of an electrolyte.
State the advantages and disadvantages of using cells and batteries.
(+) more or less cheap, some are rechargeable, a convenient source of electrical energy
(-) harmful chemicals
Describe 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 reaction in a hydrogen fuel cell? What are the half equations?
Cathode: 2 H2 → 4 H+ + 4 e−
Anode: O2 + 4 H+ + 4 e−→ 2 H2O
Overall: → 2 H2 + O2 → 2 H2O
What are the advantages and disadvantages of hydrogen fuel cells?
Advantages: no pollutants, no recharging
Disadvantages: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis
What requirements for a reaction to take place?
Particles must collide, with sufficient energy
What is a product?
The new substance formed by a chemical change
What is a reactant?
The starting material in a chemical reaction
What happen in a reaction profile when the reaction is exothermic?
Reactants are at higher energy than products
What happens to a reaction profile when the reaction is endothermic?
Products are at higher rate than the reactants
What is bond energy?
Average amount of energy that will break/make a bond between two atoms
Why do exothermic reactions happen?
More energy is given out when new bonds are formed than is taken in to break bonds
Why do endothermic reactions?
More energy is required to break bonds than is given out when new bonds formed
How do catalysts work?
They provide an alternative reaction pathway will lower activation energy
What happens to the temperature in exothermic reactions?
Temperature increase
What happens to the temperature change in endothermic reactions?
Temperature decreases
What is enthalpy?
The heat content of a system
What are the uses of exothermic reactants?
Self-heating cans and hand warmers
What are the uses of endothermic reactions?
Sport injury packs
What is an electrochemical cell?
A device containing chemicals that react to produce electricity
What is a non-rechargeable cells?
A cell in which the current stops as a reactant runs out
What is a rechargeable cell?
a cell in which the chemical reaction is reversed when a current is applied
What are the advantages of non-rechargeable cells?
Cheap
What are the disadvantages of non-rechargeable cells?
Contains toxic chemicals so are difficult to dispose
What are the advantages of rechargeable cells?
Can be reused
What are the disadvantages of rechargeable cells?
Takes time to recharge
What do you have do to do to construct a simple cell?
Join two different metals together, whilst both are in contact with an electrolyte
What factors determine a cell’s voltage?
- type of electrode
- concentration of electrolyte
- type of electrolyte
What is an electrode?
A device for conducting electricity
What is an electrolyte?
A substance containing free moving ions
What are the positive terminal of a cell?
- less reactive metal
- accepts electrons
What are the negative terminal of a cell?
- more reactive metal
- gives electrons
