chemistry energy changes

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 hydrogencarbonate. Positive sign of energy change.

What is activation energy?

Minimum amount of energy that particles need to react

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 H2 O

Overall: → 2 H2 + O2 → 2 H2 O

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