Chapter 7: Energy Changes

What is the conservation of energy principle?

Energy is conserved in chemical reactions; the amount of energy in the universe remains the same before and after a reaction.

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, increasing the temperature; examples include combustion and neutralisation.

What sign indicates an energy change in an exothermic reaction?

Negative sign of energy change.

What is an endothermic reaction?

A reaction that absorbs energy from the surroundings, decreasing the temperature; examples include thermal decomposition.

What sign indicates an energy change in an endothermic reaction?

Positive sign of energy change.

What is a reaction profile?

A graph showing the relative energies of reactants and products, along with the activation energy of the reaction.

What occurs in terms of bond energies during a chemical reaction?

Energy is supplied to break bonds and released when bonds are formed.

Describe exothermic reactions in terms of bond breaking and forming.

Energy released from forming bonds is greater than the energy needed to break them.

Describe endothermic reactions in terms of bond breaking and forming.

Energy needed to break bonds is greater than the energy released when bonds are formed.

What is the equation for enthalpy change in terms of bond energies?

Energy of reaction = sum of bonds broken – sum of bonds made.

What is a cell in chemistry?

A cell consists of two electrodes in an electrolyte solution and 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 of a cell?

The identities of the metals used as electrodes and the identity and concentration of the electrolyte.

State one advantage and one disadvantage of using cells and batteries.

Advantage: Convenient source of electrical energy; Disadvantage: Harmful chemicals.

Describe the difference between rechargeable and non-rechargeable cells.

Rechargeable cells have reactions that can be reversed with an external current; non-rechargeable cells are used up and cannot be recharged.