Energy Changes and Fuel Cells

These flashcards cover key concepts related to energy changes in chemical reactions, types of reactions, and fuel cells.

What is the conservation of energy principle?

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

What is an exothermic reaction?

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

What are examples of exothermic reactions?

Combustion, oxidation reactions, and neutralisation reactions.

What is the sign of energy change in an exothermic reaction?

Negative sign.

What is an endothermic reaction?

A reaction that absorbs energy from the surroundings, leading to a decrease in the surroundings' temperature.

What are examples of endothermic reactions?

Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate.

What is the sign of energy change in an endothermic reaction?

Positive sign.

What is a reaction profile?

A graph showing the relative energies of reactants and products, including activation energy.

What occurs in bond energies during a chemical reaction?

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

How do exothermic and endothermic reactions differ in terms of bond energies?

Exothermic: energy released from forming bonds is greater than needed to break them; Endothermic: energy needed to break bonds is greater.

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

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

What are the components of a cell?

A cell is composed of two electrodes immersed in an electrolyte solution, producing electricity through a chemical reaction.

What determines the voltage of a cell?

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

What are the advantages of using cells and batteries?

They are generally cheap, some are rechargeable, and they provide a convenient source of electrical energy.

What are the disadvantages of using cells and batteries?

They contain harmful chemicals.

What distinguishes rechargeable cells from non-rechargeable cells?

Rechargeable: chemical reactions reverse with external current; Non-rechargeable: reactants are consumed and cannot be reused.

What is a fuel cell?

Fuel cells generate electricity by oxidising fuel, supplied by fuel and oxygen.

What is the overall reaction in a hydrogen fuel cell?

2 H2 + O2 → 2 H2O.

What are the half equations in a hydrogen fuel cell?

Cathode: 2 H2 → 4 H+ + 4 e−; Anode: O2 + 4 H+ + 4 e− → 2 H2O.

What are the advantages of hydrogen fuel cells?

They produce no pollutants and do not require recharging.

What are the disadvantages of hydrogen fuel cells?

They are flammable, storing H2 is difficult, production can involve fossil fuels and toxic chemicals, and they are expensive to produce by electrolysis.

What is the temperature change in an exothermic reaction?

The surroundings' temperature increases.

What happens during the process of thermal decomposition?

It is an endothermic reaction where energy is absorbed to break down a compound.

What is the role of an electrolyte in a cell?

It enables the flow of ions, allowing the chemical reaction to produce electricity.

What can be a use of a fuel cell?

Generating electricity for vehicles or stationary power sources.

What is one drawback of non-rechargeable cells?

They cannot be reused once the reactants are depleted.

What happens to energy in exothermic reactions when bonds are formed?

Energy is released.

What happens to energy in endothermic reactions when bonds are broken?

Energy is absorbed.

What factor influences the effectiveness of a cell?

The identity and concentration of the electrolyte.

What is meant by the term 'activation energy'?

The minimum energy required for a chemical reaction to occur.

How do you calculate the energy change using the equation provided?

By subtracting the total energy of bonds made from the total energy of bonds broken.

What environmental concern is associated with hydrogen fuel production?

It can involve toxic chemicals and fossil fuel consumption.

In what applications can exothermic reactions be useful?

In processes like combustion for heating and energy production.