C7: Energy Changes

What is an exothermic reaction?

• A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase.

• Examples include combustion, neutralisation, and oxidation reactions.

• Everyday uses include self-heating cans and hand warmers.

What is an endothermic reaction?

• A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.

• Examples include thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate.

• Everyday uses include instant sports cold packs.

What is activation energy?

The minimum amount of energy that reacting particles must have to collide successfully and react.

How does a reaction profile represent an exothermic reaction?

• Products are at a lower energy level than the reactants.

• The overall energy change is negative because energy is released to the surroundings.

• Activation energy is shown as a curve rising from reactants to the peak.

How does a reaction profile represent an endothermic reaction?

• Products are at a higher energy level than the reactants.

• The overall energy change is positive because energy is absorbed from the surroundings.

• Activation energy is shown as a large curve rising from reactants to the peak.

Is bond breaking exothermic or endothermic?

• Endothermic because energy must be supplied to break chemical bonds in the reactants.

Is bond making exothermic or endothermic?

• Exothermic because energy is released when new chemical bonds are formed in the products.

How do you calculate the overall energy change of a chemical reaction using bond energies?

• Sum of energy needed to break bonds in reactants minus sum of energy released forming bonds in products.

• Energy Change = Energy In - Energy Out.

How can you tell if a reaction is overall exothermic using bond energies?

More energy is released when forming new bonds in products than is needed to break bonds in reactants, resulting in a negative overall energy change.

How can you tell if a reaction is overall endothermic using bond energies?

More energy is needed to break bonds in reactants than is released when forming new bonds in products, resulting in a positive overall energy change.

What is a simple chemical cell and how does it generate electricity?

• A cell consists of two different metals immersed in an electrolyte solution.

• A chemical reaction between the metals and electrolyte creates a potential difference (voltage) so electricity flows through the circuit.

What factors affect the voltage produced by a chemical cell?

• The difference in reactivity between the two metals; a larger difference creates a higher voltage.

• The type and concentration of the electrolyte used.

What is a battery?

Two or more chemical cells connected together in series to provide a greater total voltage.

Why do non-rechargeable cells and batteries stop producing electricity?

• The chemical reactions inside the cell are irreversible.

• Over time, one or more of the reactants gets completely used up.

How do rechargeable batteries work?

The chemical reactions inside the cell are reversible, so connecting the battery to an external electrical current reverses the reaction and restores the original reactants.

What is a fuel cell?

A device that uses an external supply of fuel (such as hydrogen) and oxygen to undergo electrochemical oxidation and generate electricity continuously.

What is the overall reaction in a hydrogen fuel cell?

2H2 + O2 -> 2H2O

What half equation takes place at the negative electrode (anode) of a hydrogen fuel cell?

2H2 -> 4H+ + 4e-

What half equation takes place at the positive electrode (cathode) of a hydrogen fuel cell?

O2 + 4H+ + 4e- -> 2H2O

What are the main advantages of hydrogen fuel cells compared to rechargeable batteries?

• They produce no pollutants or greenhouse gases, only water.

• They do not lose efficiency or charge capacity over time.

• Refueling with hydrogen is much faster than recharging a battery.

What are the main disadvantages of hydrogen fuel cells?

• Hydrogen is a flammable gas that is difficult and dangerous to store at high pressure.

• Hydrogen gas is often produced by electrolysis or fossil fuels, which require large amounts of energ