Chapter 7: Energy changes

Question 1

Exothermic and endothermic reactions

Answer 1

• Energy is conserved in chemical reactions. It is neither created nor destroyed
• A reaction in which energy is transferred from the reacting substances to the surroundings is called an exothermic reaction
• A reaction in which energy is transferred from the surroundings to the reacting substances is an endothermic reaction

Question 2

Using energy transfers from reactions

Answer 2

• Exothermic changes can be used in hand warmers and self-heating cans. Crystallisation of a supersaturated solution is used in reusable warmers. However, disposable, one-off, warmers heat the surroundings for longer
• Endothermic changes can be used in instant cold packs for sports injuries

Question 3

Reaction profiles

Answer 3

• You can show the relative difference in the energy of reactants and products on reaction profiles
• Bond breaking is endothermic, whereas bond making is exothermic.
• In reaction profiles:

  o	Reactants become before the products in the progress of reaction 
  
  o	Activation energy is a small surge of energy which happens when the reaction begins
  
  o	Exothermic reaction profiles start with reactants high in energy and products low 
  
  o	Endothermic reaction profiles start with reactants low in energy and products high 

See picture in documents

Question 4

Bond energy calculations

Answer 4

• In chemical reactions, energy must be supplied to break the bonds between atoms in the reactants
• When new bonds are formed between atoms in a chemical reaction, energy is released
• In an exothermic reaction, the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken
• In an endothermic reaction, the energy released when new bonds are formed is less than the energy absorbed when bonds are broken
• You can calculate the overall energy change in a chemical reaction using bond energies

Question 5

Chemical cells and batteries

Answer 5

• Metals tend to lose electrons and form positive ions
• When two metals are dipped in a salt solution and joined by a wire, the more reactive metal will donate electrons to the less reactive metal. This forms a simple electrical cell.
• The greater the difference in reactivity between the two metals, the higher the voltage produced by the cell

Question 6

Fuel cells

Answer 6

• Much of the world relies on fossil fuels. However, they are non-renewable, and they cause pollution
• Hydrogen is one alternative fuel. It can be burned in combustion engines or used in fuel cells to power vehicles.
• Hydrogen gas is oxidised and provides a source of electrons in the hydrogen cell, in which the only waste product is water

Advantages of hydrogen fuel cells:

• Do not need to be electrically recharged
• No pollution is produced
• Can be a range of sizes for different uses Hydrogen is difficult to store

Disadvantages of hydrogen fuel cells

• Hydrogen is highly flammable
• Hydrogen is sometimes produced for the cell by non-renewable sources (through electrolysis which needs electricity made using fossil fuels)
• Hydrogen is difficult to store

Question 7

Hydrogen fuel cells

Answer 7

• Hydrogen gas is supplied as a fuel to the negative electrode.
• It diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit
• Oxygen gas is supplied to the positive electrode. It diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from the external circuit
• If you add the two electrode reactions together, the electrons and the OH- ions on either side of the half equation cancel out. So, you are left with overall change in the hydrogen fuel cell, that is the oxidation of hydrogen.

See picture in documents