C7

Question 1

Exothermic reactions

Answer 1

• burning fuels, such as combustion of methane gas
• When methane burns it gets oxidised and releases energy to its surroundings
• the products have lower energy content than the reactants

Question 2

Endothermic reactions

Answer 2

• thermal decomposition reactions
• decomposition of calcium carbonate, when heated it forms calcium oxide and carbon dioxide by absorbing energy from the surroundings
• products have higher energy content than the reactants

Question 3

hand warmers

Answer 3

• exothermic reactions can be used in hand warmers and self heating cans
• crystallisation of a supersaturated solution is used in reusable warmers
• one off heat warmers heat up the surroundings for longer

Question 4

cooling down

Answer 4

• endothermic reactions can be used in ice packs for sports injuries
• when ammonium nitrate dissolves in water it absorbs energy from its surroundings to make it colder

Question 5

Reaction profiles - exothermic

Answer 5

the products are at lower energy level than the reactants. This means that when the reactants form the products energy is transferred to the surroundings

Question 6

reaction profile - endothermic

Answer 6

• the products are at higher energy level than the reactants.
• The reactants react to form products, energy is transferred from the surroundings to the reaction mixture.
• The temperature of the surroundings decreases because energy is taken in during the reaction

Question 7

Activation energy

Answer 7

The energy needed to start a reaction

Question 8

bonds

Answer 8

• bond breaking is endothermic
• bond making is exothermic

Question 9

making and breaking bonds

Answer 9

• in chemical reactions energy must be supplied to break the bonds between atoms in the reactants (endo)
• when new bonds are formed between atoms in a chemical reaction energy is released (exo)

Question 10

chemical cells and batteries

Answer 10

• metals lose electrons and form positive ions
• when two metals are dipped in salt solution and joined by wire the more reactive metal will donate electrons to the less reactive metal. This forms a simple electric cell

Question 11

cell voltage

Answer 11

the greater the difference in reactivity between the two metals, the higher the voltage produced by the cell

Question 12

fossil fuels

Answer 12

world relies on fossil fuels but they are non renewable and cause pollution

Question 13

fuel cells

Answer 13

• 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 fuel cell
• the only waste product is water