Exothermic and endothermic reactions (energy changes)

Question 1

What is an energy transfer?

Answer 1

• When chemical reactions occur, energy is transferred from the chemicals to or from the surroundings. Therefore, many reactions are accompanied by a temperature change.
• Exothermic reactions are accompanied by a temperature rise because they transfer heat energy from the chemicals to the surroundings.
• Endothermic reactions are accompanied by a fall in temperature because heat energy is transferred from the surroundings to the chemicals.

Question 2

What is a reversible reaction?

Answer 2

• A reversible reaction is exothermic in one direction, then it is endothermic in the opposite direction. The same amount of energy is transferred in each case.

Question 3

What happens to the atoms during chemical reactions?

Answer 3

• In chemical reactions, atoms are rearranged as old bonds are broken and new bonds are formed. For bonds to be broken, reacting particles must collide with sufficient energy.

Question 4

What is activation energy?

Answer 4

• The minimum amount of energy that the particles must have for a reaction to take place is called the activation energy.

Question 5

How are energy changes in a chemical reaction shown?

Answer 5

• The energy changes in a chemical reaction can be shown using an energy level diagram or reaction profile.
• In an exothermic reaction: energy is given out to the surroundings and the products have less energy than the reactants.
• In an endothermic reaction: energy is being taken in from the surroundings and the products have more energy than the reactants.

Question 6

What is a catalyst?

Answer 6

• Catalysts reduce the activation energy needed for a reaction. This makes the reaction go faster.

Question 7

How are new substances produced in a chemical reaction?

Answer 7

• In a chemical reaction new substances are produced because the bonds in the reactants are broken and new bonds are made to form the products.
• Breaking a chemical bond requires energy (its is an endothermic process). When a new chemical bond is formed, energy is given out (it is an exothermic process).
• If more energy is required to break bonds than is released when bonds are formed, the reaction must be endothermic. If more energy is released when bonds are formed than is needed to break bonds, the reaction must be exothermic.