Topic 5-Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction which transfers heat to the surroundings, usually be heating

Question 2

How is an exothermic reaction represented?

Answer 2

A rise in temperature

Question 3

Name 3 examples of exothermic reactions?

Answer 3

-Combustion (burning fuels)
- Neutralisation reactions
- Oxidation reactions

Question 4

Give an example of an oxidation reaction?

Answer 4

Adding sodium to water, the reaction energy and the sodium moves about on the water as it is oxidised

Question 5

Explain 2 everyday uses of Endothermic reactions?

Answer 5

-Some Hand warmers- use the exothermic oxidation of iron in air (with a salt solution catalyst) to release energy
Self heating cans- of hot chocolate and coffee also rely on exothermic reactions between chemicals in their bases

Question 6

Explain how energy is moved around in reactions?

Answer 6

• Different chemicals store different amounts of energy
• Products and reactants may have different amounts of energy either because some as transferred to or from the surroundings
  energy cannot be created or destroyed, it only moves around

Question 7

What is an endothermic reaction?

Answer 7

a reaction that takes in energy from the surroundings

Question 8

What is an endothermic reaction represented by?

Answer 8

A fall in temperature

Question 9

Give 2 examples of endothermic reactions

Answer 9

-The reaction of citric acid and sodium hydrogencarbonate
- Thermal decomposition

Question 10

Give an example of a thermal decomposition reaction

Answer 10

Heating calcium carbonate causes it to decompose into quicklime (calcium oxide) and carbon dioxide

Question 11

What is an everyday use of endothermic reactions?

Answer 11

Sports injury packs- the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer

Question 12

What does conservation of mass mean?

Answer 12

Conservation of mass means that atoms are never created or destroyed in a chemical reaction- only the bonds between the atoms change

Question 13

What can you calculate to show conservation of mass?

Answer 13

relative formula mass

Question 14

Why may the mass of the reactants appeared to have increased?

Answer 14

One of the reactants is a gas and all the products are solids, liquids or aqueous
the reaction vessel needs to be unsealed

Question 15

How does one of the products being a gas make it seem as if mass has increased?

Answer 15

Before the reaction the gas is in the air so is therefore not counted in the mass of the unsealed reaction vessel.
After the reaction it forms part of the product inside the vessel so is counted in the mass

Question 16

Give an example of how the mass of the reactants may appear to increase?

Answer 16

When a metal reacts with oxygen in an unsealed container

Question 17

Why may the mass of the reactants appear to have decreased over the course of the reaction?

Answer 17

One of the products is a gas and al the products are solid, liquid or aqueous
- The reaction happens in an unsealed reaction vessel

Question 18

How may one of the products being a gas make it appear as if the mass has decreased?

Answer 18

Before the reaction, every reactant is contained in the vessel so is counted in the mass.
Gas escapes as it is formed over the course of the reaction so its mass is no longer accounted for

Question 26

Give an example of a reaction where the mass may appear to decrease

Answer 26

When a metal carbonate thermally decomposes to form a metal oxide and carbon dioxide gas