Unit 6.1+6.3 - explaining physical & chemical changes +

Question 1

What is a physical change?

Answer 1

A change that doesn’t alter the chemical identity of a substance (its chemical composition/formula), but the physical state and appearance changes. Most physical changes can be reversed. New substances are not produced.
E.g. melting, boiling, freeing, formation of mixtures

Question 2

What is a chemical change + examples?

Answer 2

When a chemical reaction changes the chemical identity of a substance is, so the reactants are different from the products. Irreversible.
Signs that a chemical reaction is happening: colour change, effervescence (bubbles), odour, fire, precipitation (solid forms when mixing 2 solutions), temperature change, etc.
E.g. combustion, rusting, baking a cake

Question 3

What is oxidation (2 meanings)?

Answer 3

1. The gain of oxygen by a substance in a chemical reaction.
   E.g. 2Mg (s) + O2 (g) -> 2MgO(s); here magnesium is oxidized.
2. The loss of electrons
   e.g. in this reaction Mg + Cl2 -> MgCl2 the oxidation half equation is: Mg -> Mg (2+) + 2e-

Question 4

What is reduction?

Answer 4

1. The loss of oxygen by a substance in a chemical reaction.
   E.g. Mg + H2O -> MgO + H2’ here water (H2O) is reduced
2. The gain of electrons
   E.g. e.g. in this reaction Mg + Cl2 -> MgCl2, the reduction half equation is Cl2 + 2e- -> 2Cl-

Question 5

What is a redox reaction + examples + C + CO2 -> 2CO?

Answer 5

A reaction which involves simultaneous oxidation & reduction.
E.g. C + CO2 -> 2CO; here carbon is oxidized and carbon dioxide is reduced
E.g. combustion, corrosion of metals, displacement reactions
When saying which substance has been reduced/oxidised, say the full compound (not Cu, CuO)

Question 6

What in an oxidation number + example of Iron (ii) chloride + chlorine -> iron (iii) chloride ?

Answer 6

The number assigned to each element in a compound showing how many electrons it has lost (positive number) or gained (negative number). Pure elements have on oxidation number of zero. For transition metal compounds the oxidation number is indicated using Roman Numerals after the name of the metal it’s variable).
E.g. 2FeCl2 + Cl2 -> 2FeCl3
Oxidation involves an increase in oxidation number (because in oxidation the element loses electrons, resulting in a higher positive charge, therefore needing more electrons to become neutral). Reduction involves a decrease in oxidation number.

Question 7

What is a displacement reaction?

Answer 7

A reaction where a more reactive element takes the place of a less reactive element in a compound

Question 8

Write the half-equations for this reaction: 2Mg + O2 -> 2MgO

Answer 8

Oxidation half-equation: 2Mg -> 2Mg(2+) + 4e-
Reduction half-equation: O2 + 4e- -> 2O (2-)

Question 9

In reactions between metals & non-metals, which are oxidized and which are reduced?

Answer 9

Metals - oxidized
Non-metals - reduced