Chemical Reactions: Reactants, Products and Energy Change

Question 1

Define chemical reaction.

Answer 1

A process that leads to the transformation of one set of chemical substances to another.

Question 2

What do chemical reactions involve?

Answer 2

The breaking and formation of chemical bonds.

Question 3

What can chemical reactions be represented by?

Answer 3

Chemical equations.

Question 4

Examples of chemical reactions.

Answer 4

Burning Magnesium Iron rusting Acid dissolving Copper Oxide

Question 5

Define chemical equation.

Answer 5

A symbolic representation of a chemical reaction using formulae.

Question 6

How are chemical equations set out?

Answer 6

Reactants appear on the left side and products on the right with them separated by an arrow.

Question 7

Examples of chemical equations.

Answer 7

N₂_(g) + 3H₂_(g) ➝ NH₃_(g)

2Mg_(s) + O₂_(g) ➝ 2MgO_(s)

Question 8

What do balanced equations indicate?

Answer 8

The relative numbers of particles (atoms, molecules or ions) involved in the reaction.

Question 9

Define reactants.

Answer 9

The starting materials for a chemical reaction that appear to the left of the arrow in an equation.

Question 10

Define products.

Answer 10

The substances formed as a result of a chemical reaction that appear to the right of the arrow in the equation.

Question 11

Define phase change.

Answer 11

A physical change where a substance changes between the solid, liquid, gaseous or aqueous state. Also called a change of state.

Question 12

Examples of phase change.

Answer 12

Water freezing

Mercury boiling

Salt dissolving

Question 13

Labelled chemical equation diagram

Answer 13

Question 14

Define enthalpy (H).

Answer 14

The stored chemical potential energy in a substance (often called the heat content).

Question 15

What do reactions often involve?

Answer 15

A change in enthalpy

∆H = H_products - H_reactants

Question 16

Example of enthalpy.

Answer 16

Combustion of Ethanol

∆H = -1370kJ

Question 17

Law of conservation of energy.

Answer 17

The total amount of energy within a system remains constant (is conserved), although the energy within the sysem can be changed from one form to another.

Question 18

Example of the law of conservation of energy.

Answer 18

Combustion of Ethane.

Energy is released as heat but only because the products have less chemical energy than the reactants.

Question 19

Law of conservation of mass.

Answer 19

In a closed system mass will remain constant over time.

Mass will only change if matter is added or removed.

Mass (and energy) can neither be created nor destroyed.

Question 20

Example of the law of conservation of mass.

Answer 20

H₂ + O₂ → 2H₂0

4g 32g 36g

Question 21

Define exothermic reaction.

Answer 21

A reaction where the system realeases energy to the surroundings (as heat).

The reactants have more energy than the products and the exess is released as heat.

Question 22

Equation for exothermic reactions.

Answer 22

∆H = -ve

Question 23

Examples of exothermic reactions.

Answer 23

Burning coal

Adding acid to water

Mg combusing

Water freezing

Question 24

Define endothermic reaction.

Answer 24

A reaction where the system absorbes energy from the surroundings (as heat).

The products have more energy than the reactants.

Question 25

Equation for endothermic reactions.

Answer 25

∆H = +ve

Question 26

Examples of endothermic reactions.

Answer 26

Cracking an ice-pack

Dissolving NH₄NO₃ in water

Boiling water

Question 27

States of matter.

Answer 27

Solid

Liquid

Gas

Aqueous solution

Question 28

Change in state during condensation.

Answer 28

Gas to liquid

Question 29

Change in state during boiling.

Answer 29

Liquid to gas.

Question 30

Change in state during evaporation.

Answer 30

Liquid to gas (<bp></bp>

Question 31

Change in state during freezing.

Answer 31

Liquid to solid.

Question 32

Change in state during melting.

Answer 32

Solid to liquid.

Question 33

Change in state during sublimation.

Answer 33

Solid to gas.

Question 34

Change in state during deposition.

Answer 34

Gas to solid.

Question 35

Define latent heat of fusion.

Answer 35

Energy required to change state from solid to liquid.

Question 36

Define latent heat of vaporisation.

Answer 36

Energy required to change state from liquid to gas.

Question 37

Define boiling point.

Answer 37

Temperature at which vapour pressure of liquid = pressure outside liquid.

Question 38

Define melting point.

Answer 38

Temperature at which solid changes to liquid at atmospheric pressure.

Question 39

Define mole.

Answer 39

Avogardro’s number of particles.

6.02 × 10²³

Question 40

Define Avogadro’s Number (N).

Answer 40

The number of atoms in 12.01 grams of Carbon 12.

1 mol = Avogadro’s Number = 6.02 × 10²³

Question 41

Number of moles formula in relation to Avagadro’s number.

Answer 41

n=number of moles

N=number of particles

N_A=Avagadro’s Number (6.02 × 10₂₃)

Question 42

Number of moles formula in relation to mass.

Answer 42

n=number of moles

m=mass (g)

M=molar mass (g/mol^-1)

Question 43

Number of particles formula.

Answer 43

N=number of particles

n=number of moles

N_A=Avagardo’s number (6.02 × 10²³)

Question 44

Mass formula.

Answer 44

m=mass (g)

n=number of moles

M=molar mass (g/mol-1)

Question 45

Molar mass formula.

Answer 45

M=molar mass (g/mol-1)

m=mass (g)

n=number of moles

Question 46

How to calculate mass to mass relationships.

Answer 46

Question 47

Define limeting reagent?

Answer 47

The reactant that is used up first.

Question 48

Define exess reactant?

Answer 48

The reactant that is in exess.

Question 49

Define the left over?

Answer 49

A substance in limiting reagent problems that is left over from the exess.

Question 50

Left over formula.

Answer 50

n(left over) = n(start) - n(reacted)