Unit 3

Question 1

How do you calculate the mass of a substance?

Answer 1

M = n x G

Question 2

How do you calculate the number of moles?

Answer 2

n = M / G   
or  
n = C x V 
or
n = Vg / Vm

Question 3

How do you calculate the volume of solution in Litres?

Answer 3

V = N / C

Question 4

How to calculate percentage yield:

Answer 4

Actual yield
————————- x 100
theoretical yield

Question 5

How to calculate atom economy:

Answer 5

mass of desired product(s)
—————————————– x 100
total mass of reactants

Question 6

How do you calculate energy required to heat water? (Eh)

Answer 6

Eh = c x m x ΔT

Question 7

What does C stand for? (in “Eh = c x m x ΔT” equation)

Answer 7

specific heat capacity of water; 4.18

Question 8

What does M stand for? (in “Eh = c x m x ΔT” equation)

Answer 8

mass of water heated in kg

Question 9

What does ΔT stand for?

Answer 9

temperature change of the water

Question 10

What is enthalpy of combustion?

Answer 10

the enthalpy change when 1 mole of a substance burns completely in oxygen

Question 11

What does it mean when ΔH is negative?

Answer 11

an Exothermic reaction has occurred; energy is lost during the reaction

Question 12

What does it mean when ΔH is positive?

Answer 12

an endothermic reaction has occurred, energy is absorbed during the reaction

Question 13

combustion reactions are always what?

Answer 13

exothermic

Question 14

What are the reasons why all the heat released by burning an alcohol was not taken up by water? (3)

Answer 14

• the container and the surrounding became hot
• water lost heat to the surroundings
• the ethanol did not burn completely

Question 15

When an ionic substance is dissolved in water, two processes occur which include:

Answer 15

• the ionic lattice gets broken apart-this requires energy- endothermic
• water molecules bond to the ions due to the polarity of water molecules - this releases energy - exothermic

Question 16

What is enthalpy of solution? ΔHsoln

Answer 16

the enthalpy change that occurs when 1 mole of a substance dissolves completely in water

Question 17

What is enthalpy of neutralisation? ΔHneut

Answer 17

the enthalpy change per mole of water when an acid is neutralised by an alkali

Question 18

What does the enthalpy change for a reaction depend on?

Answer 18

quantity of reactants and products

Question 19

What is Hess’s law?

Answer 19

the overall enthalpy change in a reaction, or sequences of reactions, depends only on the reactants and products and not on the route taken

Question 20

What is the equation for Hess’s law?

Answer 20

ΔH1 = ΔH2a + ΔH2b

Question 21

What happens to energy when free atoms form covalent bonds? and what is this called?

Answer 21

energy is released 
Bond enthalpy (energy)

Question 22

What is bond dissociation enthalpy?

Answer 22

when the same quantity of energy has to be input to break the covalent bond in a mole of H2 molecules to produce 2 moles of free hydrogen atoms
- reverse reaction (endothermic)

Question 23

What is mean bond enthalpy?

Answer 23

when considering the energy associated with a covalent bond in compound

Question 24

What is bond enthalpy?

Answer 24

the energy released (exothermic) when one mole of covalent bonds if formed from free atoms in the gaseous state

Question 25

What is oxidation?

Answer 25

gain of oxygen or loss of electrons

Question 26

What is reduction?

Answer 26

loss of oxygen or gain of electrons

Question 27

What is a redox reaction?

Answer 27

when a reduction and oxidation reaction always takes place together as electrons transfer from one reactant to the other

Question 28

What is an oxidising agent?

Answer 28

a chemical which acts on another substance to oxidise it. an oxidising agent is reduced during the reaction

Question 29

What is a reducing agent?

Answer 29

a chemical which acts on another substance to reduce it. a reducing agent is oxidised during the reaction

Question 30

In the electrochemical series, what letter (shape) is shown when forming a redox reaction?

Answer 30

"z"

Question 31

In the electrochemical series, what happens to the higher reaction?

Answer 31

it is written in reverse and is oxidised - oxidation reaction - reducing agent (electron donor)

Question 32

In the electrochemical series, what happens to the lower reaction?

Answer 32

Its written the same way as the book and is reduced - reduction reaction - oxidising agent (electron acceptor)

Question 33

What does electron transfer involve? and when does it take place?

Answer 33

has to involve one substance donating electrons and another substance accepting electrons, it takes place during a redox reaction

Question 34

In a redox reaction, what is a reducing agent described as??

Answer 34

the electron donor

Question 35

In a redox reaction, what is an oxidising agent described as?

Answer 35

electron acceptor

Question 36

Which group are described as strong oxidising agents (electron acceptors)?

Answer 36

the halogens

Question 37

Before two ion-electron half equations can be combined, which two conditions must be met?

Answer 37

- one equation must be for an oxidation change and the other must be for a reduction change - the number of electrons in each equation must be made the same

Question 38

What are the 4 steps (in order) for creating an ion-electron equation from scratch?

Answer 38

step 1 - balance the non-oxygen symbols step 2 - balance the oxygen by adding water (H20) step 3 - balance the hydrogen by adding H+ step 4 - balance the charges by introducing electrons

Question 39

What is a standard solution?

Answer 39

a solution of accurately known concentration

Question 40

What does ⇄ mean?

Answer 40

the reaction can go in either reaction

Question 41

What does chemical equilibrium involve?

Answer 41

two-way reactions known as reversible reactions

Question 42

What is chemical equilibrium?

Answer 42

eventually the rate of the backward reaction will equal the rate of the forward reaction. from then on the concentration of all substances in the reaction remain constant

Question 43

What is chemical equilibrium described as?

Answer 43

dynamic equilibrium

Question 44

What happens to the position of equilibrium if the equilibrium mixture contains a greater proportion of products than reactants?

Answer 44

lies to the right

Question 45

What happens to the position of equilibrium if the equilibrium mixture contains a greater proportion of reactants than products?

Answer 45

lies to the left

Question 46

What is a closed system?

Answer 46

when a reaction is carried out in a sealed container to prevent any loss of chemicals, or addition of chemicals to the reaction

Question 47

What is dissociation or ionisation of water?

Answer 47

when the position of the equilibrium lies well to the left, it is 99.9999% covalent and only contains a trace amount of ions

Question 48

What are the rates of most chemical reactions altered by?

Answer 48

- changing the concentration - increasing the temperature - adding a catalyst - altering the pressure

Question 49

What is stated in Le Chatelier's principle?

Answer 49

an equilibrium system always changes to reduce the effect of any outside change made on it

Question 50

What effect on equilibrium mixture does increasing the concentration of any reactant have?

Answer 50

it increases the concentration of the products

Question 51

What effect on equilibrium mixture does increasing the temperature have?

Answer 51

it shifts the position of equilibrium in the endothermic direction

Question 52

What effect on equilibrium mixture does increasing the pressure have?

Answer 52

it shifts the position of equilibrium in the direction of smaller gas volumes

Question 53

What effect on equilibrium mixture does adding a catalyst have?

Answer 53

it has no effect on the equilibrium position but allows it to be reached faster