Thermochemistry

Question 1

Exothermic reaction meaning

Answer 1

Chemical reaction which releases heat to the surroundings

Question 2

What is endothermic reaction

Answer 2

Chemical reactions which absorb heat from the surroundings

Question 3

Exothermic reactions examples

Answer 3

Respiration, combustion, oxidation of metals, ammonia production, dissolving sodium hydroxide in water, neutralisation

Question 4

Endothermic reactions examples

Answer 4

photosynthesis, decomposition of metal carbonate when heated, decomposition of metal nitrates when heated and dissolving ammonium salts in water

Question 5

What is the name of heat released/absorbed during a reaction?

Answer 5

heat of reaction (delta H)

Question 6

Definition of heat of reactions

Answer 6

heat change of one mole of reactant that reacts or one mole of product that is formed.

Question 7

Why is energy level decrease during exothermic reaction

Answer 7

heat energy released during formation of bonds in the products is greater than the heat energy absorbed to break the bonds in the reactants. (vice versa for endothermic)

Question 8

State the types of heat reactions

Answer 8

• heat of precipitation
• heat of displacement
• heat of neutralisation
• heat of combustion

Question 9

What is contrast media liquid

Answer 9

BaSO4, drank before x-ray to scan clearly

Question 10

Heat of precipitation meaning

Answer 10

heat change when 1 mole of precipitate is formed from their ions in an aqueous solution

Question 11

thermochemical equation of formation of barium sulphate

Answer 11

Ba2+ (aq) + SO4 2- —> BaSO4 (s) delta H = -42kJmol-1

Based on thermochemical equation, 42kJ of heat is released when 1 mole of barium sulphate precipitate is formed. Therefore, heat of precipitation of barium sulphate is -42kJmol-1

Question 12

100 cm3 of 1.0 mol dm-3 lead(II) nitrate, Pb(NO3)2 solution is added to 100cm3 of 1.0 mol dm-3 of sodium sulphate, Na2SO4 solution, PbSO4. Calculate the heat of precipitation of lead(II) sulphate. (specific heat capacity of solution = 4.2 Jg-1C-1 ; density of solution = 1gcm-3)

Answer 12

delta H = -25.2kJmol-1

Question 13

What is heat of displacement

Answer 13

heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal

Question 14

Thermochemical equation forms

Answer 14

full equation or ionic equation

Question 15

An excess magnesium powder,Mg is added into 50 cm3 of 0.25moldm-3 of iron (III) sulphate, FeSO4 solution. The temperature of the mixture increases by 4.0C. Calculate the heat of displacement of iron, Fe from its solution.

Answer 15

delta H = -67.2

Question 16

What is heat of neutralisation?

Answer 16

heat change when one mole of water is formed from the reaction between an acid and an alkali

Question 17

thermochemical reaction between HCl and NaOH

Answer 17

NaOH (aq) + HCl (aq). –> NaCl (aq) + H20. delta H = -57kJmol-1

Question 18

descending order of heat of neutralisation

Answer 18

strong + strong
weak acid + strong alkali
strong acid + weak alkali
weak + weak

Question 19

Why are heat of neutralisation of weak acids or alkalis are low

Answer 19

Weak acids or weak alkalis ionise partially in water and some remain as molecules

some of the heat released during neautralisation is absorbed and used to completely ionise the weak acid or weak alkali in water

Therefore, the heat released is lower

Question 20

Complete neutralisation of which type of strong acid and a strong alkali produces more heat

Answer 20

complete neutralisation of strong diprotic acid produces twice the heat than monoprotic acid

Question 21

60cm3 of 2.0moldm-3 of sodium hydroxide, NaOH solution is added into 60cm3 of 2.0 moldm-3 of ethanoic acid, CH3COOH. The highest temperature of the mixture is 40.5C. The initial temperature of NaOH solution is 28.0C, and ethanoic acid is 28.0C. Calculate the heat of neautralisation. (Specific heat capacity of solution : c=4.2Jg-1C-1 ; density of solution = 1gcm-3)

Answer 21

-52.5kJmol-1

Question 22

What is heat of combustion?

Answer 22

heat released when 1 mole of a substance is completely burnt in excess oxygen

Question 23

relationship between carbon atoms per molecule and heat of combustion

Answer 23

the higher the number of carbon atoms per molecule, the higher the heat of combustion because more carbon dioxide and water and water molecules will be produced.

Question 24

Applications of exothermic reactions in everyday life

Answer 24

Hot Can - Hot food or beverages prepared without external heat

Thermite reactions - Welding of railway tracks

Combustion of fuel - Used for cooking food

Question 25

Examples of endothermic reactions in everyday life

Answer 25

Gel Pads - Relieves fever

Self cooling beverages - Beverages in cold cans without storing in refrigerators or using ice

Sorbet - The tongue feels cold when the sorbet is put in the mouth

Question 26

What is fuel value

Answer 26

Fuel value is the amount of heat energy released when one gram of fuel is completely burnt in excess oxygen

Question 27

Unit for fuel value?

Answer 27

kJg-1

Question 28

Choice of fuel depends on?

Answer 28

Fuel value
Cost of fuel
Availability and source of substance
Effects of the fuels on the environment

Question 29

State 2 mistakes in apparatus set-up for heat of combustion experiments (usually)

Answer 29

• use metal container (copper) instead of beaker

- use pipe clay triangle instead of wire gauze

Question 30

observation of heat of displacement reaction?

Answer 30

• change in temperature
• colour solution turns colourless
• a coloured precipitate forms

Question 31

Is sodium metal suitable to be used in hot packs?

Answer 31

No, because (sodium) metals are highly reactive and its reaction is very exothermic. Sodium metals are not as easy to handle as other metals because it does not exist freely and it only exists as compounds.