Heat is a form of energy. It is considered to be a measure of the (kinetic) energy of particles transferred between two systems/system and surroundings.

Temperature represents a measure of the average kinetic energy of particles in a system. Not all the particles will have the same kinetic energy (there will be a distribution of kinetic energies) but temperature represents an average.

The energy of a system can change. Energy can move from one system to another or from a system to surroundings. For example in an exothermic reaction heat energy moves from system to surroundings.

Thus 1 and 2 only is the correct answer.

Strictly speaking standard states do not include temperature! However, chemists tend to assume a temperature of 298K, so that is included here, and should be included in any answer given.

Standard state does assume a standard pressure of 100kPa (this used to be 1 atmosphere - 101.325kPa - but is now 100kPa). Substances are also assumed to be pure and in the correct state (standard state) at 100kPa (and 298K).

Thus 'pure substance in its standard state at 100kPa and 298K' is the correct answer.

If a reaction is endothermic, then heat energy (enthaply) is transferred from the surroundings to the system. Thus the system gains energy, ΔH is positive, and the energy of the products is higher than the energy of the reactants:

On the left; exothermic - on the right; endothermic.

If the temperature of the solution has increased, then the system has released energy to the surroundings. The reaction is therefore exothermic. Thus the system loses energy, ΔH is negative, and the energy of the products is lower than the energy of the reactants:

On the left; exothermic - on the right; endothermic.

Standard enthalpy of formation is a definition that needs to be learned:

Standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states (at 100kPa and 298K).

Thus 1 and 2 only is the correct answer.

Standard enthalpy of combustion is a definition that needs to be learned:

Standard enthalpy of combustion is the enthalpy change when one mole of a compound combusts completely (in excess oxygen) with all reactants and products in their standard states (at 100kPa and 298K).

Thus C₂H_6(g) + 3½O_2(g) → 2CO_2(g) + 3H₂O_(l) is the correct answer.

Errors shown in the other answers include:

2C₂H_6(g) + 7O_2(g) → 4CO_2(g) + 6H₂O_(l) two moles of ethane (rather than one mole);

C₂H_6(g) + 3½O_2(g) → 2CO_2(g) + 3H₂O_(g) water in the gaseous state (rather than liquid - its standard state);

C₂H_6(g) + 2½O_2(g) → 2CO_(g) + 3H₂O_(l) carbon monoxide produced (rather than carbon dioxide - to show complete combustion).

Using Q=mcΔT and a specific heat capacity of water of 4.18 J K⁻¹ g⁻¹ (given in the data book).

Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, water. Water has a density of 1 g cm⁻³ (so 200cm³ is 200g water).

38.4 − 22.0 = 16.4°C (A temperature change of 16.4°C is the same as 16.4K).

Q = 200 × 4.18 × 16.4 = 13710.4J = 13.7 kJ to 1d.p

Thus the answer is 13.7

Using Q=mcΔT and a specific heat capacity of water of 4.18 J K⁻¹ g⁻¹ (given in the data book).

Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, water. Water has a density of 1 g cm⁻³ (so 100cm³ is 100g water). A temperature change of 5°C is the same as 5K.

Q = 100 × 4.18 × 5 = 2090J

(for the 5.59g of NH₄Cl used)

Molar mass of NH₄Cl = 14.01 + (1.01×4) + 35.45 = 53.50 g mol⁻¹

Moles of NH₄Cl used = mass / molar mass = 5.59 / 53.50 = 0.104485981

For one mole of ammonium chloride the enthalpy change will therefore be:

2090 / 0.104485981 = 20003J mol⁻¹ approximately

Which is 20 kJ mol⁻¹ approximately

And the enthalpy change is endothermic because the temperature dropped, so the sign is positive.

Thus the answer is +20

Using Q=mcΔT and a specific heat capacity of water of 4.18 J K⁻¹ g⁻¹ (given in the data book).

Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, the solutions are predominently water. Water has a density of 1 g cm⁻³ (so 1000cm³ is 1000g water).

Moles of HCl (and also NaOH) = concentration (mol dm⁻³ )× volume (dm³) = 1.00 × 0.500 = 0.500 mol

(Reaction is 1:1 molar ratio)

0.5 of a mole of each are used, therefore the enthalpy change will be:

−57.6 × 0.5 = −28.8 kJ = −28800J

This value is negative, so the reaction is exothermic and the temperature change will be an increase.

Q=mcΔT

28800 = 1000 × 4.18 × ΔT

ΔT = 28800 / (1000 × 4.18) = 6.889952153K

Thus the answer is an increase of 6.9K

Using Q=mcΔT

Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, water and also copper. Water has a density of 1 g cm⁻³ (so 200cm³ is 200g water).

The temperature change (46.8 − 21.4 = 25.4) of 25.4°C is the same as 25.4K.

For the water:

Q = 200 × 4.18 × 25.4 = 21234.4J

And for the copper calorimeter:

Q = 140 × 0.385 × 25.4 = 1369.06J

Total Q = 21234.4 + 1369.06 = 22603.46J

(for the 0.40 mol of fuel burnt)

For one mole of fuel the enthalpy change will therefore be:

22603.46 / 0.40 = 56508.65J mol⁻¹

Which is 56.5 kJ mol⁻¹ approximately

And the enthalpy change is exothermic, so the sign is negative.

Thus the answer is −56.5

(An answer of −53.1 is obtained by using only the heat energy absorbed by the water)

(An answer of −90.3 is obtained by using a mass of 340g of water)

(An asnwer of −9.0 is obtained by multiplying by 0.40 (mol) instead of dividing)