Energetics

Question 1

Enthalpy change

Answer 1

ΔH is the heat energy transferred in a reaction at constant pressure. The units of ΔH are kJ mol^-1

Question 2

Exothermic reactions

Answer 2

Give out energy, so surrounding temperature increases. ΔH is negative.

Question 3

Exothermic reaction example

Answer 3

Oxidation is exothermic. Combustion of a fuel, e.g. methane.

CH_4(g) + 2O_2(g) —> CO_2(g) + 2H₂0_(l)

ΔHoc, 298 = -890 kJ mol-1

Question 4

Endothermic reactions

Answer 4

ΔH is positive. Reactions absorb energy, so surrounding temperature falls.

Question 5

Endothermic reaction examples

Answer 5

Thermal decomposition of calcium carbon carbonate

CaCO_3(s) —> CaO_(s) + CO_2(g)

ΔH^o_{r, 298} = +178 kJ mol^-1

Question 6

Breaking and making bonds

Answer 6

You need energy to break bonds, so this is endothermic (ΔH positive). Energy is eleased when bonds are formed, so this is exothermic (ΔH negative). Enthalpy change for a reaction is the overall effect of theses changes.

Question 7

Why are mean bond enthalpies not exact?

Answer 7

They are the average for a large range of molecules

Question 8

Enthalpy change of reaction calculation

Answer 8

= Total Energy Absorbed - Total Energy Released

Question 9

Define Standard Enthalpy Change of Reaction, ΔH^o_r

Answer 9

Enthalpy changed when the reaction occurs in the molar quantities shown in the equation, under standard conditions in their standard states.

Question 10

Standard enthalpy of Formation, ΔH^o_f

Answer 10

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

E.g. 2C_(s) + 3H_2(g) + 1/2O_2(g) —> C₂H₅OH_(l)

Question 11

Standard Enthalpy Change of Combustion, ΔH^o_c

Answer 11

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

Question 12

Calcualting enthalpy changes in a lab

Answer 12

1. Number of moles of the stuff thats reacting
2. Change in temperature

Some reactions can be done in a polystyrene beaker with a thermometer, while combustion reactions are trickier. The reactant is burned in air. The copper calorimeter containing a known mass of water is used. Burn a known mass of the reactant and record the temperature change of the water.

Question 13

Enthalpy change equation

Answer 13

q = mcΔT

q= heat lost/gained in joules. The same as enthalpy change if pressure is constant.

m= mass of water on the calorimeter- solution in beaker, in grams

c= specific heat capacity of wate, 4.18 Jg^-1K^-1

ΔT= change in emperature of water or solution

Question 14

q=mcΔT example

1.16g of fuel completely burned in oxygen. Heat formed raised 100g of water from 95.3K to 357.8K Calculate the ΔHoc of the fuel given that its Mr is 58.

Answer 14

Amount of heat: q= 100x4.18x(357.8-295.3) = 26,126J = 26.125 kJ This becomes negative because combustion is exothermic.

Moles: n=1.16/58 = 0.02mol of fuel

Heat produced by 1 mole of fuel: -26.125/0.02= -1306 kJ mol^-1

Question 15

State Hess’ Law

Answer 15

The total enthalpy change of a reaction is always the same, independant of the route.