EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记：3.1.3 Energetics Calculations

• In order the calculate heat energy changes you need to know the mass of the substance being heated, the temperature change and the specific heat capacity of the substance
• The  specific heat capacity (c) is the energy needed to raise the temperature of 1 g of a substance by 1 K
• The specific heat capacity of water is 4.18 J g^-1 K^-1
• The energy transferred as heat can be calculated by:

Equation for calculating energy transferred in a calorimeter

• The temperature change in Kelvin is the same as the temperature change in degrees Celsius

Answer:

The solution is assumed to have the same density as water, so 100.0 cm³  has a mass of 100 g

q = m x c x ΔT

q = 100 g x 4.18 J g^-1 K^-1 x 7.5 K = – 3135 J = -3.13 kJ

Answer:

Calculate q

q = m x c x ΔT

q = 200 g x 4.18 J g^-1 K^-1 x 30 K = – 25 080 J = -25 kJ

• Molar enthalpy change is the heat energy change per mole of substance
• The symbol is ΔH and it has the unit kJ per mole
• If is found by first determining the heat energy change for the reaction, q, and then dividing by the number of moles, n, of the substance

molar enthalpy change  = heat change for the reaction ÷ number of moles

ΔH = q ÷ n 

Answer:

Step 1: q = m x c x ΔT

m (of water) = 250 g

c (of water) = 4.18 J g^-1 K^-1

ΔT (of water) = 310 – 298 K

= 12 K

Step 2: q = 250 x 4.18 x 12

= 12 540 J

Step 3:  This is the energy released by 0.01 mol of propan-1-ol

Total energy    ΔH = q ÷ n = 12 540 J ÷ 0.01 mol = 1 254 000 J mol^-1

Total energy = – 1254 kJ mol^-1

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