Measuring Enthalpy Changes

Question 1

Standard enthalpy of formation

/\fH

Answer 1

One mole of substance is formed from its elements under SC all in standard states

Question 2

Standard enthalpy of Combustion

/\cH

Answer 2

Change when one mole of substance is completely burnt in oxygen under SC, all products and reactants in standard states

Question 3

Measuring enthalpy change

Answer 3

Enthalpy change = mass of substance x specific heat capacity x temp change

1) mass of substance
2) temp change
3) specific heat capacity of substance

Question 4

Specific heat capacity

Answer 4

Amount of heat needed to raise the temp of 1g of a substance by 1K

Units = J g-1 K-1

Question 5

Simple calorie meter

Answer 5

q = mc/\T

EXAMPLE

0.32g of methanol was burnt and the temp of the 200.0g of water rose by 4.0K

Heat change = 200.0 x 4.2 x 4.0 = 3360J
0.01 mol = 3360J
1 mol = 336 000J or 336kJ

/\cH = -340kJ mol-1

Question 6

Flame calorie meter

Answer 6

Improved version of simple one

Features

• spiral chimney is made of copper
• flame is enclosed
• fuel burns in pure oxygen, rather than air

Question 7

Measuring enthalpy changes in solutions

Answer 7

polystyrene beakers often used for the calorimeter . They are good insulators and have a low heat capacity so they absorb little heat. Specific heat capacity of dilute solutions is taken as 4.1 like water

Question 8

Neutralization Reactions

Answer 8

EXOTHERMIC

acid + alkali –> salt + water