Energetics

Question 1

What is enthalpy?

Answer 1

Measure of the amount of energy / energy content of a substance. It cannot be directly measured.

Question 2

Define enthalpy change

Answer 2

A measure of heat energy given out or taken in when a chemical or physical change occurs at a constant pressure

Question 3

What are the standard conditions?

Answer 3

25C or 298K

1atm or 101,000Pa

Question 4

What is an exothermic reaction?

Answer 4

A reaction where energy has been given out to the surroundings as the reactants change to products

Question 5

What is an endothermic reaction?

Answer 5

A reaction where energy has been taken in from the surroundings as the reactants change to products.

Question 6

Define standard enthalpy of formation

Answer 6

The enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in their standard states under standard conditions.

Question 7

Define standard enthalpy of combustion

Answer 7

The enthalpy change when one mole of a substance is completely burned in oxygen with all reactants and products in their standard states under standard conditions.

Question 8

Define standard enthalpy of neutralisation

Answer 8

Enthalpy change when one mole of water is formed in a reaction between an acid and an alkali under standard conditions.

Question 9

What are the standard conditions?

Answer 9

100,000kPa of pressure (or 1 atm)
298K temperature (or 25 degrees, about room temperature)

Question 10

Formula to find heat energy given out

Answer 10

q=mcT
q is heat energy given out (J)
m is mass of substance heated (g)
T is temperature change
c is specific heat capacity

Question 11

Define specific heat capacity

Answer 11

The amount of heat needed to raise the temperature of 1g of substance by 1K
Units: J/g/K

Question 12

How to find the enthalpy change from the heat energy given out

Answer 12

Divide the heat energy given out (kJ) by the number of moles of the substance in the reaction (not the water).

Question 13

What is the enthalpy of formation for elements?

Answer 13

0 kJ/mol

Question 14

Calculation to work out enthalpy change of a reaction using enthalpy of formation values

Answer 14

Sum of enthalpy of formation of products - sum of enthalpy of formation of reactants (remember to times by all coefficients)

(Hess’s cycle has arrows from constituent elements to reactants and products)

Question 15

Hess’s law

Answer 15

The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 16

Calculation to work out enthalpy change of a reaction using enthalpy of combustion values

Answer 16

Sum of the enthalpy of combustion of reactants - sum of enthalpy of combustion of products (remember to times by all coefficients)

(Hess’s cycle has arrows from products and reactants to oxides)

Question 17

Calculation to work out enthalpy change of a reaction using bond enthalpy values

Answer 17

Sum of the bond enthalpy of the bonds broken - sum of the bond enthalpy of the bonds formed
(Remember the substances have to be gases so if they’re not you have to add the energy to make them a gas on to the bond enthalpy before the calculation)
Always write out the displayed formula equation

Question 18

Define bond dissociation enthalpy

Answer 18

The enthalpy change required to break a covalent bond with all species in the gaseous state. The mean bond enthalpy is the average bond dissociation enthalpy for a given type of bond.

Question 19

What is important to remember when using a calorimeter

Answer 19

If you are using a spirit burner, the material containing the water should be a good conductor of heat. If the reaction is in the same container as the water, there should be as much insulation as possible to minimise heat loss to the surroundings. Record the temperature of the water a few minutes before and after the reactants are mixed so you can do an extrapolation curve to find the maximum temperature change.