3.2

Question 1

What happens in an exothermic reaction

Answer 1

• the enthalpy of the products is smaller than the enthalpy of the reactants
• heat loss from chemical system to surroundings
• △H has a - sign because heat has been lost by the chemical system
• Z shaped graph

Question 2

What happens in a endothermic reaction

Answer 2

• the enthalpy of the products is greater than the enthalpy of the reactants
• there is a heat gain to the chemical system from the surroundings
• △H has a + sign because heat has been gained by the chemical system
• S shaped graph

Question 3

Define activation energy

Answer 3

The minimum energy required to start a reaction by breaking bonds in the reactants

Question 4

What are standard conditions

Answer 4

Standard set of conditions for experimental measurements established to allow comparisons to be made between different sets of data
100kPa
298K

Question 5

What does it mean by standard states

Answer 5

All substances are in their standard states, i.e. their most stable form.

Question 6

Define enthalpy change of reaction

Answer 6

The energy change associated with a given reaction

Question 7

Define enthalpy change of formation

Answer 7

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions

Question 8

Define enthalpy change of combustion

Answer 8

The energy change that takes place when 1 mole of a substance is completely combusted

Question 9

Define enthalpy change of neutralisation

Answer 9

The energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions

Question 10

q=mc△t stands for

Answer 10

q = heat exchanged w surroundings (J)
m= mass of substance (WATER) heated or cooled 
c= specific heat capacity 
t= change in temperature measured in kelvin (K)

Question 11

Explain average bond enthalpy

Answer 11

The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission

Question 12

How do you predict enthalpy change

Answer 12

△H = Σ(bond enthalpies of reactants) - Σ(bond enthalpies of products)

Question 13

Describe how a chemical reaction takes place in terms of bonds broken and made

Answer 13

1. reactant bonds broken, takes in energy, endothermic
2. atoms rearrange to form products
3. product bonds formed, releases energy, exothermic change

Question 14

What is Hess’ Law

Answer 14

Hess’ Law states that the enthalpy change in a chemical reaction is independent of the route it takes

Question 15

What is an enthalpy cycle

Answer 15

An enthalpy cycle is a pictorial representation showing alternative routes between reactants and products