energetics 3.1.4 in all papers

Question 1

enthalpy change

what is an enthalpy change?

Answer 1

• the heat energy change measured under constant pressure.
• its measured in J/KJ per moles.

Question 2

enthalpy change

what does standard enthalpy change refer to?

Answer 2

• standard conditions, 100kpa, 25 degrees/ 298 kelvins.

Question 3

enthalpy change

what is an endothermic reaction?

Answer 3

• a reaction where heat energy is transferred from the surrondings to the reactants, tempreture decreases.

Question 3

enthalpy change

what is an exothermic reaction?

Answer 3

• a reaction where heat energy is released to the surrondings.

Question 4

enthalpy change

what is bond breaking?

Answer 4

• the input of energy, making it endothermic (+).

Question 5

enthalpy change

how do you work out enthalpy change?

Answer 5

• mass of water (g) x change in tempreture of water x specific heat capacity of water (KJ).
• assuming that all energy from the burning fuel is transferred to the water.
• density g of water is 1gcm-3, so 24.5cm3 g of water = 24.5 g

Question 6

enthalpy change

what is bond making?

Answer 6

• the output of energy, making it exothermic (-).

Question 7

enthalpy change

what is standard enthalpy of combustion?

Answer 7

• the enthalpy change when 1 mole of a substance is completely burnt in excess oxygen with all reactants being in their standard state under standard conditions.

Question 8

bond enthalpy

define mean bond enthalpy.

Answer 8

• the energy required to break 1 mole of a covalent bond averaged across many compounds containing the bond.

Question 9

bond enthalpy

which type of bonds are stronger?

Answer 9

• shorter the covalent bond, the stronger the bond.
• triple bonds are generally shorter than double or single bonds, so more energy is required to break them resulting in a positive bond enthalpy.

Question 10

what is the enthalpy change of formation?

Answer 10

• when one mole of a substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.

Question 11

how do you work out enthalpy change of formation?

Answer 11

enthalpy change = sum of the enthalpy change of formation of products - sum of the enthalpy change of reactants.

Question 12

hess’s law

what is hess’s law?

Answer 12

• states that the enthalpy change during a reaction is independent of the route taken, providing the initial and final conitions are the same.
• (the sum of the indirect route = direct route)

Question 13

hess’s law

why might you not get a 1 mole of methane in a mixture of carbon and hydrogen?

Answer 13

• carbon is highly unreactive.
• a range of other hydrocarbons could form, meaning we wont get 1 mole of methane.