1.6: Chemical Energy

Question 1

What is the energy stored within chemical bonds called?

Answer 1

Enthalpy

Question 2

What does bond breaking require?

Answer 2

requires energy, which is an endothermic process

Question 3

What does bond making do?

Answer 3

release energy, which is an exothermic process.

Question 4

What occurs in an exothermic reaction?

Answer 4

when more energy is released when bonds form than was needed to break bonds.
△H is negative.

Question 5

What occurs in an endothermic reaction?

Answer 5

When more energy is needed to break bonds than is released when bonds form.
△H is positive.

Question 6

What is bond enthalpy?

Answer 6

The energy required to break one mole of bonds in a gaseous molecule.

Question 7

How do you calculate enthalpy change △H?

Answer 7

Enthalpy of bonds broken - enthalpy of bonds formed.

Question 8

What is activation energy?

Answer 8

The energy needed to start a reaction by breaking bonds.

Question 9

How do you measure the enthalpy change of combustion?

Answer 9

A simple apparatus is set up, where a known mass of fuel is burned and is used to heat up a known mass of water (or other substance), the fuel (usually in a spirit burner) is then reweighed and the temperature rise is measured.

The temperature rise is then used together with the specific heat capacity of the water (4.18 JK-1g-1) and the mass of water (1 cm3 = 1 g of water) to calculate the amount of energy released:

Question 10

What is the equation for calculating an energy change?

Answer 10

q = mc△T
q = mass x specific heat capacity x temp change

Question 11

How do you calculate enthalpy change? (step by step)

Answer 11

1. q = mc△T
2. calcuate the number of moles, n, of fuel burnt
3. calculate the enthalpy change of reaction

Question 12

For calculations to be accurate several assumptions are made, what are these?

Answer 12

• all the heat energy released by the burning fuel is absorbed by the water.
• the fuel undergoes complete combustion.

Question 13

What is the reality of the assumptions made about heat energy?

Answer 13

• Heat energy is lost to the surroundings (air; apparatus etc)
• incomplete combustion occurs.

Question 14

What improves the accuracy of the heat energy calculation/ method?

Answer 14

• The combustion chamber is surrounded by water to maximise the absorption of the heat energy.
• A good supply of air / oxygen promotes complete combustion of the fuel.

Question 15

What is enthalpy of neutralisation?

Answer 15

The standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.

Question 16

How do you calculate the number of moles of acid or alkali?

Answer 16

using the stoichiometry / mole ratio in the symbol equation to determine the moles of water.

Question 17

What is the law of conservation of energy?

Answer 17

Energy can be neither created nor destroyed, only transferred from one form to another.

Question 18

What is Hess’ law?

Answer 18

The enthalpy change of a reaction is independent of the route taken, provided that the initial and final conditions are the same

Question 19

What applications can be made using Hess’ law?

Answer 19

as long as we start with the same reactants in the same physical states at the same temperature, and finish with the same products in the same physical states at the same temperature, then overall the amount of energy taken in or given out will be the same.

Allows us to carry out and measure q for a unmeasurable directly reaction and ones that you can’t carry out under normal lab conditions.

Question 20

What is enthalpy change of formation?

Answer 20

Def: Enthalpy Change of Formation: the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions. (298 K or 25oC and 1atm pressure

Question 21

What is the enthalpy change of combustion?

Answer 21

The enthalpy change when 1 mole of a substance is reacted with excess oxygen, under standard conditions, all reactants and products in their standard states.

Question 22

What equation shows the formation of methane?

Answer 22

C(s) + 2H₂(g) –> CH₄(g)

Question 23

What equation shows the formation of carbon dioxide?

Answer 23

C(s) + O₂(g). –> CO₂ (g)

Question 24

What equation shows the formation of water?

Answer 24

H₂(g) + 1/2 O₂(g) –> H₂O(l)

Question 25

What equation shows the combustion of methane?

Answer 25

CH₄(g) + 2O₂(g) –> CO₂ + 2H₂O(l)

Question 26

What equation shows the composition of carbon?

Answer 26

C(s) + O₂(g) –> CO₂(g)

Question 27

What equation shows the composition of hydrogen?

Answer 27

H₂(g) + ½ O₂(g) –> H2O(l)

Question 28

What is the enthalpy cycles and Hess’s law?

Answer 28

Hess’s law tells us that if we use an alternative route – via E + F,
i.e. A + B → E + F → C + D
then the overall enthalpy change will be the same as long as the initial and final conditions are the same.

Question 29

What are the steps to calculate enthalpy change, using the enthalpy change formation

Answer 29

Step 1: Write the equation for the enthalpy change you want to determine across the top.

Step 2: Construct a cycle If ΔHf data, write the elements at the bottom. Arrows from elements to compounds. Pointing UP.

Step 3: Add the data values. Check No of moles

Step 4: Show the alternative route.

Step 5: Add the values. Check arrow direction & amend signs.

Question 30

What are the steps to calculate enthalpy of formation/combustion using Hess’s law?

Answer 30

Step 1: Write the equation for the enthalpy change you want to determine across the top.

Step 2: Produce a Hess’s cycle. If ΔHc data, write the combustion products at the bottom and draw the arrows down to the combustion products.

Step 3: Add the enthalpy change data. Check No of moles

Step 4: Show the alternative route.

Step 5: Add the values. Check arrow direction & amend signs.

Question 31

What are born-haber cycles?

Answer 31

cycles that allow us to calculate the value of any of the enthalpy changes. on the cycle, provided that we know all the others. Usually, they involve calculating the lattice enthalpy or possibly the enthalpy of electron affinity.

Question 32

what is lactice enthalpy?

Answer 32

The enthalpy change when one mole of ionic solid is formed from its gaseous ions, under standard conditions.

Question 33

What is enthalpy of atomisation?

Answer 33

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions.

Question 34

What is 1st Ionisation energy?

Answer 34

The energy required to remove one mole of electrons from one mole of gaseous atoms, under standard conditions.

Question 35

What is 1st Electron affinity?

Answer 35

The enthalpy change when one mole of electrons is added to one mole of gaseous atoms, under standard conditions.