Module 3 chapter 9- Need to finish calormetry

Question 1

What is enthapy?

Answer 1

When a chemical reaction occurs there is normally a change in energy,

The enthapy change of a reaction is the heat change in a reaction at constant pressure

Question 2

UNITS of enthalpy

Answer 2

KJmol-1

Question 3

what are the standard conditions?

Answer 3

1- 100kpa pressure
2- 298K (25degrees)

Question 4

What is endothermic reaction?

Answer 4

Absorb energy from the surroundings

Products are higher in energy than reactants

Enthalpy is positive

Question 5

EXAMPLE of endo

Answer 5

Thermal decomposition of calcium carbonate

Question 6

What is exothermic?

Answer 6

• Release energy to the surroundings

-Reactants are higher in energy than the products

• Change in enthalpy is negative

Question 7

Example of exothermic

Answer 7

Combustion of ethane

Question 8

4 type of enthalpy changes you need to know of

Answer 8

1- Standard enthalpy change of reaction

2- Standard enthalpy change of naturalization

3- Standard enthalpy change of formation

4-Standard enthalpy change of combustion

Question 9

What is standard enthalpy change of reaction?

Answer 9

The enthalpy change of a reaction to the molar quantities in the equation under standard conditions

Question 10

What is a standard enthalpy change of neutralization?

Answer 10

The enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions

Question 11

What is standard enthalpy change of formation?

Answer 11

The enthalpy change when 1 mole of a compound is formed from its elements its their standard states under standard conditions

Question 12

What is standard enthalpy change of combustion?

Answer 12

The enthalpy change when 1 mole of a substance is completely burned in oxygen to make co2 and H20 under standard conditions

Question 13

Bonds making and breaking

Answer 13

Bonds are broken and made during a reaction

Question 14

What happens when bonds are broken

Answer 14

to break a bond, energy need to be absorbed

Bonds are broken in reactants and this is endothermic process so enthalpy change is positive

if more energy is need to break bonds than energy given out, this reaction is endothermic

Question 15

What happens when bonds are made?

Answer 15

When bonds are made energy is released

Bonds are made when products are being produced and this is an exothermic process so enthalpy change is negative

if more energy is released when bonds are formed than what was needed to break initial bonds the reaction will be exothermic

Question 16

mean bond enthalpy

Answer 16

Bond enthalpy varies for even the same time of bond, the energy needed to break the first C-H will be different to the energy needed to break the second C-H

We can measure the enthalpy of breaking all 4 bonds and divide by 4 to get a mean and that is our mean bond enthalpy

Question 17

What is the bond dissociation enthalpy?

Answer 17

The energy needed per mole to break attractive forces in bonds is called the BDE.
Only happens in gaseous compounds

Question 18

How to work out enthalpy change using mean bond enthalpy

Answer 18

Total energy released from forming bonds (exothermic - products)

Question 19

Calorimetry (practical technique)

Answer 19

Calorimetry is used to work out the enthalpy change of combustion of a fuel

Question 21

How can energy from calorimetry experiment be calculated?

Answer 21

q = mc(delta)T

q= heat energy lost or gained (J)

m= mass of water or/in? solution (g)

c= specific heat capacity pf water (4.18Jg-1K-1) given in exams

deltaT= change in temp (K)

Question 22

IMORTANT!

Answer 22

heat energy is different to enthalpy

Question 23

How to work out enthalpy from energy ?

Answer 23

1- work out energy (covert to KJ)

2- Work out the moles of fuel

3- Work out enthalpy (q/moles)

Question 24

What is Hess’s Law

Answer 24

The total enthalpy change of a chemical reaction is independent of the route taken

Question 25

Why do we have Hess’s law?

Answer 25

Germain Hess came up with a law to work out enthalpy changes you cant find out by doing an experiment

Question 26

What are the two Hess’s cycles?

Answer 26

1- Formation cycles
2- Combustion cycles

Question 27

One thing to remember about Hess’s cycles

Answer 27

Start anywhere on the cycle and add up the numbers until you go back where you started. FLIP THE SIGN IF YOU GO AGAINST THE DIRECTION OF THE ARROW. It should add up to ZERO