energetics

Question 1

What is an exothermic process

Answer 1

Energy released from the chemical to the surroundings during the process
The temperature of the surroundings increases

Think - Exothermic, ex -> exit

Question 2

What is an endothermic process

Answer 2

Energy is taken in by chemicals from the surroundings during the process.
The temperature of the surroundings decrease

Think - Endothermic en -> enters

Question 3

Define enthalpy change

Answer 3

Enthalpy change is the change in heat energy at constant pressure

Question 4

What is the △ H sign for exothermic processes & endothermic processes

Answer 4

Exothermic - △ H is negative -> reaction loses energy
Endothermic - △ H is positive -> reaction gains energy

Question 5

What are the standard conditions defined for enthalpy change measurements

Answer 5

100kpa pressure
298 K temperature
1.0 mol dm-3 (concentration) of all solutions

Question 6

When an enthalpy change is written under standard conditions what is it written as

Answer 6

ΔHƟ

Question 7

In order for a reaction to happen what must happen

Answer 7

Bend & Mex
Breaking of bonds - exothermic process
Making bonds - exothermic

Question 8

What is enthalpy change

Answer 8

The heat energy changed measured under conditions of constant pressure

Question 9

Define standard enthalpy of combustion (ΔcHƟ)

Answer 9

The enthalpy change when one mole of substance is completely burnt in excess oxygen under standard conditions, all reactants and products being in their standard states

Question 10

Define standard enthalpy of formation (ΔfHƟ)

Answer 10

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

Question 11

Define the term mean bond enthalpy

Answer 11

A mean bond enthalpy is the energy required to break one mole of covalent bind into gaseous atoms, averaged over a range of different compounds

Question 12

Define activation energy

Answer 12

Activation energy is the minimum energy needed to start a reaction

Question 13

What process is mean bond enthalpy

Answer 13

Are always endothermic processes and therefore have a positive sign
This is because energy is requested to break bonds
The more positive the bond enthalpy the larger the amount of energy needed or break the bond so a stronger bond

Question 14

N2 + 3H2 -> 2NH3
State why the enthalpy formation of Na(s) is zero
State why the enthalpy of formation of liquid Na is not zero

Answer 14

Na(s) an element in its standard state
Na (l) is not the standard state of Na

Question 15

Define Hess’ law

Answer 15

Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 16

What way do the arrows go for the following hess cycles

Mainly bond enthalpies
Mainly formation enthalpies
Mainly combustion enthalpies

Answer 16

Mainly bond enthalpies - down
Mainly formation enthalpies - up
Mainly combustion enthalpies - down

Question 17

What is the calorimeter equation for measuring enthalpy change & what does each letter stand for

Answer 17

Q= MC△T
△H = Q/ N

Q = energy transferred
M = mass of substance (usually water) (g)
C = specific heat capacity
△T = temperature change

△H = enthalpy change (kj mol-1)
Q = energy released or taken in (kj)
M = moles or chemicals reacting

Question 18

What is an assumption made in calorimeter

Answer 18

Energy transferred to water is equal to energy released by reaction

Question 19

What are the sources of error for both combustion calorimetry & solution calorimetry

Answer 19

Heat loss to surroundings

Question 20

What is the sources of error for combustion calorimetry

Answer 20

Incomplete combustion
Some energy used to heat
Metal calorimeter

Question 21

What are the improvements you could make to minimise sources of error for both combustion calorimetry and solution calorimetry

Answer 21

Add a lid to reduce heat loss for bath

Question 22

What are the improvements you could make to minimise sources of error for combustion calorimetry

Answer 22

Insulate inside of calorimeter
Put sleeve around flame to protect draught
Reduce distance between calorimeter and flame

Question 23

What are the improvements you could make to minimise sources of error for solution calorimetry

Answer 23

Insulate calorimeter

Question 24

Steps to measure an enthalpy change using a cooling curve (6)

Answer 24

Data collection -
Record temperature of solution every minute before adding reactants
To establish accurate starting temperature
Add reactants of 4th minute and measure temperature every minute until a cooling trend is seen

Graph plotting -
Plot graph of temperature vs time
Draw a line of best fit through cooling curve and extrapolate, back to time of addition
To calculate theoretical temperature change accounting for heat loss