Energetics

Question 1

Define Enthalpy change

Answer 1

Heat energy change at constant pressure

Question 2

Describe the enthalpy change in an exothermic reaction (2)

Answer 2

• Energy is transferred from the system(chemicals) to the surroundings
• The products also have less energy than the reactants

Question 3

What is the energy change in exothermic reactions and give examples of exothermic reactions (3 + 1)

Answer 3

• Negative
• Products are the negative stuff
• Combustion
• Respiration
• Oxidation
• Forming bonds

Question 4

What happens if there is an enthalpy change (2)

Answer 4

*Energy is transferred between the system and the surroundings
*The system is the chemicals and the surroundings is everything outside the chemicals

Question 5

Describe the enthalpy change in an endothermic reaction (2)

Answer 5

*Energy is transferred from the surroundings to the system (absorb energy)
* Requires an input of heat energy

Question 6

What is an example of an endothermic reaction (3) and what is the energy change in endothermic reactions

Answer 6

• The thermal decomposition of calcium carbonate
• The products have more energy than the reactants
• Enthalpy change is positive Reactants
• Breaking bonds

Question 7

What is the standard enthalpy change of formation

Answer 7

• The enthalpy change when 1 mole of a compound is formed from its elements
• Under standard conditions, with all substances in their standard states.

Question 8

What is the standard enthalpy of combustion

Answer 8

• The enthalpy change when 1 mole of a substance is completely burned in oxygen
• Under standard conditions, with all substances in their standard states.

Question 9

What are the 4 standard conditions

Answer 9

• 100kPa
• 298K (25C)
• Solutions at 1moldm^3
• All substances should have their normal state symbols at 298

Question 10

What does incomplete combustion lead to

Answer 10

Soot( carbon)
Carbon monoxide
Water
It will be less exothermic than complete combustion

Question 11

What is the equation for energy change

Answer 11

Energy change (J)=Mass of solution (g) x Heat capacity (Jg^-1 K^1) x Temperature change (K)

Q= Mx Cp x Delta T

Question 12

What are the errors in the calorimetric method?

Answer 12

• Energy transfer from surroundings (usually loss)
• The method assumes all
  solutions have the heat capacity of water.
• Neglecting the specific heat capacity of the calorimeter- we ignore any
  energy absorbed by the apparatus.
• Reaction or dissolving may be incomplete or slow.
• Density of solution is taken to be the same as water.

Question 13

Explain why bond breaking is endothermic and why bond forming is negative

Answer 13

• Energy is needed to break bonds so enthalpy change is positive (endothermic)
• Energy is released when bonds are formed so when the enthalpy change is negative it is exothermic

Question 14

What is calorimetry used for

Answer 14

To find how much heat is given out by a reaction by measuring a temperature change

Question 15

How can you use a graph to find an accurate temp change

Answer 15

record the temp at regular intervals beginning a couple mins before the you start the reaction
Plot a graph of the results draw two lines of best fit one through the points when u started and then another after you started
Extend both lines so they both pass the time when the reaction started
The distnace between the two lines at the time the reaction started is the accurate temp change for the reaction

Question 16

What does Hess’s Law state

Answer 16

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

Question 17

Formula for Enthalpy chnage for combustion

Answer 17

ΔH = q/n,
where ΔHf is the enthalpy of formation, q is the heat energy, and n is the number of moles.
You get q (heat energy) from q=mcDelta T

Question 18

Calculating the enthalpy change of reaction, (Delta)H from experimental data

Answer 18

General method
1. Using q = m x cp x T calculate energy change for quantities used
2. Work out the moles of the reactants used
3. Divide q by the number of moles of the reactant not in excess to give H
4. Add a sign and unit (divide by a thousand to convert Jmol-1
to kJmol-1

Question 19

you neeed to know this

Answer 19

Also assume that the solutions have the density of water, which is 1g cm-3 . Eg 25 cm3 will weigh 25g

Question 20

Signs for reactions

Answer 20

If there is a temp change that is posiitve th answer ,ust be given as a negative and vice versa wih a negative temp change

Question 21

Calculate the enthalpy change of reaction for the reaction where 25.0cm3 of 0.200 mol dm-3
copper sulfate was reacted with 0.0100mol (excess of zinc). The temperature increased 7.0 oC .

Answer 21

Step 1: Calculate the energy change for the amount of reactants in the calorimeter.
Q = m x cp x T
Q = 25 x 4.18 x 7
Q = 731.5 J
Step 2 : calculate the number of moles of the reactant not in excess.
moles of CuSO4 = conc x vol = 0.20 x 25/1000
= 0.005 mol
Step 3 : calculate the enthalpy change per mole which is often called the enthalpy change of reaction
H = Q/ no of moles
= 731.5/0.005
= 146300 J mol-1 = 146 kJ mol-1
to 3 sf
Finally add in the sign to represent the energy change: if temp increases
the reaction is exothermic and is given a minus sign e.g. –146 kJ mol-1

Question 22

Example 2. 25.0cm3 of 2.00 mol dm-3 HCl was neutralised by 25.0cm3 of 2.00 mol dm-3 NaOH. The temperature increased 13.5oC
Calculate the enthalpy change per mole of HCl.

Answer 22

Step 1: Calculate the energy change for the amount of reactants in the calorimeter.
Q = m x cp x T
Q = 50 x 4.18 x13.5
Q = 2821.5 J
Step 2 : calculate the number of moles of the HCl. moles of HCl = conc x vol
= 2 x 25/1000
= 0. 05 mol
Step 3 : calculate H the enthalpy change per mole which might be called the enthalpy change of neutralisation
H = Q/ no of moles
= 2821.5/0.05 = 56430 J mol-1 = -56.4 kJ mol-1
to 3 sf Exothermic and so is given a minus sign

Question 23

Example 3. Calculate the enthalpy change of combustion for the reaction where 0.650g of propan-1-ol
was completely combusted and used to heat up 150g of water from 20.1 to 45.5oC

Answer 23

Step 1: Calculate the energy change used to heat up the water.
Q = m x cp x T
Q = 150 x 4.18 x 25.4
Q = 15925.8 J
Step 2 : calculate the number of moles of alcohol combusted.
moles of propan-1-ol = mass/ Mr
= 0.650 / 60 = 0.01083 mol
Step 3 : calculate the enthalpy change per mole which is called cH (the enthalpy change of combustion)
H = Q/ no of moles
= 15925.8/0.01083
= 1470073 J mol-1 = 1470 kJ mol-1
to 3 sf
Finally add in the sign to represent the energy change: if temp increases
the reaction is exothermic and is given a minus sign eg –1470 kJ mol-1

Question 24

Errors in this method

Answer 24

• Energy losses from calorimeter * Incomplete combustion of fuel * Incomplete transfer of energy
• Evaporation of fuel after weighing
• Heat capacity of calorimeter not included
• Measurements not carried out under standard conditions as
  H2O is gas, not liquid, in this experiment

Question 25

Why is a mean bond energy used

Answer 25

Every single bond in a compound has a slightly different bond energy. E.g. In CH4 there are 4 C-H bonds. Breaking each one will require a different amount of energy. However, we use an average value for the C-H bond for all hydrocarbons.

Question 26

Combustion and formation

Answer 26

Combustion points down FOrmation points up

Question 27

What are the units and symbol for enthalpy change? (2)

Answer 27

- Symbol: ΔH - Unit: kJ mol-1

Question 28

What are the conditions for standard enthalpy change (ΔH°)? (2)

Answer 28

- Temperature: 298 K - Pressure: 100 kPa

Question 29

How do you balance combustion equations? (3) E.g. complete combustion of C3H8

Answer 29

1. Balance carbons on the CO2 / CO / C -> 3CO2 2. Balance hydrogens on the water molecule -> 4H20 3. Balance oxygens on the O2 molecule - 502 C3H8(g) + 5O2(g) -> 3CO2(g) + 4H20(l)

Question 30

What is the enthalpy of formation of elements? (1)

Answer 30

Zero

Question 31

Write an equation that represents a reaction equal to the standard enthalpy of formation of NAHCO3(s)

Answer 31

Na(s) + ½ H2 (g) + C (s) + 1.5 O2(g) → NaHCO3 (s)

Question 32

When is hess' law needed for enthalpy change calculations? (1)

Answer 32

For enthalpy changes that cannot be determined experimentally e.g. due to a solid dissolving etc

Question 33

How do you convert 1cm3 of water to g? (1)

Answer 33

1cm3 = 1g

Question 34

How do you calculate enthalpy change from q? (3)

Answer 34

- ΔH = kJ mol-1 (so divide q by 1000 to get it into kJ) Delta H= +or-q/n

Question 35

What are the two ways of carrying out calorimetry? (2)

Answer 35

1. A fuel is used to heat up a known volume of water 2. A reaction in soloution is carried out in an insulated container

Question 36

What is the first step in using a fuel to heat up water? (1)

Answer 36

Record the mass of the spirit burner and the fuel with the cap on

Question 37

What do you do after recording the mass of the spirit burner? (1)

Answer 37

Place a known volume of water in a beaker and record its temperature.

Question 38

How is the fuel used to heat the water? (2)

Answer 38

- The flame is placed close to the beaker of water, and heat-proof guards are used to minimise heat loss. - Stir the water while heating.

Question 39

What should you do when the water's temperature has increased by a reasonable amount? (2)

Answer 39

- Extinguish the flame by placing the lid on the burner to avoid fuel evaporation. - Monitor and record the highest temperature reached by the water.

Question 40

How do you calculate the mass of fuel burned? (1)

Answer 40

Reweigh the spirit burner with the lid on and calculate the difference in mass from the initial measurement.

Question 41

What are two reasons why the value of enthalpy change obtained is less than the data book value? (2)

Answer 41

Heat loss to the surroundings. Incomplete combustion

Question 42

What are three other reasons why the value obtained is less exothermic than the data book value? (3)

Answer 42

Non-standard conditions. Evaporation of fuel/alcohol. Heat loss to the can.

Question 43

How can poor stirring of the water affect the result? (1)

Answer 43

It can cause the temperature reading to be incorrect.

Question 44

What is used as the insulated container in method 2 of carrying out calorimetry ? (1)

Answer 44

A polystyrene coffee cup

Question 45

What is the first step in method 2 of carrying out calorimetry? (2)

Answer 45

1. Pour the aqueous reactant into the cup 2. Record the temperature every minute for three minutes to ensure the solution is at the same temperature as the room.

Question 46

What do you do on the fourth minute in method 2 of carrying out calorimetry? (2)

Answer 46

- Add the other reactant and stir to ensure the reactants mix thoroughly for a complete reaction and to ensure the temperature is even throughout the solution - Do not record the temperature

Question 47

What should you do from the fifth minute onwards? (1)

Answer 47

Record the temperature at the fifth minute and continue to record it every minute for up to ten minutes

Question 48

What is bond dissociation energy? (1)

Answer 48

The enthalpy change to break the bond in 1 mole of gaseous molecules, forming gaseous atoms.

Question 49

What is mean bond enthalpy? (1)

Answer 49

The enthalpy required to break 1 mole of covalent bonds, averaged over a range of compounds.

Question 50

Why is the enthalpy change calculated from mean bond enthalpy data not 100% accurate? (1)

Answer 50

Bond enthalpies are average values from a range of compounds.