3.1 Physical Chemistry: .4 Energetics

Question 1

what is an exothermic reaction?

Answer 1

a reaction where heat energy is given out to the surroundings

to make bonds

there is usally a temperature increase

Question 2

what is an endothermic reaction?

Answer 2

a reaction where heat energy is absorbed (taken in) from the surroundings

to break bonds

the temperature usually decreases

Question 3

in an exothermic reaction is ΔH positive or negative?

Answer 3

negative

Question 4

in an endothermic reaction is ΔH positive or negative?

Answer 4

positive

Question 5

In a endothermic reaction out of the reactants and products which one has more energy?

Answer 5

the products have more energy than the reactants

Question 6

In a exothermic reaction out of the reactants and products which one has more energy?

Answer 6

the reactants have more energy than the products

Question 7

what is meant by enthalpy change? what are the units

Answer 7

enthalpy change is the heat energy transferred in a reaction at consant pressure

units: kJ mol^-1

Question 8

what are the standard conditions for measuring the enthalpy changes of a reaction?

Answer 8

• Pressure: 100kPa
• Temperature: 298 K (25 degrees)
• 1 mole

Question 9

why is it important to have a standard of pressure for measuring energy changes?

Answer 9

• pressure affects the amount of heat energy given out by the reactions that involve gases
• if a gas is given out. some energy is required to push away the atomsphere
• the greater the atomspheric pressure the more energy is used for this
• this means that less energy remains to be given out as heat by the reaction. `

Question 10

what is the eqaution for calculating ΔH (the enthalpy or heat energy change)?

Answer 10

ΔH= Σ Er (sum of bonds broken) - Σ Ep (sum of bonds made)

Σ= sigma (sum)

Er= Energy of reactants

Ep= Energy of products

Question 11

Calculate the overall enthalpy change for this reaction:

N_2(g) + 3H_2(g) → 2NH_3(g)

Use the mean bond enthalpy valules

N=N 945 kJ mol^-1

H-H 436 kJ mol^-1

N-H 391 kJ mol^-1

Answer 11

+ΔH (bonds broken)

(N=N x 1)= 945 kJ mol-1

(H-H x 3)= 1308 kJ mol-1

Total= 2253 kJ mol-1

-ΔH (bonds made)

(N-H 391 x 6)= 2346 kJ mol-1

Total= -2346 kJ mol-1

ΔH= Σ Er - Σ Ep

ΔH= 2253 - 2346

ΔH= -93 kJ mol-1

Question 12

what is bond dissociation energy?

Answer 12

the enthalpy (heat energy) change when 1 mole of a particular covalent bond is broken, all species are in the gaseous state

under standard conditions of;

298 K

100kPa

1 mole

Question 13

give the equation for the bond dissociation of chlorine

Answer 13

Cl_2(g) → 2Cl_(g)

Question 14

what is the definition for the enthalpy of formation? ΔH_f^θ

Answer 14

this is the enthalpy (heat energy) change when 1 mole of a compound is formed from its elements, all reactants and products in their standard states when measured under standard conditions of 298 K and 100 kpa

Question 15

give the eqaution where hydrogen, nitrogen and oxygen are reacted together to make 1 mole of HNO3 (in enthalpy of formation)

Answer 15

1/2H₂ + 1/2N₂ + 3/2O₂ → HNO₃

Question 16

what is the definition for the enthalpy of combustion?ΔH_c^θ

Answer 16

This is the enthalpy (heat energy) change when 1 mole of a substance is burnt in excess oxygen, all reactants and products in their standard states when measured under standard conditions of 298 K and 100 kPa

Question 17

for the eqaution:

4Na_(s) + O2_(g) → 2Na2O_(s)

state the formula

you would use for the enthalpy of combustion

Answer 17

ΔH^θ= 4 x (ΔH_c^θ [Na_(s)]

Question 18

state the eqaution you would use to work out the heat change in a reaction

Answer 18

q= mcΔT

m= mass/grams (usually gives you in cm3 which = g)

c= specific heat capacity (for water its usually 4.18)

ΔT= temeperature change in Kelvin

Question 19

give an eqaution for the buring of Sodium in excess oxygen to make sodium oxide (enthalpy of combustion)

Answer 19

1Na_(s) + 1/4O_2(g) → 1/2Na₂O_(s) ΔH_c^θ (Na)

(you are only allowed to make 1 mole of Na because of the standard conditions)

(you get 1/4 O2 because 1/2 O produced divided by 2 O used gives a 1/4)

Question 20

describe a experiment where you use calorimetry to find out how much heat is given out by a reaction

-state the equipment needed

Answer 20

equipment needed:

• stirrer
• thermometer
• Fuel (reactant)
• water
• copper calorimeter
  1. weight the intial mass of your feul and note it down
  2. measure the temperature of the water before starting to burn and note it down
  3. heat the water to a temperature change of 20 degrees (or ΔT of your choice)
  4. measure the final mass of feul

Question 21

In a labortory experiment, 1.6 g od an organic liquid fuel were completely burned in oxygen. The heat formed during this combustion raised the temperature of 100g of water from 295 K to 358 K. Calculate the standard enthalpy of combustion ΔH_c^θ, of the fuel. Its M_r is 58.0

Answer 21

q=mcΔT

q= 100 x 4.18 x (358-295)= 26 334 J

convert jules into kilojoules (26 334 J/ 1000)= 26.334 kJ

then find out how many moles of fuel produced

number of moles of fuel = 1.16/58.0 = 0.0200

so the heat produced by 1 moles of fuel (ΔH_c^θ)= -26.334/ 0.0200

= -1320 kJ

you put a minus infront of the 26.334 because the reaction is exothermic. you know this since it raised the temperature of the water

Question 22

what are the experimental problems with all calorimetry?

Answer 22

• some heat will be absorbed by the container, rather than going towards heating up the water
• some heat is always lost to the surroundings during the experiment (however well the container’s insulated)

Question 23

what are the experimental problems with flammable liquid calorimetry?

Answer 23

• some combustion may be imcomplete- which will mean less energy will be given out
• some of the flammable liquid may escape by the vaporation (they’re usually quite volatile)

Question 24

give the definition for Hess’s Law

Answer 24

the enthalpy chnage at constant pressure is independan of the route.

The intial and final energu states are constant

Question 25

work out the enthalpy change of combustion to calculate ΔH_r^θ of ethanol (C₂H₅OH)

using the thermochemical cycle

ΔHcθ [C_(s)]= -394 kJ mol^-1

ΔHcθ [H_2(g)]= -286 kJ mol^-1

ΔHcθ [C2H5OH]= -1367 kJ mol^-1

Answer 25

start with your equation:

2C_(s) + 3H_2(g) + 1/2O_2(g) –> C₂H₅OH_(l)

use the equation ΔH₁₌ ΔH_{2 +} ΔH₃

Question 26

what is the mean bond enthalpy?

Answer 26

this is the average or mean change when 1 moles of a particular covalent bond is broken. when averaged over many different environments, all speices are gaseous state.

Question 27

explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law

Answer 27

• the values determined by Hess’s law is the ‘correct value’ because all the enthalpy of formation changes of the values have been obtained from the actual compunds involved
• mean bond enthalpy calculations also allow us to calculate an approximate value for ΔH_fθ for a compound that has never been made.