Unit 5 Energetics

Question 1

What is an enthalpy

Answer 1

term used for the heat released or absorbed by a system at constant pressure

Question 2

What symbols represents the change in enthaply

Answer 2

Delta H

Question 3

What do you call the enthaply change measured under standard conditions

Answer 3

standard enthalpy change of reaction (ΔHƟ).

Question 4

Exothermic and endothermic reactions are

Answer 4

Exo: release heat
Endo: absorb heat

Question 5

Examples of exothermic reactions

Answer 5

Combustion and Neutralisation reactions

Question 6

Explain energy profile of exothermic reaction

Answer 6

the enthalpy change (∆H) is negative because heat has been transferred from the system to the surroundings, so the system has lost heat.
The products of the reaction have lower enthalpy than the reactants, so they are more energetically stable.

Question 7

Examples of endothermic reaction

Answer 7

thermal decomposition of calcium carbonate.

Question 8

Explain energy Diagram of endothermic reaction

Answer 8

The enthalpy change is positive because the reactants gain heat from their surroundings. For this reason, the products have higher enthalpy than the reactants and are therefore less energetically stable.

Question 9

How are enthalpy changes calculated:

Answer 9

Q= mc deltaT, where…
q is heat absorbed or released in J
C is specific heat capacity of solution
M is mass of solution in gr
DeltaT is change in Temperature in C or K

Add or remove minus to results depending on exothermic or endothermic reaction.

Question 10

Calculate enthalpy change (deltaH) for a reaction

Ex:

50.0 cm3 of a 0.500 mol dm−3 solution of aqueous copper(II) sulfate was reacted with 3.00 g of zinc powder, according to the equation:

Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

The results are shown in Table 1 ​below. Use this data to determine the enthalpy change (ΔH) for the reaction.

Volume of 0.500 mol dm-3 CuSO4 (aq) used (cm3) =50.00
Mass of Zn powder added (g) = 3.00
Initial temperature of solution (C) = 21.0
Maximum temperature of solution (C) = 43.5

Answer 10

1. Determine the change in heat (q) for the reaction using the equation q=mc deltaT with values from table
   - delta T: 22.5
   - mass is 50.00 cm3 which is 50.00 gr
   -assume heat capacity is 4.18
2. Determine limiting reactant in reaction
   - calculate mol of each reactant
   - divide each amount in mol by the coefficient in balanced equation
   - lower value is limiting reactant
3. Calculate the enthalpy change per mole of limiting reagent.
   - DeltaH divided by moles of limiting reagent

Question 11

What is the standard enthalpy of combustion or the molar enthalpy of combustion

Answer 11

ΔHƟc), for a substance is defined as the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
Always exothermic

Question 12

How to calculate enthalpy change of combustion of hexane
Ex. In an experiment to measure the enthalpy change of combustion of hexane (C6H14), a student heated a copper calorimeter containing 400.0 cm3 of water with a spirit lamp and collected the following data:

Initial temperature of water: 20.0 °C
Final temperature of water: 77.9 °C
Mass of hexane burned: 2.49 g

Use this data to calculate the enthalpy change of combustion, ΔHc, of hexane.

Answer 12

1. Calculate mol of hexane burned
2. Calculate q using mc deltaT
3. Divide heat released by amount in mol of hexane burned.

Question 13

How do you calcite the experimental error

Answer 13

Percentage error=
(experimental value- theoretical value) divided by theoretical values times 100.

Question 14

What is Hess’s law

Answer 14

states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same.

Question 15

What is Hess’s law used for?

Answer 15

be used to calculate enthalpy changes for reactions that cannot be determined experimentally
Ex, enthalpy change of formation of methane bc hydrogen doesn’t react with carbon under standard conditions

Question 16

Define entalphy chnage of formation (ΔHf), or standard enthalpy change of formation (ΔH⦵f)

Answer 16

the enthalpy change when one mole of a compound is formed from the elements in their standard states under standard conditions.

Question 17

Outline the enthalpy cycle when using the enthalpy change of formation and how it can be summarized

Answer 17

Arrows going to products and reactants from intermediate elements

ΔH⦵ = ΣΔH⦵f (products) − ΣΔH⦵f (reactants)

A useful application of Hess’s Law is that the enthalpy change of any reaction can be calculated provided that we know the enthalpy of formation values of the reactants and products.

Question 18

What does the standard enthalpy of formation indicate

Answer 18

they indicate the stability of compounds in relation to their elements - the more negative the value, the greater the stability of the compound.

Question 19

How can the enthalpy change of a reaction be calculated

Answer 19

using standard enthalpy change of combustion (ΔH⦵c ) values, using the equation below:

ΔH⦵ = ΣΔH⦵c (reactants) − ΣΔH⦵c (products)

Question 20

Outline MexoBendo

Answer 20

Making bonds is exothermic
Breaking bonds is endothermic

Question 21

In terms of bonds how do you determine wether a reaction is exothermic or endothermic

Answer 21

if the energy absorbed while breaking bonds is less than the energy released when forming new bonds, the reaction is exothermic

If the energy absorbed while breaking bonds is greater than the energy released when forming new bonds, the reaction is endothermic

Question 22

Define bond enthalpy

Answer 22

the energy required to break one mole of chemical bonds in the gaseous state.

Question 23

Define average bond enthalpy

Answer 23

the enthalpy change when one mole of bonds are broken in the gaseous state averaged for the same bond in similar compounds.

Question 24

How to calculate enthalpy changes using bond enthalpies

Answer 24

ΔH = ΣE(bonds broken) − ΣE(bonds formed)

or

ΔH = Σ(bond enthalpies of reactants) – Σ(bond enthalpies of products)

Weird E means sum

Question 25

What is the equation for the Haber process

Answer 25

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Question 26

Outline relative strengths of the bonding in oxygen and ozone

Answer 26

Oxygen, O2
Bonding: 0=0 (two electron pairs on each O)
Bond energy: 498
Dissociation: UV-C. Higher energy radiation of shorter wavelength

Ozone, O3
Bonding: triangle with O, one bond between two Os, has a dotted bond.
Bond energy: 364
~Dissociation: UV-B. Lower energy longer wavelength radiation.

Question 27

Outline the formation of free radicles in stratosphere

Answer 27

The strong double covalent bond in di-oxygen (O2) can be broken in the stratosphere by ultraviolet radiation with a wavelength shorter than 242 nm, producing two free oxygen atoms (or radicals).

O2 (g) → O• (g) + O• (g) UV light, λ < 242 nm

The oxygen atoms produced each have an unpaired electron; they are highly reactive free- radicals and will react with oxygen molecules to form ozone (tri-oxygen, O3).

O2 (g) + O• (g) → O3 (g)