Chapter 4) Energetics

Question 1

Why do all reactions require energy?

Answer 1

to break bonds in the reactants, and all reactions give off energy when new bonds are made to form the products.

Question 2

What tells you whether a reaction is endothermic or exothermic?

Answer 2

The difference in the energy required to break the bonds and to make the new bonds

Question 3

Why is it possible to be able to work out the enthalpy change if you know the products and reactants?

Answer 3

Hess’s law states that whichever route a reaction takes, the enthalpy change will always be the same..

Question 4

What are the units of enthalpy change?

Answer 4

mol/KJ

Question 5

Define enthalpy change:

Answer 5

The heat change at constant pressure

Question 6

Why do open flasks lead to slight error when calculating enthalpy change?

Answer 6

as atmospheric pressure varies from day to day, this leads to slight systematic error

Question 7

Describe the enthalpy diagram of an exothermic reaction:

Answer 7

the reactants have a higher enthalpy than the products. the delta H is negative (decreasing), the x axis is labelled extent of reaction and the y axis is labelled enthalpy.

Question 8

Describe the enthalpy diagram of an exothermic reaction:

Answer 8

the reactants have a lower enthalpy than the products. the delta H is positive (increasing), the x axis is labelled extent of reaction and the y axis is labelled enthalpy.

Question 9

What is there no way to measure directly?

Answer 9

heat

Question 10

How is the heat of a reaction measured?

Answer 10

In an experiment, heat created from a reaction is transferred to a mass (often water) and the temperature change is measured.
q=mc(delta)T
This also means the specific heat capacity of the substance , mass of substance and the temperature change is recorded.

Question 11

Define temperature:

Answer 11

The average kinetic energy of the particles in a system, temperature Is independent of the number of particles present.

Question 12

Define heat:

Answer 12

A measure of the total energy of all the particles present in a given amount of substance, it is dependent on the amount of substance. Heat flows from a high temperature to a low. e.g a lukewarm bath has much more heat than a red hot nail, as the bath contains more particles.

Question 13

What is the apparatus called that is used to calculate enthalpy change?

Answer 13

calorimeter

Question 14

what are the differences between simple calorimeter and flame calorimeter?

Answer 14

Flame calorimeter is designed to reduce heat loss:

• spiral chimney made of copper
• flame is enclosed
• fuel burns in pure oxygen rather than air.

Question 15

What are the units for q=mc(delta)T

Answer 15

q=Joules
m=grams
(delta) T= kelvin or degrees Celsius

Question 16

How do you go from joules/q to mol/KJ (enthalpy change)?

Answer 16

-divide by 1000=KJ
-find the number of moles burnt
divide the number of KJ by number of moles

Question 17

Name 2 types of exothermic reactions:

Answer 17

• neutralisation

- combustion

Question 18

State Hess’s law:

Answer 18

the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products..

Question 19

Define the enthalpy change of combustion, ΔcHo,

Answer 19

the enthalpy change when one mole of a substance is burnt to completion under standard conditions.

Question 20

What do thermochemical cycles represent?

Answer 20

a reaction so that the enthalpy change can be calculated from known enthalpy change values, such as the enthalpy change of combustion..

Question 21

How do you use an enthalpy change diagram to find unknown enthalpy changes?

Answer 21

start by drawing a line at zero to represent the elements, and then build up the diagram using the enthalpy changes of formation for each compound before finding the differences in their enthalpies.

Question 22

What’s the difference between enthalpy diagrams and thermochemical cycles?

Answer 22

• Both are used to represent enthalpy changes in a chemical reaction.
• Enthalpy diagrams show the enthalpy level of reactants and products of a chemical reaction on a vertical scale, so you can compare energies.

Question 23

What can you say about a substance that has a lower energy than another:

Answer 23

it is energetically more stable

Question 24

Define bond dissociation enthalpy:

Answer 24

The enthalpy change required to break a covalent bond with all species in a gaseous state.

Question 25

Define mean bond enthalpy:

Answer 25

The average of the different bond enthalpies for the same bond, they are calculated using Hess’s law cycles and can be looked up in data books and databases. This is because there are variations in the bond enthalpies of a bond, for example C‒C, depending on the molecule it is in, so mean bond enthalpies are used when calculating enthalpy changes.

Question 26

How do you use mean bond enthalpies to find enthalpy of formation/combustion?

Answer 26

1. Draw out ALL molecules (check ratio of reactants and products).
2. list all the types of bonds and number of them on both sides of equation
3. add up mean bond enthalpy of reactant bonds
4. add up mean bond enthalpy of product bonds.
5. product’s total bond enthalpy - reactant’s total bond enthalpy= difference and sign = estimate of enthalpy change

Question 27

How can you reduce the time it takes to find enthalpy change using bond enthalpy?

Answer 27

only find the sum of bonds broken for reactants, and only find the sum of bonds forms for products.

Question 28

a formation reaction always has a xxxxx (delta) H as it is yyyyy

Answer 28

x= negative
y= as is exothermic