10 - Enthalpy

Question 1

Define enthalpy

Answer 1

How much heat is stored in a system

symbol: H
unit :
kJ / mol
or
kJ mol^-1

Question 2

Define system

Answer 2

The atoms, ions and particles in the chemical involved in the reaction

Question 3

Define exothermic and endothermic

Answer 3

An exothermic reaction is one which releases heat energy from the system into the surroundings

An endothermic reaction is one which takes in heat energy from the surroundings into the system

Question 4

Give an example of an exothermic reaction

Answer 4

Combustion reactions

Question 5

Give an example of an endothermic reaction

Answer 5

Thermal decomposition of calcium carbonate

Question 6

What is the difference in enthalpy change for exothermic or endothermic reactions?

Answer 6

Exothermic reactions give a negative enthalpy change

Endothermic reactions give a positive enthalpy change

Question 7

What is the enthalpy change of the thermal decomposition of calcium carbonate?

Answer 7

+ 178 kJ mol^-1

Question 8

Define activation energy

Answer 8

Minimum energy required to start a reaction by breaking bonds

symbol: E~a

Question 9

What equation do we use to find out how much energy is released by / stored in a given amount of substance?

Answer 9

E = nH

Energy = moles * enthalpy

Question 10

What 3 things is enthalpy affected by?

Answer 10

Temperature

Pressure

Concentration

Question 11

What are the standard conditions for measuring enthalpy change?

Answer 11

298K temperature

101kPa pressure

1 mol dm^-3 concentration

Question 12

Define standard enthalpy change of formation

Answer 12

Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with substances in standard states

Question 13

How do you calculate the equations of standard formation?

Answer 13

You write the product (like H~2 0) on the right of the reaction arrow

You then decide how many moles of the elements are needed to make that 1 mole of product

Question 14

What is the standard formation equation of ammonia

Answer 14

0.5 N~2 (g)
\+
1.5 H~2 (g)
-->
NH~3 (g)

You would then probably have to write the standard enthalpy change of formation afterwards

Question 15

What is the enthalpy of formation of just an element?

Answer 15

Always 0 kJ/mol for just elements

Question 16

What is the first way of measuring enthalpy change?

Answer 16

Transferring heat to and from water or surrounding air

You then measure the temperature of the water / air

Question 17

Define specific heat capacity

Answer 17

The amount of energy needed to heat 1g of a substance by 1K

Question 18

What is the specific heat capacity of water?

Answer 18

4.18 J g^-1 K^-1

Question 19

How do you calculate the heat energy a fuel has released?

Answer 19

Q = mc delta T

Heat energy = mass of surroundings specific heat capacity temperature change in surroundings

Question 20

What is the density of water / a solution?

Answer 20

1 g cm^-3

Remember that it is grams and centimetres

Question 21

How do you calculate enthalpy change from heat energy?

Answer 21

Delta H = Q / n

Enthalpy change = heat energy released / number of moles which released that energy

Question 22

What is direct determination?

Answer 22

When enthalpy change is calculated from one experiment

Question 23

What are the 4 steps for calculating enthalpy change from experiment results?

Answer 23

Calculate Q using Q = mc Delta T

Calculate n of the surroundings

Do Delta H = Q / n

Make enthalpy change +ve / -ve if reaction is endothermic or exothermic respectively

Question 24

Define standard enthalpy change of combustion

Answer 24

Enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions with substances all in standard states

Question 25

Define standard enthalpy change of neutralisation

Answer 25

Enthalpy change that accompanies the reaction of an acid and a base to from 1 mole of water under standard conditions with substances all in standard states

Question 26

What is the other type of standard enthalpy change?

Answer 26

Standard enthalpy change of reaction

Question 27

When is standard enthalpy change of reaction used?

Answer 27

When the reaction isn’t any of the others (formation/combustion)

Question 28

What are the 3 reasons that you can’t measure enthalpy change for a reaction?

Answer 28

There are too many possible products that will be made (we can’t choose a select one)

Rate of reaction is too slow

Activation energy is too high

Question 29

When do we use Hess’s law?

Answer 29

when dealing with reactions where we can’t measure enthalpy change

Question 30

State Hess’s law

Answer 30

If a reaction can take place by 2 routes and the start and end conditions are the same, the enthalpy change for each route is the same

Question 31

Describe what Hess’s law means

Answer 31

If we can do multiple reactions to get the same result as our original reaction (which we can’t do)

We will get the same overall enthalpy change over the multiple alternative reactions than we would doing the original reaction

Question 32

Describe enthalpy cycles

Answer 32

Reaction A goes straight from reactants to products but we can’t use this reaction

Reaction B and then reaction C get us from the reactants to the same products and we can do these reactions respectively

A = B + C is a rule we need to know

Question 33

What happens when you go in the opposite direction of an arrow on an enthalpy cycle?

Answer 33

You change the sign of the enthalpy change of that reaction

Question 34

For combustion reactions, where do the enthalpy cycle arrows point?

How do you remember this?

Answer 34

Both B and C arrows point downwards to the combustion products which are H~2 O and CO~2

combustion=point down

Question 35

For formation reactions, where do the enthalpy cycle arrows point?

How do you remember this?

Answer 35

Both B and C point upwards

formation=point up

Question 36

What do you do for a Hess law enthalpy change of reaction?

Answer 36

You are given a table of reactions and their enthalpy changes

You have to assign the reactions to the letters like A, B etc

Question 37

What happens in terms of energy when bonds are broken and when bonds are formed?

How does this relate to exothermic / endothermic?

Answer 37

Forming bonds releases energy

Breaking bonds requires energy

Making is exothermic and breaking is endothermic

Question 38

Define average bond enthalpy

Answer 38

Energy required to break one mole of bonds in a gaseous molecule

Question 39

What is the formula for enthalpy change using bond enthalpy?

Answer 39

Enthalpy change =

Sum of enthalpy changes of bonds broken

-

Sum of enthalpy change of bonds formed

Question 40

Why are bond enthalpies always positive?

Answer 40

They are the enthalpy change when bonds are formed

Forming bonds releases energy

Question 41

Describe the practical experiment for determination of enthalpy change of combustion

Answer 41

Measure a known volume of water into a beaker

Weigh the burner containing fuel

Take initial temp of water

Burner is lit and water heated until temperature has risen by a reasonable amount

Final temperature is taken

The burner is weighed to find mass of fuel burnt

Question 42

When assigning reactions to letters in an enthalpy change of reaction Hess question

What do you need to do in order to see which reaction you are looking for?

Answer 42

You have to cancel out any substances which are in reactants and products in order to find the reaction in the table

BUT DO NOT CROSS THEM OUT because you will need them later

Question 43

Define standard enthalpy change of reaction

Answer 43

Enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation under standard conditions with all substances in standard states