Enthalpy changes

Question 1

What is a system

Answer 1

The reactants and products involved in a reaction

Question 2

what are the surrounding in the context of a reaction system

Answer 2

Anything else other than the reactants and products e.g beaker , air, bunsen burner, stirring rod, (aq) water if there no H2O in the products or reactants

Question 3

Key ideas behind system and surroundings

Answer 3

Substances can both be part of the system and surrounding e.g water can be present in the surroundings by the state symbol (aq). However, water can be present in the system shown by the reactants and products.

Question 4

Explain constant pressure when the pressure is constant and when it increases

Answer 4

When the pressure the system exerts on the surroundings stay the same as the pressure the surrounding exert on the system. The pressure the surroundings exert on the system is constant. (Reaction is under constant pressure)

When the pressure the system exerts on the surroundings increases so does the pressure the surroundings exert on the system. Therefore, the pressure the surroundings exert on the system changes. (reaction is nto under constant pressure)

Question 5

Explain constant pressure when a reaction is not in a closed system

Answer 5

During reactions which are not closed gases can be released. Therefore, the system does not exert any pressure onto the surroundings. Therfore, the surroundings do not exert pressure on the system. Therefore, there is a constant pressure.

Question 6

Key ideas behind constant pressure

Answer 6

Every reaction system has air (e.g in the solution or the container from the air)

These surroundings all exert pressure in the reaction system. This happens under pressure fromt the surroundings.

The amount of pressure the surroundings exert are different depending on the molecules in the air, the solution the reaction occurs in and if the container has a lid or not.

Pressure can remain constant or change

Question 7

Explain energy changes in a reaction

Answer 7

As the reactions turn into products energy flows into the reaction system and the surroundings. This energy which is changed is called heat energy.

If heat energy flows from the system to the surroundings it gives out energy. Therefore, the surroundings get hotter

If heat energy flows from the surroundings into the reaction system it takes in heat energy. The surroundings get colder

Question 8

Explain an exothermic reaction

Answer 8

If heat energy flows from the system to the surroundings it gives out energy. Therefore, the surroundings get hotter. This is called an exothermic reaction.

During and exothermic reaction the system loses heat energy and the surroundings gain heat energy

Question 9

Explain an endothermic reaction

Answer 9

If heat energy flows from the surroundings into the reaction system it takes in heat energy. The surroundings get colder. This is called an endothermic reaction.

During endothermic reaction the reaction system gains heat energy while the surroundings lose heat energy

Question 10

Draw an energy level diagram for both an exothermic reaction and endothermic reaction

Question 11

What is enthalpy change and how does it affect temperature

Answer 11

Heat energy change under constant pressure is called enthalpy change.

Heat energy changes the temperature of the surroundings:

During an exothermic reaction under constant pressure, heat energy is released to the surroundings and the temperature increases. The system loses heat energy and the enthalpy change is negative.

During an endothermic reaction under constant pressure, heat energy is absorbed from the surroundings and the temperature decreases. The system gains heat energy and the enthalpy change is positive.

Question 12

How can you express enthalpy change

Answer 12

△H = (KJ^-1 mol)

△H = +26KJ^-1 mol) - positive enthalpy change

△H = -26KJ^-1 mol) - negative enthalpy change

Question 13

Explain how mass affects enthalpy change

Answer 13

We can’t find enthaply change change just from the change in heat energy as the mass of a substance can affect the temperature.

When experiencing the same change in heat energy. The temperature change of a material with smaller mass may be larger than the temperature change of a material with a greater mass

Question 14

Explain how specific heat capacity affects enthalpy change

Answer 14

We also need to consider the material we are measuring temperature change in

If the surroundings have a low specific heat capacity and an exothermic reaction takes place. Then it will cause a big increase in the temperature of the surroundings.

If the surroundings have a high specific heat capacity and an exothermic reaction takes place. Then it will cause a small increase in the temperature of the surroundings.

If the surroundings have a low specific heat capacity and an endothermic reaction takes place. Then it will cause a big decrease in the temperature of the surroundings.

If the surroundings have a high specific heat capacity and an endothermic reaction takes place. Then it will cause a small decrease in the temperature of the surroundings.

Question 15

What is specific heat capacity

Answer 15

Specific heat capacity is the amount of energy required in order to raise the temperature of one gram of a substance by one kelvin

Question 16

Express the unit for Specific heat capacity

Answer 16

JK^-1g^-1

Question 17

How can we find the enthalpy change

Answer 17

Q=mc ΔT

Amount of heat energy taken in or given out by a reaction = mass X specific heat capacity X change in temperature

Question 18

Explain a flame calorimeter

Answer 18

We measure the enthalpy change of the system by measuring the temperature of the surroundings. The energy released when burning ethanol in a spirit burner (the system) and the surroundings are the beaker, water and thermometer.

We measure the tempreature change in the water and calculate the enthalpy change.

Not all heat energy is released into the water and some is trasferred from the water into the air.This can be prevented by increasing the mass of water in the beaker ( if the mass of water is too big it wil be difficult to measure a change in temperature).

Question 19

Explain a bomb calorimeter

Answer 19

Chemists put the reactants in an empty steel container called a bomb. Then they put the bomb in a known mass of water then they start the ignition wires and let it run to completion. If the heat energy produced by the reaction is transferred to the steel bong and then in the water surrounded by a small layer of air and more metal. This prevents any heat energy being lost from the water into the surroundings around the calorimeter.
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Scientists calculate the change in temperature of the water and then use the results to calculate an enthalpy change of the reaction
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Bomb calorimeters are expensive and very exclusive. It is cheaper and simple to use a flame caloirmeter.

Question 20

Explain a simple calorimetry for reaction in a solution

Answer 20

We cant measure the enthalpy change of all reactions using a flame calorimeter. Therfore, for reactions in solutions we use constant pressure calorimetry.

We can use a simple method which involves a polystyrene cup a solutions with a known concentration and a thermometer. Then we add a solid and a reaction will take place. We can measure the change in temperature of the reaction. The system is the solid reacting with the solution and the surroundings are everything else apart from the reactants or products.

The polystyrene cup prevents some heat escaping from the water into the surroundings. We can also reduce heat loss by increasing the mass of the solution

Question 21

Explain a complex Calorimetry for reactions in a solution

Answer 21

A more complex method is used to gain more accurate results while minimising heat loss. It has an inner and outer vessel , a precession thermometer and a motorised stirrer. This measures the change in temperatrue of the reactions surroundings. Therefore, we can use the data in order to find the enthalpy change of the reaction.

Question 22

Express Joules per mol

Answer 22

ΔH(mol^-1) = ΔH(J)/n

Joules per mol = joules mol^-1

Question 23

Explain standard conditions

Answer 23

It is difficult to compare the enthalpy change of different reactions if they are carried out in different conditions. Therefore, chemists have created a set of conditions all reactions should take place in called standard conditions.
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The atmospheric pressure should be 100,000Pa and 298 K.
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Question 25

Express the symbol for enthalpy change given under standard conditions (standard enthalpy change)

Answer 25

ΔH^⦵

Question 26

What is a standard state

Answer 26

Substances will form into a certain state under standard conditions (state of a substance under standard conditions)

Question 27

Express the standard enthalpy of formation

Answer 27

ΔfH⦵

Question 28

Explain standard enthalpy of formation of an element

Answer 28

Standard enthalpy of formation is the enthalpy change that occurs when one mole of substance is formed from its constituent elements under standard conditions and with everything in their standard state It takes more energy in order to create the standard enthalpy of formation for an element which isnt in their standard state even if it is under standard conditions. If an element is in its standard state under standard conditons then its standard enthalpy of formation is 0KJ mol^-1

Question 29

Explain standard enthalpy of combustion of an element

Answer 29

Standard enthalpy change of combustion is the enthalpy change when one mole of a substance in its standard state is completely burnt in oxygen. (Incomplete combustion cannot give the standard enthalpy of combustion)

Question 30

How is carbon dioxide , water and metal oxide formed in a combustion reaction

Answer 30

In a combustion reaction, a substance where carbon is a constituent element reacts to form Carbon dioxide . A substance where hydrogen is a constituent element reacts to form water A substance where a metal is a constituent element reacts to form a metal oxide

Question 31

Explain complete and incomplete combustion

Answer 31

When a substance with carbon as a constituent element reacts to form carbon dioxide. The reaction has undergone complete combustion (substance burns completely) When a substance with carbon as a constituent element reacts to form carbon or carbon monxoxide. The reaction has undergone incomplete combustion.( the equation can contain both carbon and carbon monoxide and even carbon dioixde although is still incomplete combustion)

Question 32

Explain standard enthalpy of neutralisation

Answer 32

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.

Question 33

What is an alternative route

Answer 33

An alternative route only contains elements that take up the reactants and products in the direct reaction route.

Question 35

What is a thermochemical cycle

Answer 35

Thernochemical reactions show the different possible routes of a reaction starting from the reactants to final products.

Question 36

Define Hess’s law

Answer 36

The enthalpy change in a reaction at constant pressure is independent of the route taken

Question 37

Why cannot we always measure the enthalpy change of a reaction

Answer 37

Hazards: This may be due to the hazardous nature of a molecule (carbon monoxide-toxic and flammable) Unwanted by products: It is impossible to react oxygen and carbon together, and only form carbon monxide as a product. CO2 and C will be formed, so enthalpy change cannot be measured accurately Reaction to slow or fast: If a reaction is very slow, heat changes may not be easily detected. If a reaction is too fast, heat may be lost to the surroundings before it can be measured, affecting the accuracy of measuring enthalpy change Reaction impossible under standard conditions: Some reactions do not occur under standard conditions making direct measurement of enthalpy change impractical era

Question 38

Define activation energy

Answer 38

Activation energy is when the minimum energy required for a reaction takes place between two colliding reactant particles

Question 39

Express the standard enthalpy of combustion

Answer 39

Δ cH^⦵

Question 40

Express the standard enthalpy of neutralisation

Answer 40

ΔnH^⊖

Question 41

Express enthalpy changes of a reaction

Answer 41

Δ ᵣH^⊖

Question 42

Define average bond enthalpy

Answer 42

The energy required to break one mole of the stated bond in a gaseous state, under standard conditions.

Question 43

Explain how does the actual value for bond enthalpy differ from the average value

Answer 43

Average bond enthalpies are averages that may not accurately reflect specific molecules, experimental errors, and varying reaction conditions.

Question 44

Explain bond breaking and bond making in relation to exothermic and endothermic reactions

Answer 44

Exothermic reactions: Less energy is required for bond breaking than is released by bond making Endothermic reactions: More energy is required for bond breaking than is released by bond making

Question 45

How can we use bond enthalpies to determine enthalpy change

Answer 45

Add the sum of bonds broken and subtract by the sum of bonds formed in order to get enthalpy change