Energetics

Question 1

What does the first law of thermodynamics state?

Answer 1

Energy can not be created or destroyed

Question 2

What is enthalpy?

Answer 2

A measure of the heat content of a substance at constant pressure
Can’t measure actual enthalpy only enthalpy change
A thermodynamic value that is equal to the systems internal energy

Question 3

What is enthalpy change?

Answer 3

The heat energy change at constant pressure

Question 4

What equation is used to calculate energy transfer?

Answer 4

Mass x specific heat capacity x change in temperature

Question 5

How do you calculate temperature change?

Answer 5

Final temperature - initial temperature
Temperature change is equal to energy change

Question 6

How do you measure the change in energy of the surroundings?

Answer 6

By measuring the change in temperature

Question 7

What is the most common value for specific heat capacity?

Answer 7

4.18
Which is given in most exam questions

Question 8

What does the term exothermic mean?

Answer 8

Energy leaves the reaction
External temperature increases - energy of the surroundings increases
Enthalpy change decreases so is a negative value
Energy transfers from the system to the surroundings

Question 9

What does the term endothermic mean?

Answer 9

Energy goes into the reaction
External temperature decreases - energy of the surroundings decreases
Enthalpy change increases
Energy transfers from the surroundings to the system

Question 10

What symbol is enthalpy change written as?

Answer 10

Delta H
Delta = a little triangle

Question 11

What is energy change equal to?

Answer 11

Enthalpy change

Question 12

What is the equation for enthalpy change ?

Answer 12

-q/n

n= number of moles
-q= energy change

Question 13

What does a calorimeter do?

Answer 13

Measure temperature change

Question 14

What is one gram equal to?

Answer 14

One centimetre cubed

Question 15

What measurements do you need to be able to calculate enthalpy of combustion?

Answer 15

Mass of the cold water
Temperature rise of the water(= temperature rise if the fuel)
Specific heat capacity which is 4.18

Question 16

What is the enthalpy change of combustion?

Answer 16

The enthalpy change observed when a substance is burned
Enthalpy change is always negative
Endothermic reaction

Question 17

What is the enthalpy change of solution?

Answer 17

Can be endothermic or exothermic
Enthalpy change can be positive or negative

Question 18

Describe the reaction profile of an exothermic reaction

Answer 18

The reactants start with a higher energy than the finishing products

Question 19

Describe the reaction profile for an endothermic reaction

Answer 19

The reactants start off with lower amounts of energy and finish with a higher amount of energy in their products

Question 20

What are chemicals known as in a reaction?

Answer 20

The system

Question 21

What are things like air and water known as in a reaction ?

Answer 21

The surroundings

Question 22

What is internal energy?

Answer 22

Heat energy which is stored inside chemicals

Question 23

How do you calculate enthalpy change?

Answer 23

Enthalpy of products - enthalpy of reactants

Question 24

What are enthalpy changes per amount of?

Answer 24

For one mole of reactants or per mole of reactants
Must use limiting reagent only after identifying it

Question 25

What are the steps in calculating enthalpy change if a solution ?

Answer 25

Calculate energy Change Calculate moles of limiting reagent Calculate enthalpy change

Question 26

What is energy calculated in?

Answer 26

Joules that needs to be converted into kilojoules

Question 27

What does a combustion reaction do?

Answer 27

Release heat that can heat water The heated water is the surroundings of the reaction

Question 28

What are the steps in calculating enthalpy changes in combustion?

Answer 28

Calculate the energy change Calculate the moles of fuel burnt Calculate enthalpy change

Question 29

What does Hess’s law state?

Answer 29

The enthalpy change of a chemical reaction is always the same even if you carry out that chai cal reaction by a different route

Question 30

What is the enthalpy of formation?

Answer 30

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

Question 31

What does the Hess cycle for formation look like?

Answer 31

Arrows going up to show the formation reactions taking place

Question 32

What is the enthalpy of formation of an element?

Answer 32

Always zero Data needed is always supplied in the question

Question 33

What are the steps in calculating the enthalpy of formation?

Answer 33

Always draw out the Hess cycle Identify the enthalpies of formation from the question and add them in Calculate the total enthalpy of reactants Calculate the total enthalpy of products Calculate enthalpy change

Question 34

How do you calculate the enthalpy change of formation?

Answer 34

Products - reactants

Question 35

What s the enthalpy of combustion?

Answer 35

The enthalpy change when one mole of a compound reacts fully with oxygen under standard conditions in their standard states

Question 36

What are the steps in calculating enthalpy of combustion?

Answer 36

Draw out a Hess cycle Identify the enthalpies of combustion Calculate the total enthalpy of reactants Calculate the total enthalpy of products Calculate overall enthalpy change

Question 37

What does a Hess cycle for combustion look like?

Answer 37

Arrows always go downwards to the products of carbon dioxide and water

Question 38

How do you calculate the enthalpy change for a combustion reaction?

Answer 38

Reactants - products

Question 39

What does a Hess cycle actually show?

Answer 39

The alternative route

Question 40

What is the process of breaking bonds?

Answer 40

An endothermic process Energy has to go In to break the bonds apart

Question 41

What is the process of forming bonds?

Answer 41

Forming bonds is an exothermic process Energy is released when atoms are joined together as bonds

Question 42

What is bond enthalpy?

Answer 42

The enthalpy change required to break one mole of bonds under standard conditions

Question 43

What is the equation for this reaction?

Answer 43

Bonds broken - bonds formed

Question 44

What are the steps in calculating bond enthalpies?

Answer 44

Draw out the structures Identify all the bond enthalpy data in the question and add it in Calculate the total bonds broken Calculate the total bonds formed Calculate the overall enthalpy change

Question 45

What is special about bond enthalpies?

Answer 45

Bond enthalpies are averages of that bond that are found in different molecules Actual bond enthalpies in a molecule that you calculate will not be the same as a bond enthalpy listed in a data book Will be close but not exact

Question 46

What is a bond enthalpy?

Answer 46

Breaking one mole of bonds

Question 47

What is the extra definition of bond enthalpy?

Answer 47

The average enthalpy needed to break one mole of that type of chemical bond th form individual atomsAll compounds are in the gaseous state

Question 48

What is enthalpy of ionisation?

Answer 48

The enthalpy change when one mole of electrons is removed to form one mole of atoms in their gaseous state

Question 49

What type of bond is broken in a bond enthalpy?

Answer 49

Covalent bond

Question 50

Describe the reaction taking place in a solution

Answer 50

In an insulated beaker and the temperature change is measured Reaction must be fast so the maximum temperature is reached quickly and before it starts to cool

Question 51

When are flame calorimeters used?

Answer 51

Used to find enthalpy of combustion of a fuel Fuel in a bottle with a wick is burned so heat is passed to the water which it heats

Question 52

What is a major problem with calorimeters?

Answer 52

Heat loss which leads to errors in results

Question 53

What is hess’s law in simple terms?

Answer 53

The enthalpy change for a reaction is independent of the route taken