Unit 1: Section 4: Energetics

Question 1

What is enthalpy change

Answer 1

The heat energy transferred in a reaction at constant pressure

Question 2

What is the notation for enthalpy change

Answer 2

ΔH

Question 3

What are the units of enthalpy change

Answer 3

kj mol^-1

Question 4

What are the standard conditions

Answer 4

100kPa or 1 atm of pressure

Question 5

At what temperature are enthalpy changes measured at

Answer 5

298 K (25 degrees celcius)

Question 6

What type of value will the enthalpy change be if the reaction is exothermic

Answer 6

Negative because the energy is given out to the surroudings

Question 7

What type of value will the enthalpy change be if the reaction is endothermic

Answer 7

Positive because it’s taking in energy from the surroundings

Question 8

What is bond enthalpy

Answer 8

The energy needed to break a bond

Question 9

Is breaking bonds endothermic or exothermic

Answer 9

Need energy to break bonds so bond breaking is endothermic

Question 10

What is the enthalpy change

Answer 10

The overall effect of these two changes

Question 11

What is mean bond ethalpies

Answer 11

The average energy needed to break a cetain type of bond over a range of compounds

Question 12

How do you calculate the enthalpy change

Answer 12

Enthalpy change = Totale energy absorbed - Total energy released

Question 13

How do you find the total energy released by the reaction

Answer 13

Find the total energy needed to form all the new bonds

Question 14

What are the three different types of entahlpy change

Answer 14

Standard enthalpy change of formation
Standard enthalpy change of combustion
Standard entahlpy change of reaction

Question 15

Explain standard enthalpy change of formation

Answer 15

The enthalpy change when 1 mole of a compound is formed from its elements in their standard sates under standard conditions

Question 16

Explain standard enthalpy change of combustion

Answer 16

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in the ir standard states

Question 17

Explain standard entahlpy change of reaction

Answer 17

The enthalpy change when a reation occurs in molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states

Question 18

What are the three things you need to know when measuring the enthalpy change in a lab

Answer 18

Number of moles of stuff that is reacting
Change in temperature
How much stuff you’re heating

Question 19

What are experiments that measure heat given out by reaction called

Answer 19

Calorimetry experiments

Question 20

Describe the apparatus needed in when measuring enthalpy changes in a lab

Answer 20

Polystyrene beaker
Solution inside beaker
Thermometer inside solution

Question 21

Explain how you measure enthalpy change for reactions that happen in solution

Answer 21

Use the thermometer to measure the temperature at regular intervals
Use a polystryene beaker to insulate the reaction

Question 22

Describe the apparatus needed to measure the enthalpy change of combustion

Answer 22

Stirrer
Thermometer
Water
Combustion chamber
Fuel (reactan)
Air tube
Calorimeter

Question 23

Explain how you measure the enthalpy change of combustion

Answer 23

As the fuel burns it heats the water and you can measure the temperature change and find out how much heat the water has absorbed if you know the mass of the water and the specific capacity of the water

Question 24

Explain why the method for working out the enthalpy change of combustion may give inaccurate results

Answer 24

Some heat may be lost to surroundings and there might be some incomplete combustion

Question 25

Why is finding the starting value of the water and subtracting it from the final value not a good method of finding out the temperature change when measuring enthalpy change of combustion

Answer 25

Because some heat is lost fromp the calorimeter to the surroundings

Question 26

How do calculate the temperature change when measuring enthalpy change of combustion

Answer 26

Record temperature at regular intervals (begin a couple of minutes before you start the reaction) Plot a graph (temp against time) Plot two lines of best fit one for the results before the reaction and one for after The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction

Question 27

How do calculate the standard enthalpy change of combustion

Answer 27

Calculate the amount of heat lost Calculate the number of moles of ffuel that caused thsi enthalpy change (number of moles = mass/Mr) Use the equation - heat change/number of moles of fuel burned - to calculate

Question 28

What is standard enthalpies of combustion measured in

Answer 28

kj mol^-1

Question 29

What is the enthalpy of neutralisation

Answer 29

The energy change when 1 mole of water is formed by the reation of an acid and an alkali

Question 30

How do you find the enthalpy of neutralisation

Answer 30

Add a known volume of acid to an insulated conatainer Then add known volume of alkali and record the temperature at regular intervals Stir solution to make sure it's evenly heated Work out the heat energy given out by reaction using the heatc change/number of moles equation

Question 31

What is a dissolution reaction

Answer 31

When the reactants split up into it's ions and makes it an aqueos solution

Question 32

How do you measure the enthalpy change of a dissolution reaction

Answer 32

Add known mass of a solid to a known mass of water in a polystyrene cup and stir Record the temperature regularly

Question 33

How do you find out the molar enthalpy chanage of a dissolution reaction

Answer 33

Find the enthalpy change per mole of solute dissolved Divide the heat energy change for the reactoin by the number of moles of solid that you started off with

Question 34

What is Hess's Law

Answer 34

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

Question 35

What is the purpose of Hess's Law

Answer 35

If you know the enthalpy change for one route of a reaction you can use that to find out the enthalpy change of the other route of the same reaction

Question 36

What are entahlpy changes of formation

Answer 36

The heat change of going from and element to a compound

Question 37

What does increasing the concentration of molecules in a reaction do to the Maxell-Boltzmann distribution curve

Answer 37

It increases the peak of the curve