Enthalpy Changes

Question 1

What is meant by the term enthalpy

Answer 1

A measure of the heat change in a chemical system

Question 2

Define system

Answer 2

The reactants and products

Question 3

Define surroundings

Answer 3

The rest of the univers- lab and equipment

Question 4

Define exothermic

Answer 4

Energy is released form the system to the surroundings- △H = -

Question 5

Define endothermic

Answer 5

Energy is transferred from the surroundings to the system-△H = +

Question 6

Draw an energy level diagram for an exothermic reaction

Answer 6

1. y axis labelled enthalpy
2. X axis labelled progress of reaction
3. First line is higher up than final line and is labelled reactants
4. An arrow pointing down from the reactants line to the products line labelled △H

Question 7

Draw an energy level diagram for an endothermic reaction

Answer 7

1. Y axis labelled enthalpy
2. X axis labelled progress of reaction
3. First line is lower down than final line and is labelled reactants
4. An arrow pointing up from the reactants line to the products line labelled △H

Question 8

Explain why, when the reaction mixture is measured the temp has increases despite the reaction losing energy,

Answer 8

Because it is an exothermic reaction, so the system is transferring energy to the surroundings and the thermometer is part of the surroundings.

Question 9

Define the term activiation energy

Answer 9

The energy required to start a reaction by the breaking of bonds

Question 10

Why do most chemical reactions have an activation energy

Answer 10

It is necessary to break the bonds in the reactant molecule.

Question 11

Draw an energy diagram for an exothermic reaction with activation energy

Answer 11

1. The same basis as needed for a normal exothermic energy diagram
2. Have a dotted curved line which starts at the reactant line goes up and then down to the products line.
3. The height from the reactant line to the peak of the curve is the Ea (activation energy)

Question 12

Write equation of the energy change in a reaction

Answer 12

q=mc△T
m= mass of substance heated (g)
c= specific heat capacity (J g^-1 K^-1)
△T = Change in temperature (K but will be the same in Celsius)
q= heat energy change (J)

Question 13

State the assumptions made when using the q=mc△T equation

Answer 13

1. Density of solution = density of H2O
2. No heat loss or gain to surroundings or calorimeter
3. The Specific heat capacity of solution= Specific heat capacity of water ( means it works best if solution is quite dilute)

Question 14

What do you use as your measurement for mass and why

Answer 14

The mass of all the solution and not the solid

Because the spc of most solids will be closer to 0 than 4.18 so it is more accurate to not count it.

Question 15

Define specific heat capacity

Answer 15

Energy required to heat 1g of a substance by 1 degree celsius

Question 16

Write formula for enthalpy change

Answer 16

Enthalpy change = q/mol

Enthalpy change should include the correct sign(-/+) Depending on whether the temp rose or fell in the experiment.

Question 17

Outline a method for an experiment you might use to determine the enthalpy change of solution △solH of ammonium bromide

Answer 17

1. Measure out a know volume of water in measuring cylinder
2. Pour water into a polystyrene cup and measure the initial temp suing a thermometer
3. Weigh a weighing bole containing some solid NH4Br
4. Put solid in water and stir using thermometer
5. Measure the highest/lowest temp reached by the solution
6. Reweigh empty weighing bottle
7. Calculate temp change
8. Find energy change using q=mc△T
9. Find number of moles of NH4Br and use this to find enthalpy change= q/mol

Question 18

State the standard conditions

Answer 18

Temp= 25 degrees or 298K
Pressure= 100kPa
Concentration= 1 mol dm^-3

Question 19

Define Standard enthalpy change of combustion △cH (planet symbol)

Answer 19

The enthalpy change that takes place when 1 mole of substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Question 20

Define standard enthalpy change of formation △fH (planet symbol)

Answer 20

The enthalpy change that takes place when 1 mole of compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 21

Define standard enthalpy change of neutralisation △neutH (planet symbol)

Answer 21

The enthalpy change that accompanies the reaction of an acid and a base to form one mole of H2O (l) under standard conditions, with all the reactants and products in their standard states.

Question 22

State Hess’s law

Answer 22

The △rH is always the same irrespective of the reaction route.
If a reaction can take place by more than one route and he initial and final conditions are the same, the total enthalpy change is the same for each route.

Question 23

Practice drawing Hess cycles using the Hess’s law sheets

Answer 23

Do it

Question 24

Define average bond enthalpy

Answer 24

The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given bond in the molecules of a gaseous species

Question 25

What is the sign of bond enthalpy

Answer 25

+ always requires energy to be put in to break bonds. It is endothermic

Question 26

What is the sign of the enthalpy change when forming a bond

Answer 26

• Energy will be released when forming bonds.

Question 27

Describe how to find the overall enthalpy change from the mean bond enthalpies

Answer 27

1. List the bonds broken and the bonds formed
2. Attribute values to each bond to find the total energy used for forming the bonds and the total energy to break the bonds.
3. The bonds formed should have a - sign and bonds broken should have a + sign
4. Add the values together to get the overall enthalpy change △rH for the reaction.

Question 28

Explain why the value for overall enthalpy change is not always equal to the standard enthalpy change

Answer 28

Standard enthalpy change requires standard states and standard conditions. Using bond enthalpies assumes gaseous states.

Question 29

State the formula for calculating percentage error in measurements

Answer 29

Total inaccuracy in apparactus/ actual measurement *100

Question 30

State procedural errors that may have occurred when measuring the enthalpy change of combustion which may lead to differences in your result compared the standard enthalpy changes

Answer 30

1. Heat loss to the surroundings
2. Incomplete combustion of alcohol- seen by soot
3. Evaporation of alcohol or water
4. Non-standard conditions
5. Specific heat capacity of apparatus ignored