Energetics

Question 1

What is an exothermic reaction?

Answer 1

One in which energy is transferred from the system to the surroundings, heating them.

Question 2

What is an endothermic reaction?

Answer 2

When energy is transferred from the surroundings to the system and the surroundings cool down

Question 3

All substances store energy. What is this the sum of?

Answer 3

• the potential energy stored by the electrostatic forces within and between particles
• the kinetic energy stored by movement of particles from place to place and when bonds in them vibrate and rotate

Question 4

In chemical reactions when do energy transfers happen?

Answer 4

When bonds are broken and new ones are formed

Question 5

What is the energy stored in a compound called and what is it’s symbol?

Answer 5

Enthalpy with the symbol H

Question 6

What is enthaloy change and what is it’s symbol?

Answer 6

Delta H, the quantity of energy transferred when a chemical reaction happens

Question 7

If you are given a value for an enthalpy change how do you know whether the reaction is exothermic or endothermic?

Answer 7

A negative sign is used for exothermic reaction, a positive sign is used for endothermic reactions

Question 8

What is enthalpy change measured in?

Answer 8

KJ mol -1

Question 9

What is the standard enthalpy of combustion?

Answer 9

Delta cH is the enthalpy change when 1 mole of a substance burns completely in oxygen under standard conditions with all reactants and products in their standard states (solid, liquid or gas under standard conditions)

Question 10

What is the standard enthalpy of formation?

Answer 10

Delta f H is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions. All reactants and products are in their standard states

Question 11

Why is it acceptable to use fractions when writing enthalpies of combustion and formation?

Answer 11

Because you must ensure that there is one mole of the porduct formed in enthalpy of formation and 1 mole of the substance that burns in enthalpy of combustion

Question 12

What is the technique used to work out enthalpy changes of reactions?

Answer 12

Calorimetry. The apparatus used is a calorimeter and it enables temperature changes to be measured when energy is transferred during a chemical reaction

Question 13

What is the specific heat capacity of a substance?

Answer 13

The energy required to raise the temperature of 1g of a substance by 1K

Question 14

To calculate the energy that is transferred to water (normally, sometimes a different solvent) what equation do we use?

Answer 14

q=mc(deltaT)

Q= energy transferred (J)
C = specific heat capacity (usually of water)
Delta T = temperature change/ K

Question 15

If you have calculated the amount of energy transferred to the water how do you calculate the enthalpy of combustion?

Answer 15

you work out the number of moles of the substance burnt and then you divide the energy transferred by the number of moles

Question 16

What are the main reasons there are errors in calorimetry?

Answer 16

Procedural errors:
• not all of the energy released in the combustion reaction is used to heat the water. Some is transferred to the copper calorimeter, the stirrer and the air around the apparatus
• there are also errors from the measuring cylinder, thermometer and balance, but these are insignificant compared to the procedural errors

Question 17

What experiment can you use to measure the enthalpy of neutralisation?

Answer 17

E.g. hydrochloric acid reacts with sodium hydroxide solution in a neutralisation reaction.

The reaction can be carried out in a polystyrene cup with a lid and the results used to calculate the enthalpy change. Measure the temperature every minute

Question 18

What can you use as a calorimeter?

Answer 18

• a glass beaker, however there are heat losses which are significant. Insulating the beaker and putting a loose lid on it and shielding it from draughts can reduce losses
• you can use a copper can and it is a good thermal conductor and the energy released from combustion heats the water quickly, reducing heat loss to the surrounding air

Question 19

What is the teachnique for measuring an enthalpy of combustion?

Answer 19

• first measure the required volume of water into the calorimeter using a measuring cylinder.
• then measure the initial temperature of the water
• measure the mass of the spirit burner (used to combust liquids) cap and liquid. The cap is kept on the spirit burner when the liquid is not being burned to prevent loss through evaporation
• heat shields are positioned around the aparratus to help reduce heat loss (normally sheets of aluminium metal)
• then ignite the calorimeter and stir the water gently and constantly
• after a suitable temperature rise (15-20°c) the spirit burner is extinguished by replacing the cap and the final temperature of the water is taken
• the final mass of the alcohol burner, cap and liquid is measured

Question 20

What technique do you use to measure the enthalpy change in solution?

Answer 20

• measure the volumes of the liquids with a measuring cylinder
• if a solid is used weigh it on a weighing boat on a balance
• the liquid or one of the solutions is placed in the calorimeter and the initial temperature measured.
• the second solid is then added and the mixture stirred to mix the reactants
• the largest temperature change is recorded

Question 21

What is Hess’s law?

Answer 21

If a reaction can take place by more than one route, the overall enthalpy change is the same, regardless of the route, as long as the initial and final conditions are the same

Question 22

How do you work out the standard enthalpy change of combustion and formation?

Answer 22

• enthalpy of combustion = sum of products - sum of reactans
• enthalpy of formation = sum of reactants - sum of products

This doesn’t work all the time

Question 23

What is the energy needed to break a particular bond in a gaseous molecule called and what are it’s units and symbol?

Answer 23

The bond enthalpy with the units KJ mol-1 and the symbol E

Question 24

Why is the energy needed to break each C-H bond in a methane molecule slightly different?

Answer 24

Because as soon as the methane molecules begin to break down the remaining C-H bonds exist in a different environment

Question 25

What is the mean bond enthalpy and why is it more useful to use when applying data to other compounds than just bond enthalpies?

Answer 25

• the mean bond enthalpy is the average enthalpy change when one mole of bonds of the same type are broken in gaseous molecules under standard conditions
• it is more often to use mean bond enthalpy when applying data to other compounds since bond enthalpies vary with their molecular environment

Question 26

How do you use bond enthalpies to calculate the enthalpy change of a reaction?

Answer 26

1) write out the equation using structural formulae
2) for each reactant list the type and number of bonds broken. Look up the mean bond enthalpy for each bond type and calculate the enthalpy change. do this for the bonds broken and for the bonds made
• breaking bonds is an endothermic process so when bonds are broken they have positive values
• making bonds is an exothermic process so the enthalpy changes when bonds are made have negative values
3) calculate the overall enthalpy change in the units KJ mol -1

Question 27

How do you use Hess’s law to calculate enthalpy changes?

Answer 27

• if the values are swapped it’s negative
• if the values in your given equation are multiplied you must multipy the enthalpy of formation

Look at the videp because i can’t explain this properly