Topic 8: Energetics I

Question 1

What is the definition of enthalpy change and what is it?

Answer 1

Enthalpy change, delta H, is the heat energy change in a reaction at constant pressure - the units of delta H are kJ/mol - the bond breaking or making in chemical reactions cause a change in energy which is enthalpy change

Question 2

What are standard conditions?

Answer 2

• 100kPa (about 1 atm) pressure - 298K / 25 degrees C

Question 3

What are exothermic reactions?

Answer 3

• exothermic reaction give out heat energy - delta H is negative - the temperature often goes up

Question 4

What are endothermic reactions?

Answer 4

• absorb heat energy - delta H is positive - temperature often falls

Question 5

What do enthalpy (energy) level diagram show?

Answer 5

• the relative energies of the reactants and produces in a reaction - the differences ein enthalpies is the enthalpy change of the reaction

Question 6

Is substance more stable or less stable when it has less enthalpy?

Answer 6

• less enthalpy makes it more stable

Question 7

Draw an energy level diagram for an exothermic reaction and explain

Answer 7

• in an exothermic reaction, the reactants release energy to the surrounding, so the products have less enthalpy than the reactants

Question 8

Draw an energy level diagram for an endothermic reaction and explain

Answer 8

• in an endothermic reaction the reactants take in energy from the surrounding, so the products have more enthalpy than the reactants

Question 9

What do reaction profile diagrams show?

Answer 9

• show you how the enthalpy changes during reactions - activation energy, Ea, is the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction

Question 10

Draw a reaction profile diagram for endothermic reactions

Answer 10

• GO GO GO

Question 11

Draw a reaction profile diagram for exothermic reactions

Answer 11

• GO GO GOOOOO

Question 12

Define the standard enthalpy change of reaction delta(r)H

Answer 12

Standard enthalpy change of reaction is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

Question 13

Define standard enthalpy change of formation

Answer 13

Standard enthalpy change of formations is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

Question 14

Define standard enthalpy change of combustion

Answer 14

Standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is completely burned in oxygen under conditions

Question 15

Define standard enthalpy change of neutralisation

Answer 15

Standard enthalpy change of neutralisation is the enthalpy change when an acid and an alkali react together under standard conditions, to form 1 mole of water

Question 16

How can you calculate enthalpy changes?

Answer 16

• Q = mc deltaT - Q= enthalpy change in joules - m = mass of water in calorimeter - c = specific heat capacity of water 4.18 J g^-1 K^-1 - deltaT= change in temperature of water in K

Question 17

What is Hess’s Law?

Answer 17

• the total enthalpy change of a reaction is always the same, no matter which route is taken

Question 18

What type of reaction is bond breaking?

Answer 18

• endothermic - you need energy to break bonds

Question 19

What type of reaction is bond forming?

Answer 19

• exothermic - energy is released when bonds are formed

Question 20

What is bond enthalpy?

Answer 20

• the amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase

Question 21

Why is mean bond enthalpy not exact?

Answer 21

• it is an average for a large range of molecules

Question 22

What is mean bond enthalpy?

Answer 22

• the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds