energetics

Question 1

what units are exothermic and endothermic reactions measured in?

Answer 1

● joules, J

● kilojoules, kJ

Question 2

what happens in chemical reactions in term of bonds?

Answer 2

bonds are broken and they form new ones

Question 3

what happens to energy to break bonds?

Answer 3

taken in

Question 4

what happens to energy when bonds are made?

Answer 4

given out

Question 5

what does exothermic mean?

Answer 5

● bond making

● overall more energy is released in bond making than was used in bond breaking process

● overall energy is given out

● surroundings heat up

● products have less energy than starting material

● enthalpy change is negative

Question 6

what does endothermic mean?

Answer 6

● bond breaking

● overall less energy released in bond making process than was used in bond breaking process

● overall energy is taken in

● surroundings cool down

● products have more energy than starting material

● enthalpy change is positive

Question 7

what is enthalpy?

Answer 7

measure of heat content in substance

Question 8

what is enthalpy measured in?

Answer 8

● kJmol-1

● enthalpy of substance can not be measured

● but changes in enthalpy can be measured

Question 9

what is enthalpy change?

Answer 9

● heat energy change

● measured under conditions of constant pressure

● measured under standard conditions

Question 10

what are standard conditions in chemistry?

Answer 10

● 100 kPa (1 atm)

● stated temp (usually 298k)

● solutions at 1moldm-3

Question 11

what should always be included in equations?

Answer 11

state symbols

Question 12

what is some basic equipment you could use to measure enthalpy change of reaction that takes place in solution?

Answer 12

● thermometer

● polystyrene cup and lid

● stop clock

Question 13

why is polystyrene cup a good insulator?

Answer 13

● keeps heat inside

● so reduces heat loss (exothermic)

● reduces heat gain (endothermic)

● applys to lid

Question 14

what do calorimeters measure enthalpy change of?

Answer 14

● dissolving

● neutralisation

● formation

● combustion

Question 15

what are the issues of a simple calorimeter?

Answer 15

heat loss, no lid

Question 16

what is some basic equipment you could use to measure enthalpy change of a combustion reaction?

Answer 16

● thermometer

● copper calorimeter (metal can)

● heat shields

Question 17

what is the equation for energy released / heat change?

Answer 17

● Q = mcΔT

● heat change = mass of substance that has temp change x specific heat capacity x temp change (don’t need to convert degrees to kelvin)

Question 18

what is the specific heat capacity of water?

Answer 18

4.18 Jg-1k-1

Question 19

what is the formula for enthalpy change?

Answer 19

ΔHc = q/n

Question 20

explain enthalpy change in neutralisation reactions

Answer 20

● all exothermic

● add masses together

● need balanced symbol equation

● use moles of limiting reactant

Question 21

explain enthalpy change in displacement reactions

Answer 21

● use mass of solution

● all exothermic

● use moles of limiting reactant

Question 22

explain enthalpy of solution

Answer 22

● known mass of solids totally dissolved in excess water

● add masses

● exothermic

Question 23

why do we not record temperature at 4th minute while doing practical?

Answer 23

adding second reagent

Question 24

what safety precautions should be taken doing practical?

Answer 24

avoid skin contact

Question 25

what are errors which could take place during practical?

Answer 25

● some of the water could have evaporated ● room temperature could have changed ● heat transfer to and from surroundings ● neutralisation reaction - virtually instantaneous but temp on thermometer does not instantly peak time needed for thermometer to respond to change in temp - but during time heat being lost to surroundings

Question 26

how to improve accuracy of practical?

Answer 26

● use flame calorimeter - spiral chimney made of copper, flame is enclosed, fuel burns in pure oxygen rather than air ● electronic temp sensor and data logging software could be used to plot graph accurately

Question 27

state hess's law

Answer 27

● the enthalpy change of a reaction is independent of the route taken ● enthalpy change A = enthalpy change B + C

Question 28

define standard enthalpy of formation

Answer 28

● the enthalpy change when 1 mole of a substance is formed from its elements ● where substances are in their standard states at standard conditions

Question 29

what is the standard enthalpy change of formation of any element in its standard state?

Answer 29

0

Question 30

define standard enthalpy of combustion

Answer 30

● the enthalpy change when 1 mole of a substance is completely combusted in oxygen where substances are in their standard states at standard conditions ● combustion exothermic so answer negative

Question 31

what is bond dissociation enthalpy?

Answer 31

● enthalpy change required to break a covalent bond with all species in the gaseous state ● put in energy to break a covalent bond - endo ● same amount of energy is given out when bond formed - eco

Question 32

what is mean bond enthalpy?

Answer 32

● enthalpy change required to break a covalent bond averaged out over different compounds ● same type of bond (e.g. C-H) may have a slightly different value in different molecules so we use an average

Question 33

what is the calculation for energy change?

Answer 33

energy change = bonds broken - bonds made

Question 34

explain difference in values in hess cycles and bond enthalpy

Answer 34

● bond enthalpy are a mean so value is approximate ● hess cycle is calculated using data for actual compounds so is the correct answer