chemistry chapter 5

what is an exothermic reaction and do they break or form bonds?

an exothermic reaction (like combustion) is when energy is lost to the surroundings

they form bonds

what are endothermic reactions and do they break or form bonds?

an endothermic reaction is one that takes in energy from the surroundings

they break bonds

what is activation energy?

the mimimum amount of energy reactants need to collide with eachother and react

describe amount of energy released by forming bonds compared to amount of energy used to break them in endo and exo reactions

exo - amount of energy released by forming bonds is higher than amount of energy used to break bonds

endo - amount of energy used to break bonds is higher than the amount needed to form them

how are these reactions shown?

exo - rise in temp

endo - fall in temp

explain what happens to energy in endo and exo reactions

chemicals store a certain amounts of energy in their chemical store, different chemicals store different amounts

if the energy of the reactants is higher then that of the products, its exothermic, as the energy has been dissipated to the surroundings

if the energy of the products is higher than the reactants, energy has been taken in, making it endothermic

the amount of energy in the universe never changes, only moves

give examples and everyday uses of exothermic reactions

combustion/burning

neutralisation

oxidation

hand warmers - use oxidation of iron in air to release energy

self-heating cans

give examples and everyday uses of endothermic reactions

citric acid + sodium hydrogencarbonate

thermal decomposition - calcium carbonate + heat = carbon dioxide + calcium oxide (quicklime)

sports injury packs

describe the method for measuring temp change in endo and exo reactions

example - neuralisation reaction between hydrochloric acid and sodium hydroxide

• fill a measuring cylinder with 30cm cubed of dilute hydrochloric acid

• transfer acid into a polystyrene cup and place cub into a beaker - stopping it from falling

• use thermometer to measure the temp of the acid and put into a table

• measuring cylinder of 5cm cubed of sodium hydroxide solution - transfer to cup

• put a lid on cup and put thermometer for the top, ensuring the bulb is in the solution

• stir using thermometer

• watch temp change and record when it stops changing - this is the highest

• rinse and dry cup and repeat many times, increasing the sodium hydroxide concentration each time in a set interval

• do each atleast twice to create a mean highest temp reached for each conc and plot on graph

why might the reulsts begin to deacrease?

may have added so much sodium hydroxide that there’s not enough hydrochloric acid for it to react with so by adding a larger volume of solution, the energy realeased is more spread out

explain what you can do to make your results more reliable and why they work

we are measuring temp change so we want to reduce heat loses

• polystyrene = good thermal insulator, reduces heat lloss from sides and bottom

• lid - reduces heat loses from the top as it contains it and reduces energy loss by evaporation

• can add cotton wool to beaker = gives more insulation

how do you calculate bond energies?

energy released (products) - energy required (reactants)

can use this to tell if a reaction is endo or exo