C7-Energy Changes Flashcards
What are Exothermic reactions ?
Exothermic reactions transfer energy from the reacting molecules to the surroundings . Thereforre , the temperature of the surroudings increases (hotter ) .
Examples of Exothermic rections .
Combustion and certain oxidation and neutralisation reactions .
How do scientists represent energy changes that take place in a reaction ?
By drawing an energy profile diagram .
What does star one show for exothermic reaction profile ?
Products have less energy that reactants , as energy is being transferred from the reaction to the surroudnings ..
How do we figure out the energy released to the surroudings in an exothermic reaction profile ?
The difference between the energy of the reactans and the enrgy of the products tells us the energy released to the surroudnigs .
Label the energy change on the exothermic diagram .
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Wat are tewo uses of exothermic reactions .
- Hand warmers
- Self-heating cans example food and drink .
What are endothermic reactions ?
They tae in energy fromt heir surroundings . Thereofore the temperature of the surroundigns decreases (gets colder )
an example i thermal decompostion .
What does star one show for enndothermic reaction profile ?
Products have got more energy than reactants . As enegry is being taken in from the surroudnigns .
How do we figure out the energy release taken in from the surroudings in an endothermic reaction profile ?
-The difference in energy between the reactants and products , tells us the amount of energy taken in by the reaction .
Similarirites in both endo and oexothermic reactions ?
-In both cases eergy rises to a peak . This is the activaiton energy . WHICH ARE THE PEAKS OF THE CURVES , LABEL IT ON ENERGY PROFILE .
-The minimum amount of energy that paricles must have in order to react is called he activation eenrgy .
FROM THE PEAK TO THE CURVE CHECK HE SHEET AND LABEL ON PROFILES .
What is the RQP for invesigating temp chage ?
- Investigating the temperature change in an exothermic reaction .
- This is a reaction between hydrochloric acid and the alkali sodium hydroxid e.
- We are going to add incresing volumes of sodium hydroxide solution to hydrochloric acid . In each experiment , will measure the maximum temperature reached .
What is the independant variable ?
Volume of sodium hydroixide solution .
What is the dependant variable
The maximum temperature reached .
What is the control variable ?
VOLUME OF HYDROCHLORIC ACID AND CONCENTRATION OF BOTH SODIUM HYDROIXDE SOLUTION AND HCL .
Stage one for RQP for investigation temp change ?
Use a measuring cylinder to measure 30cmcubed of dilute hydrochloric acid .
Stage two for RQP for investigation temp change ?
Transefr acid into a polystyrene cup .
Stage three for RQP for investigation temp change ?
Stand the polstyrene cup inside a beaker , this stops the cup from falling over .
Stage four for RQP for investigation temp change ?
Next we use a thermometer to measure the temperture of the acid and we record this in a table .
Stage five for RQP for investigation temp change ?
Now we use a measuring cyclinder to meassure 5cm cubed of sodium hydroxide solution and we transfer this to the polystrene cup .
Stage six for RQP for investigation temp change ?
Fit a plastic lid to the cup and plce thermometer through the hole in the lid .
-The bulb of the thermometre must be in the solution
Stage seven for RQP for investigation temp change ?
Use the thermometer to gently stir the solutoin .
-Because this reaction is exothermic , it releases energy . This means that the temperature of the solution will increase.
Stage eight for RQP for investigation temp change ?
Now we look carefully , at the temperature rise , on the thermoemeter .
-When the reading on the thermomeer sops chaning , we record the highest temperature reached .
Stage Nine for RQP for investigation temp change ?
Rinse out the polstyrene cup . Repea the whole experiemnt using 10cm cubed of sodium hydroxide solution .
-Carry out the experiment severla more times , each tie increading the volume of sodium hydroxide solution b 5cm cubed until we reach a maximum of 40 cm cubed of sodium hydroixde solution
Stage ten for RQP for investigation temp change ?
Repeat the experiemnt one more time , so e have wo ses of results . Use this to calcualte a mena caue , for maximum temperature reached for each volume of sodium hydroxide soluon .
Check the graph for the RQP , what does star one show ?
-As we increase the volume of sodium hydroxide solution , the maximum temperature reaced increases .
BECUASE - as we add more particles of sodium hydroxide ,t hey react iwth hydrochloric acid .
-This is an EXOTHERMIC reaction , so more enrgy is releaseed and the maximum temperature reached increases .
Check the graph for the RQP , what does star twos hwo ?
At a certian volume of sodium hydroxide solution , the maximum temperature reached starts to decrease .
- At this point we are adding so much sodium hdroxide ,that there is not enough hydrochloric acid . Meaning some of the sodium hydroxide is unable to react .
- Because of this ,t he amoutn of energy released by the reaciton has reached a maximum .
Why does the temperature actually decerase from start two ont he RQP graph ?
- In each experiemnt , we are adding a greater volume of solution .
- So energy relesed is now spread out into a greater volme .
- Because of this ,when we dd large volumes of sodium hydroixde solution , the maximum temperature reached decreases .
EXAM QESTION - , why are we using a polystyrene cup ith a lid ?
- As we are measuring the temperature in the ecperiment . This means , we want to reduce any heat loss .
- Polystyrene is a good thermal inslator , reducing heatloss htrough the sides and the botto , and the lid reduces heat loss into the air .
What can we do if we ake two different metals and place them into an electrolyte ?
-We can produce electricity .
What is an electrolyte ?
-AN electrolyte is a solution that can doduct electricty , e.g a solution of an ionic compound .
Look at the setup on the page ,
what is the setup for a cell ?
- a strip of copper nad magnesium is palced inside an electrolyte soluton .
- If you connect the two metals with a voltmetre , you cn see a potetntial difference and an electric current will flow through the wire .
- A chemical reaction is taking place on the surface of the two metals , and these reactions are producing electircity , this setupp is called a CELL .
- Cells can be used to generate electricity .
Facts about a cell
- A cell can only produce electicity for a certian period of time .
- Eventually the chemicals in the cell runout and the reaction stops .
- Cells only produce electicity if we use metals with different reactivities .
Reactivity and votage ?
- The bigger the difference in reactivity in metals , the larger potential difference produced by the cell .
- THe electrolyte also affects the potential difference .e
A battery
-A battery cotins two or mroe cells connected ins eries , to produce a greater voltage
Alkaline batteries ?
- In alkaline batteries at some point , the reactants in the batteries run out and no more electricity is produced .
- There is now way we can reverse these reactions so these are NON - RECHARGEABLE BATTERIES.
Rechargeable batteries ?
- Rechargeable batteries can be rechrged .
- As we can reverse the chemical reactions when we apply an electric current .
What happens in a fuel cell ?
- We react a fuel (hydrogen) with either pure oxygen or air .
- Inside the fuel cell ,a chemical reaction reaction takes place , producing an electric current , the only waste prodcut is water .
Draw the diagram .
:)
What happens at the negative electrode of the reaction ?
-The hydrogen molecule spilts into hydrogen ions and electrons .
-These electrons pass through the wire and that is the electric current .
See the equation on the page and learn .
What happens at the positive electrode of the reaction ?
- Oxygen molecules combine with electrons and hydrogen ions produced at the negative electrode .
- See equation on the page and learn .
TO CONCLUDE , what happens at the hydrogen fuel cell ?
-In a hydrogen fuel cells , hydrogen is combined with oxygen (Hydrogen is oxidiised .)
The overal equaion is (check book too )
Nh2+O2—>2H20 (Hydrogen is being oxidised )
What is one advantage of hydrogen fuel cells ?
Hydorgen fuel cells will produce electricity for AS LONG AS YOU PROVIDE HYDROGEN .
What is another advanatage of hydrogen fuel cells ?
Hydrogen fuels cellsdo not get less efficent the logner they are run .
What is the last advantage of hydorgen fuel cells ?
Hydrogen fuel cells can be a source of drinkable water e.g on a spacecraft .
What is one disadvantage of a hydrogen fuel cell ?
Hydrogen fuel cells runon hydrogen which is an explosive gas and it is very difficult to store safely .
What is another disadvanatge of hydrogen fuel cells ?
-Hydrogen fuel cells produce a realtively low potential diffeence , or voltage so several are needed together
What is one advantage of rechargeable batteries ?
-No dangerous fuels are required with rechargeable batteries .
(Some types of rechargeable batteries can catch fire if not manufactured correctly )
What is another advantage of rechargeable batteries
Rechargeable batteries can produce a greaer potential difference than a hydrogen fuel cell .
What is one disadvantage of rechargeable batteries ?
-Rechargeable batteries run out , and need to be recharged .
What is another disadvantage of rechargeable batteries ?
-Rechargeable batteries can store less electricity the more charginc ycles they go through and eventually need replacing so are not that efficent .
yay fuel cells
-dont produce ppllutants whereas fossil fuels do .
petrolc omesfrom fosilf uels - combustion of that porduces co2 atmosphere .
-fossil fuels re a finite supply , whereas getting hydorgen from water wont run out .
hydorgen fuel cells noo
Hydrogen is a gas and requires a much larger storage volume compared to fossil fuels like petrol.
no efficent wya to produce hdyorgen , actually still genrared from burnign fosisl fuels .
electrolysis for h ydrogen fuel cells is expensive .,
