C7.1 - C7.6 Energy Changes✔️ Flashcards
What is an exothermic reaction?
Chemical reactions which transfer energy to the surroundings often heating them up they are a very common type of reaction - causes a rise in temperture
What is an endothermic reaction?
Chemical reaction which transfers energy from the surroundings to the reacting chemicals - causes a fall in temperture
Examples of exothermic reactions?
-combustion of methane gas - realeses energy into surroundings
-neutralisation reactions between acids and alkalis
Examples of endothermic reactions?
-citric acid and sodium hydrogencarbonate
-photosynthesis
How do you measure exothermic reactions?
Measure rise in temperature
How do you measure endothermic reactions?
Measure fall in temperature
What reactions,products have a higher energy content than the reactants?
Endothermic - products have higher energy content than the reactants so energy is transferred from the surroundings
Method for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?
-measure 25 cm^3 of sodium hydroxide - pour into polystyrene cup.
-stand cup in 250cm^3 beaker.
-measure temp of sodium hydroxide every 30 seconds while stirring.
-after 2 mins add 25cm^3 of hydrochloric acid and stir record temp every 30 s for 10 min
-repeat
repeat with copper(II) sulfate and iron fillings
repeat with potassium hydroxide and nitric acid
Equipment for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?
-measuring cylinder
-polystyren cup - less heat loss
-thermometer
-stopwatch
-stirrer
-hydrochloric acid
-sodium hydroxide
Results for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?
-all exothermic - showed increase in temperture - energy transfered from reacting chemicals heat up surroundings
Conclusion for investigating temperature change of a chemical reaction - classify it as exothermic or endothermic ?
All three exothermic but copper sulfate most exothermic reaction - largest temperature change
uses of exothermic reactions?
-hand/body warmers
-self heating cans
Uses of endothermic reactions?
-cold packs
-chill cans of drinks
How does a reusable hand warmer work?
Based on formation of crystals from solutions of salts - supersatured solution is prepared by dissolving as much salt as possible in hot water - solution then allowed to cool - metal disk in pack when pressed releases small metal particles starts of crystalisation - crystal transfer energy to surroundings - work for 30 min
How does a 1 time handwarmer work?
-uses energy transferred to surrounding in the oxidation of iron - iron turns into hydrated iron(III) oxide in exothermic reaction - sodium chloride used as catalyst - lasts for hours but can not be reused
How are exothermic reactions used in self-heating cans?
Press button on base of can - breaks seal lets water and calcium oxide mix - exothermic reaction can begin
Reaction used to heat up food/drink in self-heating cans?
Calcium oxide + water - calcium hydroxide
How do chemical cool packs work?
Endothermic reaction used - ammonium nitrate and water kept seperate in the pack when squeezed-bag inside water pack breaks - releases ammonium nitrate - usually work for about 20 min- only used once - same used for cooling cans/drinks
What is activation energy?
Minimum amount of energy needed to start a reaction
Explain why bond breaking is an endothermic process?
Energy has to be supplied to break chemical bonds - energy taken in from surroundings - so its endothermic
Explain why bond making is an exothermic process?
When new bonds are formed - energy is transferred to surroundings - energy is released as it has to be stabalised
Endothermic reaction profile ?
Products at higher energy level then reactants as reactants react to form products - energy transferred from the surroundings - surroundings get colder
Exothermic reaction profile?
Products at lower energy level of the reactants - when reactant form products energy transffered to surrounding - surrounding gets hotter
What is bond energy?
The energy needed to break bonds between two atoms
How do you know if it as endothermic reaction?
Energy released when new bonds are formed is less then energy absorbed when bonds are broken
How do you know if it as exothermic reaction?
Energy released when new bonds are formed is greater then energy absorbed when bonds are broken
What is energy needed for in a reaction?
Energy must be supplied to break the bonds between atoms in the reactants
What happens to energy when bonds are formend?
When new bonds are formend between atoms in a chemical reaction energy is released
What happens to energy when bonds are broken?
Energy has to be supplied to break chemical bonds this means it is an endothermic process - energy is taken in from the surroundings
Equation for caculating overall energy change?
Energy change = break - make
What can determine the voltage produced between two metals?
The greater the difference in reactivity between the two metals used the higher the voltage produced
What happens when two metals , conected by a wire are dipped in a salt solution?
The more reactive element will donate electrons to the less reactive metal which forms a simple electrical cell - because metals tend to loose electrons to form ions
Word equation for the reaction of hydrogen and oxygen?
Hydrogen + oxygen - water
Benefits of using hydrogen fuel?
does not produce carbon dioxide so there is reduced impacts on global warming and less greenhouse gases being released
Issues with using hydrogen fuel?
-problems with storing it safely
-supplying the hydrogen to burn in car engines
-making hydrogen using electrolysis - requires electricity from non renewable fossil fuels which release carbon dioxide
Explain what a fuel cell is?
Electrical cell thats supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently
Issues with developing and replacing engines powered by fossil fuels with cleaner energy sources?
The issue is trying to match the performance,convinence and price with the petrol/diesel car and make it a economical descion for the customer
What happens at the negative electrode or anode (opposite for fuel cells) in a fuel cell?
Hydrogen gas supplied as fuel to the anode - it diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to the circuit
What happens when fuel enters the fuel cell?
The fuel becomes oxidised and sets up a potential differene within the cell an example is the hydorgen-oxygen fuel cell
What is the positive electrode?
Anode - electrolysis
Cathode - fuel cells
What is the negative electrode?
Cathode - electrolysis
Anode - fuel cells
What happens at the positive electrode or cathode (opposite for fuel cells) in a fuel cell?
Oxygen gas supplied to the positive electrode and diffuses through the graphite and reacts to form hydroxide accepting electrons from the circuit
equation at negative electrode - fuel cells?
2H₂(g) + 4OH-(aq) —— 4H₂O(I) + 4e-
How are the electrodes adapted in fuel cells?
The electrodes are made of porous carbon (have lots of tiny holes) and also contain a catalyst to speed up the reaction
Described the structure of a fuel cell?
In the centre there is an electrolyte (most common is potassium hydroxide) - either side there is the electrodes (negative anode on left and positive cathode on right) connected by a wire - on the outside of the electrodes there is the anode compartment and cathode compartment with each having an inlet at the top - outlet on the cathode compartment
What is the half equation for the oxidation of hydrogen at the negative electrode (anode)?
H₂ - 2H(+) + 2e(-)
What is the half equation for the reduction of oxygen at the positive electrode (cathode)?
O₂ + 4H(+) + 4e(-) - 2H₂O
What happens to the H+ ions produced through the oxidation of hydrogen at the negative electrode (anode) ?
H+ ions in the electrolyte move to the cathode (positive electrode) where the oxygen gains the electrons transfered via the wire from the anode and react with the H+ ions to form water
How do the electrons travel to the opposite electrodes?
Via wires
Advantages of hydrogen fuel cells?
-do not need to be electrically recharged
-no pollutants are produced
-can be a range of sizes
Disadvantages of fuel cells?
-hydrogen is highly flammable
-hydrogen is sometimes produced for the cells by non renewable ways
-hydrogen is hard to store
What are primary cells?
First mass produced batteries - they cannot be recharged (the dry cells with electrodes made of zinc and carbon are non-rechargeable) and once the reactant has run out the cells stop working and has to be disposed off
How can some batteries be reused?
The battery is connecetd to a power supply that reverses the reactions that occur at each electrode which regenerates the origional reactants
