energy changes (topic 5)

march-

define exothermic reactions

and examples (chemical reactions (4)+ uses (2))

reactions that release thermal energy. These surroundings get hotter

examples

chemicla reactions- oxidation, combustion, neutralisation .displacemnet

uses- self heating cans, hand warmers

define endothermic reactions

and examples (chemical reactions(1) + uses (1))

reaction that absorb energy from the surroundings. Surroundings get colder

examples

chemical reactions- thermal decomposition

uses- single- use coolpacks for sports injuries

why do single-use cool packs get cold (3 marks)

-because it is an endothermic reaction

-that absorbs energy from the surroundings

-as ammonium nitrate in cool paco dissolves

-surrounding get colder (for 3 marker only)

define activation energy

the energy required for a chemical reaction to happen

the minimum amount of energy the reactant particles require in order to collide with each other and react

endothermic and exothermic reactions energy profiles

how do we know which energy profile is which

endothermic→ reactants have less energy than he products so the energy must have been absorbed

exothermic→ reactants are higher than the product so the energy must have been released

If an exothermic reaction occurs in a closed container, what would happen to the temperature inside the container?

The temperature would increase

If an endothermic reaction occurs in a closed container, what would happen to the temperature inside the container?

The temperature would decrease

Which of the above arrows indicates the energy change of the reaction?

A

The energy change of the reaction is the difference in energy between the reactants and the products. 

Which of the above shows the activation energy?

B

The activation energy is the difference in energy between the reactants and the top of the curve. 

combustion of ethane word equation and symbol equation

ethane + oxygen → carbon dioxide + water

2C₂H₆+7O₂ → 6H₂O + 4CO₂

display formular of combustion of ethane

what are the steps to calculate bond energy (4)

1. count the number of bonds broken

2. work out energy needed to break bonds (times number of bonds by that bond and add with the breaking bonds)

3. work out energy to make bonds (times number of bonds by that bond and add with product bonds)

4. find out overall energy change (enerfy brocken- energy made, exothermic if neg)

Does breaking bonds release energy or require energy? 

Is breaking bonds exothermic or endothermic?

Require energy 

Endothermic

bond energy define

the amount of energy required to break one mole of a particular covalent bond.

What is the energy change of the above reaction?

1. count the number of bonds

2. times the number of bonds the bond energy

3. add the amount of energy for the products breaking, then the reactants forming

4. then work out energy needed to make these bonds (total energy(Kj/m) =energy brocken-energy formed )

   exothermic→ neg

   endothermic →pos

ANSWER: -114 Kj/m exothermic reaction

what is a cell

cells contain chemicals which react to produce electricity

what do cells contain

a simple cell can be made by connecting 2 different metals in a constant with an electrolyte

what is a battery

batteries consist of 2+ cells connected together in a series to provide a greater voltage

why can some cells only be used once

in non rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up

which type of battery is non rechargeable

alkaline batteries

how are some types of batteries rechargeable

rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical ccurrent is supplied

whats different about the performance of rechargeable and non rechargeable batteries

non rechargeable batteries output gradually reduces as the chemicals in them get used up whereas rechargebable batteries have a consgtant output until just before they need recharging

pros and cons of a non rechargeble battery (5,5)

pros

-cheap

-lifespan in low current appliences e.g clocks is long

-doesnt need a powersource to recharge

-energy stored is higher

-can be stored for up to 10 years

cons

-lifespan in moblike phones is very short

-long term cost of buying is expensive

-performance gradually reduces as chemicals are uused up

-doesnt work in higher power devises e.g electric drills

-environment damage ot be thrown away is high due to large quantity

pros and cons of rechargeable battery ()

pros

-lifespan in low current appliences e.g clocks is high

-lifespan in mobile phones is long

-performance is constant until just before recharging

-works in high power devises e.g drills

cons

-expensive

-long term cost of buying, recharging ect is high

-need a power source to recharge

-less energy is stored

-can only be stored for 2-3 years

-high environmental damage when trowing away due to metal content

what affects the bvolayage of a cell (3)

1. The metals used for the two electrodes. The greater the difference in reactivity of the two metals, the greater the voltage will be.

2. The type and concentration of the electrolyte used.

3. The conditions, such as temperature.

how are some batteries non rechargeable

In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. These are the batteries usually used in smoke alarms and tv remotes.

What do we call the pieces of metal in an electrochemical cell?

electrodes

how do hydrogen fuel cells work

+half equations

-hydroghen gas is supplied as a fuel to the neg electrode

-it diffuses through the graphite electrode and reacts with hydroxide ions to form water (waste product) and provides a source of electrons to an external circuit

-oxygen gas is supplied to the pos electrode - cathode (opposite to electrolysis)

-it diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from external circuit

half equations

2H₂(g)+ 4OH^- (aq) →4H₂O(l) + 4e^- ——> cathode

O₂(g) + 2HO(l) +4e’→4OH^- (aq) ——→anode

Which direction do the electrons flow in a hydrogen-oxygen fuel cell? 

From the anode to the cathode

What is the overall reaction for a hydrogen oxygen fuel cell? 

hydrogen + oxygen ➔ water

are the anode pos or neg

is the cathode pos or neg

is this the same or different to electorolysis

In fuel cells the anode (which is drawn on the left) is negative, while the cathode (drawn on the right) is positive.

This is the opposite way around to the anode and cathode in electrolysis, so be careful not to mix them up. 

What are the electrodes in fuel cells made from?

Porous carbon

When hydrogen gas enters a fuel cell, it loses electrons to become hydrogen ions. Is the hydrogen gas oxidised or reduced? 

Oxidised 

What are the main advantages and disadvantages of hydrogen-oxygen fuel cells? (3,3)

pos

• Hydrogen and oxygen are both renewable 

• Fuel cells last longer than batteries 

• The reaction doesn't produce any pollutants 

neg

• Hydrogen gas requires a large space to store

• It requires energy to produce hydrogen 

• Hydrogen is highly flammable, so dangerous to store 

what are fuel cells

a type of electrochemical cell - meaning they convert their chemical energy (fuel and oxygen) into electrycal energy that we can use to power things e.g hydrogen, oxygen fuel cell -forms water and electrical energy

whats the structure of a fuel cell

1. centre theres an electrolyte (solution ions can move through)

2. either side are the electrodes (made of porous carbon, has tiny holes and a catalyst to speed up the reaction) - neg electrode on the left (anode), pos on the right (cathode), connected by a wire on the top - allows electrodes to flow from the anode around to the cathode

3. on the outside of the electrons we have the anode and the cathode compartments,with an inlet at the top of each and an outlet at the bottom cathode - hydrogen enters the anode compartment, oxygen enters the cathode compartment, all the water and heat leaves out the cathode compartment

how does a fuel cell work

1. hydrogen comes in through the anode compartment, oxidised by the anode

2. electrons pass around the wire to the cathode and the hydrogen ions move through the electrolyte to the cathode

3. electrons and hydrogen ions then react with the oxygen coming from the cathode compartment, to create water, water leaves via cathode outlet

as a fuel eneters the cell, it becomes oxidsed and this sets up a potential difference across the cell to generate electricity

What is the purpose of the temperate changes practical?

to investigate the variables that affect the temperature change in chemical reactions

2 hazards and risks and control measures to the temperate changes practical

(Hazard) copper sulfate is corrosive to eyes and irritant to skin (risk) damage to eyes and irritation to skin (control) wear safely spectacles and clear spills immediately, wash hands after practicle

(hazard) stirring and taking temp of a solution in a polystrene cup (risk) piercing the cup/tipping over- leads to spillage (control) place cup in a glass beaker for stability and to contain spills

CID variables for temp changes rp

control

-mass of zinc

-volume of copper sulfate

independant

-concentration of copper sulfate

dependant

-temp change

method for temp changes rp

1. put 10cm³ copper sulfate solution into 50cm³measuring cylinder

2. top measuring cylinder up to 50 with water

3. transfer diluted copper sulfate solution to a polystrene cup inside the glass beaker and use the thermometer to measure temp. record initial temp

4. use the balance, measure 1g of zinc powder on scrap paper

5. add zinc powder to the cup and stir vigorously

6. continue stirring until reading on themometer stops changing, write final temp in results table

7. repeat steps for different dilutions of copper sulfate, ensuring the total volume is 50 each time

8. calculate temp change for each experiment

9. plot a graph

how do you ensure you measured the temp of the solution accurately

looking at eye level and stiring vigourously

how do yuou measure the volume of the solution accurately

i looked at eye level and used a measuring cylinder

how could you ceck the repeatablity of your results

why would this improve the repeatablilty of your investigation

we could repeatability 3x and find a mean so any anomalies are ignored

what are the benefits of looking at other groups results

to see if your results are accurate and therefore repeatable

What is the purpose of the polystyrene cup and lid?

to prevent heat loss to the surroundings