Chapter C7 Energy Changes Flashcards
What is an exothermic reaction?
Energy is transferred from the reaction to the surroundings
What is an endothermic reaction?
Energy is transferred from tens surroundings to the reaction
What happens to the temperature of surroundings in endothermic and exothermic reactions?
Endo- gets colder
Exo- gets warmer
What energy content do the products of an exothermic reaction contain?
A lower energy content than the reactions as energy was transferred to the surroundings
What energy content do the products of an endothermic reaction contain?
A higher energy content than the reactants
Give examples of uses of exothermic reactions
Hand and body warmers
Self heating cans
How do reusable hand warmers work?
Salt crystals are dissolved in hot water, and then allowed to cool
A metal disc is inside the solution, which when clicked start off the crystallisation process. The crystals spread throughout the solution resulting in exothermic energy. They can then be reused by redissolving the crystals
Give examples of endothermic reactions
Chemical cold packs
How does a chemical cold pack work?
The ammonium nitrate and water are in a bag but separated. When the bag is squeezed, the ammonium nitrate is released, taking in temperature from the surroundings and resulting in a cold compress.
Describe the exothermic reaction profile
Reactants are high on the left, and the products are low on the right.
This means that when the reactants form the products energy is lost to the surroundings
Draw the reaction profile of an endothermic reaction
Products
Activation [ /\
Energy [ / \
Required[ / \\_\_\_\_\_\_\_\_\_\_\_\_
[ |
[ |
[ |
[ |
[ |
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_|
ReactantsDescribe the reaction profile of an endothermic reaction
Reactants are low on the left, whilst the products are high on the right. As the reactants form products, energy is transferred from the surroundings to the reaction mixture
What is activation energy?
Minimum amount of energy needed before colliding particles of reactants can cause a reaction
What is breaking bonds?
An endothermic process- energy has to be supplied to break bonds
What is making bonds?
Exothermic process- energy is transferred to surroundings
Do bind breaking and making processes happen at the same time?
Yes
What is bond energy?
Energy needed to break between two atoms
What are bond energies measured in?
KJ/mol
What do bond energies show?
Work out energy changes for chemical reactions
What similarities do bond energies have?
Braking and making bonds require the same amount of energy
What does an exothermic bond energy calculation always have?
A negative final result
What is a redox reaction?
Change in charge of ion
What is an electric current?
A flow of electrons
What does a greater difference in reactivity between two metals result in?
A higher voltage produced
Why does a primary cell run out?
Yeh reactants are used up
What happens in the recharging process of a battery?
Reversed reactions that occur at the electrode and so regenerates initial reactants
Why is hydrogen a good fuel?
It burns well and produces no pollutants
What is an advantage to hydrogen as a fuel
Produces no pollutants as so reduces impact on global warming.
What is a disadvantage to hydrogen as a fuel?
Proteins with safety and storage
Supplying the hydrogen through electrolysis burns fossil fuels so carbon dioxide is still produced
What is a fuel cell?
Cells fed with hydrogen and oxygen and produce water- energy released transferred to electrical energy
What is a challenge of hydrogen fuel cells?
They just match the performance, convenience and price of diesel powered alternatives
Explain how a hydrogen fuel cell works
Hydrogen gas is supplied at the cathode. It diffuses through the graphite electrode and reacts with hydroxide ions to form water and provide electrons to the external circuit (as hydroxide is negatively charged ion)
Oxygen gas is supplied at the anode. It diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from the external circuit (provided by the hydrogen and hydroxide reaction)
When added, these reactions cancel out and so you are left with the overall change in the hydrogen cell, which is the oxidation of hydrogen, with a wasted product of water
Advantages of hydrogen fuel cells
Do not need to be electrically recharged
No pollutants
Can be a range of sizes for different uses
Disadvantages of hydrogen fuel cells
Hydrogen is highly flammable
The hydrogen needed is produced via non renewable sources
Hydrogen is difficult to store
Hydrogen needs to be compressed- requires large tank
Doesn’t work at low temperatures
Energy calculations
Normally do reactants - product
What is a battery also called?
A cell
By product of a fuel cell?
Water
