C5- Energy Changes Flashcards
What is an exothermic reaction?
A reaction where energy is transferred to the
surroundings.
Give examples of exothermic reactions.
Combustion, respiration and many neutralisation reactions
What happens to the temperature of the
surroundings during an exothermic reaction?
They increase
What is an endothermic reaction?
A reaction where energy is transferred from the
surroundings.
Give examples of endothermic reactions.
Thermal decomposition reactions, the reaction between
citric acid and sodium hydrogencarbonate and respiration
What happens to the temperature of the
surroundings during an endothermic reaction?
They decrease
State two uses of exothermic reactions
Self-heating cans, hand warmers
State two uses of exothermic reactions
Some cooling sports injury packs
What is a reaction profile?
A diagram which shows whether the reactants have more
or less energy than the products and the overall energy
change of a reaction.
What is the activation energy?
The energy needed to break bonds to start a reaction
Why does the reaction profile always go up at the start?
A reaction starts with bonds being broken
Why does the reaction profile always go down at the end?
A reaction always ends with bonds being made
If the reactants have more energy than the products,
what kind of a reaction must have taken place?
An exothermic one. Energy has been transferred to the surroundings.
If the reactants have less energy than the products,
what kind of a reaction must have taken place?
An endothermic one. Energy has been taken in from the surroundings.
Is breaking bonds endothermic or exothermic?
Exothermic. Energy is released when chemical bonds are formed (like how two
magnets move together when close and generate kinetic energy)
How do we work out the overall energy change of a
reaction?
Work out the difference between the energy needed to break all the bonds in
the reactants and the energy released to form all the bonds in the products.
How do we work out the energy change from a reaction profile?
The difference between the energy of the reactants and the energy of the
products
How do we work out the activation energy from a reaction
profile?
The difference between the energy of the reactants and the highest point on
the profile
What is meant by bond energy?
The amount of energy required to break a particular type of bond.
What is a chemical cell?
A unit which contains chemicals which produce electricity
through a reaction
What are the main components of a chemical cell?
An anode, a cathode (made of 2 different metals) and an
electrolyte
What is an electrolyte?
A solution containing ions
What does the voltage of a cell depend on?
The type of electrode and the electrolyte.
What is a battery?
Two or more cells connected in series
What happens to the electrodes in a chemical cell?
The more reactive metal is used up and the less reactive
one increases in size.
In non-rechargeable cells, why do the chemical
reactions stop over time?
One of the reactants becomes used up.
How can certain cells be recharged?
Applying an external electric current
How does the reactivity of the metal electrodes
affect the size of the potential difference?
The greater the difference in reactivity, the greater the
voltage
Which way do electrons move in a cell?
From the more reactive metal to the less reactive metal
What is a fuel cell?
A cell which uses a fuel and oxygen (or
air) to generate electricity.
What are the products in a hydrogen
fuel cell?
Water
State advantages of hydrogen fuel
cells.
Do not need to be recharged, no
pollutants are produced
State three disadvantages of
hydrogen fuel cells.
Hydrogen is highly flammable,
hydrogen is often made from fossil
fuels, hydrogen is difficult to store
What reaction occurs at the negative
electrode of a hydrogen fuel cell?
H2 → 2H + 2e⁻
Which reaction occurs at the positive
electrode of a hydrogen fuel cell?
O2 + 4H+ + 4e- → 2H2O
What must be supplied for a bond to break
Energy
What is the conservation of energy principle?
Energy is conserved in chemical reactions. The amount of energy in the universe
at the end of a chemical reaction is the same as before the reaction takes place
What is an exothermic reaction? Give examples
A reaction where energy is transferred to the surroundings so that the
surroundings temperature increases – combustion, oxidation reactions and
neutralisation (acid + alkali) reactions. Negative sign of energy change.
What is an endothermic reaction? Give examples
A reaction where energy is taken in from the surroundings so the surroundings
temperature decreases – thermal decomposition, reaction of citric acid and
sodium hydrogencarbonate. Negative sign of energy change.
What is activation energy?
Minimum amount of energy that particles need to react
What is a reaction profile?
Reaction profile is a graph which shows the relative energies of reactants and
product, as well as activation energy of the reaction.
What occurs in a chemical reaction in terms of
bond energies? Describe exothermic and
endothermic reactions in terms of bond
breaking/forming.
Energy is supplied to break bonds and energy is released when bonds are made;
exothermic – energy released from forming bonds is greater than that needed to
break the bonds; endothermic – energy needed to break bonds is greater than
energy released making them
What is the equation to find enthalpy change in terms of bond energies?
Energy of reaction = sum of bonds broken – sum of bonds made
What is a cell?
A cell is composed of two electrodes dipped in an electrolyte solution. It produces
electricity from a chemical reaction.
What is a battery
A battery consists of two or more cells connected in series.
What determines the voltage obtained from a
cell?
Identities of metals used as electrodes and the identity and concentration of an
electrolyte.
State the advantages and disadvantages of
using cells and batteries
(+) more or less cheap, some are rechargeable, a convenient source of electrical
energy
(-) harmful chemicals
Describe rechargeable and non-rechargeable cells
Rechargeable – chemical reactions are reversed when an external current is
supplied
Non-rechargeable – reactants are used up, cannot be recharged
What is a fuel cell? What is the
overall reaction in a hydrogen
fuel cell? What are the half
equations?
Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate
electricity.
What is the overall reaction in a hydrogen fuel cell? What are
the half equations?
Cathode: 2 H2 → 4 H+ + 4 e−
Anode: O2 + 4 H+ + 4 e−→ 2 H2O
Overall: → 2 H2 + O2 → 2 H2O
What are the advantages and disadvantages of hydrogen fuel
cells?
Advantages: no pollutants, no recharging Disadvantages: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis
