C5 core questions
Types of Reactions Questions
Answer
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 endothermic reactions
Some cooling sports injury packs
What are reactants?
The substances involved in a chemical reaction
What are products?
The substances formed when reactants have a chemical reaction
What is a reaction profile?
A diagram which shows whether the reactants have more or less energy than the products.
Why does the reaction profile always go up at the start?
A reaction starts with bonds being broken
What is the activation energy?
The energy needed to break bonds to start a reaction
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?
Endothermic. Chemical bonds are strong so require energy to break (like when you have to put energy in to separate magnets from each other)
Is making 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
Electrochemical Cells Questions
Answers
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