Energy Changes

What two things are conserved during chemical reactions?

Two things conserved during chemicals reactions: ★ Mass ★ Energy

What is a reactant?

A reactant is the substance that takes part in and undergoes change during a reaction (the left side of the chemical reaction)

What is a product?

Products are elements or compounds produced by a chemical reaction (the right side of a chemical equation) What is an exothermic reaction?

What are examples of exothermic reactions?

Examples of exothermic reactions: ★ Combustion ★ Many oxidation reactions ★ Neutralisation reactions

What is the key characteristic of an exothermic energy profile diagram?

★ A key characteristic of exothermic energy profile diagrams is the reactants have more energy than the products ✔︎ This is because energy has been transferred from the reaction to the surroundings

In energy profile diagram, what is the energy transferred to the surroundings?

In an energy profile diagram, the energy transferred to the surroundings is the [energy of the reactants] - [energy of the products]

What are two uses of exothermic reactions?

Two uses of exothermic reactions: ★ Hand warmers ★ Self-heating cans e.g. for food or drink

What is an endothermic reaction?

An endothermic reaction is one that: ★ Takes in energy from the surroundings ➜ So the temperature of the surroundings decreases.

What is an example of an endothermic reaction?

An example of an endothermic reaction is thermal decomposition

What is a key characteristic of an endothermic energy profile diagram? Why?

★ A key characteristic of an endothermic energy profile diagram is the products have more energy than the reactants ✔︎ This is because energy has been taken in by the surroundings

When can a chemical reaction occur? What does this require?

★ Chemical reactions can only occur when the particles collide with each other ✔︎ They need sufficient energy for this

What is activation energy?

Activation energy is the minimum amount of energy that particles must have in order to react

How do you draw activation energy on an energy profile diagram? What must you make sure to do?

★ To draw the activation energy on an energy profile diagram, you draw an arrow from the reactants to the peak of the curve ⚠︎︎ Make sure to do this properly. You can't just draw an arrow pointing up

How does a reaction work? [Answer in terms of endothermic and exothermic processes]

When a reaction takes place: ★ Energy is supplied to break bonds ✔︎ This is an endothermic process (e.g. it takes energy to break a plate) ★ Energy is released (to the surroundings) when new bonds are formed ✔︎ This is an exothermic process

What is the symbol for activation energy?

The symbol of activation energy is: Eₐ

What is enthalpy?

Enthalpy is the heat content of a system

What is the symbol for enthalpy?

Symbol for enthalpy: ΔH

What is the y-axis on an energy profile diagram?

Y-axis on energy reaction profile: ★ Enthalpy ★ Energy

What is the x-axis on an energy profile diagram?

X-axis on an energy profile diagram: ★ Time (of reaction) ★ Reaction pathway

What is a catalyst?

A catalyst is: ★ A substance that increases the rate of a chemical reaction ★ Without itself undergoing any permanent chemical change

How does a catalyst work?

A catalyst work by: ★ Offering an alternative reaction pathway ★ Of lower activation energy

When are energy changes negative?

★ Exothermic energy changes are shown as negative ✔︎ This is because energy has been transferred from the chemicals

When are energy changes positive?

★ Endothermic energy changes are shown as positive ✔︎ This is because energy has been gained

What is bond energy?

★ Every chemical bond has an energy value (called bond energy) ✔︎ This tells us he energy required to break that bond

What is the unit for bond energy?

The unit for bond energy is kJ/mol

How can you produce electricity with two different metals?

★ If you have two different metals ★ And then place them into an electrolyte ✔︎ You can produce electricity with two

How do two metals placed in an electrolyte solution generate electricity?

Two metals placed in an electrolyte solution

What is an electrolyte?

An electrolyte is a solution that can conduct electricity

What is an example of an electrolyte?

Any solution of an ionic compound is an electrolyte

What is a cell?

★ If you connect two metals in an electrolyte solution using a voltmeter ✔︎ You will see a [voltage/potential difference] go between the two metals ✔︎ An electric current will also flow through the wire ★ A chemical reaction is taking place on the surface of the two metals ➜ These reactions are producing electricity ✔︎ This setup is called a cell and you can use them to conduct electricity

Are GCSE cells the same as the cells used in chemistry?

★ Cells used in chemistry are much complicated than GCSE cells ★ GCSE cells would not produce electricity for very long ✔︎ However, the core principle remains the same

What are the limitations of cells?

Limitations of cells: ★ Cells can only produce electricity for a certain period of time ✔︎ Eventually, the chemicals in the cell run out ➜ Which makes the reaction stop ➜ Which makes no more electricity ★ Cells can only produce electricity if we use metals with different reactivities

What determines the voltage of a cell?

★ The greater the difference between then reactivity of two metals ➜ The higher the potential difference produced by the cell ★ The electrolyte also affects the potential difference

What is battery?

Batteries consist of two or more cells connected together in series to provide a greater voltage

What are non-rechargeable cells and batteries?

★ In non-rechargeable cells and batteries the chemical reactions: ✔︎ Stop when one of the reactants has been used up and no more electricity is produced ✔︎ There is also no way that we can reverse these reactions ✔︎ Alkaline batteries are non-rechargeable

What are examples of non-rechargeable batteries?

Alkaline batteries are non-rechargeable

What are rechargeable batteries?

★ Rechargeable batteries can be recharged ✔︎ This is because we can reverse these reactions when we apply an electric current (unlike non-rechargeable batteries)

What is the dependent variable in the Temperature Changes RP?

The dependent variable in the Temperature Changes RP is the maximum temperature reached

What are the control variables in the Temperature Changes RP?

Control variables in the Temperature Changes RP: ★ Volume of hydrochloric acid (HCl) ★ concentrations of both: ★ Hydrochloric acid (HCl) ★ Sodium hydroxide solution (NaOH)

What are the purposes of the beaker in the Temperature Changes RP?

Purposes of the beaker in the Temperature Changes RP: ★ Stops cup from falling over ★ Acts as a thermal insulator (keeps heat in)

What must we do with the thermometer in the Temperature Changes RP?

We must make sure the bulb is in the solution in the Temperature Changes RP

Is the reaction between HCl and NaOH [endo/exo]thermic?

The reaction between HCl and NaOH is an exothermic reaction

Why does the maximum temperature initially increase with the volume in the Temperature Changes RP?

★ The maximum temperature initially increase with the volume in the Temperature Changes RP ✔︎ This is because the reaction is exothermic ➜ So more energy is released 𐬽 The maximum temperature reached increases

Why does the maximum temperature then DECREASE with the volume in the Temperature Changes RP?

★ The maximum temperature then DECREASE with the volume in the Temperature Changes RP ✔︎ This means some of the sodium hydroxide was unable to react ➜ The energy released by the reaction is now a maximum ✔︎ The same amount of energy is spread out at a greater volume 𐬽 The maximum temperature decreases

Why do we use a polystyrene cup with a lid in the Temperature Changes RP?

We use a polystyrene cup with the lid in the Temperature Changes RP: ★ We are measuring the temperature in this experiment ✔︎ This means we want to reduce any heat losses ✔︎ Polystyrene is a good thermal insulator ➜ It reduces heat loss through the sides and the bottom ★ The lid reduces heat loss to the air

How do you determine the difference in voltage in a battery?

To determine the voltage in a battery, simply find the difference of reactivity between the metals (sign matters, swapping a positive voltage battery will yield a negative voltage)

What happens in a fuel cell?

In a fuel cell: ★ React a fuel e.g. hydrogen with: ★ Pure oxygen OR ★ Air ★ Inside a fuel cell, a chemical reaction takes place ➜ Which generates an electric current ✔︎ These provide an alternative to rechargeable batteries

What is the half equation for the negative electrode (anode) in a hydrogen-oxygen fuel cell?

Half equation at the negative electrode (anode) in a hydrogen-oxygen fuel cell: 2H₂ ➜ 4H⁺ + 4e⁻

What is the half equation for the positive electrode (cathode) in a hydrogen-oxygen fuel cell?

Half equation at the positive electrode (cathode) in a hydrogen-oxygen fuel cell: O₂ + 4H⁺ + 4e⁻ ➜ 2H₂O

What is the overall equation in a hydrogen-oxygen fuel cell?

Overall equation in a hydrogen-oxygen fuel cell: 2H₂ + O₂ ➜ 2H₂O

What is the waste product in a hydrogen-oxygen fuel cell?

The only waste product in a hydrogen-oxygen fuel cell is water (H₂O)

What are some advantages of hydrogen fuel cells? [Hence, include disadvantages of rechargeable batteries]

Advantages of hydrogen fuel cells: ★ Hydrogen fuel cells produce electricity as long as you provide them with hydrogen ⚠︎︎ Rechargeable batteries run out and need to be recharged ★ Hydrogen fuel cells do not get less efficient the longer they run ✔︎ Rechargeable batteries store less electricity the more charging cycles they go through ➜ They eventually need to be replaced ★ Hydrogen fuel cells can be a source of drinkable water e.g. on a space-craft

What are some disadvantages of hydrogen fuel cells? [Hence, include advantages of rechargeable batteries]

Disadvantages of hydrogen fuel cells: ★ Hydrogen fuel cells which runs on hydrogen ✔︎ Which is an explosive gas ➜ It is very difficult to store safely ✔︎ No dangerous fuels are required for rechargeable batteries (while some rechargeable batteries do catch fire if they are not manufactured properly) ★ Hydrogen fuel cells produce a relatively low potential difference ➜ Several are needed together ✔︎ Rechargeable batteries produce a much greater voltage

What is usually the electrode in the fuel cell?

The electrode in a fuel cell is usually KOH (potassium hydroxide)