Energy Changes

What is an exothermic reaction?

A reaction that gives out energy to the surroundings

What is an endothermic reaction?

A reaction that takes in energy from the surroundings

What happens to temperature in an exothermic reaction?

Temperature increases

What happens to temperature in an endothermic reaction?

Temperature decreases

Give an example of an exothermic reaction.

Combustion, neutralisation, respiration

Give an example of an endothermic reaction.

Thermal decomposition, photosynthesis, citric acid + sodium bicarbonate

What is an energy profile diagram?

Graph showing energy change during a reaction

Where is the activation energy shown on a diagram?

The energy difference between reactants and the highest point

What is activation energy?

The minimum energy needed to start a reaction

Where is energy in exothermic reaction profile?

Products are lower than reactants

Where is energy in endothermic reaction profile?

Products are higher than reactants

What is bond energy?

Energy required to break one mole of specified bonds

Is bond breaking exothermic or endothermic?

Endothermic (energy absorbed)

Is bond forming exothermic or endothermic?

Exothermic (energy released)

How do you calculate energy change in a reaction?

Sum of energy needed to break bonds minus energy released forming bonds

What do exothermic reactions feel like?

Warmer

What do endothermic reactions feel like?

Colder

What is the energy change (ΔH) for exothermic reactions?

Negative

What is the energy change (ΔH) for endothermic reactions?

Positive

What is a simple practical for exothermic reactions?

Measuring temperature rise with acid and alkali

What practical shows endothermic reactions?

Measuring temperature fall in dissolving salt in water

What are examples of everyday exothermic processes?

Hand warmers, self-heating cans

What are examples of endothermic processes in life?

Cold packs, ice on injuries

What does a catalyst do in a reaction?

Lowers activation energy

On an energy profile, how does a catalyst change the diagram?

Lowers the activation energy peak

How is energy measured in reactions?

Using a thermometer to record temperature change

How do you improve accuracy in heat experiments?

Insulate container, use lids, reduce heat loss

What is the equation for energy change using SHC?

Q = m × c × ΔT

What is the common unit for energy change?

Joules (J)

Why might experimental energy change be less than calculated?

Heat lost to the environment

What is a calorimeter?

Apparatus to measure heat energy change

How do you calculate bond energies?

Add all bonds broken, subtract all bonds formed

What is breaking a C-H bond?

Endothermic process

What is forming an O-H bond?

Exothermic process

How are combustion reactions usually classified?

Exothermic

Why are cold packs useful in sports?

Absorb heat (endothermic), reduce swelling/pain

How does a hand warmer work?

Exothermic reaction (e.g., iron oxidation)

What is a reversible reaction?

Reaction that can go both ways, products can react to form reactants

Are reversible reactions always energy changing?

Yes, one direction is exothermic, the other endothermic

In the energy profile for a reversible reaction, which direction is exothermic?

Forward or reverse, depending on which releases energy

How is bond energy data found?

Experimentally determined and compiled in tables

Does dissolving ammonium nitrate in water take in energy?

Yes, endothermic

What is the energy change when bonds form?

Energy is released (exothermic)

If more energy is released making bonds than breaking them, is the reaction exothermic or endothermic?

Exothermic

If more energy is needed to break bonds than is released, is the reaction exothermic or endothermic?

Endothermic

What is the energy level of reactants in an exothermic reaction?

Higher than products

What is the energy level of reactants in an endothermic reaction?

Lower than products

What does ΔH stand for?

Enthalpy change (overall energy change)

What is a chemical cell?

Device usually containing two metals in a solution; releases energy by a redox reaction

What is a fuel cell?

A cell that releases energy by reacting fuel (like hydrogen) with oxygen

Give one advantage of hydrogen fuel cells.

Only product is water (no pollution)

Why are fuel cells considered sustainable?

Use hydrogen and oxygen, produce water

Give one disadvantage of hydrogen fuel cells.

Hydrogen is hard to store and transport

What happens at the anode in a hydrogen fuel cell?

Hydrogen loses electrons (oxidation)

What happens at the cathode in a hydrogen fuel cell?

Oxygen gains electrons (reduction)

What type of reaction occurs in a fuel cell?

Redox reaction

What is the half-equation for the anode in a hydrogen fuel cell?

H₂ → 2H⁺ + 2e⁻

What is the half-equation for the cathode in a hydrogen fuel cell?

O₂ + 4H⁺ + 4e⁻ → 2H₂O

Why are alkaline batteries non-rechargeable?

Reaction cannot be reversed

What is the main environmental concern with batteries?

Waste, toxins, difficulty recycling

What is the main difference between a rechargeable and non-rechargeable cell?

Rechargeable can be reversed by passing current through

How do you test if a reaction is exothermic?

Measure temperature before and after mixing reactants

How do you test if a reaction is endothermic?

Measure if temperature decreases during reaction

Name a solid that dissolves endothermically in water.

Ammonium chloride

Why are reaction mixtures often insulated in practicals?

To improve accuracy by minimizing heat transfer

What are the two products in hydrogen fuel cell reaction?

Water and electrical energy

Write the overall equation for a hydrogen fuel cell.

2H₂ + O₂ → 2H₂O

Where is the energy stored in chemical reactions?

In chemical bonds

What affects the voltage of a cell?

Metal reactivity difference and electrolyte concentration

Why are high-voltage cells made with metals far apart in reactivity series?

Greater difference, bigger voltage

What does SHC stand for?

Specific heat capacity

How do catalytic converters in cars use exothermic reactions?

Convert harmful gases into safer ones, releasing heat

What is a system in energy terms?

The reactants, products, and surroundings

Why are insulated beakers used in energy change practicals?

Reduce heat loss/gain for more accurate results

How can you modify a calorimeter experiment to improve data?

Add insulation, cover, use digital thermometer

What is the difference between open and closed system in reactions?

Open: energy can be lost/gained with surroundings; Closed: contained

Why might a hand warmer using a physical change (crystallization) be reusable?

Process is reversible by reheating

What is heat of reaction?

Overall energy absorbed or released during a process

What type of chemicals are used in cold packs?

Chemicals undergoing endothermic reactions when mixed

How does reaction mixture mass affect measured energy change?

Larger mass usually absorbs/releases more energy

What practical safety measure should you use in energy change experiments?

Wear goggles, handle hot/cold carefully

How do you ensure reactions are complete in energy change experiments?

Stir thoroughly, use excess reactants if needed

Give a real-life industrial use for exothermic reactions.

Cement production, self-heating cans

What is the main risk of exothermic reactions in industry?

Overheating, fire, explosion

How can energy change per mole be calculated?

Calculate energy for whole sample, divide by moles of limiting reactant

What is necessary for combustion to occur (apart from reactant)?

Oxygen

What might cause experimental value to differ from theoretical?

Heat loss to surroundings, incomplete reaction, measurement error

In a reversible reaction, if the forward is endothermic, what is the reverse?

Exothermic

What does a decrease in temperature do to an exothermic equilibrium?

Shifts equilibrium towards exothermic side (more product)

What does a decrease in temperature do to an endothermic equilibrium?

Shifts equilibrium towards reactants (reverse)

Why are bond energies average values?

Bond strength varies in different environments

What is the general formula for energy change?

Energy in (bonds broken) - Energy out (bonds made)

What safety device uses exothermic reaction?

Airbag inflation (rapid combustion)