Unit 3 - Lesson 13: Endothermic and Exothermic Reactions Flashcards
Define an exothermic reaction.
An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature of the surroundings.
Define an endothermic reaction.
An endothermic reaction is one which takes in energy to the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings.
What happens to particles when applying thermal energy (heat) to them?
Thermal energy causes particles to move (gain kinetic energy). The more thermal energy, the more kinetic energy. The more particles move, the more intermolecular forces and/or bonds can break.
What is a reaction if it needs heat or speeds up when heat is applied?
Endothermic.
Give an example of endothermic reactions.
Sublimation. Apply heat to a solid and the particles will move so much, the intermolecular bonds break, forming a gas.
Is thermal decomposition of a metal carbonate endothermic or exothermic?
Endothermic. Heat breaks down the substance.
What kind of reaction is it if there is a temperature decrease?
Endothermic reaction.
What do we use endothermic reactions in? (Think daily life)
Handwarmers, cold packs & self-heating food/mugs.
Are combustion reactions exothermic or endothermic?
Exothermic. They produce thermal energy. For example:
2Mg + O2 –> 2MgO
S + O2 –> SO4
H + O2 –> H2O
Give three examples of reactions that are exothermic.
- Reactions between metals and acid
- Neutralisation reactions
- Displacement reactions
What are displacement reactions?
Displacement reactions are two elements ‘fighting’ for something (displace it from a compound). The most reactive will win.
What does the thermite reaction involve?
Two metals ‘fighting’ for an oxide. For example:
Aluminium + Iron (III) Oxide –> Iron + Aluminium Oxide
2Al + Fe2O3 –> 2Fe + Al2O3
Aluminium is higher up the reactivity series than iron, so it steals the oxide (displaces the iron). Pure iron is produced. Iron can be extracted with carbon. This is a redox reaction. Iron is reduced. Aluminium is oxidised.
What kind of reaction (endothermic/exothermic) is it if there is a temperature increase?
Exothermic reaction.
What is ‘Enthalpy’?
Enthalpy is the total heat energy in a reaction. This is the overall change in energy during the reaction.
What is enthalpy measured in?
Joules (J)
What letter stands for enthalpy?
H
What does the Greek letter ‘delta’ (Δ) stand for?
‘Change in’
How can enthalpy change be calculated?
Energy in product - Energy in reactant = Enthalpy change
Do exothermic reactions lose or gain energy?
b) What does this say about their ΔH?
Lose.
b) Therefore, their ΔH is always negative.
Do endothermic reactions lose or gain energy?
b) What does this say about their ΔH?
Gain.
b) Therefore, their ΔH is always positive.
What is enthalpy change (ΔH) measured in?
kJ/mol (kilojoules per mole). This is the amount of kJ lost or gained for every 1 mole of reactant. Occasionally, it’s written as kJ mol^-1.
Why are endothermic ΔH positive?
Because they take in energy.
Why are exothermic ΔH negative?
Because they release energy.
