Energy Changes
Energy Changes
Learning Objectives…
• Describe the term exothermic as a process or chemical reaction which transfers energy, often in the form of heat, to the surroundings and may be detected by an increase in temperature, e.g. the reaction between sodium hydroxide and hydrochloric acid
• Describe the term endothermic as a process or chemical reaction which takes in energy, often in the form of heat, from the surroundings and may be detected by a decrease in temperature, e.g. the dissolving of ammonium nitrate in water
Introduction
• Endo = In• Exo = Out
• Thermic = Heat
What is an ENDOTHERMIC reaction?
• A reaction that takes in heat (thermal energy)• Causes a drop in temperature in the
surroundings.
What is an EXOTHERMIC reaction?• A reaction that gives out heat (thermal
energy)• Cause a temperature rise in the surroundings.
Heat of reaction
ΔHHeat of reaction
Unit: kilojoules (kJ)
The amount of heat energy given out or taken in (absorbed) during a
chemical reaction.
Exothermic reaction Endothermic reactionHeat energy lost Heat energy gained
- ΔH + ΔH
Example 1
Reaction: Magnesium and hydrochloric acidEquation: Mg + 2HCl MgCl2 + H2 [ΔH = -462 KJ]ΔH ΔH = -462 KJExothermic/Endothermic reaction:
Explanation: Negative value of ΔH shows that the reaction is exothermic. 462 kJ means that 462 kilojoules of energy is given out.
Example 1
Reaction: Magnesium and hydrochloric acidEquation: Mg + 2HCl MgCl2 + H2 [ΔH = -462 KJ]ΔH ΔH = -462 KJExothermic/Endothermic reaction:
exothermic reaction
Explanation: Negative value of ΔH shows that the reaction is exothermic. 462 kJ means that 462 kilojoules of energy is given out.
Example 2Reaction: Decomposition of calcium carbonateEquation: CaCO3 CaO + CO2 [ΔH = +222 KJ]ΔH ΔH = +222 KJExothermic/Endothermic reaction:
endothermic reaction
Explanation: Positive value of ΔH shows that the reaction is endothermic. 222 kJ means that 222 kilojoules of energy is given out.
CHECKPOINT!
1. DETERMINE IF THE REACTIONS STATED BELOW ARE ENDOTHERMIC OR EXOTHERMIC REACTIONS. (A) COMBUSTION OF METHANE: EXOTHERMIC/ENDOTHERMIC REACTION
CH4 + 2O2 CO2 + 2H2O [ΔH = -882 KJ]
(b) Decomposition of copper(II) carbonate: exothermic/endothermic reaction
CuCO3 CuO + CO2 [ΔH = +46 kJ]
(c) Formation of nitrogen oxide in a car engine: exothermic/endothermic reaction
N2 + O2 2NO [ΔH = +181 kJ]
(d) Reaction of sodium hydroxide and hydrochloric acid: exothermic/endothermic reaction
NaOH + HCl NaCl + H2O [ΔH = -57 kJ]
1. DETERMINE IF THE REACTIONS STATED BELOW ARE ENDOTHERMIC OR EXOTHERMIC REACTIONS. (A) COMBUSTION OF METHANE: EXOTHERMIC/ENDOTHERMIC REACTION
CH4 + 2O2 CO2 + 2H2O [ΔH = -882 KJ]
(b) Decomposition of copper(II) carbonate: exothermic/endothermic reaction
CuCO3 CuO + CO2 [ΔH = +46 kJ]
(c) Formation of nitrogen oxide in a car engine: exothermic/endothermic reaction
N2 + O2 2NO [ΔH = +181 kJ]
(d) Reaction of sodium hydroxide and hydrochloric acid: exothermic/endothermic reaction
NaOH + HCl NaCl + H2O [ΔH = -57 kJ]
Exothermicreaction Exothermic reaction
only occurs when the chemical energy present in the substance is converted to thermal energy (and sometimes light and sound energy).
1. Change of state
• Particles move closer together, at a lower energy level
• Excess energy is lost in the form of heat energy
freezingcondensation
2. Combustion reactions
• Combustion is the reaction between a substance and oxygen.
• Energy is released in the form of heat (sometimes light and sound energy).
• Respiration is also a type of combustion reaction.
3. Neutralisation
• Acid + Alkali Thermal energy given out– Combining of H+ and OH- ions to form H2O
– H+ (aq)+ OH- (aq) H2O (l)
Endothermic reaction
Endothermic reaction occurs when heat energy is absorbed.
1. Change of state
• Particles move further apart, breaking forces of attraction holding them together
• Energy is absorbed and used to break bonds• Surrounding temperature drops.
2. Decomposition of compounds
• Most compounds require heating to be decomposed. – Thermal energy absorbed– Compound is decomposed
bond-making vs. bond-breaking
Heat changes in a reaction
heat of reaction, ΔH
Overall heat change (heat of reaction, ΔH)= heat given out in making bonds + heat
absorbed in breaking bonds