Energetics

Define exothermic reactions

A reaction that releases heat energy

What happens to the temperature in an exothermic reaction?

The temperature of a substance increases

How does a graph for an exothermic reaction look like?

• Products are lower than reactants

What are some examples of exothermic reactions?

1) Neutralisation

2) Combustion

3) Displacement (can be endothermic as well)

What are everyday uses of exothermic reactions?

1) Heat pack

Define endothermic reactions

A reaction that takes in heat energy

What happens to the temperature in an endothermic reaction?

The temperature of a substance decreases

How does a graph for an endothermic reaction look like?

• Products are higher than reactants

What are some examples of endothermic reactions?

1) Displacement (can be endothermic as well)

2) Dissolving

What are everyday uses of endothermic reactions?

1) Ice pack

What equation is used to calculate heat energy transferred between two objects?

• Q = m c ∆T

• Heat energy [J] = mass [g] x specific heat capacity [J/g°c] x temperature change [°C]

What is enthalpy?

Total energy stored within a system (reactants + products of a reaction)

What equation is used to calculate enthalpy?

• ∆H = Q/mol

• Enthalpy = Heat energy/mole [J mol⁻¹]

In what reaction is enthalpy change negative, and why?

1. Exothermic reactions release heat as bonds are formed in products

2. Thus, products have a lower enthalpy than reactants started with

3. ΔH (enthalpy change) = Products - Reactants

   → negative number

In what reaction is enthalpy change positive, and why?

1. Endothermic reactions take in heat as bonds require breaking

2. Thus, products have a higher enthalpy than reactants started with

3. ΔH (enthalpy change) = Products - Reactants

   → positive number

What are the reasons for lower energy released in a calorimetry experiment?

1) Heat lost to surroundings

2) Incomplete combustion

What are the reasons for lower enthalpy change in a calorimetry experiment?

1) Heat lost to surroundings

2) Heat is absorbed by calorimeter

What is the method of the calorimetry experiment for combustion?

1. Using a measuring cylinder, measure 100 cm³ of water into a copper can

2. Measure and record the initial temperature of water

3. Filling the spirit burner with test substance, measure and record its mass

4. Place burner under copper can and light the wick

5. Constantly stir the water and heat until temperature rises 20 - 30°C + blow out flame

6. Measure and record the highest temperature of water

7. Measure and record the final mass of burner and remaining alcohol

What is the method of the calorimetry experiment for displacement, dissolving, and neutralisation?

1. Using a measuring cylinder, measure 25 cm³ of Solution 1 into a Polystyrene cup

2. Measure and record the temperature of Solution 1

3. Add a measured amount of reactant (solid for dissolving, solution 2 for displacement and neutralisation) into the Polystyrene cup and stir the mixture

4. Measure and record the highest temperature reached by the mixture