Standard Enthalpy Change of Neutralisation

Standard Enthalpy Change of Neutralisation

What is the standard enthalpy change of neutralisation, ΔneutH°?

The enthalpy change when one mole of water is produced by the neutralisation of an acid with an alkali under standard conditions.

What pressure is used for standard enthalpy change of neutralisation?

100 kPa.

What temperature is usually used for standard enthalpy change of neutralisation?

298 K.

What does the standard enthalpy change of neutralisation refer to in terms of water?

The formation of one mole of water.

What is the unit of standard enthalpy change of neutralisation?

kJ mol⁻¹.

Is neutralisation usually exothermic or endothermic?

Exothermic.

What sign does ΔneutH° usually have?

Negative.

Why is ΔneutH° negative?

Heat energy is transferred from the reacting system to the surroundings.

What is the equation for the neutralisation of hydrochloric acid by sodium hydroxide?

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).

What is the ionic equation for neutralisation between a strong acid and strong alkali?

H⁺(aq) + OH⁻(aq) → H₂O(l).

Why are ΔneutH° values for different strong acids and strong alkalis very similar?

Strong acids and alkalis are fully ionised, so the reaction is essentially always H⁺(aq) + OH⁻(aq) → H₂O(l).

What is a typical value for ΔneutH° for a strong acid reacting with a strong alkali?

Approximately −58 kJ mol⁻¹.

What apparatus is commonly used to measure enthalpy change of neutralisation?

An expanded polystyrene cup in a beaker, with a lid and thermometer.

Why is a polystyrene cup used in a neutralisation experiment?

It is a good thermal insulator, reducing heat transfer to the surroundings.

Why is a lid used on the polystyrene cup?

To reduce heat loss to the surroundings.

Why is the polystyrene cup placed inside a beaker?

To support the cup and keep it stable.

What should be measured before mixing the acid and alkali?

The initial temperatures of both solutions.

What should be measured after mixing the acid and alkali?

The maximum temperature reached.

Why should the mixture be stirred after adding the alkali?

To ensure the solutions mix completely and the temperature is uniform.

How can the starting temperature be found if the acid and alkali have slightly different initial temperatures?

Calculate the mean of their initial temperatures.

How is the temperature change, ΔT, calculated?

ΔT = maximum temperature − mean starting temperature.

What equation is used to calculate the heat energy transferred during neutralisation?

Q = mcΔT.

What does m represent in Q = mcΔT?

The mass of the solution heated, in grams.

What value is normally assumed for the specific heat capacity of the solution?

4.18 J g⁻¹ K⁻¹.

What density is normally assumed for dilute aqueous solutions?

1.00 g cm⁻³.

How can the mass of solution be estimated from its volume?

Assume 1.00 cm³ has a mass of 1.00 g, so the mass in grams equals the total volume in cm³.

What does ΔT represent in Q = mcΔT?

The temperature change in K or °C.

How do you calculate the number of moles of an acid or alkali in solution?

n = concentration × volume in dm³.

How do you convert cm³ to dm³?

Divide the volume in cm³ by 1000.

How do you determine the number of moles of water formed?

Use the balanced equation and the amount of the limiting reactant.

For H⁺ + OH⁻ → H₂O, what is the mole ratio?

1 : 1 : 1.

How is ΔneutH calculated experimentally?

ΔneutH = −Q ÷ moles of water formed.

Why is a negative sign used when calculating ΔneutH?

The solution gains heat released by the reaction, so the reaction itself loses energy and is exothermic.

What are the main calculation steps for ΔneutH?

Find ΔT; calculate Q = mcΔT; calculate moles of water formed; calculate ΔneutH = −Q/n.

If 25.0 cm³ of 1.00 mol dm⁻³ HCl is neutralised, how many moles of HCl react?

0.0250 mol.

If 0.0250 mol of HCl reacts completely with excess NaOH, how many moles of water form?

0.0250 mol.

If 25.0 cm³ acid and 25.0 cm³ alkali are mixed, what total volume is heated?

50.0 cm³.

Assuming a density of 1.00 g cm⁻³, what is the mass of 50.0 cm³ of solution?

50.0 g.

What are important sources of error in a neutralisation calorimetry experiment?

Heat transfer to the surroundings, heat absorbed by the thermometer and cup, and measurement uncertainties.

How does heat loss to the surroundings affect the measured ΔneutH?

The measured temperature rise is smaller, so the calculated ΔneutH is less negative than the true value.

Why is some heat absorbed by the thermometer and polystyrene cup a source of error?

Q = mcΔT normally only accounts for energy absorbed by the solution.

How could heat loss in a neutralisation experiment be reduced?

Use an insulated polystyrene cup with a lid and take temperature measurements promptly.

Why should an experimental ΔneutH usually be quoted to an appropriate number of significant figures?

The precision of the answer is limited by the precision of the experimental measurements.

What is a thermometric titration?

A titration in which the endpoint is indicated by a temperature change.

How is a thermometric titration carried out?

Add portions of one reactant to another, stir after each addition and record the temperature after each addition.

What happens to the temperature before the equivalence point in an exothermic thermometric titration?

The temperature increases as neutralisation occurs.

What happens to the temperature after the equivalence point?

The temperature begins to decrease because the added reactant is in excess and no additional neutralisation occurs.

How can the equivalence point be found from a thermometric titration graph?

Plot temperature against volume added and determine the volume corresponding to the maximum temperature or intersection of the trend lines.

Why can weak acids give different ΔneutH° values from strong acids?

Weak acids are not fully ionised, so energy changes associated with ionisation also affect the overall enthalpy change.

Define standard enthalpy change of neutralisation.

The enthalpy change measured at 100 kPa and a stated temperature, usually 298 K, when one mole of water is produced by neutralisation of an acid with an alkali.