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Measure 50ml of acid using a burette and put it into the polystyrene cup

Make lid out of foil to prevent heat from escaping.

Accurately weigh out powdered solid

Start the stopwatch

Use a thermometer to measure the starting temperature of the acid, multiple times to ensure a constant

Add the solid, stir until dissolved and put the lid back on quickly

Measure the temperature of the solution at regular intervals (30 seconds)  until the temperature begins to fall

Plot data on graph to calculate the change in temperature (Δt)

ΔH= -mcΔt ÷ n for MgO

Repeat experiment for MgCO₃

ΔH₃ = ΔH₁ - ΔH₂

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The temperature is recorded for a few minutes before mixing to make sure it constants

The time of mixing (starting the reaction) is noted on the graph

The temperature is recorded for a number of minutes after mixing.

Lines are extended forward and back to the time of mixing. This is called extrapolating the data.

The temperature change is measured between the two extrapolated lines at the time of mixing.

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If we add a solid to a solution and there is an enthalpy change we must work out how many moles of solid went into the solution.... OR... we use the volume and concentration of a solution to get the number of moles.
It is the chemical that isn’t in excess that is used to get n and then work out enthalpy changes. This chemical is said to be the limiting reagent.

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Exothermic: ΔT positive and ΔH negative
Endothermic: ΔT negative and ΔH positive

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Breaking bonds requires energy therefore it is endothermic.
Making bonds give out energy therefore it is exothermic.

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ΔH = - mcΔt ÷ n
Units - Jmol-1
M = mass of solution (g)
N= no. of moles that reacted
C= specific heat capacity - energy to heat 1g by 1 °C
ΔT - temperature change (°C)

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Formation - arrows up
Breaking bonds - arrows down
Combustion - arrows up/down

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The total enthalpy change for a reaction is independent of the route taken from reactants to product.

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The enthalpy change in a reaction (performed under standard conditions) between the number of moles of reactants shown in the chemical equation.

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The average bond enthalpy is the average value of the enthalpy required to break a covalent bond in a gaseous molecule.

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The enthalpy change when one mole of a substance is completely combusted in oxygen (under standard conditions).

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The enthalpy change when one mole of substance is formed from its constituent elements in their standard states (under standard conditions).

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Substances in standard states

Temperature = 298K (25°C)

Pressure = 1 atm (101,000 Pa)