反应热公式
反应热也称为反应焓，是在恒定压力下化学反应的焓值之差。它是热力学测量单位，用于确定反应中每摩尔产生或释放的能量总量。因此，化学反应的热量可以定义为在恒定压力和温度下发生反应时释放到环境中或吸收的热量。焦耳 (J) 是用于测量接收或释放的热量总量的单位。
In chemical reactions, heat is mostly transferred between the reacting system as one medium and the surrounding as the other one. Before and after the chemical transformation, the amount of heat energy is the same. In other words, the heat acquired or lost in a reacting system is equivalent to the heat gained or lost in the surroundings.
什么是反应热？
简而言之，反应热是实现给定反应所需的热量，对放热反应为负，对吸热反应为正。对于吸热反应，ΔH 为正，而对于产生热量的反应，ΔH 为负。
当给定的反应在恒定体积下进行时，影响反应所需的热量只不过是通过 ΔH/ΔU 增加的内能 (ΔU) 对吸热反应为负，对放热反应为正。
Formula for Heat of Reaction:
Q = m × c × ΔT
where,
- Q = Heat of Reaction,
- m = mass of medium,
- c = specific heat capacity of the reaction medium,
- ∆T = difference in temperature of the medium.
Besides we also have another equation as,
Heat of Reaction = ΔH (products) – ΔH (reactants)
where,
- ΔH = change in heat value
示例问题
问题 1：假设水的比热容为 4.2J，计算当指定量的物质在空气中燃烧以将 200 克水的温度从 28 摄氏度提高到 42 摄氏度时，乙醇燃烧时发生的热变化/gK
解决方案：
It is given that,
c = 4.2 Jg-1K-1,
m = 200 g,
ΔT = 42 – 28 ,
i.e. ΔT = 14 °C or 14 K
Here in the question, it is mentioned that a certain amount of ethanol is burned in order to raise the temperature of the water, implying that heat absorbed by water is evolved by the ethanol combustion process. The amount of heat lost in the combustion process is equal to the amount of heat gained by the water.
The amount of heat that has been changed can be determined by using formula,
Q = m × c × ΔT
Q = 200 × 4.2 × 14
Hence, Q = 11760 J
问题2：当氯化钠溶解在25℃的100克水中时，适当搅拌后所得溶液的温度为21℃。如果假设溶液的比热容为 4.18 J/g °C，计算溶解过程中的热量变化。
解决方案：
Here it is given that,
c = 4.18 J / g °C,
m = 100 g,
ΔT = 25 – 21,
i.e. ΔT = 4 K
The process results in a temperature drop, indicating that the salt dissolution tends to heat absorption from the system. Since, the heat lost by water is same as the heat absorbed by salt,
We have,
Q = m × c × ΔT
Q = 100 × 4.18 × 4
Hence, Q = 1672 J
问题3：240克铁从90摄氏度冷却到25摄氏度，放出多少热量？ （给定：c = 0.452 J / g °C）
解决方案：
We have,
m = 240 g,
Specific heat capacity of Iron (c) = 0.452 J / g °C,
ΔT = Final temperature – Initial temperature = 25 – 90 = -65 °Celsius
We have the formula,
Q = m × c × ΔT
By putting given values in above equation we get,
Q = 240 × 0.452 × (-65)
hence, Q = -7051.2 J
i.e. Q = -7.05 KJ
Hence, 7.05 KJ heat is released when the process takes place.
问题 4：650 KJ 的能量，多少碳可以从 20 摄氏度加热到 100 摄氏度？ （给定：c = 4.184 J / g °C）
解决方案：
Here we are given with,
c = 4.184 J / g degrees C,
q = 650 KJ = 650000 J
ΔT = 100 – 20 = 80 Degrees Celsius
We are asked to find the Mass ( m ) so we have the formula,
Q = m × c × ΔT
the above equation will give us,
m = Q / ( c × ΔT )
by putting given values in above equation, we will get the actual mass of carbon required,
m = 650000 / ( 4.184 × 80 )
m = 1941.9 g
i.e. m = 194 kg
问题 5：当加入 968 J 的能量时，将 60 克物质从 30 °C 加热到 40 °C，其比热容是多少？
解决方案：
It is given in the question that,
m = 60 g
ΔT = 40 – 30 = 10 degrees Celsius
q = 968 Joules
We have to find the Specific heat capacity ( c ) so we have the formula,
Q = m × c × ΔT
the above equation will give us,
c = Q / ( m × ΔT )
by putting given values in the above equation we will get,
c = 968 / ( 50 × 10 )
c = 1.936 J / g °C